Anaplasma spp are a rickettsiales (family Anaplasmataceae) bacteria which causes anaplasmosis (tick-borne fever), anaemia and abortion in cattle. Anaplasmosis is of significant economic concern to the beef and dairy cattle industries.
Found in tropical and subtropical regions worldwide, Anaplasma species are obligate intraerythrocytic parasites of cattle. Common species which cause disease include:
- A. marginale - worldwide
- A. centrale - mild disease
- A. phagocytophilum (formerly Ehrlichia phagocytophilum & E. equi) - non-pathogenic
- A. platys (previously E. platys)
Anaplasma spp are transmitted to cattle via insect vectors, primarily ticks including Rhipicephalus microplus (formerly Boophilus microplus), Dermacentor spp, Ixodes spp, Hyaloma spp and Ornithodorus spp. The bacteria circulate within cattle blood, do not cross the blood-brain barrier but may cross into the placenta, allowing vertical transmission and birth of infected calves. A. marginale and A. phagocytophilum can also cause persistent infection in adult cows.
Mechanical transmission by contaminated needles, during dehorning and by Culicoides spp midges and some other biting flies is believed to also occur.
Anaplasmosis may also be spread through the use of contaminated needles or dehorning or other surgical instruments.
Clinical signs appear to correlate with the duration of exposure and cattle breed (Bos indicus relatively resistant). Young calves are often subclinically infected unless coinfected with other diseases such as bovine viral diarrhoea or infectious bovine rhinotracheitis. Anaplasmosis usually manifests in cattle over 1 year of age and can be fatal in aged cattle.
Signs are related to erythrocytic phagocytosis, and anaemia is a common presenting signs. Fever, dyspnea, depression, lethargy, anorexia and recumbency are also observed in acutely affected cattle. Pregnant cows may abort. Haematological tests reveal a regenerative hypochromic, macrocytic anaemia. Icterus is often a terminal sign.
In contrast to Babesia spp infections, haematuria and hemoglobinuria are rarely observed.
Diagnosis is based on circumstantial evidence of clinical signs referable to tick-borne fever, supported by laboratory identification of intraerythrocytic parasites. Identification of Anaplasma spp in the laboratory can be confirmed by use of light microscpy using Giemsa staining, supported by ELISA, complement fixation and card agglutination. Serological cross-reactivity appears to be common and PCR testing is required for definitively establishing speciation of parasite.
Subunit vaccines have been tested experimentally in cattle but do not appear effective at preventing anaplasmosis.
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