Fig 1: Heartwater organisms as seen under the electron microscope. (magnified about 2000 times )
Dr W J Grobler BVSc
Heartwater has been around since the introduction of domestic live stock from Europe to South Africa. Before the advent of Sulpha drugs and tetracyclines it used to create havoc amongst small and large stock alike. Even in these modern times millions of rands are spent each year in preventing and treating the disease and despite this animals still die from it.
The disease is caused by an organism called Cowdria ruminantium. (Fig 1) The organism is only about 2µm (two thousandths of a millimetre)in size and is transmitted by the bont tick (Amblyomma hebraeum)(Fig 2) to susceptible ruminants. When an uninfected tick feeds on a sick animal, some of the heartwater organisms land in the tick's intestines where they develop further. The mature stages of the tick will then transmit the organism via the tick's salivary glands to a susceptible animal.
Fig 1: Heartwater organisms as seen under the electron microscope. (magnified about 2000 times )
Fig 2: Bont tick male.
During the next 8 to 30 days the organisms grow in the animal's lymph nodes and then in the inner lining of blood vessels. At this stage the animal develops a high fever (40-41,5ºC) and various symptoms related to leakage of fluid from the damaged blood vessels. The two organ systems most commonly affected are the brain and the lungs. Leakage of fluid from the brain vessels leads to increased pressure on the brain and this causes the animal to become very sensitive to outside stimuli — the ears twitch, the eyes blink excessively and animals walk with a high stepping gait as if trying to walk over obstacles. At times animals chew continuously and in advanced cases become prostrate while the legs are making running movements and the head and neck is pulled backwards.
Damage to the blood vessels of the lungs causes lung oedema (water on the lung). Breathing is initially rapid, but in extreme cases animals may drown owing to the large amount of fluid. This is the fluid that runs out the nose after an animal has died from heartwater (Fig 3). In some breeds, notably Jerseys, diarrhoea may occur.
Fig 3: Lung oedema and white froth in the trachea
On post mortem examination the lungs are heavy with fluid and a clear fluid will seep out of cut surfaces. White foam in the trachea is a very strong indication that heartwater was the cause of death. An animal that died due to excess fluid on the lungs will have a blue tinge to the mucous membranes of the eyes and mouth (cyanosis). A clear fluid is often found in the heart sac, in the thoracic cavity and sometimes in the abdomen (Fig 4).
Fig 4: Excess fluid in the heart sac and chest cavity. (point at organs to view description)
A definitive diagnosis is made by examining a brain smear under the microscope — the organisms can usually be seen in the cells of the inner lining of blood vessels (Fig 5). Organisms can also be shown up in the brain and large blood vessels using a special technique (immunofluorescent staining).
Fig 5: Heartwater organisms seen in a brain smear (arrow). They are magnified approx. 1000 times!
There are numerous drugs on the market containing Oxytetracycline as 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 the drug should be given slowly into the vein, but it is very important not to stress such an animal since that may precipitate sudden death. Do not use long acting drugs for treatment of acutely sick animals, since high enough blood levels may be attained too late and long acting drugs may not be injected into the vein. These products are however a good idea for follow up treatment when a degree of recovery has occurred.
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 stabilise the damaged blood vessels to prevent further leakage of fluid and also aids in relieving excess pressure on the brain.
Diuretics may help in severe cases to rid the animal of the lung oedema, but these products may cause sheep to deteriorate. Feel free to discuss the use of these drugs with your veterinarian.
Nursing of sick animals is quite important — place the animal in a shaded area, prevent it from lying on its side and bloating, provide soft, green feed and water within reach and turn the animal four times daily on the other side to prevent bed sores. Animals that have been sick for a few days may need oral rumenotorics i.e. drugs to stimulate movement of the rumen e.g vinegar/sugar/yeast or one of the commercial products. Vitamin B-complex injection can be used to stimulate appetite and help the nervous system to recuperate.
All domestic 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.
Heartwater blood is collected from sick animals, usually sheep, and then frozen at -190ºC. When the blood has thawed the organisms are still alive for about 30 minutes and it is very important that the blood be given intravenously within this period. Young animals younger than three weeks should not become ill after being given blood, but any animal older than three weeks has to be monitored closely. The safest way to go about treating these animals is to take rectal temperatures twice daily from about the fifth day after having given blood. When the temperature rises to above 40ºC the animal should be treated as described above. Cortisone is not administered at this stage so as not to impede an immine reaction. Pregnant animals should preferably not be given blood as a fever reaction may cause abortion.
Ruminants that are moved from non-heartwater areas to heartwater areas(see fig 6) have to be given blood and treated as described above. Calves born from such cows and which are bitten before three weeks of age will develop an immunity. Calves born in winter months when tick numbers are low have to be monitored closely and should preferably be given blood and treated early the next summer.
Fig 1, 3-5: Bristol Biomedical Image Archives © University of Bristol 2000
Fig 2, 6: Scientific pamphlet No. 393, Dept. Agricultural Technical Services
1983
