Comparative pathogenicity study of Leptospira interrogans serovar pomona strains

The comparative pathogenicity study of two Leptospira interrogans serovar pomona strains isolated from pig herds of different epizootiological status is reported. Using monoclonal antibodies (Mabs), the isolates were identified as Leptospira interrogans serovar pomona. The results obtained for the reference strain of L. pomona were identical with those of the two isolates, with the exception of monoclonal serum designated 61-7, which gave a 1:30 titre with the reference strain. In a pig herd comprising 1,000 sows, strain XVIII caused abortion in 300 animals. In the hamster pathogenicity test, this strain killed the hamsters on postinoculation (PI) days 5 or 6. The pathogen was reisolated from their organs. Strain XV did not cause abortion in a herd of 600 sows; it only elicited seroconversion in a proportion of the animals (26 out of 274 blood samples were positive, corresponding to a seropositivity rate of 9.5%). This strain did not kill the hamsters and could be demonstrated from their organs neither by isolation nor by histological examination. The blood serum of two pregnant sows infected with strain XVIII contained high titres of antibodies (1:1,600 and 1:3,200) already on PI day 10. The first urine sample of these animals, taken on PI day 10, also contained large numbers of leptospires. The pathogen was reisolated from urine samples taken at 10-day intervals and from the kidneys of the slaughtered sows. One of the two infected sows delivered four non-viable and a viable piglet. From the kidneys of the non-viable piglets, leptospires were demonstrated by histological examination. In contrast, none of the four pregnant sows inoculated with strain XV aborted. The sows did not produce antibodies detectable by the microscopic agglutination test (MAT). The pathogen could not be isolated from urine samples and from kidney samples taken from the sows at slaughter. Histological demonstration of leptospires from the sows' kidneys also failed.