Heartwater (HW) is an acute, febrile, tick-borne disease of cattle, sheep, goats, and wild ruminants characterized by nervous signs and high mortality. The disease is caused by a rickettsia agent, Erlichia ruminantium, formally classified as Cowdria ruminantium. The disease is transmitted by several ixodid ticks of the genus Ambylomma. Chemoprophylaxis has been used as a method to facilitate the movement of heartwater susceptible stock into heartwater endemic areas while allowing them to acquire immunity by limited tick exposure.
TopIntroduction
Heartwater (HW) is an acute, febrile, tick-borne disease of cattle, sheep, goats and wild ruminants characterized by nervous signs and high mortality (Camus et al. 1996). The disease is caused by a rickettsia agent, Ehrlichia ruminantium (Dumler et al. 2001). The disease was formerly classified as Cowdria ruminantium (Cowdry, 1925). The disease is transmitted by several 3-host ticks of the genus Ambylomma (Henning, 1956). Although therapeutic drugs are available, treatment often fails because the disease usually runs a very rapid course.
Heartwater was first diagnosed in South Africa by Lois Trichardt in 1838 in his sheep 3 weeks after they had a heavy tick infestation. In Kenya the disease was first reported By Daubney (1939-40). Documentation of heartwater in Kenya like most other African countries is negligible and comprises annual Veterinary reports and from authors who have attempted to carry out some studies on heartwater (Kocan et al. 1987).
Heartwater is not a notifiable disease in most countries and is overshadowed by the threat from East Coast fever, which is the major tick-borne disease limiting livestock production in many countries in East, Central and Southern Africa. Few actual cases of heartwater are reported occasionally by field Veterinarians and other workers as a by- the way even in areas where Ambylomma ticks are endemic.
Since the tick vector is extremely sensitive to acaricides, strict tick control has caused the vector to all but disappear, together with the disease from farmlands where a majority of ruminants are kept.
In Ambylomma-endemic areas, the disease incidence in indigenous animals is low and most farmers are not encouraged to keep exotic animals, creating the impression that heartwater is unimportant. Coupled with the difficulty of diagnosing the disease clinically and at post mortem, most cases go unreported even with experienced staff.
Background
With the advent of ECF immunization, it is anticipated that farmer’s response to strict regular acaricidal control of ticks will be reduced in favor of strategic control (Purnell, 1984). Ambylyoma ticks which had been eradicated in high potential areas are therefore likely to re-establish and spread heartwater in susceptible populations and cause massive livestock deaths. The Abylomma ticks are also likely to spread in other areas suitable for the tick survival. There is therefore the need to develop more effective vaccines for heartwater bearing in mind that there may be different antigenic variations of Ehrlichia parasites in different areas.
Aetiology And Life Cycle
The causative agent of heartwater is a rickettsia with a trans-stadial transmission cycle in the Amblyomma ticks and a wide ruminant host population. It has been shown that the rickettsial organism is transmitted between ruminant and vector host while undergoing a simple development cycle in distinct reticulate bodies (Rbs), elementary bodies (EBs) and intermediate bodies (IBs), (Jongejan et al. 1991a). It is thought that single elementary bodies are picked up by the tick while feeding. These invade the gut epithelium where they multiply and invade other organelles via the haemolymph. They find their way to the salivary glands and from there, they are injected into the ruminant as single organisms that eventually find their way to the endothelium of blood vessels. In endothelium tissue culture, the infectious cycle lasts 5-6 days (Jongejan et al. 1991). They multiply greatly in the host endothelium and are released by disruption into circulation from where they are picked up by the tick as EBs colonies of the organism have so far been demonstrated in gut, salivary gland cells, haemocytes and malpighian tubules of infected Amblyomma ticks (Kocan and Bezuidenhout, 1987). With the ELISA test, the parasite was revealed in various tissues and haemolymph of adult A. hebraeum. The gut, salivary gl ands, hypodermis and synganglion showed the highest concentration of the parasites in females while in males it was in the salivary glands and gut (Viljoen et al. 1988).