Insects and disease

113

"INSECTS

AND

DISEASE."

By

J. S.

C

Elkinqton,

m.d., d.p.h,

Chief Health

Officer of

Tasmania,

(Read

9th

May,

1904.)

Dr.Elkington said:

—

The

_subject

upon

whichIhave been asked to address

you

thiseveningisonewhich has, witbin the pastsixyears,produced aprofound

alter-ation in several branches of science.

Here

in

Tasmania

we

arefortunatelyfree

fromthosescourges of

humanity

which

areasyet

known

todepend whollyfor their

spread

upon

theagencyof insectlife, but

even with us theproblem is not wholly oneofoutsideinterest, as I willendeavor

to

show

youin relation to typhoid fever.

Inthis case,however, the

method

of

con-veyanceispurely a mechanical one, due

toparticles of infectedmatter clinging to

the legs and bodies of flies, and

subse-quently deposited

upon

food material,

whence

it is taken into the alimentary

canal of the victim. It is advisable,

therefore, to clearly distinguish between

thetwo methodsinwhich insects play a

part inspreading disease

amongst

human

beings,whether

—

(a)

As

carryingagentspure

and

simple, theinfectingagentundergoing no change andnot being dependent onthe insect in

any way.

(b)

As

hosts,intermediateor definitive,

the infective organism being dependent

upon

theinsectand undergoing an

extra-corporeal phase of development in its

tissues.

The

first group is of distinct import

ancefromthesanitarian's point of view,

in that it teaches us to guard against

a real danger in times of prevalence

of certain infectious diseases, but the

second one far outweighs it. Into this

second group fall

some

of the chief

scourgesofmankind, in tropical regions

at

any

rate, andof these the greatest is

malaria. Nextto tuberculosis,malariais

probablythe greatestcause of death

and

of ill-healthwith which

mankind

has to

contend.

Some

of

you

have, no doubt,

visited or lived in places where malaria, orevenYellow fever, have claimed iheir

yearlytoll of livesandhealth, have seen

the long white-washed wards

and

wide

verandahs of an Indian

Cantonment

Hospital filled with pasty-faced

fever-strickensoldiers, and have heard dayafter

day the drone of the regimental bands

along thedusty white roads to the

ceme-tery.

To

that

supreme

critic of all our

actions, the

"man

inthestreet,"the proof

oftheconveyanceofmalariabytheagency

of certain species of mosquitoes

was

merely aninterestingpieceof knowledge,

buttothose

who

know

what

the disease

inquestionreallymeans,it

was

a fact of startling importance, presaging the

ap-proachof effective

means

of checking an

enemy

whichintheyear 1897 alone laid

75,821 British soldiers by the heels in

hospital outofatotal strengthof178,000

114

2

Anglo-Saxon civilisation to progress in

places where evensixyearsbacka

new-comer

couldcounthisreasonable

expecta-tion of life on the fingers of one hand.

Tropical medicine has

assumed

such

greatproportions thatitpracticallyforms

the pivot

upon

which turns the whole

guestion of

European

colooisation in the

tropics,and,practicallyspeaking, tropical

medicine is verylargely based

upon

the spreadof disease by insects. There,

free-dom

from malaria practically

means

freedom fromill-health.

The London

and

Liverpool schools have received noble

recognition from

Mr

Chamberlain, and

from hard-headed business

men who

can

see into the future.

Many

a Peter the

Hermit

is leading a crusade against

dis-easewith knowledge, andoftenwithfunds

suppliedin this way,enabling the British racetogeton a stepfurtherwith colonisa-tion.

America

has taken up the

work

withespecially striking results in relation

toYellow fever,

and

ahandfulof workers succeededin clearing

Havana

of thisfell

diseaseinninety days

—

after it

had had

an uninterrupted reign for 140 years.

The

discoveries which have been

made

are,Ithink,but aforetaste ofthose which

are tocome,buttheir histories, with the patient recordofconstanttoil overminute

details, ofthe sacrifice of leisure and

re-creation, and even in

some

casesofthe

iives ofthe workersforthefurthermentof

knowledge^form,to

my

mind, oneofthe

6<)bl3st

romances

everwritten.

The work

of

Manson,

Eoss,

McCallum,

Reid, and

others have revolutionised the

methods

ofresearchinto

human

disease, andthere

islittledoubt that

we

are onthevergeof

further great discoveries in which the

lower formsoflife will be foundto play

anessentialpartindisease affecting

man-kind.

But

this

may

appear tobe

some-thing of a digression fromthe practical

treatment of

my

subject. I trust,

how-ever, that I have

shown you

to

some

smallextent theimportanceofthe results

obtained, and thepossibilitieswhichthey

openup. I will

now

endeavor,withthe

aid ofafew lantern slides, to indicate in

some

measurethe

manner

inwhichthese

discoverieshave been made,the

difficul"-ties against which the workers had to

contend,and the

method

in which they

are practically' applied.

The

classicalexampleis,ofcourse,

sup-pliedbymalaria,orpaludism,asitisoften

termed.

As

Ineedhardly pointout,both

thesetermsindicate the association with

the presence of marshes,

swamps,

and

other wet-soiledlocalities, which has been

forcenturiesnoted inconnectionwiththe

disease, malaria

meaning

merely

"bad

air," andpaludismthe condition induced by marshes.

Now,

about the middle of lastcentury, certainobserversnoted the

presence of certain

brown

and black

granules in great

numbers

inthe blood

and organs of malarial patients,

and

in 1880, . Dr, Laveran, a French

military isurgeoh in Algeria,

made

the

discovery that the black granules, which

are

known

asmelanin, were produced by

the action of

immense numbers

of tiny

parasites living in the red blood

cor-puscles. This

was

a very notable

dis-covery, as it naturally caused enquiry

intothe

manner

in which the parasites gotinto the blood corpuscles, and once

there

how

they produced the fever.

