1 The order of appearance of categories of hazards listed (mycotoxins etc.) is not intended to indicate any ranking of relative importance.
Mycotoxins
Infectious agents
Veterinary drugs and agricultural and other chemicals
Mycotoxins are secondary metabolites produced by fungi of various genera when they grow on agricultural products before or after harvest or during transportation or storage. Some fungi such as Fusarium spp. typically infest grains before harvest, others such as Penicillium spp. can invade grain after harvest, while Aspergillus spp. can grow on grains both before and after harvest. It must be emphasised that the presence of the fungi does not necessarily imply that mycotoxins can be found. Conversely, the absence of fungi does not necessarily mean the absence of mycotoxins.
Both intrinsic and extrinsic factors influence fungal growth and mycotoxin production on a given substrate. The intrinsic factors include water activity, pH, and redox potential whereas extrinsic factors which influence mycotoxin production are relative humidity, temperature and availability of oxygen.
Many mycotoxins, with different chemical structures and biological activities, have been identified. Mycotoxins may be carcinogenic (e.g. aflatoxin B1, ochratoxin A, fumonisin B1), oestrogenic (zearalenone and I and J zearalenols), neurotoxic (fumonisin B1), nephrotoxic (ochratoxins, citrinin, oosporeine), dermonecrotic (trichothecenes) or immunosuppressive (aflatoxin B1, ochratoxin A, and T-2 toxin). Much of the published information on toxicity concerns studies in experimental animals and these may not reflect their effects in humans and other animals. In addition, the significance of the presence of most mycotoxins in foods of animal origin is not completely understood.
Mycotoxins are regularly found in feed ingredients such as maize, sorghum grain, barley, wheat, rice meal, cottonseed meal, groundnuts and other legumes. Most are relatively stable compounds and are not destroyed by processing of feed and may even be concentrated in screenings.
Different animal species metabolise mycotoxins in different ways. For example in pigs, ochratoxin A can undergo entero-hepatic circulation and is eliminated very slowly while it is rapidly excreted by poultry species. The polar mycotoxins, such as fumonisins, tend to be excreted rapidly.
Mycotoxins, or their metabolites, can be detected in meat, visceral organs, milk and eggs. Their concentration in food is usually considerably lower than the levels present in the feed consumed by the animals and unlikely to cause acute intoxications in humans. However residues of carcinogenic mycotoxins, such as aflatoxin B1 and M1, and ochratoxin A, when present in animal products pose a threat to human health, and their levels should be monitored and controlled.
In most instances the principal source of mycotoxins for humans is contaminated cereals and legumes rather than animal products. This means that the exposure to mycotoxins may be greater in developing countries in which cereal grains and legumes form the staple diet and the intake of animal products, including meat, is low.
There is little information available regarding the occurrence of mycotoxin residues in animal products intended for human consumption. Some examples are summarised in Table 1. Examples of maximum levels in force in various countries include 0.05-1 ppb for aflatoxin M1, 5 ppb for aflatoxin B1, 25 ppb and 50 ppb ochratoxin A in porcine kidneys and cereals respectively and, depending on the country, 30-1,000 ppb for zearalenone in corn and foods (1). The levels of mycotoxins detected are usually below the maximum levels accepted in most countries.
Table 1. Examples of food of animal origin which may be naturally contaminated with mycotoxins
|
Mycotoxin |
Potential effects on humans |
Occurrence |
Maximum level reported |
Reference |
|
Aflatoxin B1 |
Hepatic cancer |
Eggs |
0.4 ppb |
(2) |
|
Pig liver |
0.5 ppb |
(3) |
||
|
Pig muscle |
1.04 ppb |
(4) |
||
|
Pig kidney |
1.02 ppb |
(4) |
||
|
Aflatoxin M1 |
1 |
Cow's milk |
0.33 ppb |
(5) |
|
Ochratoxin A
|
Renal damage
|
Pig liver |
98 ppb |
(6) |
|
Kidney |
89 ppb |
(7) |
||
|
Sausages |
3.4 ppb |
(7) |
||
|
Zeralenone
|
Oestrogenic |
Pig liver |
10 ppb |
(8) |
|
Pig muscle |
10 ppb |
(8) |
1 There is insufficient evidence to describe aflatoxin M1 as a human carcinogen although it is a potent carcinogen in rodents.
