Page 271 - The Veterinary Laboratory and Field Manual 3rd Edition

240 Susan C. Cork and Roy Halliwell

Mycotoxins 4.7 Virology
M. Faizal Abdul-Careem, Susan C.
Mycotoxin production in crops (for example, Cork and M. Sarjoon Abdul-Cader
groundnuts, maize and oil seed) can pose a sig-
nificant health hazard. The presence of fungi, Viruses are widely distributed in nature and cause
and subsequent toxin production, is dependent a broad range of diseases in plants and animals
on the environmental conditions before and (including bacteria). Many hundreds of viruses
after harvesting and can also occur in feed stuffs have now been described.
after processing. Although mycotoxins are pro- Credit for the first discovery of a virus is
duced by fungal organisms, the absence of fungi given to Dmitri Iwanowski, a Russian bota-
in animal feeds does not imply the absence of nist, who in 1892, presented a paper to the St
mycotoxins. Aflatoxin, produced by Aspergillus Petersburg Academy of Science demonstrating
sp. fungi is particularly common in humid con- that the agent that produced the mosaic dis-
ditions but also occurs in temperate climates ease of tobacco plants could be freely passed
especially where feed becomes damp during through bacteria-retaining filters. In 1898,
storage. There are several forms of naturally Friedrich Loeffler, working in conjunction with
occurring aflatoxins (including B1, B2, G1 and Paul Frosch in Germany, identified a filterable
G2). Aflatoxins M1 and M2 are hydroxylated agent, smaller than any bacteria previously dis-
metabolites of B1 and B2 and can be excreted covered, that caused foot-and-mouth disease.
in the milk of animals that have consumed the With the advent of the electron microscope in
toxins. Aflatoxin type B1 is the most common 1939, by Ruska and Knoll, our understanding of
and one of the most toxic mycotoxins. Even the morphological structure of viruses rapidly
low levels of aflatoxins can cause cumulative progressed. In 1931, Ernest Goodpasture, an
liver damage where ingestion of spoiled feed is American pathologist, discovered that viruses
prolonged. could be propagated in embryonating chicken
Other toxins that can be found in feed include eggs. Later, in 1954, John Franklin Ender was
ergot (Claviceps purpurea), Zearalenone (Fusarium able to culture poliovirus in vitro. These develop-
spp.), Ochratoxin (Penicillium spp. and Aspergillus ments began an era of research where virus–cell
spp.). For the purposes of diagnosis, the fungal interactions could be studied in the laboratory
toxin levels that should be considered signifi- at the cellular level. Subsequent to this, the
cant depend on the amount of the contaminated use of molecular tools has helped us to better
diet which will be fed and the livestock species understand the genetic structure and phylogeny
involved. For example, in poultry < 100 µg afla- of viruses. Some DNA and RNA viruses of vet-
toxin/kg feed can be fatal and in pigs 140 µg erinary importance are listed in the next section
aflatoxin/kg may produce liver damage. These (see Tables 4.7a and b).
species are very susceptible to aflatoxicosis In this section, we will cover the traditional
but ruminants and horses are more resistant. approach to diagnosing diseases caused by
Mycotoxin contamination can be avoided by viruses. The molecular tools used in the areas
making sure that the raw ingredients of animal of bacteriology and virology will be considered
feeds are of good quality and that concentrate in the supplementary section at the end of this
feeds are stored in dry conditions. chapter.

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