It

was

found that they could not be

grown

on artificial media, even from

bloodwhich

swarmed

with them, and

other discoveries soon

made

it clear that

they represented a distinct form of

animallife, andnotavegetable one, like

the bacteria for instance.

Some

five

yearslateranotherdiscovery

was

made, namely,that these parasites breedin the

blood,andthat the attacks of fever coin-cidewiththebreaking

up

of thecluster of

spores so formed. It

was

also found

that threevarieties at least of the

or-ganismsinfect

men,

and that yet others

were tobe obtained from the blood of

birds, bats, cattle, monkeys,

and

other animals.

(Apicture.was then

shown

ofone form

ofmalarialorganismasitappears under themicroscope inthevarious stages of

development, whichare traceable in the bloodsolongasitretainsitsvitalityafter

removal.)

Under

the microscope, the strange octopus-like creature,

which

represents

thelaststage of development in blood

specimens,would attract your attention

even

more

than in the lantern pictures, sinceits

arms

are in vigorous motion fromthetime they shoot out from the

surface. Ifkeptmoist and

warm

another

strange

phenomenon

occurs.

One

or

113

3

rate itself from the body

and

to shoot vigorouslyaway. Sovigorously, and with such apparentintention doesitdoso,that it often sets the surrounding corpuscles

bobbinglike a

row

ofcorks in a tideway.

For

years a discussion raged over this

phenomenon.

Why

did the organism throwout these

arms

only after leaving thebody?

The

octopus-like creature, or

male

gamete,

was

never seen in blood

immediately after its removal, but

de-velopedfromcertain of theparasites, but notall,ina shorttime

when

kept

warm

andmoist. Ifnotkept

warm

and moist

itdidnot develop. Moreover,

why

was

it that only

some

ofthe amoeloid organisms threwout

arms and

not all ?

And

why

didoneor

more

ofthe

arms

when

thrown

out cast loose from the rest

and

dart

away?

Upon

these

and

similar facts Dr.

Manson,

thenof

Hong

Kong, formulated

in 1894 that fine piece of inductive

reasoningwhichwillalways be associated

with his

name.

Earlier observers

had

fromtimetotime

made

suggestions of a similar nature, but it

was

reserved for

Manson

to formulate from the

known

factsatruescientific hypothesis. Being

a parasite, he argued, the malarial

organism must,like

most

other parasites, live forpart of its existence outside the

human

body in another host.

The

octopus-likemale gamete does not

come

into existence until after leaving the

body, andwhileinthe body the parasite

is incapableofliberating itselfbyits

own

efforts.

No

traces of the parasite could

be found in any of the discharges

of the patient. It

was

there-fore probable that it

was

re-moved

from the body by

some

blood

eatingcreature.

Manson

had

previously

performed

some

brilUant researches into

the conveyance of certain other blood parasitesbymosquitoes,and he

was

led tothe conclusion that

some

distmctkind

ofmosquito

was

the probable agent, and

that themale gamete formedthefirststage

ofthe extra corporeal development of the

parasite inits

new

host. Ineedscarcely

remind you that in all its

main pomts

Manson's hypothesis has been proved

correct.

Then

Captain (now Major) Ross, of

the Indian Medical Service, took upthe

work

ofinvestigating thedevelopmentin

themosquito. It

must

be

remembered

thathe^began with practically no

know*

ledge of the classification, structure,or

habits of mosquitoes, since at that date

(1895) comparatively little

was

known

evento dipterologists, and that

most

of his

work

was

done in an up-country Indian station far from theequipment andfacilitiesofa

modern

laboratory. For two anda-half years he workedon,

dis-secting and examining, cell by cell,

hundreds of mosquitoes without result,

and attempting feeding experiments in vain. Itisnatural that aftersuchan

ex-tended series of negative results he

was

beginningto suspect a flaw in ManFon's

theory.

Then

came

light.

A

peculiar

species of mosquito, which he had once

before noticedin avery malariouslocality

aspossessing dappled wings and laying

boat-shapedeggs,

was

fed

upon

theblood

ofamalarialpatient, and on dissecting it

hefound,forthefirsttime, in itstissues

granules of that coal black substance

which

I mentioned as being called

mela-nin, and as chara'^teristic of malarial

fever,and in its stomach walls certain

ovalcells. These discoveries practically

solved theproblem.

The

dappledwinged

mosquito withboat shaped eggsbelonged

tothegenus Anopheles, andthe oval cells

in thestomach wall were the developing parasitesofremittent malarialfever.

The

rest

was

comparatively easyonce

thesefacts were

known.

The

plague scare

amongst

the natives

made

it necessary for Rossto pursue his

researches into the development of the

organisminthemosquito by

means

of a closely similar organism conveyed by a

Culexto birds, sinceblood feeding

experi-ments

from

human

beings

became

practi-callyimpossible,buthe hadsolved thetwo

greatproblemsinthe

way

ofthe

triumph-ant proof of Manson's hypothesis,

and

thathypothesis

was

provedtobe soundly

founded.

He

had,moreover,distinguished

the true malefactor

—

the Anopheles

mosquito—

fromthe innocent Culex,

and

hisprevious difficulties were to

some

ex-tentexplained by the silent retiring and

unobtrusive habitsof the former genus.

The

differences between Culex and Anopheles are clear enough nowaday?,

butitisproverbiallyeasytobewise after

the event.

