Heartwater is an infectious, noncontagious, rickettsial disease caused by Ehrlichia ruminantium.
This heart disease affects cattle in regions of Africa south of the Sahara and the islands of the Comores, Zanzibar, Madagascar, Sao Tomé, Réunion, and Mauritius. Seroprevalence rates have been recorded at up to 50% of some cattle herds.
The disease is characterized by cerebral edema and an outpouring of fluid in the thorax.
The causative organism, Ehrlichia ruminantium, is transmitted by Amblyomma ticks to cattle. Like most tick-borne diseases, infection occurs by biting and transmission of the parasite into the blood.
Subclinical infections are common. In peracute cases, fever, hyperesthesia, lacrimation, and convulsions are observed. In the acute form, cattle show anorexia and nervous signs such as depression, a high-stepping stiff gait, exaggerated blinking, and chewing movements. Both forms terminate in prostration and convulsions. Diarrhea is occasionally seen.
Indigenous African cattle breeds (Bos indicus) appear more resistant than B. taurus breeds. Lower mortality rates are observed in young calves with a maximum mortality rate seen in 1-year-old bovines. This suggests that maternal colostrum may play a protective role.
Diagnosis is based on presenting clinical signs supported by post-mortem analysis of euthanased or dead animals. Lesions are associated with functional injury to the vascular endothelium, resulting in hydrothorax, hydropericardium, edema and congestion of the lungs and brain, splenomegaly, petechiae and ecchymoses on mucosal and serosal surfaces, and occasionally hemorrhage into the GI tract, particularly the abomasum. The typically straw-colored effusions are so high in large-molecular-weight proteins, including fibrinogen, that the fluid readily clots on exposure to air.
Clinical cases must be differentiated from a wide range of infectious and noninfectious diseases, especially plant poisonings, that manifest with CNS signs.
For treatment, oxytetracycline at 10 mg/kg or doxycycline at 2 mg/kg usually is effective. Corticosteroids have been used as supportive therapy (prednisolone, 1 mg/kg), although there is debate as to the effectiveness and the rationale for using potentially immunosuppressive drugs in an active infectious disease.
In South Africa, a doxycycline implant is available for SC deposition in the fat cushion at the caudal ear base at the time of IV infection. Young calves (<6–8 wk old) and lambs and kids (<1 wk old) are fairly resistant and may recover spontaneously from natural or induced infections.
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- Merck Vet Manual
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