Bacterial Pathogens

A number of pathogenic bacteria have been implicated in seafood-borne dis-eases. These include Salmonella spp., Shigella spp., pathogenic E. coli and Campylobacter spp., Vibrio cholerae, V. parahaemolyticus, Aeromonas spp., Plesiomonas spp., Yersinia enterocolitica, Clostridium botulinum, and Listeria monocytogenes. The leading cause of food-borne illness during the last few

TABLE2.4

Pathogenic Bacteria Indigenous to the Aquatic Environment and Naturally Present on Fish

Pathogen Primary habitat Carrier seafood Quantitative level Clostridium

Source: Adapted from Lee, J.S. and Hilderbrand, S. Jr., Hazard analysis and critical control point applications to the seafood industry. Oregon Sea Grant, ORESU-H-92-001, Corvalis, OR.

years was salmonellosis followed by shigellosis, staphylococcal intoxication, and gastroenteritis.⁸⁷ Salmonella spp. including S. paratyphi and S. enteritidis have been detected in processed shrimp and bivalves.⁹⁰ Shellfish have been a common carrier of this pathogen. Others included fecal coliforms, S. aureus and V. parahaemolyticus in addition to Norwalk virus and Hepatitis A virus.⁹¹ Many seafood-importing countries do not permit the presence of the organisms in imported consignments.⁹²The major pathogens of concern are listed in Table 2.4.

Presence of Salmonella spp. in seafood indicates contamination with sewage.

There is a higher prevalence of Salmonella in tropical than in temperate waters, although seasonal variations occur. E. coli strains that colonize the tract are generally harmless. However there are at least four types of pathogenic E. coli strains. These can be enteropathogenic, enterotoxigenic, enteroinvasive, and enterohemorrhagic strains. E. coli O157:H7 is both enterohemorrhagic and cytoxin producing strain and hence, hazardous. Where animal manure, particularly bovine

manure, is used as pond fertilizer in aquaculture, there is a risk that patho-genic strains of E. coli O157:H7 may be present in pond water. Shigella spp.

have also been isolated in aquacultured systems. Poultry guts invariably harbor Campylobacter spp. and hence use of poultry-based manure may pose hazards in inland and coastal aquaculture. Nevertheless, the risk associated with consumption of cultured fish infested with this bacterium is low.

Currently, more than ten species of Vibrio are known to be involved in human infections acquired by consumption of contaminated food and water.

These organisms being salt tolerant, occur in marine and estuarine waters, while V. cholerae and V. mimicus also occur in freshwater. All members of this group show an increase in abundance in warmer waters and an apparent reduction in numbers during cooler months. Some Vibrio spp. are both human and fish pathogens. The diseases associated with Vibrio spp. are characterized by gast-roenteritic symptoms varying from mild diarrhea to cholera. The hazards are associated with consumption of raw fishery products, particularly farmed finfish and crustaceans harboring V. cholerae and V. parahaemolyticus and they have been major causes of gastroenteritis in Japan.⁹³ Plesiomonas spp. and A. hydrophila, which are common in estuarine waters, have also been isolated from fishery products. The former has been implicated in the outbreaks of gastroenteritis by fish consumption.

C. botulinum type E, naturally found in soil and aquatic sediments, is often isolated from fish. Human botulism is a serious but rare disease and is due to the intoxication caused by a toxin released by the bacterium. P. shiglloides, L. monocytogenes, and A. hydrophila have not so far been unequivocally incrim-inated in disease outbreaks involving fish products. However, Listeria spp. have been isolated from tropical fish, shrimp, crab, lobster tail, and surimi-based products.^94,95Several of these pathogenic organisms including L. monocytogenes, Y. enterocolitica, and A. hydrophila, are capable of survival even at refriger-ated temperatures, posing a threat to the safety of refrigerrefriger-ated products.^90,91 L. monocytogenes is frequently reported from fish items, particularly aquaculture products from temperate regions; however, it is rarely detected in tropical waters.

Fishery products that are not processed under strict hygienic conditions may harbor pathogens. Screening of 1264 samples of individually quick-frozen (IQF) peeled and deveined raw and 914 samples of cooked ready-to-eat shrimp commer-cially produced from farm raised black tiger (Penaeus monodon) were shown to be contaminated with E. coli, coagulase positive Staphylococcus, and Salmonella spp.

The coliform count in raw products ranged from 1.0× 10¹to 2.50× 10³cfu/g.

Although the cooked shrimp samples were free of coagulase positive staphylo-cocci, E. coli, and Salmonella spp., the frozen raw shrimp samples tested positive for these pathogens.⁹⁰

There is recent evidence that viable but noncultivable (VBNC) state of microbes may be formed, in the stressing environment of food processing. This is a cause of concern because the microbial pathogens in such a state may retain the capacity to cause infection after consumption by the consumer, des-pite their inability to grow under conditions employed for determining their

presence in food. VBNC organisms can cause potentially dangerous public health problems.⁹⁶

2.5.2 Viruses

The prominent food-borne viruses are Hepatitis-type A, Norwalk virus, and small round viruses (calicivirus, astrovirus, and parvovirus).^83,91The symptoms of dis-eases caused by these viruses include paralysis, meningitis, respiratory illness, and myocarditis among others.⁹⁷Consumption of raw molluskcan bivalves is a major cause of viral disease as a result of filtration of water by these shellfish.

