Project: MLMMI 02-HERS-05
To gather and summarize existing knowledge regarding food-, water-, and air-borne enteric diseases in Canada and to assess the risk presented by intensive livestock operations.
University of Manitoba
Funding Partners: who have contributed to MLMMI in support of this project:
MRAC - $14,805
Manitoba Pork Council and industry groups - $29,741
Amount Funded: $44,546.00
Performer Funded: $0.00
Total Cost: $44,546.00
First Progress Report received Apr 2/02
Second progress report received June 27/02
Final Report due & received on Oct. 1/02
A scan of literature revealed a large amount of research as well as some gaps in knowledge pertaining to the following: sources and transmission of enteric disease, pathogen survival and transport in the environment, pork production and its impact on human health, reliability of pathogen detection methods, swine manure treatment and handling, and environmental legislation addressing intensive livestock operations. Important findings for each of these issues considered are briefly summarized below.
Canadians are exposed to the risk of enteric illness via foodborne, waterborne, and airborne sources, or via direct contact by infected persons or animals. Foodborne illness (FBI) is the most common route of transmission for enteric pathogens, and meat and poultry products are the largest contributor of FBI outbreaks. Food animals carry some of these human pathogens with no clinical symptoms and there seems to be a continuous re-infection among animals and between them and the environment in which they reside. Fruits and vegetables are vulnerable to microbial contamination during production. Produce can be contaminated with enteric pathogens from soil, fertilizer, irrigation water, pesticide spray, wild and domestic animal waste, or field workers. Later, minimal processing (e.g. mechanical) can further magnify the contamination level. Seafood-associated enteric infection is usually the result of fecal contamination of the marine or freshwater environments followed by consumption of raw or undercooked seafood. Enteric infection due to dairy foods often implicates raw milk, improper pasteurization, and post pasteurization contamination as sources of the problem. Enteric illness outbreaks associated with drinking water are a result of either poor protection of the source water or inadequate water treatment. Sources of pathogens in recreational waters often are the users themselves, in particular the young who are also the victims of the diseases. Major sources of pathogens causing airborne transmitted enteric illness (sometimes referred to as an occupational hazard) include human activity (such as sneezing, coughing, and talking), animal movement and waste, sewage and sewage treatment plants, and dust. Children's farms (petting zoos) and dairy farms are recognized as a risk setting for animal-person disease transmission. Person to person contact during sporting events, high-risk sexual practices, child play activities, elevated household densities, and even general circulation of currency, can also spread enteric disease. There is little evidence to suggest that hog manure contributes in a meaningful way to the transmission of pathogens to humans in North America. Data indicate that this status could change in the future.
Canada is the world's leading exporter of pork, but recent increases in production contrast with constant domestic levels of pork consumption and declining levels of foodborne illness caused by pork. Survival of some pathogens (E. coli O157:H7, Salmonella, Campylobacter, Yersinia, Cryptosporidium, and Giardia) in soil, manure and water indicate significant variability in resistance to environmental challenge which is characteristic of the organisms themselves. Generally, pathogens survived longer in environmental samples at cool temperatures. Of those pathogens considered, E. coli O157:H7 was the most persistent organism in cattle manure regardless of the temperature and manure form (solid or slurry). We hypothesize that holding manure at 25°C for 3 months will render it free from the pathogens considered above.
The assessment of the potential impact of pork production on human health is complicated by several factors. Cases of pork-associated FBI in major hog-producing countries appear to be more influenced by the amount of pork consumed, the prevalence of pathogens in pigs, hygiene at the slaughterhouse level, or even cooking habits, rather than by pork production or the pig density. In all hog-producing countries (Canada, U.S., Denmark, Holland, and Taiwan) studied, pork production did not increase national pork consumption, instead it increased exports. Pork consumption is the highest among all meat consumed in Denmark and Holland and pork is a major source of human enteric illness. All human Yersinia and most S. Typhimurium infections in both countries are attributed to pork consumption and both pathogens are highly prevalent in domestic swine. Although pork consumption is also the highest in Taiwan, pork does not represent a major source of human illness. The high pig density and pork production in this country have not increased known cases of pork-associated human illness because of thorough cooking and use of tertiary treatment for hog manure. Increased pork production in Canada and the U. S. did not seem to increase cases of pork-associated human illness. Lower pork consumption and thorough cooking of pork may have contributed to the lower incidence rates in North America. Given the currently available data, pork is not considered a major source of foodborne infection in Canada and reports of illness from any food contaminated by hog manure do not exist in North America. However, the potential exists for manure to be an important vector.
