AN OUTBREAK OF HEARTWATER IN WEST AFRICAN DWARF LAMBS ON PASTURE

An outbreak of Heartwater disease in young West African Dwarf ram/lambs on pasture in Ile-Ife is reported. The outbreak which started 2 weeks after being put to pasture lasted 4 weeks. It affected seven out of 24 lambs put to an ex- perimental pasture. The remaining animals in the unit (about 250) were apparently not af- fected. In 3 of the cases Cowdria ruminantium organisms were demonstrated in the endothelial cells of the jugular vein. Ticks removed from the animals and ground in sterile saline produced Heartwater disease in susceptible goats.

Heartwater and Control

Heartwater (HW) is an acute, febrile, tick-borne disease of cattle, sheep, goats, and wild ruminants characterized by nervous signs and high mortality. The disease is caused by a rickettsia agent, Erlichia ruminantium, formally classified as Cowdria ruminantium. The disease is transmitted by several ixodid ticks of the genus Ambylomma. Chemoprophylaxis has been used as a method to facilitate the movement of heartwater susceptible stock into heartwater endemic areas while allowing them to acquire immunity by limited tick exposure.

The pCS20 PCR assay for Ehrlichia ruminantium does not cross-react with the novel deer ehrlichial agent found in white-tailed deer in the United States of America

White-tailed deer are susceptible to heartwater (Ehrlichia [Cowdria] ruminantium infection) and are likely to suffer high mortality if the disease spreads to the United States. It is vital, therefore, to validate a highly specific and sensitive detection method for E. ruminantium infection that can be reliably used in testing white-tailed deer, which are reservoirs of antigenically or genetically related agents such as Ehrlichia chaffeensis, Anaplasma (Ehrlichia) phagocytophilum (HGE agent) and Ehrlichia ewingii. Recently, a novel but as yet unnamed ehrlichial species, the white-tailed deer ehrlichia (WTDE), has been discovered in deer populations in the United States. Although the significance of WTDE as a pathogen is unknown at present, it can be distinguished from other Ehrlichia spp. based on 16S rRNA gene sequence analysis. In this study it was differentiated from E. ruminantium by the use of the pCS20 PCR assay which has high specificity and sensitivity for the detection of E. ruminantium. This assay did not amplify DNA from the WTDE DNA samples isolated from deer resident in Florida, Georgia and Missouri, but amplified the specific 279 bp fragment from E. ruminantium DNA. The specificity of the pCS20 PCR assay for E. ruminantium was confirmed by Southern hybridization. Similarly, the 16S PCR primers (nested) that amplify a specific 405-412 bp fragment from the WTDE DNA samples, did not amplify any product from E. ruminantium DNA. This result demonstrates that it would be possible to differentiate between E. ruminantium and the novel WTDE agent found in white tailed deer by applying the two respective PCR assays followed by Southern hybridizations. Since the pCS20 PCR assay also does not amplify any DNA products from E. chaffeensis or Ehrlichia canis DNA, it is therefore the method of choice for the detection of E. ruminantium in these deer and other animal hosts.