Also known as
Nagana, nagana pest, sleeping sickness, sleeping disease, bovine malaria
Description
Animal Trypanosomiasis is a disease of all species of animals, but especially cattle, and caused by a protozoan parasite that gets into the bloodstream via biting flies. It is most common in Africa and is not seen in North America.
Animal Trypanosomiasis is a progressive disease and usually fatal. The protozoan parasite multiplies rapidly in the bloodstream of the infected animal, causing blood clotting and blockage of vessels and capillaries, with subsequent anemia and lack of oxygen to organs and body tissues, causing damage to all organs.
The parasite also passes through the placenta in pregnant animals and into the fetus, causing some cows to abort or give birth to premature calves. Infection of the brain sometimes occurs, occasionally as a relapse after an animal seems to have recovered following treatment.
Wild animals serve as reservoirs of infection, as well as some domestic animals that act as carriers--some with chronic disease. Most wild animals and some domestic ones seem to establish a balance with the parasite and serve as relatively healthy carriers.
Some breeds of cattle native to West Africa, such as the N’Dama (a Bos Taurus breed) and the Baoule have a natural tolerance and are more resistant to the disease than West African zebu (Bos indicus) cattle. By contrast, the Orma Boran zebu has superior resistance. Some lines of cattle within breeds have more resistance than others.
Signs
- Dullness
- Loss of appetite
- Discharge from eyes
- Significant weight loss Anemia
- Fever
- Edema of throat and underline
- Inflammation/swelling of lymph nodes
- Dermatitis (inflammation of the skin)
- Nervous disorders
- Diarrhea (sometimes)
- High mortality rate
Causes
The disease is caused by trypanosomes (parasitic protozoa) of several species in the genus Trypanosoma. The tsetse fly in Africa is commonly associated with spread of this parasite, which is transmitted via the mouthparts of these biting flies. The parasite spends part of its life cycle in the fly. Some areas of Africa are not very conducive to cattle raising because of the prevalence of tsetse flies.
Trypanosoma vivax causes a disease called nagana, mainly in West Africa, although it has now spread to Central and South America. This particular parasite can be spread mechanically by other types of biting flies (such as horse flies and stable flies) in the absence of tsetse flies.
The trypanosomes infect the blood of the host animal, causing fever, weakness, lethargy and loss of appetite, which lead to weight loss and anemia; in some animals the disease is fatal unless treated.
Most types of trypanosomes protozoa can develop in tsetse flies (Glossina spp.), the biological vector. When an infected fly bites an animal, the parasites are transmitted through the fly’s mouthparts and saliva. They can also be spread by surgical instruments, needles, and syringes that contact the blood of an infected animal and then a susceptible animal.
After exposure, the incubation period ranges from 4 days to 8 weeks. The immune response of some animals may be unable to eliminate trypanosomes completely, and the host may become an inapparent carrier. These hidden infections can be reactivated if the animal is stressed.
Severity of disease may depend on the strain of the protozoan, the number encountered, and susceptibility of the host animal. There may be an acute episode lasting for a few days, after which the animal dies or goes into a chronic stage, or it may be chronic from the beginning—with periodic bouts of severe illness or sometimes eventual recovery. Fever may be intermittent and last for a long time.
This disease cannot be diagnosed with certainty (since it resembles other choric diseases and has no definitive clinical signs or postmortem lesions) except by detecting the parasites by microscopic examination of blood, lymph node fluid or cerebral-spinal fluid, or by use of various laboratory tests. The indirect fluorescent antibody test (IFAT) has been used in detection of trypanosomal antibodies in animals and humans. Antigens are usually prepared from blood smears.
Prevention
Control of the tsetse fly is the best prevention, but difficult; it requires a diligent regimen of spraying the animals, spraying foliage in pastures that might harbor the flies, and brush control to remove the flies’ habitat. Most attempts at control are aimed at prophylactic dosing (for prevention) of animals periodically with certain drugs that are effective against the parasite. This can help protect the animals during times they are exposed to the disease.
If an outbreak is detected early, it might be halted by quarantines, control of cattle movement, and euthanasia of infected animals. Tsetse fly populations can be reduced or eliminated by use of fly traps, insecticides, and by treating infected animals with anti-parasitic drugs.
Prophylactic treatment every few months may be the only possible strategy in areas with large populations of infected tsetse flies during the main fly season. The duration of effectiveness of any drug used will be extended if the animal also develops some protective antibodies—sometimes providing protection that might last as much as 5 months. However, most cattle producers in at-risk regions give their animals 4 or 5 treatments per year.
Treatment
There are several trypanocidal drugs that are very effective but they also have some challenges. Since they are often used in healthy animals as a preventative, they may be used a number of times in the same animal, leading to drug resistance in the protozoa. Various different drugs may need to be rotated (used alternately) to avoid resistance to the primary drug.
Safety range for these drugs is narrow, and many of them may cause severe local reactions. Relapses are also common if treatment was not begun early in the course of the disease or the dose rate was too low.