2.5.3 Parasites

Parasites such as flatworms, roundworms, and protozoa infest the gills, viscera, and skin of marine, freshwater as well as farm raised fish and shellfish, and can pose health hazards to consumers.^91,98,99The most common parasites associated with fish that can infect consumers include nematodiases, trematodiases, and cest-odiases. Fish-borne nematodiases (roundworms) can be detected in humans as incidental infections, whose natural definitive hosts include marine mammals, birds, and pigs. The mode of infection is ingestion of fish containing the infective larvae. Anisakids (particularly A. simplex) are among the most common nemat-odes in marine fishes.⁸⁷Others include Ascaris lumbricoides, Trichuris trichura, T. spiralis, Capillaria philippinensis, and Pseudoterranova decipiens.¹⁰⁰ Cod, whitefish, and salmonids can carry T. spiralis. Nematodes in fish can be killed by thoroughly cooking the fish, freezing, or curing with salt and acetic acid.⁴

Fish-borne trematodiases are major diseases in various parts of the world, causing morbidity and complications leading to death. Clonorchis and opsthorchis are the two major genera that are of great concern for human health. Clonorchi-asis, caused by clonorchis, is endemic in some countries in East Asia. For liver fluke species such as Clonorchis sinensis, Opisthorchis felineus, and O. viverrini, the intermediate hosts are snails and freshwater fish, while dogs, cats, and humans are the final hosts where the fluke lives and develops in the bile ducts of the liver.¹⁰¹ Other trematodes of importance are Nanophetus salmincola⁹⁴ and Crytotyle lingua,⁴ the latter existing under the skin of gadoids such as herring and mackerel. Cestodiases (tapeworms), which mature in the small intest-ine, cause cestodes infection in humans. The disease is not very pathogenic.

Diphyllobothrium spp. cause another major human cestodiasis and is transmitted by various species of freshwater, marine, and anadramous fish.

The smallest of the parasites are the protozoa, which are single-celled organ-isms. The three most important ones found in food are Entameba histolytica, Giardia lamblia, and Toxoplasma gondii. A well-known protozoan infection is caused by mixosporidian Chloromyscium thyristes. It occurs in a number of spe-cies of fish although its presence is difficult to detect in fresh fish.⁴During ice storage, the flesh of badly infected fish becomes softened by proteolytic enzymes produced by the parasite.⁴The abnormal color of shrimp may be due to infestation

of a protozoan that decomposes the meat, giving it a soft and white appearance.

Strict regulations regarding the presence of parasites in the edible portions of fish are in force in countries such as Germany.¹⁰²

Compared with capture fisheries, aquacultured fishery products are more exposed to hazards, both biological as well as chemical, due to possibilities of their contamination through agents from both freshwater and coastal ecosystems.

In addition, the use of antibiotics such as chloramphenicol to control diseases has resulted in the accumulation of residues of these compounds in shrimp and other products. The high prevalence of bacterial pathogens in coastal and inland aquaculture environments than in open seas has been recognized. Salmonella spp.

have been reported in fish ponds, which are usually scavenged by birds, and there-fore, have been detected in the gut of cultured tilapia and carp.^93,101Surveys have revealed that 21% of Japanese eel culture ponds, 5% of North American catfish ponds, and 22% of shrimp ponds in one of the major shrimp-exporting countries in Southeast Asia are contaminated with Salmonella spp.¹⁰³Viral disease has been a major reason for the setback in shrimp aquaculture during the last few years. The disease spread mostly through unhealthy seeds. The safety issues associated with aquaculture products have been addressed by a Joint FAO/NACA/WHO Study Group.¹⁰¹The Study Group evaluated options for implementing risk management strategies to reduce or eliminate risks associated with products from aquaculture systems and recommended food safety measures to control these hazards (see also Chapter 12).

2.5.4 Insects

A number of insects such as flesh flies (Saracophagidae), beetles (Dermestes, Cornestes, and Necrobia spp.), and mites (Lardoglyphus and Lyrophagus spp.) infest fish, particularly during sun drying.¹⁰⁴The most destructive pest is the hide beetle, D. maculatus Deg. A case study of the losses in traditionally cured fish showed the infestation of blowfly as the major cause of losses in dried sardine products.¹⁰⁵

2.5.5 Biotoxins

A large proportion (approximately 80%) of seafood-borne disease outbreaks is caused by biotoxins and histamine.⁸⁷ The main types of fish poisoning include ciguatera, paralytic shellfish poisoning (PSP), diarrheic shellfish poisoning (DSP), amnesic shellfish poisoning (ASP), and puffer fish poisoning.^94,106PSP is asso-ciated with consumption of clams and mussels harvested during occurrence of

“red tide,” when these animals concentrate the dinoflagellates (usually species of Ganyaulax). Such poisons or toxins are not destroyed by heat or processing.

Ciguatera poisoning, which causes gastrointestinal and neurological symptoms, is associated with a number of fish species, most notably reef fish such as groupers, sea basses, and snappers. Scombroid poisoning has been recognized as the greatest cause of seafood-associated disease linked to post-harvest contamination and

improper storage of fish.⁹⁴Scombroid poisoning is associated with the consump-tion of fish of the scombroid family (mackerel, tuna) and is due to ingesconsump-tion of appreciable amounts of histamine, and the characteristic symptoms are headache, dizziness, nausea, vomiting, and urticarial eruptions.^72,73 The maximum con-centration of histamine permissible in 100 g fish flesh as per the E.U. and U.S.

regulations, and as per the Codex Alimentarius Standards is 20 mg. Contamina-tion of aquacultured fishery products with mycotoxins may also be possible. These are transmitted through agricultural components used in feed, which are contamin-ated with fungi such as Aspergillus flavus, A. parasiticus, A. rubur, and Penicillium veridicatum.