Use of an animal model in the assessment of the risk of human viral disease transmission associated with surface application of treated wastewater onto lawns

Land application of treated wastewater is increasing in the United States and developing countries. One of the major barriers to use of treated wastewater for irrigation is the uncertainty associated with potential disease transmission from contact with human enteric viruses in wastewater. This study evaluated the feasibility of using pigs as an animal model to assess the risk of human viral infection from land application of treated wastewater. The model system consisted of 3-week-old pigs and porcine enterovirus-serotype 2 (PE2). Pigs were used because the porcine digestive tract and the associated porcine enteroviruses are similar to those of man. Oral challenge inoculation experiments were carried out to examine antibody production in pigs as a result of infection with PE2 virus. Piglets free of antibody were intragastrically inoculated with 0, 100 or 100,000 CCID₅₀ of PE2 virus. Serum was monitored to detect antibody to PE2 at 3, 7,10 and 13 days post administration. By 13 days post administration all pigs challenge inoculated with as little as 100 CCID₅₀ of PE2 virus had seroconverted. To evaluate the potential for using pigs to assess the risk of viral infection from contact with virus on lawns, piglets free of antibody to PE2 were placed on grass plots seeded with known concentrations of PE2 virus and serologically monitored for production of PE2 antibody. A total of 52 pigs were exposed for two hours to concentrations of 4 x 10² to 4 x 10⁵ CCID₅₀ virus seeded to 0.36 m² grass plots. Incidence rates of infection were calculated for each exposure concentration in order to examine the nature of the dose-response relationship resulting from a single exposure to virus seeded to grass. The incidence rate of infection among pigs exposed for two hours to 4 X 10⁵ CCID₅₀ PE2 virus seeded on grass was 66.7. The incidence rate dropped off dramatically as the virus concentration seeded onto plots decreased. No pigs exposed to grass plots seeded with 400 CCID₅₀ of PE2 virus became infected. The results demonstrated that pigs provide a simple, efficient means for providing an indication of the risk of viral infection to humans who may come in contact with virus in wastewater applied to lawns. Information gained from studies using pigs for this purpose can be used to examine the dose-response relationship and establish best practicable management procedures which will limit potential health and environmental effects from use of treated wastewater for irrigation.