Volume 43 Number 1
Agricultural Wastes on Groundwater Quality -- The Microbial Impact
While we've known for some time domestic animals can carry human protozoan and bacterial pathogens (i.e., Cryptosporidium, E. coli, Salmonella), viruses tend to be more host-specific-or so we thought. Continued improvements in genetics and use of molecular methods for pathogen monitoring and identification has greatly advanced our knowledge of disease trends and future implications. Recent genetic evidence suggests viruses of domestic animals are highly homologous to known human viruses and thus may be transmitted from animals to humans.
The question of whether or not animals can pass viruses to humans isn't new. Consider the ongoing debates of the origin of AIDS (associated with testing polio vaccines on African monkeys) or the safety of xenotransplantation (using animals as organ donors to humans). Each of these topics includes elements of viruses crossing over to humans from animals. The question is how? Do the viruses first have to randomly mutate from a closely related host, i.e., related strains of HIV are found in primate hosts or, can the viruses just be passed as is, maybe not causing noticeable illnesses in their new hosts for years?
Root of the problem
Enteric-or intestinal-animal viruses present in the environment are an obvious concern to livestock handlers, as animal-to-animal transmission of pathogens may result in reduced production or loss of animals. In addition, recent studies suggest several viruses of animals may actually be able to cross the species barrier, potentially leaving humans exposed to viral infections transmitted by animals. Specifically, hepatitis E viruses, found in pigs, and the caliciviruses of cattle and swine, are very closely related to those causing illness in humans,7,9,10,11 suggesting animal wastes may play a more important role in contaminated groundwater than previously thought. Calciviruses-including the Norwalk virus-are spread to humans by the fecal oral route, causing an acute gastroenteritis, and have been found in contaminated ground and surface water. They are listed on the U.S. Environmental Protection Agency's (USEPA) Contaminant Candidate List for regulatory consideration in drinking water.
While the potential for groundwater contamination by human enteric viruses from municipal wastes has been studied, no information is available on the potential of animal enteric viruses in manure to contaminate groundwater. Also, no studies exist for some of the emerging human pathogens in municipal wastes to be transported through soil. Of special importance are the caliciviruses that may be the most common cause of diarrhea in the developed world.5
What we do know
Past studies have shown poliovirus and echovirus present in the sludge-amended soil for up to six months following application.3 The transport potential of viruses is highly dependent on the characteristics of the viruses themselves. For example, poliovirus has been shown to migrate less than 10 centimeters (cm) in 10 days compared to coxsackie B3 that has been isolated 18 m below the soil surface following wastewater discharge.16
Viral contamination of groundwater supplies is known to be exacerbated by heavy rainfall events.8 The presence of viruses were monitored in soil and groundwater wells as deep as 27.5 m when secondary sewage effluent was used to irrigate to agricultural crops.6 Human and porcine enteroviruses were isolated from the majority (70 percent) of samples collected from a river in Quebec.14 The contamination source was attributed to a massive pig-raising activity in the area.
Viruses of concern Caliciviruses infect both humans and animals. These viruses are now believed to be the major cause of viral gastroenteritis in the world13 and are common causes of food and waterborne disease. Little is known about the occurrence and environmental fate of these viruses because they cannot be grown in cell culture; however, molecular methods are available for their detection in environmental samples. Porcine caliciviruses can be grown in cell culture and have been used as models for human calicivirus survival and removal by water treatment processes.
The USEPA is very interested in preventing calicivirus contamination of water and is considering regulatory action aimed at controlling caliciviruses in drinking water. Before regulatory decisions can be made, significant information is needed regarding the source, prevalence, fate and transport of caliciviruses in the environment. Currently, there's no data on the occurrence of caliciviruses in animal wastes, or on their treatability.
Although molecular methods have provided an extensive database of calicivirus presence, information on survival and viability is imperative for risk assessment. In addition, the possibility that animal caliciviruses, carried by pigs and cattle, may be harmful to humans is of great concern. One study found that 45 percent of calf herds tested positive for Norwalk-like caliciviruses.10
Hepatitis E virus (HEV) is responsible for large epidemics of acute hepatitis and a proportion of sporadic hepatitis cases in southeast and central Asia, the Middle East, parts of Africa and México. The majority of swine in the Midwestern U.S. test positive for HEV and humans appear to be at risk of infection.11 In addition, questions remain concerning the high rate (up to 28 percent) of HEV antibodies in non-endemic areas and whether or not this is due to exposure to infected animals.12
Rotaviruses are a leading cause of gastroenteritis in children worldwide and a major cause of hospitalization of children in the United States.4 Rotaviruses have been responsible for both water and foodborne outbreaks in the United States. Currently, no data is available on its occurrence in animal wastes but evidence suggests close homology between animal and human strains.
Conclusion The discovery of close genetic relationships between human and animal viruses is a major development. More research is needed to determine if animals are a natural reservoir for human disease causing viruses. With the high prevalence of outbreaks associated with groundwater and the many sources for groundwater pollution, it can no longer be relied upon as a safe and direct water source.
Nearly 50 percent of all waterborne disease is due to unknown agents. A significant portion of these illnesses are thought to be due to pathogens that aren't yet recognized or for which detection methods haven't yet been developed. Because animal viruses aren't being targeted as causes of human disease, their impact may go unrecognized. The long-term goal of public health protection agencies should be to more closely evaluate the contribution of agricultural practices to the presence of animal and human pathogens as well as to determine potential for transport of these pathogens through the environment.
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