|Livestock to 2020 - The Next Food Revolution. 2020 Vision for Food, Agriculture, and the Environment. Discussion Paper 28. (IFPRI, 1999, 79 p.)|
Livestock production is a source of risk to human health in both low- and high- intensity production systems. Risks exist not only from the uncontrolled endemic diseases found in developing regions, but also from those that appear in highly developed production systems when animal concentrations are high, feeds contain contaminants, or meat and milk are improperly handled. Humans are exposed to these risks through several pathways. Zoonotic diseases, those shared by humans and animals, can mutate and spread in animal hosts before passing to humans. Animal wastes can carry disease or chemical toxins into the environment. And milk and meat can expose humans to disease and toxins contained in the animals or produced by improper handling and processing.
Although great progress has been made in controlling endemic diseases of animals, intensification of both traditional and modern livestock production systems has brought about new risks. High concentrations of animals, often moving between locations for different stages of production, can become breeding grounds for disease. Proximity of these facilities to high human populations can magnify the potential damage of an outbreak.
Livestock pose a particular risk in developing regions, where animal concentrations are often located near and around cities because of limited transportation facilities and infrastructure. The risks are compounded by inadequate or nonexistent health infrastructure, regulations, monitoring, or enforcement. Zoonotic diseases such as tuberculosis and brucellosis, which are nearly or entirely controlled in developed regions, continue to pose major problems in these regions.
Health risks in developing countries are still often monitored only at the household level. As livestock production levels rise, the ability of people to gauge the quality of the animal food products they purchase in market places becomes increasingly difficult. The classic worry in developing countries is the microbial contamination of food. Bacilli or bacteria such as Salmonella, Escherichia coli, Clostridium botulinum, and Staphylococcus are the most frequently observed causes of food-borne diseases. They are usually related to improper food preparation or inadequate refrigeration.
The World Health Organization (WHO) reports that hundreds of millions of people worldwide suffer from diseases caused by contaminated food and that products of animal origin rank at the top of the list of causes (WHO 1997). More than 3 million children under five years of age die each year because of diarrhea. Contaminated water and food-borne pathogens cause much of this diarrhea. Salmonella infections are also on the rise and threaten to become major public health concerns, especially in big Asian cities where large bird flocks are kept for food and fecal contamination is hard to control.
Risks from the intense industrialization of livestock production are also appearing in developed countries. Movements of animals for birthing, weaning, and fattening have aggravated health risks because animals from different locations are exposed to each other and to humans. Risks of transmission are further increased by greater international trade of livestock and livestock products.
Intensive livestock production units can also breed new and more virulent strains of zoonotic diseases. Antibiotics used in intensive animal production of poultry have led to the emergence of Salmonella, Listeria, and E. coli resistant to antibiotics. There is increasing concern about new strains of influenza developing in pigs and chickens that could be transmitted to humans, such as the avian flu strain that forced eradication of chickens in Hong Kong in 1997. Some researchers go so far as to say that the next lethal flu pandemic might emerge from Europes crowded pig barns (MacKenzie 1998).
Despite long-established food quality assurance systems in developed countries, new food contamination risks have emerged. According to WHO, seven food-borne pathogens (Campylobacter jejuni, Clostridium perfringens, E. coli O157:H7, Listeria monocytogenes, Salmonella, Staphylococcus aureus, and Toxoplasma gondii) are responsible for an estimated 3.3 to 12.3 million infections and 3,900 deaths annually in the United States (WHO 1997). Furthermore, global surveys by WHO indicate that food-borne diseases may occur 300-350 times more frequently than reported.
Salmonellae are blamed for more than 50,000 cases of bacterial food poisoning in the United States every year (WHO 1997). Transmission of this microbe usually occurs through insufficiently cooked meats and eggs. Chickens are a major reservoir of salmonella. Ingesting foods contaminated with significant amounts of salmonella can cause intestinal infection.
