West Nile virus (WNV) infects weanling pigs. These infections appear to be mostly inapparent but can result in a reduction of weight gain during the time that WNV is found in the blood. The lesions that were found in the brain and spinal cord of infected pigs have the potential of causing neurological symptoms. Studies involving laboratory animals currently underway in other laboratories indicate that different isolates of WNV can vary in their ability to cause disease. Furthermore, WNV is an RNA virus. A general characteristic of RNA viruses is their high rate of mutation. Consequently new variants of WNV can emerge that may be more virulent for pigs than the WNV that is currently circulating in the field.

The failure of WNV to cause an adverse affect on developing fetuses in a sow during the second trimester does not rule out the possibility that WNV could be a reproductive pathogen. The WNV is closely related to the Japanese encephalitis (JE) virus that is not currently in the USA. The JE virus is a known swine reproductive pathogen. Additional laboratory experiments with sows in the first and second trimester are necessary in order to more fully evaluate potential adverse affects on reproduction. These studies are expensive because they require BL3 containment facilities. Alternatively field studies could be conducted to determine if there is a direct relationship between exposure to WNV and reproductive losses. If such a relationship is demonstrated, a vaccination program may be justified.

Characterization of WNV in weanling pigs demonstrated that viremias generally persist for 3 to 5 days in individual pigs at levels that occasionally are sufficient to infect blood-feeding mosquitoes. The relative importance of the pig as a source of WNV for mosquitoes is judged to be low because the highest levels of WNV that occur in pigs are generally not sufficient to efficiently infect blood-feeding mosquitoes. However, the pig could be a source of infection for people that work in meat processing facilities and those, including consumers, who handle uncooked pork products, if viremias in WNV-infected market hogs are similar to those that occur in weanling pigs. Consequently it is important to characterize WNV in market-age pigs to determine if a risk exists.