We

must

always

rememter

116

4

The

difteirenees aife well exemplified

in the lantern Blides, which I will

now

show

you, and they extend throughtheinsects'hves from begioniog

toend. In thefirstplace they

may

be

readily enough distinguished by the

attitude

assumed

in resting. Anopheles seems alwaystobe endeavoring to stand

on its head, while Culex rests parallel

withthe surfaceina

hunched

up position.

This,ofcourse,is arough test, but after

alittle whileone

becomes

very expert in

distinguishing the

two

in a locality

where Anopheles are present. I

may

statethat Idonot

know

that

any

species of Anopheles existing in Tasmania,

al-though one species has been recently

described as existing in Victoria. I

have not

come

across a

member

ofthe

genus in a large

number

of mosquitoes

lately

examined

inTasmania. All species ofAnopheles arenot capable ofconveying

malaria, a fact which explains the

presence of this genus in non- malarial

localities where cases are from time to

time introduced.

The

female, by the

way,isusually the blood-sucker

amongst

mosquitoes, amealof blood being stated

tobeessential for thefertilisation of the

eggs.

The

male, exceptin thestegomyia

and

afewothers, is an innocent creature,

preferring avegetable diet of fruit juices,

ifindeed hedoes feed atall,and not, like

hispartner,varying it with animal food

extracted in a voracious

and

annoying

manner.

Both

male

and

femalearehere shown,

but, taking the latter first,

you

will note

the length of the palpi

—

as long or

longer than the proboscis. This is in

itself, in the female, a

mark

of the

genus.

The

wings are spottedin both

specimens

—

another fairbut notinfallible

guide-mark to the Anopheles.

Turn-ing to the male, you will note that the palpiare also long, but they are always

long in the male, whether of Anopheles

orCulex. These, however, are clubbed

atthe ends, andin this particularspecies are plumose at the ends.

The

dis-tinguishing

marks

of the male in this

case, as in others, are the large plumose

antennae

—

regularwhiskers, as befits the

sex.

The

adult msects can thus be dis

tinguished,butforcompleteidentification this isnotenough. Variousmarkings of

thelegsand palpi, and the arrangement

ofthewing and body Bcales, are used to

identify species.

The

shape of the eggs

isdifferentinthetwogenera, thoseofthe

Anophelespossessing adistinctboat shape, whileCulex eggsareblunterforthe

most

part.

They

are laid inmassesoffrom 200

to400. These eggsdevelop inthecourse

offrom 12hoursto severaldaysintolarvae,

which are

known

to every boy as the "wigglers,"foundineverytank and pool

in fairly

warm

weather.

Here

we

note

another difference in the genera, the

Anopheleslyingparallel with the surface

owingtothe absenceofthelong breathing

tube,whichenables Culexto

hang

down

fromthe surface

—

the direct opposite of

the position

assumed

byihe adultinsects,

as

you

will recollect.

The

arrangement

ofthehairs

upon

the different segments

enables species to be determined. In a

week

forCulices or

more

forAnopheles

—

thetimevariesconsiderably in these

pro-cesses for different species and under

differentconditions

—

the active wiggling larvaafter develops into a pupa, which

again differsinthe twogenera, and after

a varying period of from two to 10 or

more

daysthe

pupa

casesplits

down

the back, the adultinsect emerges, and after

balancingitselfon the

empty

case for a

whilein ordertodryitswings,flies

away

topropagateitskindinturn.

To

sum

up

the points which I have

endeavored to briefly indicate

we

have

the following facts toconsider wherever malaria is to be dealt with. There is firstly a blood parasite, which is the

cause of the disease,

and

next

come

particular species of a particular genus

of mosquito, whose tissues afford

the only place wherein this parasite

can completeits life cycle. Malaria can

beconveyed from

man

to

man

by

inject-ing blood in which the organism

is, but this need not be considered in

practice. Therefore,intacklingthe

ques-tion ofstampingoutmalaria

we

naturally

attack the mosquito.

The

adult

insect

may

he guarded against by

mos-quito curtains and other measures to

prevent its biting

—

a particularly useful

one by the

way

consists in wearing two

pairs of socks after dark, a thick paii

underneath and a thin pair on top, since

more

people are perhaps infected

at the dining tabic than in almost

any

other way, the Anopheles loving the

dark lurking-places under ihat cheerful

117

5

fumigation with sulphur or

pyrethrum

powder. This, however,is an incomplete way,

and

since the Anopheles breeds

by preference insmallpuddles

and

shal-lowstretches of

weedy

swamp

or slowly

moving

waterfreefromsmallfish, its de-structionisrather an engineeringthan a

medical question.

Much may

bedoneby

pouring kerosene and paraffin on the

surface in the proportion of about

a tablespoonful to every square yard

ofsurface, so as toforma thinfilm,which

clogs the air tubes of the larvae and

pupae, and also kills the adult female

while depositingeggs. This

remedy

will

be found efficient by those of you

who

are worried by the local Culices, which

evinceabloodthirstydisposition in

New-town, atany rate. Culices areespecially

fond of smallcollections of waterin pots

and

tanks—

the saucerofa flowerpotona

window

sill

may

providesufficient

mos-quitoesto

make

the inmates'life a misery, whileanordinary400gallontankwillstock aneighborhood.