The increased size of production units and frequency of contact between animals greatly increases the overall impact of outbreaks of illness. Six million pigs were destroyed in Holland to eradicate classical swine fever in 1997. As this report was being written, Malaysians were in the process of destroying a million pigs in their largest pork producing region to stem an outbreak of a new form of viral encephalitis that has killed more than 100 people in six months (Washington Post 1999; ProMed 1999).
Use of new feed resources has also brought new risks. Insufficient temperatures used in rendering animal tissue into feed has been clearly linked to the appearance of BSE (bovine spongiforme encephalopathy), or mad-cow disease, in cattle. The infectious prions involved are similar to those that cause CJD (Creutzfeldt-Jakob disease). More than 20 human victims in the United Kingdom are thought to have contracted the disease through ingestion of animal nerve tissue.
Infection with Escherichia coli O157:H7 (E. coli) was first described in 1982. E. coli has emerged rapidly as a major cause of diarrhea and acute renal failure. The infection is sometimes fatal, particularly in children. Outbreaks, generally associated with beef, have been reported in Australia, Canada, Japan, the United States, various European countries, and southern Africa (WHO 1997).
Beef that was contaminated in the slaughterhouse is the principal cause of E. coli infection in humans. Bacteria are transmitted when infected meat is consumed raw or undercooked. Ground meats, such as hamburgers, are particularly associated with infections because the infected material is mixed throughout the product during the grinding process.
Hygienic slaughtering practices will reduce contamination of carcasses but will not guarantee the absence of microbial contamination from products. Similarly, the contamination of raw milk on the farm cannot be completely prevented. The only effective method of eliminating dangerous microbes is to heat (thoroughly cook or pasteurize) or irradiate food.
Listeria monocytogenes (Lm) has been recognized as a risk only recently. It infects cheese and meat that has been refrigerated for a long time. Listeria monocytogenes infections can cause miscarriage in women and septicemia (blood poisoning) and meningitis in infants and persons with weakened immune systems.
Nonmicrobial contamination of food also poses a risk. Food-borne chemical toxins increasingly prevalent in rapidly developing countries include pesticide residues (often containing mercury or arsenic), metals (zinc, cadmium, and copper), and heavy metals (especially mercury from pigs eating fungicide-treated corn).
Livestock feeding is partly to blame as well for the accumulation of toxic levels of heavy metals in the environment. Trace elements are often added to animal feeds to supply micronutrients and enhance feed conversion efficiency. Copper and zinc are deliberately added to a variety of animal feed concentrates, while heavy metals like cadmium are introduced involuntarily via feed phosphates. During digestion the elements are concentrated, resulting in manure and slurries that can contain high levels of the toxic elements. Soils on which pig and poultry manure are regularly applied at high rates can accumulate large amounts of heavy metals, which can in turn contaminate crops and pose a health risk to humans. High concentrations of toxic elements can also accumulate in the meat of animals fed feeds with trace metals.
Residues of growth hormones, antibiotics, and insecticides are increasingly found in the tissues of animals raised in industrial production systems. The presence of antibiotics in animal food products can cause allergies. Overdoses of antibiotics are often used illegally in feed to promote growth where enforcement systems are weak. Considerable debate (and trade dispute) also exists about the safety of growth hormones such as BST (bovine somatotropin). The illegal use and misuse of growth promoters such as DES (diethyl-stilbetrol) and clenbuterol can also create problems.
Traditionally the health risks from livestock and livestock products have been borne by consumers and farmers. With intensification has come recognition that many risks cannot be controlled at the household and farm levels alone. Consumers cannot adequately judge the safety of food that was produced and processed far from the household. Farmers acting individually cannot fully protect their animals from diseases that spread from farm to farm. In many cases institutions are required to develop, monitor, and enforce quality standards across the marketing chain. Unfortunately, government services are being curtailed in this area in many poor countries as the size of the overall public sector is being reduced.