Economically speaking, heartwater is one of the most important tick-borne diseases in cattle, sheep, and goats in Zimbabwe. It is an acute, fatal, infectious but non-contagious, tick-borne disease of ruminants caused by Ehrlichia ruminantium (formerly Cowdria ruminantium) and transmitted by Amblyomma ticks, and the bont tick in particular.
The disease can cause a high mortality of up to 90% in domestic ruminants. Goats and sheep are more susceptible than cattle, and European breeds are generally more susceptible than indigenous African breeds.
The disease can cause considerable losses to farmers, mainly through mortality, treatment, and control costs. Like other tick-borne diseases in Zimbabwe, heartwater has historically been controlled through acaricide dipping to control the ticks that transmit the disease, but it has become costly.
The disease is limited to the geographical limits of the distribution of these ticks. The spread of the disease may be related to the increased movement of cattle and wildlife and the frequency, or the reduced intensity of dipping.
Heartwater has historically been limited to the southern and western low velds of Zimbabwe. Since 1986, however, cases of heartwater have been diagnosed more often in the central and eastern regions of the previously heartwater-free highveld plateau.
At present, the distribution of heartwater and its carrier ticks – or vectors – in the highveld is still largely restricted to the central and eastern regions. The northern regions of the highveld appear to be mostly uninfected, though it is likely that heartwater will eventually spread further, which may have a considerable impact on livestock production in Zimbabwe.
Heartwater is widespread in all southern African countries except Namibia, which is too dry to support Amblyomma ticks. In other countries, these ticks are most often concentrated in low-lying bushveld and along river valleys. In Malawi and Madagascar, Amblyomma variegatum is recognised as the exclusive vector of heartwater as A.hebraeum and A.pomposum are absent. In South Africa, A.hebraeum is the only major vector species.
The symptoms are noticeable in sheep for seven to 35 days, and nine to 29 days in cattle after infection. In most cases, heartwater causes a sudden rise in body temperature, which may exceed 41°C within a day or two after the onset of fever.
Fever is followed by a loss of appetite, sometimes listlessness, diarrhoea, particularly in cattle, and laboured breathing that is an indication of lung oedema, which means excessive liquid accumulation in the tissue and air spaces of the lungs.
Nervous signs develop gradually. The animal is restless, walks in circles, makes sucking movements, and stands rigidly with tremors of the muscles under the skin. Cattle may push their heads against a wall or behave aggressively or anxious.
Eventually, the animal falls to the ground, pedalling with its legs and showing an abnormal posture caused by spasms or contractions in the muscles of the neck, back, and legs. The eyes may move rapidly and uncontrollably, and the animal makes chewing movements. Following such an attack, the animal usually dies.
Acute and less acute forms of the disease occur, and some cases may end in death, while there is a higher rate of recovery in others. Recovered animals become carriers of infection. Certain wild animals, like springbok, are susceptible and can experience high mortality, but if they survive, they can play a role as a reservoir for the disease.
Diagnosis is difficult because there are no obvious stages in the blood of infected animals; the principal area where the disease develops is in the blood vessels of the brain.
The resultant brain damage causes typical nervous symptoms like an unnatural high-stepping gait, but a positive diagnosis is usually obtained by examination of smears taken from the brain.
Cattle are prone to heartwater when young home-bred calves are exposed to heavy Amblyomma tick presence at the start of the spring rains, or when susceptible young animals are brought into a farm where heartwater is endemic.
Regular dipping at set intervals, as well as vaccination, can be used to prevent the disease. Start treating the animal with an antibiotic containing oxytetracycline as soon as the disease has been diagnosed and give supportive treatments to help the recovery process.
A short series of injections of Terramycin/LA may provide cover against the disease during the first month of exposure to ticks and may render the cattle immune to further infections.
Contact the local veterinarian or veterinary extension officers to confirm the disease, to provide a prognosis and to suggest treatment protocols.
Lifecycle of the bont tick
To understand the disease, it is important to take note of the bont tick’s life cycle and the role it plays in the spread of the disease.
Amblyomma hebraeum is known as the bont tick because of the brightly coloured pattern on the scutum or shield on the back and the banded or striped legs. Bont ticks are large ticks with long mouthparts.
The bont tick gets infected when it feeds on infected or carrier animals and then transmits the disease to a susceptible animal when they feed on the next host.
The bont tick is a three-host tick, which means the larvae, nymph and adult stages attach to different animals. In favourable conditions, the lifecycle can be completed within one year, otherwise, it can stretch over three years.
The female ticks lay up to 18,000 eggs. These can take up to five months to hatch into larvae, which is the first active stage. The larvae attach to the host and feed for seven to fourteen days before they turn into the nymphal stage.
After moulting, the nymphs may remain inactive for some months before attaching to a host. They feed for seven to fourteen days and once engorged, drop off the host animal to change into adult ticks.
The adults attach to their preferred hosts and feed for about seven to nine days, before dropping off and start laying eggs.
Bont tick attachment sites
- The larvae, nymphs and adults attach to different areas on the animal.
- The larvae attach to the animal’s feet, legs, muzzle, and head.
- The nymphs attach to the feet, legs, groin, and neck.
- The adult ticks attach in clusters on the hairless parts of the body underline, the groin, axillary cavity, dewlap, belly, perineum, and peri-anal region.
With heavy infestations, the bites inflicted with the long mouthparts can cause wounds which may develop secondary bacterial infection. Sometimes the wounds become infested with cattle blowflies or maggots.
Bont ticks occur throughout the year. Dip animals strategically and frequently to reduce the tick infestation. Spot treatment can be used by spraying only the sites on the body where ticks usually attach.
In peak seasons, dipping intervals need to be increased to reduce tick numbers. Use a 5,5,4-day dipping interval strategy when there are many ticks.
The bont tick also transmits the single-celled protozoan that causes benign theileriosis in cattle and Rickettsia conori and Rickettsia africae that cause tick-bite fever in humans.
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