Introduction
Mycotoxicosis refers to all of those diseases caused by the effects of toxins produced by moulds. Disease is often subclinical and may be difficult to diagnose. Problems occur worldwide, but especially climates with high temperature and humidity and where grain is harvested with high water content. Economic impact is considerable in some countries.
A number of different types are recognised: aflatoxins are produced by Aspergillus flavus; T2 fusariotoxins by Fusarium spp. (mouth lesions and thin eggshells); ochratoxins by Aspergillus ochraceus (interferes with functions of kidney, proventriculus and gizzard); rubratoxin by Penicillium rubrum (interferes with thiamine metabolism and causes symptoms of deficiency). Other mycotoxins certainly occur.
Mortality is variable but all are detrimental to bird health and are resistant to heat inactivation. The following species may be affected, in decreasing order of susceptibility: ducks, turkeys, geese, pheasants, chickens. The route of infection is by ingestion of fungal spores, which are readily carried in the air. High grain humidity, and damage due to insects, as well as poor storage conditions are major predisposing causes.
Once toxins have been formed it is difficult to avoid their biological effects; they also increase susceptibility to bacterial diseases. Both fungal spores and formed toxins are generally highly resistant. Affected flocks return to normal mortality by 7 to 15 days after removal of the toxins. Some believe that mycotoxicosis is an important factor in fatty liver syndrome.
Aflatoxins are known to inibit the synthesis and transport of lipids in the liver. Deficiencies of fat-soluble vitamins (A, D, E, and K) are also sometimes seen in aflatoxicosis. Multiplication of moulds in cereals requires selenium and this element is also important for the production of hepatic lipases. Aflatoxins have been shown to be carcinogenic in rodents so there may be public health issues relating to the effective control of these problems.
Signs
- Signs vary with the species affected, the mycotoxin, the dose ingested and the period of exposure.
- Diarrhoea.
- Paralysis or incoordination.
- Reduced feed efficiency.
- Reduced weight gain or egg production/hatchability.
- Increased condemnations.
- Pale shanks, combs, bone marrows.
Post-mortem lesions
- Lesions also vary in accorance with the same factors as signs.
- Mycotoxins can cause damage to mucosae with which they come in contact.
- They can also be absorbed and affect blood coagulation, resulting in petechiae and larger haemorrhages in various tissues.
- Liver and kidney lesions – livers may be enlarged and fatty or show bile retention or tumours.
- Enteritis of variable degree may be seen.
- Hydropericardium.
- Pale bone marrow.
- Regression of the bursa of Fabricius.
- Gizzard erosions.
Diagnosis
In severe cases a presumptive diagnosis may be based on the history, signs and lesions. Histology may be beneficial in some cases, as may identification and quantification of toxins in samples of feed or feed residue.
Differentiate from poor nutrition, poor management, physical damage to tissues, and infectious bursal disease.
Treatment
The most effective treatment is removal of the source of toxins. Addition of antifungal feed preservatives is also helpful. Increasing protein level in the feed until mortality reduces may also be beneficial. Administration of soluble vitamins and selenium (0.2 ppm), along with finely divided copper sulphate in the feed 1kg/ton for 7 days (where approved) has been used.
Prevention
Mycotoxicoses may be prevented by careful choice of feed raw materials, reduction in water content of the raw materials and hygienic storage.
Antimycotic feed additives may also be used but may not deal with toxins already formed.
Feeds with high levels of fishmeals are particularly susceptible and should not be stored for more than 3 weeks. Pelletising feed may reduce fungal counts but does not affect toxins. Certain minerals additives have been shown to bind mycotoxins and reduce their effects. Good stock control, management of feeders and bins, and avoidance of feed spillage are all important.