Transmissible spongiform encephalopathies in ruminants
Salmonella enterica
Toxoplasma gondii
Trichinella spiralis
The transmissible spongiform encephalopathies (TSEs) are non-febrile neurological diseases. They have a long incubation period and are ultimately fatal. TSEs are associated with incompletely defined agents currently termed prions which are resistant to normal heat treatments of feed and food. The TSEs recognised in food producing animals are BSE and scrapie. Sheep scrapie has been recognised for over 250 years. BSE was first recognised in the UK during 1986. The BSE infectious agent enters feed primarily through infected tissues (notably the central nervous system and the reticuloendothelial system) rendered under conditions of insufficient heat treatment to reduce the concentration of the infectious agent to an ineffective dose.
In the case of sheep scrapie, infection is naturally maintained by transmission between sheep. It is likely that humans have been exposed to the scrapie agent by eating brain and other tissues from infected sheep although there is no evidence that the occurrence of either CJD or nv-CJD has been associated with scrapie. With respect to BSE, humans can potentially be exposed through consumption of the infected tissues. The occurrence in humans of nv-CJD has raised the possibility of an association with the BSE agent. At present, with the limited number of diagnosed cases, there is no proven link between nv-CJD and the possible transmission of the infective agent from bovine tissue to humans.
There are over 2,000 salmonella serotypes and these can be divided arbitrarily into three unequally sized groups. These include:
1) the species specific serotypes such as S. dublin (cattle) and S. gallinarum and S. pullorum (poultry);2) the invasive serotypes which may cause septicaemic disease in several animal species (e.g. S. enteritidis and S. typhimurium); and
3) the non-invasive serotypes which tend not to result in septicaemia. Members of the first group are not recognised as feedborne pathogens.
The third group is by far the largest and may be associated with subclinical infections in farm livestock. Occasionally they can cause disease and are associated with food poisoning in humans. The principal manifestation of human salmonellosis is gastroenteritis. Septicaemia occurs in a proportion of patients. The case mortality rate is low with the young, old or immunocompromised being most susceptible.
Salmonellae arc widely distributed in nature, and feed is only one of many sources for farm animals. Feed ingredients, of both animal and plant origin, are frequently contaminated with salmonellae although the most common serotypes isolated are rarely the most prevalent in animals including man. The two most important serotypes associated with human disease, S. enteritidis and S. typhimurium, are rarely isolated from feed. Feed can be contaminated by contact with raw ingredients after processing.
The protozoan Toxoplasma gondii is found in cats, and based on serological surveys, in birds, and in domesticated species including sheep, pigs, goats, and horses. The primary source of infection for animals is feeds contaminated with cat faeces.
Cats are an important source of infection for humans, with the handling or consumption of raw meat also being implicated. Pregnant women who become infected may abort or give birth prematurely, and infants often develop central nervous system disorders and ocular disease. Immunocompromised patients are at particular risk.
Trichinella spiralis is a nematode which parasitises the intestinal tract of mammals, particularly pigs. The larvae encyst in the tissues, particularly the muscles, which act as a source of infection for humans who consume raw or undercooked meat. The clinical manifestations include fever, muscle pain, encephalitis, meningitis, myocarditis and very occasionally, death.
The cysts can be killed by freezing infected carcasses at -18°C for 20 days. They are also heat sensitive and are killed by traditional rendering temperatures. Effective cooking of raw meat and table scraps before feeding to farm animals will eliminate this hazard.
Veterinary drugs may be administered in animal feeds. If the concentration used results in foods of animal origin with residues exceeding the established Maximum Residue Limits (MRLs) such as those established by Codex, there may be a potential risk to human health. Codex MRLs should not be exceeded if concentrations used are correct, withholding times are observed and Good Agricultural Practices (GAP) and Good Veterinary Practices (GVP) are applied.
The potential hazards may include excessive residue levels of herbicides, pesticides, and fungicides and industrial/environmental or other extraneous contaminants such as the polychlorinated biphenyls (PCBs) and heavy metals including mercury, lead, or cadmium. Cereals and treated seeds are the most likely source of these contaminants. The most significant hazards to human health are those chemicals that accumulate in animal tissues or are excreted in milk or become incorporated in eggs at levels in excess of established limits such as the Codex MRLs for pesticides or maximum levels for contaminants in a food or feed.
