Heartwater

After an incubation of usually two to three weeks (10 days to a month), a sudden high fever, which can exceed 41 degrees C, is followed by loss of appetite, listlessness, diarrhoea (particularly in cattle) and laboured breathing from lung oedema. Nervous signs develop gradually: restlessness, circling, sucking or chewing movements, a rigid stance with muscle tremors, and in cattle head pressing or aggression. The animal finally collapses, paddling with opisthotonos and nystagmus, and usually dies during such a fit. Peracute cases die suddenly. Necropsy shows hydropericardium, hydrothorax, pulmonary oedema, epicardial and endocardial petechiae, brain congestion and moderate splenomegaly.

Heartwater (cowdriosis) is caused by Ehrlichia ruminantium, an obligate intracellular rickettsial bacterium of the family Anaplasmataceae, formerly named Cowdria ruminantium. It has a two-stage cycle: infectious elementary bodies enter host cells, turn into reticulate bodies that divide inside a vacuole, and accumulate as clusters (morulae) before reverting and leaving the cell. Its key target is the vascular endothelium, including the capillaries of the brain, where the colonies can be seen in stained smears. Damage to the endothelium makes vessels leak, so fluid collects around the heart and in the chest, the lungs become oedematous, and the brain congests, producing the nervous signs and sudden deaths typical of the disease.

All domestic and wild ruminants can be infected; goats and sheep are more susceptible than cattle, European breeds more than indigenous African breeds, and farmed deer and springbok can suffer heavy losses. The disease is non-contagious and spread only by ticks of the genus Amblyomma, principally A. variegatum and A. hebraeum; Rhipicephalus microplus has recently been identified as a vector in West Africa. It occurs in nearly all sub-Saharan African countries where Amblyomma ticks live, on Indian Ocean islands including Madagascar and Reunion, and on three Caribbean islands, from which it threatens the American mainland. Recovered animals remain carriers. Human infection is disputed: WOAH describes only suspected cases based on molecular detection, while a 2023 review states human cases have been reported. Heartwater is WOAH-listed.

Tentative diagnosis rests on the presence of Amblyomma ticks, nervous signs and fluid in the pericardium and chest at necropsy. Conditions to consider are cerebral babesiosis and theileriosis in cattle, anaplasmosis, botulism, haemonchosis in small ruminants, rabies and poisonings. Confirmation is by finding reddish-purple to blue E. ruminantium colonies in capillary endothelial cells on Giemsa-stained brain smears, or by PCR on blood or organs; serology is of little use because febrile animals are still seronegative.

Vascular endothelium, brain, pericardium and heart, pleural cavity and lungs, mediastinal and bronchial lymph nodes, spleen, intestine

Control of Amblyomma ticks; infection-and-treatment immunisation with infected blood where still used; inactivated, attenuated and DNA vaccines remain experimental and are limited by strain diversity; check herds and ticks before importing animals.

Acute heartwater is usually fatal; mortality reaches up to 90% in susceptible or naive ruminants. Treatment with sulfonamides or tetracyclines is effective only early in the disease. Recovered animals become carriers.