Last update: November 22, 2010 01:52:15 PM E-mail Print

 

Heartwater in Southern Africa

 

P. G. Marais

 

Grootfontein Agricultural College, Private Bag X529, Middelburg, 5900 Eastern Cape, South Africa

 


Heartwater (cowdriosis) is a tick-borne disease of cattle, sheep, goats and some wild ruminants and is caused by a rickettsia, Cowdria ruminantium. The economic importance of heartwater as a disease of domestic ruminants in Africa is surpassed only by East Coast fever and trypanosomiasis. In the endemic areas in South Africa, mortalities due to heartwater are three times greater than the combined deaths caused by babesiosis and anaplasmosis. The losses from heartwater in South Africa during the period 1981 - 82, represented 30.3 % of 1.54 million head lost to the three major tick - borne diseases; heartwater, anaplasmosis and redwater. The cost incurred from losses due to heartwater was estimated to lie between 21.2 and 54.5 million Rand. Animals exposed to heartwater respond in a variety of ways, depending on a number of factors, including species, breed, age, immune status and stock of Cowdria . Exotic breeds of sheep and goats are of uniformly high susceptibility and mortalities of 50 % or greater due to heartwater have been seen in sheep  and cattle imported into sub-Saharan Africa. Merino sheep are highly susceptible. Goats, particularly Angoras, are the most receptive to heartwater of all domestic ruminants, with mortalities surpassing 90 % in imported stock. ln contrast, annual mortality, in indigenous goats of  an endemic area has been estimated at around 10% . Wild ruminants may either be refractory or receptive to infection, but of 11 species of non-African ruminants tested, all responded to infection with clinical signs and/or death. The innate resistance of indigenous breeds in endemic areas has not been fully examined and, in many instances, it is not possible to judge whether resistance is a characteristic of the breed, or if the  resistant state has been acquired, through natural selection that favours survival of  the least susceptible animals upon prolonged exposure. Heartwater is one of the major obstacles (in some instances the most important one) to the introduction of high-producing animals into Africa with the aim of upgrading or replacing local stock. It is also of particular importance when susceptible animals are moved from heartwater-free to heartwater-infected areas and it remains a cause of mortality even in endemic areas, especially amongst small stock.