The

moralof which, asCaptainCuttle

is recorded to have remarked, lies

in the application thereof. I need not

say

more

than that the notoriously

malarious localities of Sierra

Leone

and Freetown, formerly

known

as the " white

man's

grave" in

West

Africa,

have,comparativelyspeaking,beenturned

into health resorts within two years of

organised effort against mosquitoes, and

that at Ismailia, in Egypt, a previously

notoriously malarial town, the average

number

of cases has been reduced from 2000 to 200 per

annum

by one season's

work. Practically, there were no fresh

cases of malaria once the

work was

got

going,anditis

now

possible to sleepthere

n

safetywithoutamosquitonet, probably

forthefirst timeinthe historyofthetown. Probably everyonehere to-night has been

at one time or another subjected,

un-asked, to thegastronomic attentionsofa

femalemosquito,andthe

method

in which

she performs the operation aftera

pre-liminary song

may

be a

more

or less

in-teresting

memory

onthenext occasionyou

commit

culicide. (The lecturer here de-scribed with a lantern diagram the

manner

in whichthemosquitofeeds).

So far

we

have considered the

cor-poreal life of the malarial organism,

that is, the part of its life-cycle which

is passed in the blood of

man,

with

the single exception of that octopus-like

male gamete which,as Ipointedout,only developes afterleaving the

human

body.

Turning

now

tothe second,or

extra-corpo-realstageofitslifehistory

—

i.e.,that stage

which, as

we

now

know,is passedin the

mosquito

—

we

find that there are really

two forms ofcell developed

amongst

the

organisms, one the octopus-like

male

gamete, the other a spherical cell.

Now,

itisonly

when

the Anophelestakes

inbloodcontaining parasites at, orabout

this particularstageofdevelopment, that it becomes infected with malaria, and

capableof conveying it to fresh

human

beings, a fact which accentuates the im-portance of protecting malarial patients

by

means

of mosquito nets wherever

possible.

Having

taken in the blood,

however, by

means

of the complicated sucking apparatus, which I have de-scribed, the gametes develop into the

sexual form,

and

the true function of

the male's loose

arm

becomes evident

sinceitjoins with a sphericalcellformed

byanother gamete, and impregnates it.

It

was

thefemale gamete whichit

was

in

searchof

when

it cast loose.

Then

the fer-tilised cellorzygote attachesitself tothe

stomach wall of the mosquito, boresits

way

through,growsgreatlyinsize,and

pro-ceeds to divide into a great

number

of

tiny cells. It becomes a cyst,

which

finally bursts and sets free the tiny structureswithin,which have

meanwhile

developed, andthese, set free inthe body

cavity,settleingreatpart in the

veneno-salivary gland at the base of the

pro-boscis, as tiny thread-like bodies,termed

zygotoblasts or sporozoits.

The

gland

in which they lie is that

which

secretes the irritating substance of

which

we

all

know

the effects in

con-nection with a mosquito bite, and ia

the caseof an infected Anopheles, they

are injectedwith this secretion along the

proboscis.

The

process for their

de-velopmentinthemosquitotakes about12

days fromthe date ofthemealofinfected

blood, before the threadlike bodies

appearinthe gland. Afterinjection,along

withthe secretion,the sporozoitsinfectthe bloodcorpuscles, growrapidly,

and

break

up

intosporeswhich in time infect fresh corpuscles, until a huge

swarm

of

para-sites,with alife cycle of either 48or72

hours is produced in the blood.

The

118

6

the fever-producing substances which

produce the

symptoms

ot malaria.

I have given but a rough outline of

the process,but it willserveto

show

the

main

points of this complicated

life-history.

It will be seen, however, that the

process of infection by Anopheles is

7iot a simple mechanical transference of

blood from infected to healthy people,

that

human

cases can only infect the

Anopheles at a particular stage in the historyoftheparasite in their blood, and,

particularly, that an Anopheles which

does not bear in its poison gland the

thread-likesporesof malariarepresenting the laststageofthe extra corporeal cycle ofthe organism passed wiihin its body

is asharmless asan

empty

gun.

It will be readily understood that the

work

ofRoss andhiscolleagues in other partsoftheworld gave ahuge impetus to

the studyofinsect-borne diseases, and it

has borne noble fruit.

The

connection

of Yellow fever with

Havana

is almost

proverbial,andtheaverage deathsfromthat disease alonewereover800in each year. Ithas beenestimated that

some

36,000 persons diedofitfrom 1853to1900,

and

in addition the islandof

Cuba

and

ad-joining parts of Brazil acted as an

endemicfocus, whence,atvarious times, the disease spreadto

many

parts of the world,mcluding Spain,Africa, and even England. In 1897, there were 6000

deaths fromit in Cuba. In Bio from

1868-1897, 41,726 deaths.

The

Southern

StatesofAmerica have also experienced dreadful visitations from it. "Yellow

Jack"

_was

_{a dreaded}

_name

_throughout

South America, the

West

Indies, andthe

Southern United States.

With

the open-ingofthe

Panama

Canal and the

conse-quent shortening of the voyage from

theselocalities toAustraliaand the

Far

East,itisvery probable thatYellowfever

will

some

daybeintroducedto this side of

the world. Threeor fouryearsago,

Aus-tralian sanitarians regarded this

possi-bilitywith

awe

and

foreboding; to-day

theylook

upon

itcertainly as a serious

problem,butas one which

would

give

comparativelylittle trouble

compared

to

the introductionofa fewcases of small-pox. I willendeavortoexplain

how

this

remarkable change in views has

come

about.

In1900,after the occupation of

Cuba

by the

American

forces, a commission

of four medical

men

was

appointed

toinvestigatethealarming prevalence of

Yellowfever there,and to devise, if

pos-sible,

means

for combating ifc»

A

great

deal ofwork

had

been done by various observersinseekingforthecause of it ;

butnothing

was

accurately

known,

and

theearlyworkofthecommission, devoted

to the examination of the blood

and

tissues of patients,only served todisprove

previoustheories, and not to discover

anything new. Absolutely nothing

was

apparent in the blood or

elsewhere as a result of the

most

careful bacteriological research.

