Bovine anaplasmosis

In a herd, a dead cow is often the first thing noticed. Affected adults lag behind, stop eating and lose condition and milk yield quickly; fever, pale then yellow mucous membranes, rapid heart and breathing rates, weakness and incoordination follow. Oxygen-starved cattle may turn aggressive, and pregnant cows may abort. Peracute cases can die within hours of signs appearing. Recovery is slow. At necropsy the carcass is markedly anaemic and jaundiced, with an enlarged soft spleen, mottled liver, a distended gallbladder of thick bile, fluid in body cavities and pinpoint haemorrhages.

Bovine anaplasmosis is caused by Anaplasma marginale, an obligate intracellular rickettsial bacterium (order Rickettsiales, family Anaplasmataceae) that lives inside red blood cells, where it appears in stained smears as dense dots near the cell margin. The organism multiplies in the bloodstream and, at peak, a large share of red cells can be parasitised. The immune system removes infected red cells, and uninfected ones with them, outside the blood vessels, so the result is a progressive anaemia without free haemoglobin in blood or urine. Jaundice appears late in the disease. Poor oxygen supply to the brain is linked to the aggression some cattle show. Animals that survive stay infected for life as carriers. A related species, A. centrale, causes only mild infection.

Only cattle are covered here; A. marginale has not been reported in people. It occurs in most tropical and subtropical countries and some temperate regions. Transmission is biological through ticks, notably Rhipicephalus and Dermacentor species, and mechanical through biting flies such as Tabanus and through blood on reused needles or surgical, dehorning and castration equipment; transplacental infection also occurs. Susceptibility rises with age: calves under a year usually show little or nothing, and severe disease is concentrated in adults. Bos taurus breeds appear more susceptible than Bos indicus. In northern Algeria, 4.0% of 299 clinically ill cattle presented to veterinary clinics carried A. marginale, with cases peaking in July during the tick season and some co-infected with Theileria annulata. Carrier cattle are the reservoir.

Babesiosis, often endemic in the same areas, is the main confusion: it produces haemoglobin in blood and urine, which anaplasmosis does not. Theileriosis and leptospirosis also cause anaemia or jaundice. Confirmation in every case rests on finding the organism: Anaplasma marginale inside red cells near the margin on a Giemsa-stained blood or organ smear, or by PCR; A. centrale sits toward the cell centre.

Blood (red blood cells), spleen, liver, gallbladder, lungs, mucous membranes, reproductive tract (abortion)

Tick and biting-fly control, a fresh or sterilised needle and clean instruments for every animal, testing herd additions by ELISA, and live A. centrale vaccination of calves in countries where it is permitted.

Untreated newly infected adult cattle may suffer 30-50% mortality (Illinois Extension); calves rarely die. Early tetracycline treatment helps, but late anaemic cases may not tolerate handling. Survivors recover slowly and stay lifelong carriers.