Public Health Professionals

Human Health Clinicians

Veterinary Clinicians

Animal Feed and Plant Fertilizer

The first stage of food production involves supplying safe feed for food animals and safe fertilizer for plant crops. The two concepts are interrelated. Animal feed may include both plant and animal products, and plant fertilizer may contain livestock manure and other animal wastes.⁷ At this stage of the production chain, the health risks are primarily to the food animals, with some risk also to agricultural workers directly handling animal feed and fertilizer. Risks to animals include both toxic and infectious agents. Examples of toxic exposures have included dioxin contamination in pigs⁸ and poultry; exposure of cattle to the fire retardant PBB (polybrominated biphenyl) through inadvertent contamination of feed; and heavy metal poisoning from cadmium, lead, or mercury contamination of fertilizers applied to crops and pastures.⁹ Infectious agents threatening the human and animal food supply include bovine spongiform encephalopathy, which appeared in cattle and other animals after being fed meat and bone meal prepared from animals infected with transmissible spongiform encephalopathy (TSE) (see Chapter 9). This risk has been greatly reduced through animal feeding bans. Less clear cut are the risks from the use of antibiotics and hormones in animal feed. Such additives may be used to reduce infection and increase meat or dairy product production (growth promoters).¹⁰ There is evidence that these practices can increase the development of antibiotic-resistant pathogens.¹¹ However, most animals treated with such agents show no ill effects and may have increased meat and dairy product output.

Not all animals who have ingested either toxicants or infectious agents show evidence of disease and therefore may pass the risks along the food production chain. A pathogen that has proved difficult to control is Salmonella, many strains of which are adapted to a wide number of animal hosts (see Chapter 9). There is evidence that Salmonella may enter the food supply chain through rendered animal protein used as animal feed.⁶

Occupational exposure to animal feed and manure fertilizers can result in disease in agricultural workers and persons engaged in small-scale (backyard) food production. Manure used as fertilizer on fields may come from animals who shed pathogens in their feces. Such contaminated feces can then be a risk to workers. For example, contact with infected chicken manure for use in a backyard garden has been linked to a fatal human case of avian influenza.¹² Commercial farm workers who have direct contact with infected human or animal wastes used as fertilizer on crops may also be at risk of infection.¹³ Another hazard of animal feed to workers is allergic and other reactions to grain, which can include “farmer’s lung,” grain mite allergy, and endotoxin exposure (see Chapters 7 and 12). Stored silage can be a source of toxic nitrogen dioxide. This gas forms nitric acid when in contact with the respiratory mucosa and creates acute lung damage, known as silo filler’s disease. This is also a risk for livestock housed near silage; outbreaks of respiratory disease and death in cattle from silo gases have been reported (Figure 11-2), potentially providing warning to human beings of the toxic risk (see Chapter 12).¹⁴

Figure 11-2 Open-mouth breathing in a 5-year-old cow.

(From Divers TJ, Peek SF: Rebhun’s diseases of dairy cattle, ed 2, St Louis, 2008, Saunders Elsevier.)

A number of measures can be implemented to prevent disease at the level of feed and manure production. In the case of bovine spongiform encephalopathy, banning the use of animal protein for cattle feed effectively reduced the incidence of new cases of the disease. Similarly, weighing the risks and benefits of feed additives that include hormones and antibiotics is an ongoing assessment process between agricultural and public health agencies. Inspections of animal feed can further improve the safety at this stage.

On a clinical level, surveillance for sentinel disease in both farm animals by veterinarians and workers handling feed and manure by occupational medicine professionals can detect emerging problems at this stage of the food production chain.

Farm and Field

In the farm environment, animal-animal, animal-human, and human-animal transmission of infectious agents can occur. In addition to the feed and manure exposures listed, environmental exposures (e.g., water, mosquitoes, and other vectors) and contact with wildlife can introduce pathogens into the food chain if meat is consumed raw or undercooked or if dairy products are unpasteurized. For example, a recent multistate outbreak of salmonellosis in Serrano peppers was traced to contaminated irrigation water in Mexico.¹⁵ Rift Valley fever is an example of a vector-borne disease in which infected cattle can then pose an occupational risk to abattoir (slaughterhouse) workers.