The

nextstep

was

toenquireintoits

pro-pagation.

Now,

one attack of |Yellow

fever safeguards the patient against a

second one,producinga condition of

im-munity, asitiscalled. It

had

been noted

that patients could be safelynursedby

non-immune

nurses, and that patieats dischargedafter anattack did not cause fresh outbreaks of diseasein those

non-immune

persons they

came

in contact

with. It

was

obviously, therefore, not contagious,nordid the infection persist ina virulent form inthe discharges.

The

next matter

was

to settle the disputed questionofthemfectivityofclothing and beddingwhich

had

been in contact with

patients.

To

this end, a small hut

was

erected, verydeficientin ventilation, and

speciallyconstructedso as to maintain a hot moist atmosphere within it.

The

windows

and doors

—

please note this

—

were carefully screened with fine wire gauze,ofsmall

enough

gauge to prevent the entryofmosquitoes. Into thishouse were piled sheets, blankets, mattresses,

pillows, and clothing of all description

from the Yellow fever hospitals,

and

amongt

them, in the beds

and

covered

with the bedding of patients,

who

had

justdied ofYellow fever, even wearing

the clothing taken from the bodies, and

stainedwith the bloodand " blackvomit"

discharge, seven

young

non-immune

Americans, including

members

of the

commission,slept twoorthreeat a time

foratotalperiodof sixty-three nights in

all.

Not

a single one of these

non-im-mune men

developed the disease. This

effectuallyprovedthatclothing and

bed-ding alone were incapable of conveying

1

The

nextstep

was

to enquire into the questionof insecttransmission.

A

com-mon

domestic mosquito of

Cuba

is the

Stegomyia fasciata, a strikingly

marked

insect ofvoracious habits, and suspicion

had

already attached itselfto it.

Both

sexesof itsuckblood, and itis probably

the

most

widely distributed of all

mos-quitoes, being found practically all

throughthe tropical world, and also in

many

sub-tropical localities. Twelve

plucky

young

Americans, two of

whom

had

taken part in theexperiments with

clothing previouslydescribed,v©lunteered

as subjects. Allthese were living under

exactlythe

same

conditions as scores of

other

non-immune

residentsofthe camp,

and

the

two

I have mentioned

had

been

free for30 days from their self-imposed

imprisonmentinthe infected clothes hut.

All were deliberately subjected to the

bites ofspecimensof thismosquito,which

had

been previously allowedto bite early cases of Yellowfever. After periods of

from three and a-half to sixdaysfrom

bemg

bitten, ailexcept

two

developedtrue attacksofYellow fever.

One

experiment in the batch

was

of especial interest.

A

newly- erected

building

was

divided into two parts

by fine-wire screens. In one partwere

let loose 15 infected mosquitoes, and

in theother half, separated only by a

wire screen,

two

non-immune

men

lived

and

slept.

The

volunteer subject entered the partin which werethe mosquitoeson

threeoccasionsof about twenty minutes

each, and

was

freely bitten on

eachoccasion.

On

the fourth day afcer

hisvisithedeveloped a severe attack of

Yellowfever, while thetwo

men

inthe partontheotheL'side ofthe gauze screen continuedfor 18 nights to sleep in and

breathe the

common

atmosphere of the

room

without any ill effect. This

was

prettyfairevidenceevenfor thescientific

mind

that the mosquitoes were the con-veyors ofthedisease, and subsequent

ex-periments provedthis to be so beyondall

doubt. \

The

organism which is the actual

cause of Yellow fever has not been

discovered, probably because it is so

minute as to escape the highest

powers of the microscope. This is the

more

likely since, as in the case of

malaria, the injection of blood from an

acute casewillproduce the disease in a

non-immune

subject,but, unlike malaria, thebloodremainssimilarly infective after

ithas been passed throughthefinest

Berk-feld filter, thus proving the extreme

minutenessoftheorganism, since such a

filterwill stopthe smallestof

known

bac-teria. Thatit IS not a bacteria, but an organism

more

ofthetypeofthe malarial

organismisvery probable from the fact

that theinfection cannot be obtained by

the mosquitofrom the blood, except in

the first three days of illness, and the

mosquitodoes not

become

capableof

in-fectingfreshpersonsuntil12daysor

more

afterithasfed

upon

a case at this

par-ticular stage.

The

factthat an outbreak

ofYellowfevertook a]Jfortnight or so to light upafter the introduction of a case

into alocality

had

longbeen noticed,

and

this explained thereasonthereof.

Having

obtained these facts, the

battle against Yellow fever

assumed

a

new

aspect.

Huge

sums

had

been spent during previous years in

indiscriminate disinfecting of articles

which

had

been in contact with

Yel-low fever patients—quite harmless as

we

now know

—

and in other expensive

measures, absolutely without avail.

The

campaign was

now

begun by awholesale

destruction of mosquitoes and their

breeding places in

swamps

and puddles,

and

by the careful screening of

Yellow fever patients for the three

firstdangerousdays,from mosquito bite inorderto prevent the loadingup offresh

wingedcarriers of disease. Quarantine

of cases

was

givenup

—

it

had

been

en-forced for

many

years without avail,

—

and

the only quarantine order

was

that

of the mosquito net.

The

resuU

was

amazing. Inninetydays from the

com-mencement

of operations

Havana

was

freed from Yellow feverfor thefirsttime

in 140 years. Repeated introductions

from without, where the measure?

were not in force and where Yellow

fever

was

raging unchecked, as it

had

donefor decades pastatthatseason,were promptly stampedout, and the fact

was

realised that although every effort

known

to sanitary science

had

been

putforthwithout avail

up

toMarch,1901, thechangeeffected in the measures by

the recognition of the mosquito as

the carrying agent

had

enabled the

au-thorities to obtain in 13 weeks a

120

8

more

than ten times as

many

years.

