A sequential study of experimental porcine paramyxovirus (LPMV) infection of pigs: immunostaining of cryostat sections and virus isolation

La Piedad Michoacan Paramyxovirus (LPMV) is a recently recognized paramyxovirus infecting pigs throughout Mexico. Disease syndromes observed in field cases associated with LPMV infection include neurologic, respiratory, and reproductive disorders. Clinical signs and the distribution of LPMV virus and antigen in tissue samples from pigs experimentally infected with LPMV by natural routes were studied. Severe neurologic disease and death occurred following experimental inoculation of 3- and 17-day-old pigs. All of the pigs inoculated at 3 days of age were either dead or moribund by 8 days after inoculation, whereas 30% of the pigs inoculated at 17 days of age were affected. Virus was consistently recovered from or demonstrated in tissues from the respiratory tract of both groups of pigs. LPMV and antigen were also demonstrated in central nervous system (CNS) tissues from these pigs; however, differences in virus distribution within the CNS were demonstrated in the 2 groups. In the pigs inoculated at 17 days of age, isolation of LPMV was restricted to the olfactory bulb and midbrain. In contrast, in the pigs inoculated at 3 days of age, isolation of LPMV was more widespread throughout the CNS tissue examined. Virus excretion studies indicated that nasal spread of LPMV was more important than fecal spread. Comparatively large quantities of infectious LPMV were consistently recovered from urine samples of experimentally infected pigs.