Heartwater
By Dr Mackie Hobson BSc(Agric),BVSc

1. Diseases

Spread of Heartwater

The Angora goat is highly susceptible to Heartwater (HW) so it is concerning to see the disease spreading from the marginal areas due to the favourable seasons we have been having. The ‘Bont’ tick Amblyomma hebraeum has been increasing its distribution range and outbreaks have occurred this autumn in the Jansenville district. The ‘bont’ tick has also been identified on game species within a few kilometres of Graaff- Reinet.

 Amblyomma hebraeum (‘Bont’ tick) male and female (Madder & Horak, 2009).        

The ticks spend 98% of their life cycle off the host and so are affected by environmental conditions (vegetation, temperature and rainfall) more than host distribution. Hosts include livestock and wildlife (more importantly kudu, scrub hares, guinea fowl and tortoises). The ticks do not tolerate cold weather so tend not to survive longer than a season in a newly introduced area but are more likely to remain in mountains and kloofs. In endemic areas only 7-10% of the ticks carry the Ehrlichia ruminatium organism which causes HW. Larvae and nymphs suck blood from carriers and then drop off and develop into nymphs and adults respectively (4-6 weeks). They then attach to another host and transmit the disease (3 host life cycle). Preferred sites of attachment in the goat are the belly and groin (sometimes peri-anal area) while the nymphs are found on the feet and legs and larvae are found on the feet, legs and ears.

Clinical signs:

When a tick transfers ER to a susceptible goat the organism replicates every 4 days. The numbers or ER increase to a point where they are high enough to cause clinical signs or death. The time from the tick bite to a rise in temperature (>40C) is usually 7-35 days (average of 14 in the Angora goat). The disease in Angoras usually progresses so quickly that they often collapse, go into convulsions and die without the farmer seeing other clinical signs. The first signs are a poor appetite, lethargy, fever, lagging behind the flock and as the condition deteriorates the goat can have difficulty breathing and may stand with its tongue protruding, bleat, the tail and eyelids can twitch and usually lies down. Chewing movements can sometimes be seen. The goat starts paddling with its leg and its head pulls back before dying.

Post Mortem:

On post mortem variable amounts of straw or blood tinged fluid can be found in body cavities, especially in the thorax and around the heart. This is due to the parasite invading the cells of the blood vessels, organs and brain leading to leakage of fluid. Bleeding on the muscle walls of the heart can be seen and the lungs are usually congested and foam can be  found in the trachea. In many cases in Angoras death may be so quick that few post mortem changes are present.  A definitive diagnosis is made by checking brain samples under microscope after staining with a special stain (Giemsa).

Paddling and head turned back before dying     - Straw coloured fluid in the chest cavity

If treated early there is a good chance of recovery but once severe signs are seen treatment is usually not effective. Tetracyclines, especially oxytetracycline is most effective. In the past a dose of 5mg/kg was used which is now seldom effective and a higher dose rate 10-20mg/kg is used. Doxycycline at a lower dose of 2mg/kg has also been effective. Sulphonamides have been used where oxytetracycline has failed. In addition betamethasone, prednisolone (not pregnant ewes) or DMSO as well as diuretics have been used when treating valuable animals. Please discuss treatment with your local vet.

If you have never had Heartwater on your farm but you are in a fringe area and suspect HW deaths WHAT SHOULD YOU DO?

 IMMEDIATELY:

1. Inject the whole flock with tetracycline 10mg/kg.
2. Apply tick control or dip.
3. Get a fresh goat that has just died to your local vet for a PM to confirm HW. (Get clearance by phoning the SAMGA vet on 082 860 0406 to get a reference number.)

If HW is confirmed on Post Mortem then an on-going management strategy will need to be discussed with your local or SAMGA vet.

Remember to quarantine animals that are being introduced onto your farm from HW areas. They should be blocked and treated for ticks.

 (Nothing to do with HW but new introductions should also be checked for worm egg counts and dosed accordingly. Faecal samples should be checked again 10 days after dosing for resistant strains of worms being introduced onto your farm.)

Tick control follows one of two strategies.

• In an endemic area strategic tick control is used to allow natural infection at an early age and maintain some immunity by repeated infections.
• In more marginal areas tick treatment is carried out to control all stages of the tick in an attempt to prevent disease transmission and so the animals will develop no immunity.

‘Blocking’ by injecting tetracyclines follows one of two strategies.

• The farmer ‘blocks’ less thn14 day intervals so  the HW remains sterilised and the goat develops little immunity..
• The farmer ‘blocks’ every 17 days to allow the development of some immunity. The treatment prevents HW infection for 2-4 days and if infected immediately after this the incubation period (average of 14 days) will result in a fever reaction which is treated by the next injection. Farmers using this strategy have initially suffered losses but in the long term allowed their goats to develop a better natural resistance.

There is concern as to the development of resistance to tetracyclines by the HW infection.

The future- vaccination?

The only vaccine that is currently available is still the cryopreserved sheep blood that contains the HW infection (Ball 3 strain) that involves intravenous administration and then a blocking treatment. A vaccine is needed that does not cause the disease and is easy to administer.

This has resulted in the exciting development of the live attenuated vaccine which has progressed to the point of field trials by the Animal Research Centre - Onderstepoort Veterinary Institute (ARC-OVI). Dr Latif is the project leader with co-workers Spickett, Josemans and Troskie.

To get to the point where we are today has involved a great deal of work by ARC-OVI:

• Firstly developing the attenuated Ehrlichia ruminantium (Welgovonden) vaccine.
• Secondly doing the various trials

(A very brief idea of what these trials have involved to date):

1. The developed attenuated vaccine was tested on sheep using as an intravenous injection (i/v). The vaccine proved effective.
2. The vaccine was then given by subcutaneous (under the skin) and intra muscularly routes. The intramuscular injection (i/m) proved effective and the subcutaneous(s/c) not.
3. The vaccine was then tested by i/v injection on Angoras at various doses. This was effective.
4. The vaccine was then tested by i/m route and challenged with the disease using ticks and i/v infection. This was done on both sheep and goats. The vaccine proved effective.
5. The vaccine was then tested at different strengths to find the right dose level using both i/v and i/m injections. An effective dose was determined.
6. Testes then had to be done to ensure that there would be no reversion back to virulence. (Ticks feeding on vaccinated animals then had to pass on the vaccine Wegevonden strain through 3 passages to ensure it did not revert back to disease).
7. ………….and finally the field trials that are currently being carried out..

• If these field trials are effective the vaccine can be registered. This may be the frustrating part and may take some time (2 years?).

There appears to be light at the end of the tunnel as far as HW control is concerned.