Epidemics of Yellow fever, thanks to

those able

and

heroic

American

doctors,

area thingof thepast in civilised

coun-tries,and the

name

of Dr. Lazear,

who

contracted the diseaseand died of it dur-ing the enquiry,anotherof the

numerous

martyrs

whom

science has claimed, will

always occupy an honored place in

the scrollofhygienic fame.

I willnotdo

more

thangivetheoutlines ofcertain other

work

which has been car-ried out in this direction.

The

recent investigationswhich have been

made

into

Sleeping sickness

—

that strange disease

which

overwhelms

its victims with

an increasing languor and somnolence

until they perishof an apparentfailure

of all the faculties—has received a

good deal of notice in the press.

The

researches of Dr. Castellani, Dr.

Bruce,and others, go to

show

that the

conveyingagentin thiscaseisthe Tsetse

fly, previously well

known

forthe fatal effects of its bite on horses and

cattle, but only lately

known

to be

connected with disease in

human

beings.

The

mortalityfrom Sleeping sickness in certainpartsofAfricahas been alarming,

and whole districts have been decimated byitsravages.

The

causativeagentappears

tobeaformoforganism

known

as a

try-panosome, or,as itshould be

more

cor-retly termed, a herpetomonas, which

livesintheblood andfluids ofthe victim.

Further

work

yet requires to be done,

but little doubt remainsthat the curious

andfatal diseasewhich almost invariably

kills its victim, and which has been

known

asa scourge ofWesternAfricafor

over100 years, has been brought within thegrasp of science,andwillbedealtwith

as effectually asmalariaand Yellow fever

arebeingdealtwith.

Whether

the

organ-ism undergoes a part of its life history within the Tsetse fly's body is not yet

definitely settled by actual observation,

butitseemsprobable thatitdoesso.

The

presenceofplagueinAustralia ren-dersits causation andspread amatterof

perhaps

more

acuteinterestthantheexotic diseases with whichIhave beendealing.

The

causativeagent is,of cour«e, a

well-known

bacillus, but the

means

wherebyit

ishabituallyconveyed to

human

beingsto infect

them

have beenwidely discussed

amongst scientific

men,

and there is a

growing tendency onthe partofthose

who

have to deal with this diseasetoregard

more

and

more

suspiciously a certain

insect which already possesses an evil

reputation in domestic sanitation; I

allude to theflea.

A

good deal of

work

has been done on these insects of late

years, andit

may

surprise you to learn that

more

than 130 distinct species of flea

have been classified, and that

new

ones

are

coming

into knowledge

month

after

month.

Eemarkable

differences are

found in their structure and habits,

but in the particular connection with which

we

have to consider

them

to-night, one genus attains prominence

—

the true pulicesof which thetypeisthe

human

flea.

Now,

in considering the histories of

plagueoutbreaks in different partsof the world, one is at once struckbya curious

fact, namely, that while preceding or

simultaneous outbreaks of the disease

amongst rats and other small rodents,

haveformed a

marked

feature ofplague

epidemics in the East, nothing of the

kind has attracted the attention of the chroniclersof the

European

outbreaks of

themiddleages,

and

of the greatplague

of

London

in 1665.

The phenomenon

is

strikingenoughto attract the attention of

any

ordinary observerwhereitoccurs,and

asignificant allusion toitistobefoundin

theFirst

Book

ofSamuel. Other and even

more

definite allusions occur scattered

through Eastern literature from a very

early date,so that it appears tohave

attracted attention, and could scarcely

haveescaped the acute chroniclers

who

have left us such a vivid picture of

medieval andseventeenth century plague.

Itisneedlesstoremind youthatthe

same

phenemenon

occurs in Australia andin

South Africa at the present day.

The

domesticrats of the East, of Australia,

and SouthAfricaareverysimilar tothose

ofEurope, andall readilycontract plague

when

infected experimentally.

But

on examination of their parasitesa curious

fact

was

noticed in that the

common

rat fleas of Australia, of India, andofSouth

Africaare widely different insects from

the

common

rat flea of Europe,bothin

structure and habits.

They

are very

closely allied to the

human

flea, so

closely, infact, that iheir relative identi-ficationis a matter of extreme difficulty,

without previous acquaintance with the

varieties,will,on ooeasiots, aoli as para^

sites to

man, and

will bite vigorously.

The

common

European

rat flea, or

cera-tophyllusfa8ciatu8,ontheotuerhand,differs

inimportant structural respects,and can, only with greatdifficulty,beinducedtofeed

on

man.

Does

it not appear to be

something

more

than a coincidence thatwhere

we

find a ratflea closely allied to the

human

flea,there

we

have plague

amongst ratsas well as

man,

whereas in

places where the ratflea varies widely

from the

human

flea the occurrence of

epidemicrat-plagueis,atany rate,notan important

phenomenon.

The

theory

was

for long held that the plague baccilus lives and multiplies in

the earth, but out of innumerable

at-tempts at its recovery from the earth-floors of plague-infectedEastern hovels,

iiot a single success has resulted under

natural conditions.

As

in the case of

Yellow fever,it hasbeen stated to have beenspread byinfectedclothing. It

pro-bablyis sospread,but in

some

other

way

thanthe

mere

bacillaryinfection of such

clothing, for theplaguebacillushas never beenrecoveredfrominfectedclothing,

un-less in cases of grossexperimental

con-tamination. Plague is particularly a

disease oflocality, and especiallytendsto infect those

who

sleep insucha locality,

sparing those

who move

activelyabout

duringtheir visits andsleep or rest

else-where. Itchooses dark, squalid, vermin-hauntedvicinities, andavoids to a great extentairy, well-litplaces.

