Others. Parasites. Viruses. Mycotoxins

1. Parasites. Parasites are defined as animals that require a second animal host for their livelihood. They grow and replicate only within the host, although they can survive for varying lengths of time outside of the host. In foods, they do not replicate, nor can they be grown in standard microbiological media and conditions. As such their detection usually employs a direct inspection of the food, frequently using a microscopic-based format. Examples of parasites include Tricbinella, Giardia, Cryptosporidium, and more recently Cyclospora. Historically Tricbinella contamination of pork was an issue, though more recently attention has been focused on the contamination of water supplies with Cryptosporidium, perhaps the result of fecal material originating from domestic or wild animals.

2. Viruses. Viruses are similar to parasites in that they require a host for their replication. They are simple in their structure and biology, possessing only the necessary nucleic acid (either DNA or RNA) to encode their structural proteins and perhaps a few proteins for replication. Foods are just one means through which viruses are transmitted. The viruses that are typically transmitted through foods are not usually life-threatening, hence an accurate survey of their significance is not available. The symptoms of food-borne viral infections usually include gastrointestinal distress.

Where food is a major vehicle for transmission to humans at least two viruses are of significance. The rotaviruses cause gastrointestinal illness and are frequently associated with day care facilities. They are approximately 75 nm in diameter and have a genome that is double-stranded RNA. Another virus of importance in foods is the Norwalk-like virus, named after the city where it was isolated. As with many viruses, Norwalk-like are a collection of very similar viruses that can, however, be distinguished immunologically. This virus causes a gastroenteritis similar to that produced by bacteria, including Staphylococcus. It is 30-35 nm in size and contains single-stranded RNA.

3. Mycotoxins. Mycotoxins are metabolic end products of fungi and are typically secondary metabolites produced during the sporulation stages of growth. Their toxicity has not always been accurately determined owing to differences in sensitivity of animal models to a particular mycotoxin. Many mycotoxins are carcinogens but typically require activation by one or more mammalian enzymes. Their primary target in animals is therefore the liver, where they are activated in, ironically, the body’s attempt to detoxify them. Their true food safety impact is difficult to assess because of longterm potential health problems.

The most studied mycotoxin is aflatoxin, which is produced by certain strains of the fungus Aspergillus flavus. It is the most potent hepatocarcinogen known to humans, although exacting measurements of its carcinogenicity have not been carried out. Four different aflatoxins have been reported, designated aflatoxin B1, B2, G1, and G2. The biosynthetic pathway for the aflatoxin has not been completely elucidated but its precursors originate from the polyketide synthesis.

Aspergillus flavus is ubiquitous in the environment but tends to proliferate under conditions that are not favorable to bacterial growth. Growth and aflatoxin production are most common on dried grains, including corn, especially when the drying process is not adequate to reduce the water activity below the requirements for fungal growth. Aflatoxin is therefore a problem not only because of the contamination of food destined for human consumption but because of its presence in animal feed. Losses from aflatoxin contamination of animal feed are again difficult to estimate as they are sometimes manifested in reduced growth rates and feed conversion efficiencies. There is also a secondary problem in the transmission of aflatoxin through animals into foods destined for human consumption. Specifically, aflatoxin M1 is a metabolic derivative of aflatoxin B1, which is found in milk. It is still toxic and carcinogenic, although the extent of its actual occurrence in milk is not known.