DUAL INFECTIONS OF PRRSV INFLUENZA OR PRRSV ACTINOBACILLUS-PLEUROPNEUMONIAE IN THE RESPIRATORY-TRACT/

To study the effect of a previous porcine respiratory and reproductive syndrome-infection (PRRS) of the respiratory tract on influenza virus and Actinobacillus pleuropneumoniae (App) infections, 3-week-old spec ific-pathogen-free (spf) piglets were intranasally infected with PRRS virus. One week later, when the lung alveolar macrophages of PRRSV inf ected pigs were lowest in number, a second infection was applied by in tranasal aerosol of influenza virus H3N2 or by endobronchial instillat ion of a mildly virulent App. The first experiment consisted of two gr oups (only influenza infection or dual PRRSV/influenza infection). A s econd experiment consisted of 4 groups (only influenza infection, only PRRSV infection, dual PRRSV/influenza infection and uninfected contro ls). At day 2, 4, 14 and 21 after influenza infection, two pigs were k illed and sampled for virological and histopathological examination. I nfluenza H3N2 virus caused only a mild inflammation of the smaller bro nchioli. Previous PRRSV infection did not influence clinical signs dur ing influenza infection. Next, we studied in two experiments the effec t of dual PRRSV/App infection during the acute stage at two days after App infection. In a third experiment, the influence of PRRSV on more chronic stages of App infection was studied at two weeks after the App infection. At the end of the experiments, the pigs were killed. Lungs were ranked according to size and kind of the lesions. Lesions were c ut and measured, samples were taken for virological and histopathologi cal examination. Statistical analysis of the ranked lung-lesions in th e first experiment showed a distinct but small effect of previous PRRS V infection on the development of App-lesions. In PRRSV infected pigs, App produced a more severe disease. The second and third experiment h owever failed to show any influence of the previous PRRSV infection on the App infection. We conclude that previous PRRSV infection of the r espiratory tract of spf pigs does not necessarily enhance the severity of secondary infections of the respiratory tract. (C) 1997 Elsevier S cience B.V.