The

experience

of plague hospitals is a curious one, for whilst, inancient days,evena short visit

toa pest-house

was

attended with great danger,it is rare for an attendant in a

modern

plague-hospital to be attacked.

Thatthe diseaseitselfhasnot variedfrom

ancienttimes

we

know

from contemporary

records, but whilst the

modern

hospital isespecially builtwitha viewto cleanli-ness, lightand airiness, the ancient

hos-pitals

seem

to have

combmed

all the hygienicoffences ofitsday.

To

sum

up

we

havecertain facts which

seem

to pointin a particular direction. Plague does not,

apparently, thrive in the soil,nor is it

known

toenterthebodywith thefood, in

human

cases at least. In the ordinary

form—

excluding plague

pneumonia—

it

enters by the skin, and in thebubonic

form itcertainlyentersinthe lymphatic

area drained by the first affected

gland, Wfttei? play* no parfc ia

its dissemination. _{Meteorological}

fac-tors have no influence except in

one significant indirect instance—that

inIndia the plague mortalityisnoticed to rise materially after a cold night, or a

heavyfallof dew, and especially after rains during the dry season,

whereby

peopleare driveninto their

homes

instead

ofsleeping outside asusual. Clothing is

apparently capable of conveying the

disease; but thebacillusis not

recover-ablefromclothing.

And

lastly, a

marked

associationof the rat with plague

out-breaks has been noticed since remote

periods inpartsoftheworld where the

rat flea closelyresemblesthe

human

flea,

and no such association where the rat

fleashows,at anyrate atthe present day,

widestructural differencesfromthe

human

flea.

This circumstantial evidenca appears

to incriminate the insect, but

circum-staniial evidence is not enough. Dr.

Simond, in 1897,

had

proved that

by infecting into a

mouse

broth in

which fleas had been emulsified, after

having fed on a _{plague-stricken animal,}

plague could be produced in that

mouse. This experiment

was

success-fullyrepeatedonseveral occasionsby

my-self

and

others,

and

the results obtained

were sufficiently encouraging toinduce

further work. Fleas of certain species

werefedonplague-infected animals,

and

aftervarying periods of starvation were

allowedto bitehealthyrats under

condi-tions which excluded any probability qf

the disease being contracted otherwise

than byfleabite. These rats died of un-deniable plague,andit

was

foundthat the

fleacouldconveyplaguein this.

way

up

to at leastthreedaysafteramealofinfected blood. Thereisno reason

why

itshould

notdoso forvery

much

longer. It

was

foundthatinone case, at least, this

con-veyancefrom rat to rat

was

effected by a

human

flea. In other cases the rat-pulex

was

used.

A

further experiment

was

performedin whichplague

was

conveyed from a

human

beingtotworatsby

means

offleas. Thislatter experiment

was

of especial significance, since, on

subse-quent examination, it

was

found that while threeof the insects used for the

purpose werethe pulex palladus of rats,

one

was

a pulex irritans, or

human

flea.

1*

10

meal of infected blood, and examining

them

undera microscope, large

numbers

of bacilli indistinguishable from plague

bacilli intheirmicroscopical

characteris-tics,were foundin apeculiarorgan form-ing a partoftheoesophagus;whileinfleas,

which

had

beenfedon healthy blood, no suchbacilli werefound.

The work which

I have alludedto

was

performedinIndiacertainly with allthe

facilities ofone of the biggest bacterio-logicallaboraties in theworld,butwithout

much

assistancefrom special literatureon

thesubject, forthe verygood reason that

none existed to indicate the technical

difficultiestobeovercome.

As you

may

readilyimagine, these were considerable,

for tosecuresuch an active and minute

creatureasthefleainsuch a

manner

as toallowitto feed, and yettoensure that itshould not get

away

afteritsdangerous

meal

of plague infected blood,

was

an

operationofnolittle difficulty, and also of

some

danger, bothtothe experimenter and to hisneighbors. After a good deal

of trial, the device

was

hit

upon

of

confining

them

in a glass tube

—

an

ordinarytesttube

was

used

—

and

covering

theopenend with a cap of finegauze,

through the

meshes

of whichtheflea

could protrudeitsproboscis,but could not escape. This difficulty overcome, it

was

necessary to find

some

distinguishing

marks

by which to identify the species

used. This

was

affected after a

time, andthefew species employed were

readily

enough

identified.

The

habitualspread of plague by fleas ofthetypeIhave

shown you

isnot to be

takenasa universally accepted fact, but

strong evidence exists that this will

eventually prove to be, at anyrate, an importantfactor in epidemics.

You

may

recollectanallusion

made

tothe

work

ofDr.,

now

Sir,Patrick

Manson,

m

relation to certain blood

worms,

which

are fairly

common

amongst

human

beings

inthe tropics.

No

lessthanfivedifi'erent

kindsof these

worms

are

known,

and

one inparticular, the filaria nocturna, so called because it is only found in the blood during sleeping hours, is of par-ticular interest, in thatit causesa good

deal of illness by blocking up the

deli-cate lymphatic ducts, andisalmost cer-tainly in this

way

the cause of that strange disease elephantiasis. Filariae

occur amongst other tropical and

subtropical places in Queensland and

Northern Australia to about the

lati-tude of Brisbane, and the

manner

in

which the filaria noctutna completes

its Hfe cycle is of interest.

The

parent female

worm

is

some

4in

long, and lives in

some

part of the

lymphaticsystem, in

company

withthe

male, which is slenderer and smaller.

