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This article was published in 1945
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
TICK FEVER
THE DETECTION OF THE CARRIER ANIMAL.
Grahame Edgar. B.V.Sc., Senior Veterinary Research Officer, Veterinary Research Station, Glenfield.
The term Tick Fever is the popular name given to the condition affecting cattle which may be due to infection with any of the following genera of pathogenic protozoa: Piroplasma, Babesiella or Anaplasma.
Like many other protozoal infections of both man and animals, every animal which becomes infected does not succumb, a percentage survive and many of these become "carriers" of the infection. Their natural defences are stimulated by the primary infection and a degree of immunity is developed, enabling them to tolerate the pathogenic organism and so inhibit its destructive effect on the erythrocytes or red blood cells. This type of "carrier" animal remains as a reservoir to infect the vector tick, which In Australia is Boophilus microplus, and which in turn possesses the capacity to pass the infection through its eggs to the next generation of tick and so become potential vectors of the infection to cattle which they may infest subsequently.
The concentration of protozoa in the general circulation of "carrier" animals is rarely high, and if blood from such an animal is inoculated into a healthy bovine Tick Fever rarely develops. It seems that this it due to the inhibiting influence of the reticulo-endothelial system, the protozoa inoculated in the blood of the "carrier" animal being destroyed before they can multiply in the circulation, attack the red cells and produce the general systemic disturbance and characteristic syndrome of Tick Fever. The reticulo-endothelial system is located chiefly in three main sites of the body; the liver, the spleen, and the bone marrow. Research has shown that when the influence of the reticulo-endothelial system is curtailed, the inoculation of blood from a "carrier" animal gives rise to Tick Fever in the inoculated animal. This is effected by removing one of the principal sites of the reticulo-endothelial system, namely, the spleen; or in other words, by performing the operation of splenectomy.
A large number of young cattle have been splenectomised at the Veterinary Research Station and are held as a reservoir for testing any stud cattle, the owner of which has applied for permission to remove them from Tick Fever areas within the Quarantine Area on the far North Coast of New South Wales.
In performing the operation the following technique is employed: Food is withheld from the animal for 48 hours as it is very important that the rumen should not be distended, otherwise the success of the operation may be jeopardised. An hour before the operation chloral hydrate is administered as a drench and is given at the rate of four grams per 50 lb. body weight, dissolved in a quart of syrup solution prepared either with sugar, honey or golden syrup in water. In approximatply thirty minutes the animal is well narcotised and incapable of resisting any form of restraint. The animal is then placed on a table 2 feet 6 inches from the floor, lying on its right side. An area approximately two inches below the transverse processes of the lumbar vertebrae, and just behind the last rib, is shaved and the skin cleaned with alcohol. General anaesthesia is then administered; a mixture of 50% chloroform and 5O% ether being used. Successful deep anaesthesia is produced within a few minutes and seems to be assisted by the initial narcosis produced by the chloral hydrate. In our experience the animal can be kept anaesthetised for 30 minutes by the use of from 1.5 to 2 ozs. of the general anaesthetic mixture. Laparotomy is performed by an incision made about 11 inches behind the last rib on the left side and commencing two inches below the transverse processes of the lumbar vertebrae. The size of the incision is dependant upon the size of the hand and arm of the surgeon. After incising the skin, two layers of muscles and the peritoneum, the hand is inserted into the abdominal cavity and passed downwards and forwards over the arch of the rumen to the lower lip of the spleen, which is unattached. By means of blunt dissection with the fingers the lower attachments of the spleen are broken down over approximately half its length. The upper tip is then freed in a similar matter and a long handled clamp passed round the splenic artery and vein near their entrance to the hilus of the spleen. Having firmly attached the clamp, the spleen then can be withdrawn from the abdominal cavity and the splenic vessels ligated with catgut below the clamp. Ligation completed, the spleen can be detached by incising the vessels between the clamp and the spleen. The stump of the splenic vessels is replaced in the abdominal cavity and the clamp released carefully. It is well to remember that the splenic artery arises direct from the posterior aorta. The peritoneum and muscles then are sutured with catgut and the skin with nylon; and a cotton wool collodion dressing placed over the incision. The animal is lifted from the table and usually regains its feet some thirty minutes after completion of the operation. Post-operative complications are rare and after removal of the skin sutures in seven days the animals appear quite normal. Some females at the Research Station, splenectomised as calves, have gone through gestation and reared good calves.
"Carriers" are detected by taking approximately 20 mls. of blood into an anticoagulent, where it is well shaken. It is essential that the blood be kept as cool as possible after collection; being kept out of the sun and dispatched under cool conditions to the Veterinary Research Station, where it is inoculated into a splenectomised beast. The temperature of the inoculated animal is taken daily for forty days, the reason being that if the suspect animal happens to be a carrier of Babesiella, the temperature of the inoculated splenectomised animal will rise to the vicinity of 105ºF. and be accompanied by the general manifestations of Tick Fever within from five to seven days after inoculation. In the case of Anaplasma infection, however, evidence of infection in the splenectomised beast may take from 30 to 36 days; hence the necessity for keeping the animal under observation for forty days.