Heartwater disease is a disease of importance to the livestock producers. It is a rickettsial () disease of ruminants in Africa. It is a tick-borne disease that can significantly decrease productivity in regions where it is endemic. Wild ruminants can also be infected. Heartwater is introduced into new regions by infected animals or ticks. Known and potential host ticks are widely distributed, and can be found on a variety of animals including reptiles. Once the tick vector becomes established, eradication of heartwater is difficult.
Heartwater results from infection by Ehrlichia (formerly Cowdria) ruminantium, a small, Gram negative, pleomorphic coccus in the family Anaplasmataceae and order Rickettsiales. This organism is an obligate intracellular (lives inside the cells of the animal) parasite.
- ruminantium mainly infects domesticated and wild ruminants. Cattle, sheep, goats and water buffalo can be affected. Among wild ruminants include blesbok, black wildebeest, blue wildebeest, African buffalo, eland, giraffes, greater kudu, sable antelope, lechwe, steenbok, springbok, sitatunga, Timor deer and chital.
Heartwater is endemic in most of Africa south of the Sahara desert, as well as in surrounding islands such as Madagascar, and in the Caribbean.
- Heartwater is transmitted by ticks in the genus Amblyomma. Ticks become infected as larvae or nymphs, and can transmit the disease as nymphs or adults. Ticks become infected by feeding on acutely ill or subclinically infected animals. Experimentally infected carrier sheep can infect ticks for at least seven months. Cattle can infect ticks for a minimum of eight months. Blesbok, black wildebeest, blue wildebeest, African buffalo, eland, giraffe, greater kudu and sable antelope can also become carriers. E. ruminantium is very fragile and does not survive outside a host for more than a few hours at room temperature. However, cows may transmit the infection to their calves in colostrum.
- The incubation period in natural infections is usually two weeks, but it can be as long as a month. The incubation period after intravenous inoculation is seven to 10 days in sheep and goats, and 10 to16 days in cattle.
- Peracute (severe and short period) disease is usually seen in Africa in non–native breeds of sheep, cattle and goats. Heavily pregnant cows are particularly susceptible to this form. Peracute disease is characterized by sudden death preceded by a brief interval of fever, severe respiratory distress, hyperesthesia, lacrimation and, in some breeds of cattle, severe diarrhea. Terminal convulsions may be seen. This form of heartwater is relatively rare.
- Acute disease is the most common form of heartwater in domesticated ruminants. This syndrome is seen in non–native or indigenous cattle, sheep and goats. The symptoms begin with a sudden fever, anorexia, listlessness and dyspnea. Some animals, particularly cattle, may also develop diarrhea. These symptoms are followed by neurological signs that may include chewing movements, protrusion of the tongue, twitching of the eyelids, and circling, often with a high-stepping gait.
- Affected animals sometimes stand rigidly with muscle tremors.
- Some animals may become aggressive or anxious.
- As the disease progresses, the neurologic signs become more severe, and the animal goes into convulsions. In the terminal stages, lateral recumbency with paddling or galloping movements, opisthotonos, hyperesthesia, nystagmus and frothing at the mouth are common.
- Animals with the acute form of heartwater usually die within a week.
- On rare occasions, heartwater occurs as a subacute disease with a prolonged fever, coughing and mild incoordination. CNS signs are inconsistent in this form. In subacute disease, the animal either recovers or dies within 1 to 2 weeks.
- Mild or subclinical infections may be seen in young calves, lambs or kids; partially immune livestock; some indigenous breeds; and some wild ruminants. The only symptom may be a transient fever. This form of the disease is known as “heartwater fever.”
Control and prevention
- E. ruminantium cannot survive outside a living host for more than a few hours at room temperature. For this reason, heartwater is usually introduced in infected animals, including asymptomatic carriers, or in ticks. In heartwater-free countries, susceptible ruminants from endemic regionsare tested before importation.
- All animals that may carry Amblyomma, including non-ruminant species, must be inspected for ticks before entry. In addition, ticks may be carried into a country on illegally imported animals or migrating birds.
- Outbreaks are usually controlled with quarantines, euthanasia of infected animals and tick control. During an outbreak, ticks should not be allowed to feed on infected animals. Iatrogenic transfer of blood between animals must also be avoided.
- In endemic areas, animals with heartwater can be treated with antibiotics. Tetracycline is effective during the early, febrile stages of this disease, but animals often die before treatment can be administered. Antibiotic treatment alone is not always successful in later stages.
- In endemic regions, heartwater can be prevented by tick control and vaccination.
- Alternatively, animals moved into endemic areas may be protected by prophylactic treatment with tetracycline.
- Vaccination currently consists of infection with a live E. ruminantium strain, then treatment with antibiotics when a fever develops.
- Alternatively, the vaccine may be given to young kids or lambs during their first week of life, or to calves less than 5 to 8 weeks of age; young animals possess non-specific resistance to infection, and do not always require treatment. Vaccination does not protect animals from all field strains, and revaccination is risky due to the possibility of anaphylactic reactions.
- Improved vaccines are in development. Intensive tick control may increase the susceptibility of animals to heartwater, because it eliminates the immune boosting effect of persistent exposure to small doses of organisms.
- Heartwater can be eradicated from a region by eliminating its vectors. Amblyomma ticks can be difficult to eradicate due to their high rate of reproduction, the wide variety of hosts they infest, and acaricide resistance. A regional program is effective in eradicating this disease and the vectors.