Tick Bite fever (Ehrlichiosis)
Dr W J Grobler BVSc
Few people are aware that there is a vast difference between biliary fever and tick bite fever. It is quite easy to diagnose biliary on a blood smear, but it is often very difficult to confirm a diagnosis of tick bite fever. The disease is caused by a bacterium that is transmitted by the Kennel tick (Rhipicephalus sanguineus). This bacterium called Ehrlichia canis needs live cells to survive, a characteristic of only a small number of bacteria.
There are usually three stages in the course of the disease:
- Acute stage
- Only lasts a few days after which temporary or permanent recovery occurs.
- Subacute stage
- This stage may last weeks or months during which bouts of illness are followed by bouts of recovery.
- Chronic stage
- After a period of months or even years the dog's immune system is broken down until death due to secondary infection or haemorrhage is inevitable.
The following symptoms may be seen at any stage and in any combination during the course of the disease:
- High fever
- Total or partial loss of appetite (dogs will usually prefer meat or table scraps)
- Pussy discharge from the nose and/or eyes
- Bluish discolouration of the cornea
- Excessive salivary flow
- Bleeding from the nose or gums (in chronic bleeding the teeth may appear black)
- Vomiting and diarrhoea
- Swelling of the hind limbs and scrotum
- Weakness or paralysis especially of hindquarter
- Haemorrhages in the skin especially on the abdomen
- Acute death due to severe internal bleeding (Fig. 1 and 2)
Fig. 1 Bleeding in the lungs in a case of tick bite fever
Fig. 2 Haemorrhages in the kidneys in a case of tick bite fever
Only in a small percentage of cases the organisms are seen in a blood smear (Fig. 3). The cluster of organisms is called a morula (mulberry). In the majority of cases, especially subacute and chronic cases the organisms are seldom seen and other diagnostic methods necessary to confirm a diagnosis. Antibodies of two types occur in sick animals — the first indicates a recent infection (IgM), while the other (IgG) shows that the dog was infected some weeks or months before. In the event that both types are found, it means that the dog is in the subacute stage of the disease. If only IgG is found, it shows that the dog was infected at some time in its life, but it doesn't necessarily indicate that an active infection is present.
Fig. 3 Tick bite fever organisms (morula) in a white blood cell (arrow).
At present a test picking up minute quantities of DNA is available. This test is extremely sensitive, especially if a biopsy from the spleen or lung is tested.
Only one antibiotic has been proven without doubt to kill the tick bite fever organisms when used at practical doses. One of the drugs used against biliary has been reported to work, but it has never been proven. The treatment needs to be given for extended periods usually starting off with three days' intravenous antibiotics followed by oral treatment for another one to three weeks. In cases where pneumonia or other infections are evident, additional antibiotic cover may be needed.
Vitamins are prescribed to stimulate appetite. If bleeding occurs, coagulants may be tried, but in severe bleeding a fresh blood transfusion is indicated. Anabolic steroids are sometimes administered to build the dog's strength and to help with the repair process. Good nutrition is important to give the dog the best chance to fight off the infection.
There is no vaccine available against tick bite fever. It is likely that a vaccine may never be developed as the natural immunity against tick bite fever is not very effective. The most important preventive measure is to keep animals as free from ticks as possible. This means regular dipping (weekly in summer) or making use of long-acting tick collars or tick sprays. Since it is not always possible to keep dogs free from all ticks, it is important to have a sick dog seen to as soon as possible.
Tick bite fever is generally a chronic disease, although acute deaths may occur. In most cases an early diagnosis and treatment will lead to recovery, but diagnosis is difficult in a large proportion of cases. Dog owners are requested not to delay taking a sick dog to the vet as early, timely treatment is important to keep animals in good shape.
Fig 1, 2 en 3: Bristol Biomedical Image Archives © University of Bristol 1999