Heartwater infectious disease

General info with focus on South Africa. 

Heartwater is an acute infectious disease responsible for severe losses among susceptible cattle, sheep and goats. Its causal organism, Cowdria ruminantium, is transmitted by the bont tick, Amblyomma hebraeum. In nature the disease can be transmitted only by this tick in its nymphal and adult stages and occurs only where this tick is present.

The bont tick prefers warm and moist lowveld and bushveld areas and is found in Mpumalanga, Kwa-Zulu-Natal, the Northern and Northwest Provinces, Eastern and Western Cape Provinces, as far south as Mossel Bay. It also occurs in parts of Swaziland and Botswana.

Bont tick distribution in South Africa.

Affected animals have a fever of between 40 and 42 °C, are listless, loose their appetite and lag behind the rest of the herd. As the disease progresses, movement becomes more impeded. Animals often develop a high-stepping gait. Breathing becomes more labored and animals may push their heads up against firm objects. The affected animals eventually lie on their side with the head pulled backwards and the legs extended. They are sensitive to intensive light and twitch their eyelids rapidly when exposed to it. The eyes roll from side to side in the orbita. The body temperature rapidly drops to below normal prior to death.

A postmortem examination reveals varying quantities of straw-colored fluid, which may sometimes be blood-tinged, in the chest cavity and the heart sac (hence the name of the disease). The lungs are heavy and have a soaked appearance. White foam is often abundant in the air passages. Some or all of these changes may, however, also be observed in animals dying from other causes and only a veterinarian can make a final diagnosis by microscopical examination of a smear prepared from brain tissue.

Heartwater occurs throughout the year. The incidence varies according to the tick population and its activity and the disease is therefore less frequently encountered during winter. The disease occurs when an infected tick feeds on a susceptible animal. Bont ticks become infected with the heartwater organism when they feed on an infected animal in which the organism is circulating in the blood at that particular time. Engorged larval and nymphal ticks drop from the host, moult within 4 to 6 weeks and are ready to seek new hosts on which they can feed as nymphae and adults, respectively. If such a host is susceptible to heartwater they will transmit the infection to it. Infected adult female ticks play no further role after having engorged, dropped and laid their eggs, because they then die and the heartwater organisms are not transmitted through the eggs to the next generation of ticks. Only 2 to 5 % of bont ticks found in heartwater areas are infected.

The movement of animals carrying infected ticks into or through an area free from the disease, can result in losses among susceptible stock if they are infected by ticks which drop from the infested animals. The ticks, however, seldom survive longer than one season in an area which is climatically unsuitable for them.

Apart from cattle and small stock, large and small game animals, hares and some ground dwelling birds (eg guineafowl) can also become subclinically infected with heartwater, and although they may show no clinical signs of the disease, they may have the organism circulating in their blood and therefore be able to infect ticks that feed on them. As healthy carriers of the infection, these animals and birds play an important role in the maintenance of the infection in ticks and consequently also in the occurrence of the disease. Because they cannot be dipped like cattle, these animals and birds also regularly serve as hosts on which the ticks can feed. It is clear that the bont tick, and therefore heartwater, cannot readily be exterminated on a farm where large and small game are present.

Cattle, particularly indigenous breeds, older than 1 year and newborn calves up to an age of 4 to 6 weeks, have nonspecific resistance to the disease and when infected many of them show no clinical signs, or at most develop a mild fever reaction.

Heartwater organisms seen in a brain smear (arrow).

They are magnified approx. 1000 times!

Cattle and small stock may be resistant to infection by infected ticks for two reasons: animals may be immune (either as a result of an earlier infection or through vaccination) or they may have a varying degree of natural resistance against the disease. Animals born in a heartwater area are usually, but not necessarily always, exposed to infected ticks (and become infected) when they are still young and have an inborn natural resistance. In this way they acquire an early immunity, which is thereafter boosted by regular reinfection by ticks. 

Losses as a result of heartwater most often occur when animals born and reared in areas free from the tick and the disease are moved into areas where the disease occurs (endemic areas) or when animals in endemic areas loose their immunity, because they are not exposed to reinfection by ticks (eg as a result of a too intensive dipping program).

Prevention

Ruminants younger than three weeks of age have a natural resistance to heartwater infection. This resistance does not depend on colostrum intake. When a young animal is bitten by an infected tick, it will build up an immunity, but not become ill. If. however, it is bitten for the first time when it is older than three weeks, it will fall ill and may die if not treated appropriately. Animals that are immune have to be bitten at regular intervals since they may lose their immunity if not bitten within six to nine months. It is thus of the utmost importance that animals in heartwater endemic areas (areas where it occurs naturally) should not be kept free from ticks for extended periods.

Treatment

There are numerous drugs on the market containing Oxytetracycline as the active substance. All of these drugs will be effective against the heartwater organism provided it is used timeously and a high enough dose is given. The earlier treatment is begun the higher the chances of success.

In severe cases, cortisone may be given with the Oxytetracycline. These drugs, however, have serious possible side-effects and are only available on prescription from a vet. If given to pregnant animals, abortion may ensue. Cortisone helps to stabilize the damaged blood vessels to prevent further leakage of fluid and also aids in relieving excess pressure on the brain.