EFFECT OF TIME OF EXPOSURE TO RAT CORONAVIRUS AND MYCOPLASMA-PULMONISON RESPIRATORY-TRACT LESIONS IN THE WISTAR RAT

The effects of time of exposure on the progression of pulmonary lesion s in rats inoculated with Mycoplasma pulmonis and the rat coronavirus, sialodacryoadenitis virus (SDAV) were studied, using six groups of 18 SPF Wistar rats (n = 108). Rats were inoculated intranasally as follo ws: Group 1, sterile medium only; Group 2, sterile medium followed one week later by 150 TCID50 SDAV; Group 3, sterile medium followed by 10 (5.7) colony forming units of Ill. pulmonis; Group 4, SDAV followed on e week later by ill. pulmonis; Group 5, Ill. pulmonis followed one wee k later by SDAV; Group 6, M. pulmonis followed two weeks later by SDAV . Six rats from each group were euthanized at one, two and three weeks after the final inoculation. In a separate experiment, six additional animals were inoculated in each of groups 3, 5 and 6 (n = 18) and wer e sampled at five weeks after they had received Ill. palmonis. Broncho alveolar lavage and quantitative lung mycoplasma cultures were conduct ed on two-thirds of the rats. Histopathological examination and scorin g of lesion severity were performed on all animals. Based on the preva lence and extent of histopathological lesions, bronchoalveolar lavage cell numbers, neutrophil differential cell counts and the isolation of M. pulmonis, the most severe disease occurred in the groups that rece ived both agents. There was no significant difference in lesion severi ty between the groups receiving both agents other than in those examin ed during the acute stages of SDAV infection. Based on these results, it is evident that SDAV enhances lower respiratory tract disease in Wi star rats whether exposure occurs at one week prior to or at various i ntervals following M. pulmonis infections.