Detection and identification of pathogens from animals and the environment are complicated by many factors including intermittent shedding of pathogens by animals, variable sensitivities of different assays, lack of reproducibility between users and on repeated testing of animals, ambiguity of negative results, and choice of the right sample. Culture methods are standard for the detection of bacteria despite their insensitivity, expense, and labour intensity. Their sensitivity is also affected by sample handling, type of enrichments, and the VBNC (1) condition of some bacteria. The standard method for detection of viruses is cell culture assay and for protozoans is fluorescence microscopy. While cell culture assay is expensive and time consuming, the microscopy method is highly unspecific and success is dependent on user experience. For protozoan pathogens there is also a major problem with determination of cyst and oocyst infectivity which requires more research. Serological methods, in general are a more sensitive approach and can identify more than one serotype of a pathogenic species. The major drawback is that an antibody response to a pathogen does not indicate an active infection in the animal or that fecal shedding is occurring. The PCR (2) technique is the most sensitive and specific detection method available but is not widely used due to the extensive sample preparation required, significant capital cost, and requirement for trained users.
Every swine producing country has its own way of handling livestock manure depending on the local needs and conditions (climate, land availability, capital and labour costs, and public concerns). Depending on regional pressures, some countries have adopted more expensive treatment systems (Taiwan, Denmark, and Holland) than others (United States and Canada). Animal density has driven most of these initiatives, however, other factors are involved (human population density, climate). Countries or states with the highest hog densities (expressed in terms of size, hogs/km2, relative to Manitoba) include North Carolina, 12.1; Denmark, 13.6; Taiwan, 21.1; and Holland, 38.6, and they have each developed very different systems to deal with manure problems. Hog densities in Quebec, Iowa and Ontario are 5.6, 3.8 and 2.4 times greater, respectively, than in Manitoba at present. An animal density threshold could be used to trigger the need for change in the way manure is handled in Manitoba, however, such a threshold is difficult to establish from the experience of other regions with high levels of hog production. Given the biological stability of hog manure while stored in the Manitoba climate, it is unlikely that the province could sustain the level of hog density currently reported for Quebec. Of other regions considered, the climate in Quebec is most like that in Manitoba.
Both field and laboratory investigations have demonstrated that pathogens can migrate for significant distances and at high rates in the environment. Some studies have also shown that pathogens from manure spread using currently acceptable application practices can travel through soil and reach receiving waters which are subsequently used as public water sources. Although it is impossible to predict the mobility of these pathogens in soil in Manitoba, it is clear that the potential exists for microorganisms to be transported in the environment and contaminate regional water supplies.
A comparison of the legislation and its requirements in a group of hog producing provinces, states and countries revealed that nutrient and odour concerns instead of pathogens have essentially formed the basis of the legislation addressing intensive livestock operations. Nutrient use (especially nitrate emission/loss) is more intensely regulated in European producing countries than in North American. One of the similarities among all jurisdictions is they all strongly discourage spreading manure on frozen land. In addition, regulations are increasingly aiming at the construction and monitoring of manure storage systems in order to prevent manure leakage. A major difference among them is the great variety of definitions of intensive livestock farming which is in part influenced by the various definitions of animal unit in each of the countries considered or provinces in Canada.
1. VBNC = viable but non-culturable
2. PCR = polymerase chain reaction
This project was supported by funds from the Manitoba Livestock Manure Management Initiative and the Manitoba Rural Adaptation Council.
The Full Report may be made available by contacting the MLMMI Office.