Trypanosoma cruzi
Holly Frisby, DVM, MS
Veterinary Services Department, Drs. Foster & Smith, Inc.
Ever hear of 'kissing bugs?' Read on. Trypanosoma cruz is a protozoan (one-celled) parasite that causes a disease called American trypanosomiasis or Chaga's disease in man. The insects that pass T. cruzi from one host to the other are called kissing bugs.
T. cruzi is found in South and Central America where it is a significant cause of disease in humans. It is estimated 16-18 million people are infected in South and Central America. T. cruzi is found rarely in the southern United States. Small animals such as cats, dogs, guinea pigs, rats, and opossums can serve as reservoirs for the parasite. They can be very important in the spread of the parasite. T. cruzi generally does not cause the significant disease in animals as it does in people.
How does T. cruzi reproduce?
The life cycle is fairly complex and we will not go into all the names of the various immature forms. Basically, kissing bugs feed on the blood of mammals like mosquitoes do. The bug generally bites at night and the bite is not painful. The bug ingests the mature form of T. cruzi from the infected mammal. The parasite goes through several developmental stages in the bug's digestive system. Unfortunately, the hygienic practices of the bug are not too appealing — while feeding, it defecates on the mammal and T. cruzi enters the skin through the bite wound or perhaps an abrasion. It can also enter the mammal through the lips, nose, or eyes that may be contaminated with the bug's feces.
Inside the mammal, T. cruzi enters muscles and other cells and starts reproducing by dividing in two. After a few days, the cells rupture and T. cruzi can either enter the circulation and be ingested by the bugs, or enter other cells where they repeat the process of reproducing and rupturing the host's cells.
Rarely, perinatal infection can occur, both through in utero transmission (across the placenta), and transmission through ingestion of milk from infected females. You get three points for remembering another parasite that can be transmitted like this. (Hint: the most common intestinal worm in puppies.)
How do cats and dogs enter into the life cycle?
Animals can become infected through bites of the bug, eating infected bugs, or eating smaller infected animals. T. cruzi can be spread from animals to man and back again through the bites of the kissing bug.
What harm does T. cruzi cause to cats and dogs?
Generally, the reservoir hosts such as cats and dogs do not show signs of the infection. In dogs, however, weakness, anemia, and an enlarged spleen and lymph nodes may develop in severe infections. Rarely, if large numbers of the parasite entered the heart, an inflammation of the heart may occur, and the dog may suddenly collapse and die. In others, the diseased heart may fail slowly and could be confused with more common types of heart disease. Some dogs show profound weakness and incoordination. Cats may have convulsions and paralysis of the hind limbs. If a large number of the host's cells rupture at the same time, fever may occur.
What harm does T. cruzi cause to humans?
This is a very serious disease in South and Central America. In the acute phase, people have fever, lethargy, and enlarged lymph nodes and spleens. It can be life threatening if the heart or brain is involved. The acute form of the disease most commonly occurs in children. Adults tend to have a more chronic form that is characterized by heart disease, and abnormalities of the digestive system. These symptoms occur 10-20 years after the person is infected.
How is infection with T. cruzi diagnosed?
Often, the numbers of the parasite in the blood are very small. Sometimes, however, by making a thick smear of blood on a microscope slide and looking at it under high magnification, the organisms can be seen. Using a similar procedure, looking at cells from the lymph nodes may reveal the parasite.
Serologic tests have been developed for the diagnosis of trypanosomiasis.
In animals, the parasite may be found in the heart muscle post mortem.
A very odd, but effective way of diagnosing infections with T. cruzi is called xenodiagnosis. In this procedure, kissing bugs are kept in the laboratory and fed blood or tissue from a person or animal suspected of having the parasite. The bug is then killed and its digestive system is examined for the parasite.
How is infection with T. cruzi treated?
There is no known cure for T. cruzi infection in cats and dogs. Because of this and the extreme danger to people, it is generally recommended that infected animals be euthanized.
The most effective means of control is the elimination of the kissing bugs. This is not always easy because the bugs tend to hide in small cracks and crevices of buildings, often safe from the application of pesticides.
References
Georgi, JR; Georgi, ME. Canine Clinical Parasitology. Lea & Febiger. Philadelphia, PA; 1992;62-72.
Griffiths, HJ. A Handbook of Veterinary Parasitology. University of Minnesota Press. Minneapolis, MN; 1978;37-38.
Hendrix, CM. Diagnostic Veterinary Parasitology. Mosby, Inc. St. Louis, MO; 1998;24-25.
Meurs, KM; Anthony, MA; Slater, M; Miller, MW. Chronic Trypanosoma cruzi infection in dogs: 11 cases (1987-1996).
Sherding, RG. Toxoplasmosis, Neosporosis, and Other Multisystemic Protozoal Infections. In Birchard, SJ; Sherding, RG (eds): Saunders Manual of Small Animal Practice. W.B. Saunders Co. Philadelphia, PA; 1994;142.
Sousby, EJL. Helminths, arthropods and protozoa of domesticated animals. Lea & Febiger. Philadelphia, PA; 1982;516-525,537-543.
