There are various toxic substances produced by fungi which contaminate milk and animal feeds.
These include mycotoxins, which are fungal chemicals produced by endophytic fungi at pre-harvest, during plant growth and by saprophytic fungi at post-harvest during storage, transport, processing and feeding.
In animals, the most frequently harmful mycotoxin is aflatoxin produced by Aspergillus flavus and parasiticus fungi, which occur in feeds.
Aflatoxins present dietary risks to people who consume raw milk.
In dairy animals, susceptibility of individual animals to aflatoxins varies considerably depending on species, age, sex and nutrition.
But chronic consumption of lower levels over long periods can cause liver damage, decreased milk production and recurrent infection as a result of immunity suppression.
Aflatoxins occur in many animal feed concentrates, including cereal grains, soybean products, oil cakes from groundnuts and fishmeal.
Pasture, hay, straw, and silage are prone to mycotoxin contamination.
Mould growth and mycotoxin production in field crops is related to environmental conditions such as heat, water and insect damage that cause plant stress and predispose crops to mycotoxin production.
After harvest, temperature, moisture content and insect activity are the major factors influencing mycotoxin contamination of feed grains and foods.
Relative humidity levels most favourable for fungal growth are between 65 per cent and 80 per cent. Growth is rapid when relative humidity in the air is above 85 per cent or moisture content in the grain is above 16 per cent.
The risks of aflatoxins in dairy products
Lactating dairy cows fed on aflatoxin contaminated feeds excrete into their milk the less toxic substances which are metabolised in the liver.
Heating, cold storing, freezing or drying during processing can also make dairy products susceptible to aflatoxin.
Aflatoxins may also be present in yoghurt, cheese and other dairy products due to their stability to processing conditions.
Milk and milk derivatives are consumed daily and, moreover, they are of primary importance in the diet of people, thus, farmers must watch against aflatoxin.
How to avoid aflatoxin contamination
Control of mould growth and mycotoxin formation is dependent on on-farm and storage management of grains and animal feeds.
The major factor for mould growth in hay is moisture. Mould will attack hay stored too wet. Therefore, hay should be properly dried.
In silage, the limiting factor for mould growth is the pH.
If silage is stored too dry, or insufficiently packed and covered, infiltration of air allows for microbial activity, which depletes acids, allowing the pH to rise and moulds to grow.
Because most moulds are aerobic, high moisture concentrations that exclude adequate oxygen can prevent their growth.
Farmers should also buy commercially prepared concentrates from certified agrovet dealers to prevent feeding their animals cereal feeds made from highly contaminated grains.
Currently, there is no proven system to control this risk in animal feeds and dairy products in Kenya. Many African countries, including Kenya, have adopted the set up maximum admissible levels of 0.05 (parts per billion) ppb by the European Union for aflatoxin in dairy products.
In countries where regulation for aflatoxins in animal feeds exists, the total permissible levels range from 0 to 50ppb with an average of 20ppb recommended by the Food and Agricultural Organisation.
Awareness and routine monitoring of animal feeds and milk is necessary to reduce risks to animal and to humans.
Makau is a member of the Dairy Team, Egerton University