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A disease of ruminants caused by a specialized group of gram-negative bacteria of the order Rickettsiales, family Anaplasmataceae, genus Anaplasma. Anaplasma is an obligate intracellular parasite infecting erythrocytes of cattle, sheep, goats, and wild ruminants in most of the tropical and subtropical world. The most important species is A. marginale, which causes anemia and sometimes death in cattle. Anaplasmosis is one of the most important diseases of cattle and results in significant economic losses.Rickettsioses

Transmission of Anaplasma occurs biologically by species of hard ticks. Rickettsiae are ingested with a blood meal and undergo complex development beginning in the tick gut. Anaplasma is transmitted from the salivary glands while ticks feed. Mechanical transmission occurs when the mouthparts of biting flies become contaminated while feeding on infected cattle and then quickly move to uninfected animals. Contaminated needles and instruments for dehorning, castration, and tagging may also transmit the organism throughout a herd.

The average prepatent period is 21 days postinfection, but animals may not exhibit clinical disease for as long as 60 days after infection. In acute anaplasmosis, cattle exhibit depression, loss of appetite, increased temperature, labored breathing, dehydration, jaundice, and a decrease in milk production. Death may result from severe anemia, and abortions may occur. Recovered animals gradually regain condition but remain chronically infected and are subject to periodic relapses.

The clinical manifestations of anaplasmosis can be halted or prevented if tetracycline antibiotics are administered early. The drug inhibits rickettsial protein synthesis but does not kill the organism. Tetracyclines may also be added to feeds to prevent clinical symptoms. Controlling fly and tick populations by chemical spraying or dipping may be used to reduce the spread of disease. Vaccination with killed A. marginale from erythrocytes provides protection against the acute disease but does not prevent infection. Boosters must be given annually. The less virulent species, A. centrale, is used as a live vaccine in some countries and provides some protection against A. marginale, but it can revert to a virulent form.

Once cattle are infected with Anaplasma, they remain persistently infected and may serve as reservoirs of infection for other cattle or tick vectors. During this time the parasitemia fluctuates and at times may be undetectable. The spread of anaplasmosis may occur when these carrier cattle are moved to nonendemic areas. Ticks have been shown to transmit new infections after feeding on animals with undetectable parasitemias. Carrier animals may also have relapse infections, producing new herd infections.

Cattle which recover from clinical anaplasmosis are protected from subsequent exposure to the same geographic strain of Anaplasma. Calves have a natural immunity to clinical disease if exposed within their first year. Protection is attributed to a complex combination of antibody and cell-mediated responses.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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