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Heartwater

Martinez D., Vachiery N., Jongejan F.. 2012. In : Manual of diagnostic tests and vaccines for terrestrial animals : (mammals, birds and bees). Paris : OIE, p. 174-187.

Heartwater (also known as cowdriosis) is an acute, fatal, non-contagious, infectious, tick-borne rickettsial disease of ruminants caused by Ehrlichia ruminantium (formerly Cowdria ruminantium) and transmitted by Amblyomma ticks. it occurs in nearly all sub-Saharan countries of Africa, in its neighbouring islands, and also in the Caribbean, threatening the American mainland. The disease can cause high mortality (up to 90%) in susceptible domestic ruminants. Goats and sheep are more susceptible than cattle, and European breeds are generally more susceptible than indigenous African breeds. Clinically, the disease most commonly presents as an acute form characterised by a sudden high fever, depressed demeanour, nervous signs and a high mortality. Hydropericardium and hydrothorax and lung oedema are commonly associated post-mortem signs. Acute and peracute clinical forms of the disease occur: In the former, there are high death rates without many clinical manifestations and in the latter, there is a higher recovery rate. Recovered animals become carriers of infection. Certain wild animals can play a role as reservoir; Rusa deer, white tailed deer, and springbok are susceptible to this infection and can experience high mortality. Identification of the agent: The specific diagnosis of heartwater is based on the observation of colonies of E. ruminantium in capillary endothelial cells of the brain. In the absence of adequate tools, a piece of cerebellum can easily be removed with a curette through the foramen magnum after cutting off the head, while a sample of cerebral cortex can be obtained through a hole made in the skull with a hammer and a large nail. Brain smears are prepared by crushing to a paste and spreading thinly a small piece of cerebral or cerebellar cortex between two microscope slides. The capillaries are spread out in a single eel/ layer by drawing one slide across the other. The smears are air-dried, fixed with methanol and stained with Giemsa. With fast stains, the smears can be fixed and stained in less than 1 minute. The colonies (clusters) are reddish-purple to blue, and very often close to the nucleus of the infected endothelial cell. They can be scanty and difficult to find, particularly in peracute cases, but they are always present in the brain of a ruminant that died from heartwater, if not treated with drugs. Colonies are not likely to be detected in animals that were treated with antibiotics. The colonies are still visible 2 days after death in a brain that has been stored at room temperature (20-25°C) and up to 34 days in a brain that has been stored in a refrigerator at 4°C. Fresh whole blood collected from suspect animals can be inoculated intravenously into a susceptible sheep or goat. The development of clinical signs and the demonstration of E. ruminantium in the brain of the inoculated animal during the febrile reaction are diagnostic for heartwater. Due to animal welfare concerns, this method should be avoided. Ehrlichia ruminantium can be isolated from the blood of an infected host using cultivation on ruminant endothelial cells. When a cytopathic effect consisting of plaques of cell lysis appears, the presence of characteristic morulae is confirmed by staining the cell monolayer with eosinmethylene blue or by immunofluorescence or immunoperoxidase techniques using a specific antiserum. DNA probes and especially the more sensitive polymerase chain reaction (PCR) techniques are available to reveal the presence of E . ruminantium in the blood of animals with clinical signs, and in the tick vectors, to a lesser extent in the blood or bone marrow of carrier animals. Apart from diagnosis, PCR is widely used for research on the E . ru minantium genome and for epidemiological studies. Serological tests: Serological tests available include indirect fluorescent antibody tests, enzymelinked immunosorbent assays (ELISAs) and Western blot. However, when the whole E . ruminantium is used as antigen, cross-re...

Mots-clés : ehrlichia ruminantium; vaccin; immunologie; Épidémiologie; test biologique; technique immunologique; technique analytique; identification; diagnostic; amblyomma; ruminant; ehrlichia; production de vaccin

Thématique : Maladies des animaux

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