At this stage of the food production chain, preventive care by veterinarians is crucial to improved herd health and also improved public health for both workers and future consumers. Early detection, prevention, and treatment of disease in herds are key. Agricultural extension services and other veterinary preventive services can encourage improved biosecurity in farming methods that reduce potential for disease outbreaks.

Harvest and Processing

When animals are slaughtered and processed for human and pet consumption, any pathogens that have entered the food chain can be spread to contaminate other carcasses and pose a risk to abattoir workers and butchers. Meat inspection at this point often may involve both veterinary and agricultural professionals as well as public health inspectors. Occupational medicine professionals and other human health clinicians can play an important role in providing medical surveillance of abattoir workers and detecting sentinel cases of disease that have implications for both animal and human health.

Although most processing of animal meat in industrialized countries occurs in specialized facilities with infection control policies and adequate refrigeration, food animals in other countries are slaughtered in backyards or households, farmyards, or open air markets. In these settings biosecurity is low, meat inspection and refrigeration are often not available, and the potential for infection consequently is greater. In addition, hunting for subsistence bushmeat consumption as well as hunting of deer, waterfowl, and other game often involves both slaughter and dressing of the wild animal carcass in a field or other improvised location. Such activities, even in developed countries, may take place without availability of running water or the use of gloves or other types of personal protection. Transmission of simian viruses has been documented in bushmeat hunters,¹⁶ and toxoplasmosis, tularemia, and brucellosis have been reported in U.S. hunters who field-dressed their carcasses without personal protection. In these informal settings there is generally no process of organized inspection or medical surveillance of at-risk individuals. Recently there has been a call for improved surveillance of bushmeat hunters¹⁷ to detect emerging pathogens and other foodborne diseases that could trigger disease outbreaks in human beings. Occupational medicine surveillance of farmers and workers in processing facilities and marketing retail chains can help detect sentinel health events in human beings (e.g., a Q fever or brucellosis outbreak),¹⁸ indicating the presence of the disease in the livestock as well and a likely problem with food safety (see Chapter 12).

To reduce the possibility of disease transmission, dairies generally use pasteurization methods on waste milk fed to calves. Although milk pasteurization for human consumption occurs at the farm level in farms that produce milk and milk products, most U.S. dairies ship milk for human consumption to processing plants, where it is pasteurized and then shipped to distributors.¹⁹ Pasteurization practices may vary between farms and regions. Although there is a consumer market for unpasteurized dairy products, which are considered by some to be more “natural,” such products have resulted in a number of foodborne illness outbreaks.^20,21

Retail

In the retail sector, handling of meat carries a risk of pathogen exposure similar to that faced by workers in abattoirs, although exposures tend to be lower and reported disease cases are fewer. If vegetables (e.g., peppers contaminated with Salmonella and spinach contaminated with E. coli)²² and fruit (e.g., raspberries with Cryptosporidium)²³ have been contaminated with pathogens from animal manure, there is some risk of retail workers becoming sickened by handling such produce.

A special case of retail exposure is the live animal market, many of which exist even in developed countries such as the United States. In these markets animals of different species are often housed in the same or immediately adjacent cages, allowing pathogen spread among species and to human beings. In addition, contamination of such markets with feces, feathers, and other animal waste can be an indirect source of exposure for market workers (as well as consumers visiting the markets). Animal slaughter taking place in such markets exposes workers to the same pathogens faced by workers in abattoirs but with less potential for biosecurity measures.

Agricultural agencies have an important role to play in such markets by veterinary surveillance of live animals and animal products. Workers in such markets should also receive ongoing medical surveillance for infectious diseases as well as training in risk reduction measures. The occurrence of a zoonotic disease in a market worker is sometimes an important sentinel health event indicating the presence of risk for diseases such as avian influenza. Unfortunately, such workers may not be included in ongoing preventive programs.