Experimental design

Study animals originated from four different farms (Farm A to D) (refer to Chapter 3, section 3.2.2, page 47 for details).

PMWS-free study animals

The study animals diagnosed as free of PMWS were sourced from two properties (Farm A and B) with no history of PMWS infection. Pigs free of PCV2 were obtained from Farm A, a herd comprised of pigs of an archaic genotype (Auckland Island pigs). This herd is free of the most common and exotic pig pathogens, including PCV2, porcine parvovirus (PPV), Aujeszky’s disease (AujD) and porcine reproductive and respiratory syndrome virus (PRRSV). Pigs obtained from Farm B, a high health status, commercial New Zealand piggery, were known to be endemically infected with PCV2, but free of other pathogens (PRRSV, PPV, and AujD). The PMWS-free study animals did not fulfil the animal-level criteria listed above and did not show clinical signs consistent with PMWS and were therefore deemed free of the disease.

PMWS-affected study animals

PMWS-affected study animals were sourced from two properties (Farm C and D) which had previously been confirmed to have PMWS and fulfilled the above criteria, with recent and historical post weaning mortality rates regularly in excess of 20%. The study pigs were diagnosed as PMWS-positive based on clinical signs of wasting with or without dyspnoea.

Study design

Table 4.1: Summary of experimental groups of a natural transmission study on PMWS outlining origin, number of pigs per group, age (weeks) and PCV2 and PMWS status, at the commencement of the experiment.

Experimental group Farm of origin N Age (weeks) PCV2 status PMWS

status Study housing

1 A1 _{6 4 – – Outdoor shelter}2

2 A 3 13 – – Outdoor shelter

3 B3 _{10 4 + – Outdoor shelter}

4 B 10 12 + – Outdoor shelter

5 B 10 4 + – Outdoor shelter

6 B 6 4 + – Commercial pig farm

7a A 6 4 – – Indoor housing4

7b B 6 4 + – Indoor housing

Exposure C_D5 5 4 ₁₂ 8 + ₊ + ₊ Outdoor shelter

1 _{Private research herd, archaic genotype, fully-enclosed housing.}

2 _{Purpose-built outdoor research facility at Massey University, Palmerston North, New Zealand,}

located on a grazing paddock previously used for ruminants only. Shelter was made of plywood and straw bales with separate pens for Groups 1 to 5.

3 _{Commercial piggery, modern genotype, fully-enclosed housing.}

4 _{A separate room on a Massey University research farm without any other pigs.} 5 _{Commercial piggeries, composite genetic mix, outdoor and indoor housing.}

– = negative + = positive

The exposure group (PMWS-affected pigs) consisted of 16 weaners with typical clinical signs of wasting from Farm C and D serving as a direct source of PMWS to healthy pigs via direct transmission. Pigs of the exposure group were mixed with healthy pigs of Groups 1 to 4 for up to 56 days, with each group housed in separate pens. The PMWS- affected weaners were initially stratified by farm of origin and randomly allocated to groups, ensuring that pigs from each PMWS-affected farm were represented per group. These exposure pigs were then rotated between Group 1 to 4 after the first week in an attempt to minimise any variation in shedding between PMWS-affected pigs within groups. Group 5 was not directly exposed to PMWS-affected pigs but was housed 2 metres adjacent to Groups 1 to 4 in a separate pen, serving as an on-site negative control group. Group 6 remained on the source farm (Farm B) for the duration of the exposure period and served as an off-site control group.

Group 7a and 7b comprised of six pigs from each of Farms A and B, which were held in the same pen for a period of 81 days. These pigs were exposed to a 12-week-old pig from Farm B and to faeces collected from 12-week-old pigs from Farm B. Both groups acted as internal study comparisons for Groups 1 and 3.

Animals of Groups 1 and 7a, which were at the at-risk age for PMWS (4 weeks), were PCV2-negative on day 0 of the study and were assumed to become infected with both PMWS and PCV2, respectively, during the study period. Pigs of Groups 3 and 7b, which were at the same at-risk age (4 weeks), were PCV2-positive on day 0 of the study but it was expected that only Group 3 would become infected with PMWS after exposure to PMWS-affected pigs.

Two unforeseen deaths occurred within the first week of the study which were not related to PMWS. One pig (ID 22) had to be euthanased on day 3 after developing acute meningitis, leaving 9 animals in Group 3, and one pig (ID 6) died of a blood sampling accident on day 5, leaving 5 animals in Group 1. Both pigs were excluded from any further investigation.

Data collection

All study animals, with the exception of the Group 6 pigs, were observed twice daily and rectal temperatures and respiratory and heart rates recorded once daily. Rectal temperatures and heart rates were measured during the morning feeding. Respiratory rates were recorded while animals were resting in the early afternoon. Pigs were assessed and given a body condition score based on the amount of fat and/or muscle covering, after first signs of weight loss became apparent, with scores recorded at three day intervals. All healthy study animals had a body condition score of 3.5 on day 0 of the study. A body condition score rating system of 1 (poor) to 5 (fat), according to the New Zealand Animal Welfare Code 2005 for pigs (National Animal Welfare Advisory Committee 2005) (Appendix I) was utilised. A pig assessed to have a body condition score of <3 was diagnosed as wasting.

Pigs with an elevated temperature (≥40˚C) for at least two consecutive days prior to their first signs of weight loss, with or without development of scruffy hair coats, were defined as clinical cases of PMWS.

Clinical outcomes

The occurrence of clinical signs indicative of PMWS is outlined in Chapter 3, section 3.3.1, page 56. Table 4.2 illustrates the groups showing clinical evidence of PMWS during the study period (Groups 1, 3 and 5). Case fatalities were observed in Group 1 and 3. There was no evidence of PMWS in any of the other groups.

Table 4.2: Summary of clinical PMWS cases and case fatalities, stratified by experimental group, in a natural transmission study of PMWS. Groups 1 to 4 were exposed to PMWS- affected pigs, while Groups 7a and 7b had exposure to PCV2-contaminated faeces and a PCV2- positive pig from a PMWS-free herd. Groups 5 and 6 served as on-site and off-site study controls, respectively. Experimental group Clinical PMWS cases/number of animals in group Characteristics of case development Number of case fatalities Study day of case fatality occurrence

1a _{5/5 Fast and fulminant 5/5 18, 21, 22}

2b 0/3 - 0/3 -

3c _{6/9 Slow and more varied 3/6 21, 33, 44}

4d 0/10 - 0/10 -

5e _{7/10 Moderate and uniform 0/7 -}

6f 0/6 - 0/6 -

7ag _{0/6 - 0/6 -}

7bh _{0/6 - 0/6 -}

a _{PCV2-negative, PMWS-negative, 4-week-old, directly exposed to PMWS-affected pigs.} b _{PCV2-negative, PMWS-negative, 13-week-old, directly exposed to PMWS-affected pigs.} c _{PCV2-positive, PMWS-negative, 4-week-old, directly exposed to PMWS-affected pigs.} d _{PCV2-positive, PMWS-negative, 12-week-old, directly exposed to PMWS-affected pigs.} e _{PCV2-positive, PMWS-negative, 4-week-old, on-site control group.}

f _{PCV2-positive, PMWS-negative, 4-week-old, off-site control group.}

g _{PCV2-negative, PMWS-negative, 4-week-old, exposed to PCV2-positive pig and faeces.} h _{PCV2-positive, PMWS-negative, 4-week-old, exposed to PCV2-positive pig and faeces.}