Epizootology of wildebeest-derived malignant catarrhal fever in an outbreak in the north-western Transvaal: indications of an intermediate host.

The investigation involved 37 herds of cattle numbering 6,280 animals and 5 groups of blue wildebeest (Connochaetes taurinus), consisting of 30-330 wildebeest per group. All the cases of wildebeest-derived malignant catarrhal fever encountered were associated with wildebeest and not with other game animals. Six per cent of the cases were encountered in late summer when the wildebeest calves were 3-4 months old, whereas 73% occurred in spring, when the wildebeest calves were 8-11 months old and did not excrete virus. The incidence of the disease among cattle born and reared in the vicinity of wildebeest was less than 0.5%. Among intermittently and directly exposed cattle the incidence was 5.2%, but the highest incidence was encountered in cattle kept in camps separated from wildebeest by a distance of approximately 100 m. Alcelaphine herpesvirus-1 (AHV-1), the causal agent of malignant catarrhal fever (MCF) was isolated from the tears, blood and nasal mucus of 8 out of 14 wildebeest calves during their 4th-6th month (April-June), but not subsequently. No sampling was possible before the age of 3 months. The occurrence of the disease from September-November, when wildebeest calves could not be incriminated because they no longer excreted virus, suggests the involvement of another host or an intermediate host capable of acquiring the infection from young wildebest calves, harbouring the infection until August-September, and then transferring it to cattle.