Neospora caninum in cattle in Sweden Isolation of the parasite and studies of its transmission

Stenlund S. 2000. Neospora caninum in cattle in Sweden. Isolation of the parasite and studies of its transmission. Doctoral thesis. ISSN 1401-6257. ISBN 91-576-5940-0 Neospora caninum is a coccidian parasite with a two-host life cycle including a canine definitive host. In cattle the parasite is an important cause worldwide of abortion, stillbirth and calves born weak. This thesis, which deals with neosporosis in Swedish cattle, is based on five separate publications. The brain of a stillborn calf, seropositive to N. caninum and born to a seropositive cow, was homogenised and cultured on Vero cells, where growth of Neospora-like tachyzoites was detected after 8 weeks. Electron microscopy, serology, immunoblot and polymerase chain reaction (PCR) revealed no dissimilarity between this new Swedish bovine isolate and previously published Neospora isolates, confirming that it was N. caninum. The isolate was designated Nc-SweB 1. In a herd of Swedish dairy cattle N. caninum antibodies were detected in 17 (29%) out of 58 animals. All seropositive cattle were descendants of two cows that were no longer alive. Evidence of congenital transmission of N. caninum and the apparent lack of other forms of transmission indicate the ability of N. caninum to be transmitted from dam to foetus for several generations. Neospora caninum antibody levels of 13 cows were monitored during two pregnancies, and of five cows during one pregnancy. The mean antibody titres rose by 1.5-2.5 dilution steps to reach a plateau 4-5 months before parturition. This pattern was consistent in both pregnancies indicating a reactivation of the parasite rather than a reinfection at midgestation. Four seronegative newborn calves were given cell culture-derived tachyzoites of N. caninum with their first feed of colostrum. Two calves fed by feeding bottle developed a lasting antibody response while the two calves dosed via stomach tubes remained seronegative throughout the study. This suggests that an oral route of N. caninum infection is possible in newborn calves. A N. caninum-infected herd of 110 Swedish dairy cattle was studied over 5 years to monitor the transmission within the herd and the abortion rate. During the study the proportion of seropositive animals increased from 63% to 87%. A large part of this increase was attributed to vertical transmission, but 16 animals sero-converted apparently due to an additional horizontal route of infection, possibly including infective stages of the parasite spread by a definitive host. The abortion rate was more than twice as high as that of the average Swedish herd. An increase in abortion rate was the main detectable effect in this herd where a combination of vertical and horizontal transmission had led to N. caninum infection in almost all animals. In this series of investigations the presence of N. caninum in Sweden is confirmed and it is shown how the parasite may be transmitted within herds. Processes in infected cows are demonstrated, and the potential clinical importance of N. caninum in Swedish dairy herds is illuminated.