There she produces a great

number

of

embryos, which

make

iheir

way

intothe

blood, and live during, the day in the

vessels of the internal organs.

About

5.

or6 p.m.,ininfectedpersons

who

dotheir

sleeping at night,they begin to appearin

gradually increasing

numbers

in the

peri-pheralcirculation, or vessels in the ex-tremities and at the surface, increasing tillaboutmidnight, and thendecreasing,

untilabout8 or 9 in the morningthey

have alldisappearedfrom the peripheral blood for the day. It is acuriousfact

that in infected persons,

who

dotheir

sleeping during the day and remain

awake

at night, the embryos reverse the timeof their appearance, and turn up

during the day in the peripheralvessels,

disappearing at night.

Each embryo

is

about l*80in long, and is containedin a loosesheath or sack

somewhat

longer

thanitself.

They

are very active organ-isms, andwriggle about strongly,but the

sheathkeeps

them

from changingtheir

position

much

; in other words, theyare

not locomotory.

The

function of

the sheath is to keep

them

from

using the strong boring apparatus which theyhave ontheir heads, and so

escap-ing from the blood vessels.

As

I have

said, they appear in the surface blood

vessels at night as a rule, and this is obviouslyfortheconvenience of the

con-veyingmosquito,which principally feeds atnight, and into

whose

stomach

some

ofthe

embryos

are takenwith the meal

ofblood.

The

ordinarymosquito so

act-ingisaspecies ofCulex,andinitsstomach

the blood becomes

much

thickerby

co-agulation.

The embryo

within thesheath is thus able to get a purchaseonthe

sheath, and eventually to

ram

its

way

outthrough one end by butting

vigor-ously while the sheath is stuck fast in

the thickened blood. Of course, while

theblcodwasfluid in the

human

host's vessels, it could not do this because the

sheath could only be

bumped

along by

11

Onee loose it bores through the

stomach wallof the mosquito, gets into

the muscles, and changes its formvery

considerably. Finally, after

some

16

days,

h

appears in the head and

pro-boscis,notinthepoison glandsas didthe

sporozoites of malaria, but coiled

up

under the pharynx and in that part of

the proboscis

known

asthe labium.

They

can remainherefor an indefinite period

awaiiing the chance to pass into the

tissues of a warm-blooded vertebrate

when

ibemosquitolakesitsnext mealof blood. Apparentlytheycandiscriminate,

forno

amount

offeeding oftheinsect on banauas or other fruit (upon which it

ordinarily subsists, will cause

them

to

come

out.

When,

however, the insect

ticks its proboscis into a

human

being thefiiariaefind their

way

througha

weak

spot which exists in the labium where

the labellae splay out, and pass along thetrackoftheproboscis into thetissues of the victim.

The

filariae apparently

endeavor to emerge in pairs, male and

female,andupon establishingthemselves

intbetissues of iheir

new

host, they set

up

housekeeping, and begin again the

process of which

we

know

the results

undfir ibe

name

of the filariasis, be it

€leph**Diiasisorany ofthe other peculiar

conditions by which

we

recognise the

blockios of the lymphatic channels by

their offspring.

I trust that I have been able to give

some

indication of the very

promi-nent part played by certam insects

in certain exotic diseases, but there is

little doubt that they also assist in

disseminating others which are with

us already.

The

ordinary house fly isubiquitousinitshabitsand unpleasant

in his historyand associations. That it

is capable of conveying on its feet

and body the germs of disease under

experimental conditionshas been

demon-strated and thatitfrequently does so in

natureis freelyadmitted. In a

more

en-lightened age the housewife will regard

flies wirb the

same

horror and disgust as

she

now

regards bugs and fleas,

and most thinking folk will even

now

cordiallyagreewith herintheoryifnotin

practice.

From

the time inwhich it is

engendered in a heapof

manure

to the

fatefulhour in which it

commits

suicide inthe milkjug,or perishes inthe sticky

recessesofthe

summer

butterpat,thelife

of the average fly ispassed amidst

more

than questionable surroundings. Its

ubiquity renders the whereabouts of its

last alightingplace a subjectfor

uncanny

speculation

when

it settles on an article of food, and I have little doubt that

domestic flies, so-called, are responsible

for afairproportionofcasesof

communi-cabledisease, especiallyperhapsoftyphoid

fever. Itspossibilities, however, do not

end there; exanthematous diseases,

especially perhaps smallpox,

may

be

readilyconceivedasspreadbythismeans.

I confessin fact toadoubtas to whether

therealcauseofthe aerial convection of

smallpoxisnot partly or wholly due to insectlife.

Many

pointsin the available

evidence

upon

itappeartorender such a

hypothesistenable,and

£y

proof door

and

windows

coverings should certainly form

a partofthefurnishing of isolation

hos-pitals for this disease.

The

possibilities of this line of research

are, however,boundless, and in time to

come

the labors of tbe biologist

and

naturalist will

become

of luore and

more

importance fromthestandpointof

human

disease.

The

extension of research in

connection with cancer in the lower

animalshas resultedinitsdiscovery in a

large

number

of creatures,includingeven

fish. Similarresultshave been obtained

with the tubercle bacillus,

whose

range

appearstobepracticallyuniversal.

The

diseases ofwhichIhave spoken tonight

do notby

any

means

include all those

inrespect ofwhichinsectsare

known,

or

suspected,toplayan important or

essen-tialpart, but theywill serve as illustra-tions ofthe pioneering

work

which has

been, andis dailybeingdone by

many

in-vestigators in differentparts of theworld

for the benefit of

humanity

and the

advancement

ofcivilisation.

The

lecture

was

illustratedby lantern