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
More than half of the waterborne disease outbreaks documented every year in the United States are caused by contaminated groundwater.2 Of particular concern are viruses. Because of their relatively small size, viruses have the greatest chance of all pathogens to reach groundwater. A recent nationwide survey found a third of all drinking water wells used by utilities contained evidence of contamination by pathogenic human enteric viruses.1 Others have found that finished water with measurable levels of free residual chlorine and meeting standards for coliform bacteria and turbidity cannot be assumed to be virus free.15

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
The fate and transport of microbes in the subsurface are controlled by physiochemical characteristics of the specific microbe and the soil. The size of the microbe, inactivation rate and surface properties play key roles in addition to soil type, flow velocity, organic content, temperature, pH, mineral composition and other chemical characteristics of the subsurface. In general, protozoan and bacterial pathogens are large enough to be filtered through the soil and usually don't reach groundwater from land application of wastes. Viruses, however, are another story. Though high organic content-such as that found in many agricultural sites-favors removal and adsorption of many viruses, they have been shown to travel up to 1,600 meters (m) in channeled limestone.

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.

References
1. Abbaszadegan, M., P. Stewart and M. LeChevallier, "A strategy for detection of viruses in groundwater by PCR," Applied and Environmental Microbiology, 65: 444-449, 1999.

2. Calderon, R.L., and Craun, G.F., "Epidemiology of waterborne outbreaks, 1971-1996," 1998

3. Damgaard-Larsen, S., et al., "Survival and movement of enterovirus in connection with land disposal of sludges" Water Research, 11:503-508, 1977.

4. Gerba, C.P., J.B. Rose and C.N. Haas, 1996, "Sensitive populations: who is at the greatest risk?" International Journal of Food Microbiology, 30:113-123.

5. Glass, R.I., et al., "The epidemiology of enteric caliciviruses from humans: a reassessment using new diagnostics," Journal of Infectious Diseases, 181(Suppl 2): S254-61, 2000.

6. Goyal, S.M., B.H. Keswick and C.P. Gerba, "Viruses in ground water beneath sewage irrigated cropland," Water Research, 18:299-302, 1984.

7. Guo, M., et al., "Molecular characterization of a porcine enteric calicivirus genetically related to Sapporo-like human caliciviruses," Journal of Virology, 73: 9625-9631, 1999.

8. Hejkal, T.W., et al., "Viruses in a community water supply associated with an outbreak of gastroenteritis and infectious hepatitis," Journal of the Air and Waste Management System, 74:318-321, 1982.

9. Jagannath, M.R., et al., "Characterization of human symptomatic rotavirus isolates MP409 and MP480 having 'long' RNA electropherotype and subgroup I specificity, highly related to the P6[1], G8 type bovine rotavirus A5, from Mysore, India," Archives of Virology, 145:1339-57, 2000.

10. Koopmans, M. J., et al., "Molecular epidemiology of human enteric caliciviruses in the Netherlands," Journal of Infectious Diseases, 181: S262, 2000.

11. Meng, X.J., et al., "Genetic and experimental evidence for cross-species infection by swine hepatitis E virus," Journal of Virology, 72:9714-21, 1998.

12. Meng, X.J., et al., "A novel virus in swine is closely related to the human hepatitis E virus," Proceedings of the National Academy of Sciences of the United States of America, 94:9860-9865, 1997.

13. Monroe, S.S., T. Ando and R.I. Glass, "Introduction: Human Enteric Caliciviruses -- An emerging pathogen whose time has come," Journal of Infectious Diseases, 181(Suppl 2):S249-51, 2000.

14. Payment, P, "Presence of human and animal viruses in surface and ground water," Water Science and Technology, 21:283-285, 1989.

15. Rose, J. B., et al., "Isolating viruses from finished water American Water Works Association: 56-61," 13, Hurst, C. J., Manual of Environmental Microbiology, 1986.

16. Straub, T.M., I.L. Pepper and C.P. Gerba, "Comparison of PCR and cell culture for the detection of enteroviruses in sludge-amended field soils and determination of their transport," Applied and Environmental Microbiology, 61:2066-2068, 1995.

About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds also has been a member of the WC&P Technical Review Committee since 1997.