TRYPANOSOMA CRUZI

Trypanosoma cruzi is a protozoan parasite that causes Chagas disease, or American trypanosomiasis. The parasite is endemic throughout Mexico and Central and South America, and an estimated 7.7 million people are infected, with 3 to 3.3 million symptomatic cases and an additional 108.6 million people at risk.¹⁶ However, with the increased movement of people from endemic areas, tourism, and pet travel, Chagas disease has become an important public health issue in the United States. Even though human prevalence within the United States is low, the blood supply is now routinely tested for evidence of Chagas infection because the parasite can be transmitted through blood transfusion and organ transplantation.^17,18

Life cycle

Indirect transmission involves mammalian definitive hosts and triatomine bug intermediate hosts, such as assassin bugs. Mammalian hosts within the United States include opossums, wood rats, raccoons, armadillos, and coyotes.^17,19 An infected triatomine bug will defecate as it feeds and pass organisms in the feces. Trypanosoma cruzi can then invade various cells at the bite wound site, form into amastigotes, and undergo asexual reproduction. They then transform in a nonreproducing stage in which they leave the cell and either invade new tissues or are ingested by a different triatomine bug as it takes a blood meal. If not ingested by an intermediate host, they transform again into intracellular amastigotes, reproducing in reticuloendothelial, neural, and glial cells and cardiac and smooth muscle cells.

Within the newly infected intermediate host, the parasite multiplies and undergoes metamorphosis into a new form in the hindgut. When the bug feeds again on a different animal, it will defecate as it feeds, and the parasite is transmitted in the feces. It then enters the body by penetrating the oral, nasal, or conjunctival mucosa or by rubbing the infectious bug feces into abrasions, such as when the bug bite site is scratched. Ingestion of an infected bug will also result in transmission of the organism.²⁰

Signalment

The age range of clinical cases in dogs has been reported to be 6 weeks to 13 years, with about half the cases in animals < 1 year old. No sex predilection has been reported. Cases have been reported in 48 breeds of dogs, with most in the sporting group, likely as a result of lifestyle factors. Cases have also been reported in dogs from both urban and rural areas.¹⁹

Clinical findings

Acute disease in dogs is characterized by lymphadenopathy and clinical signs associated with acute myocarditis, such as pale mucous membranes, lethargy, ascites, and tachyarrhythmia.^19,20 Dogs surviving the acute phase enter the indeterminate phase characterized by the lack of clinical signs. Circulating parasites can only be demonstrated by blood culture or xenodiagnosis, which is when uninfected intermediate hosts are purposefully fed on presumed infected animals and subsequently examined for the presence of the organism. An electrocardiogram is usually normal at this stage, although ventricular-based arrhythmias can be induced with exercise.²⁰ Some dogs may then progress to chronic disease with clinical signs related to congestive myocardial failure.^19,20

In a recent report characterizing Chagas disease in dogs, about half of dogs < 1 year of age presented with acute death.¹⁹ In nonacute death cases, the duration of apparent illness ranged from one day to six weeks and depended on when the client sought veterinary attention. Cardiac dysfunction, represented by cardiac enlargement and myocarditis, was the primary problem reported in both puppies and adults. Conduction disturbances, including premature ventricular contractions and atrial fibrillation, were reported in about one-fifth of the animals.

Diagnosis and treatment

Diagnosing Chagas disease is difficult. The first step is to suspect the infection. Chagas disease should be included on the differential diagnosis list for dogs presenting with signs of myocarditis or cardiomyopathy, particularly in endemic areas or if a dog has lived at any time—even years before presentation—in an endemic area.²⁰ Diagnosis has traditionally depended on demonstration of parasites in peripheral blood or amastigotes in tissue biopsy samples.

Several serologic and molecular methods are also available, which may be particularly useful during the indeterminate and chronic phases.^20-23 Immunochromatographic assays for the detection of antibodies in dogs in-clinic have been described and appear useful as screening tools.^21-23 The primary problem with most serologic methods is the cross-reaction with Leishmania infantum, another protozoan parasite endemic to the same areas of the United States.^20,23 Recommendations are to confirm screening tests with another diagnostic method available through specialized laboratories.

Because most cases are diagnosed during the chronic stage, treatment is unrewarding. Supportive cardiac therapy becomes the mainstay of treatment.²⁰

Prevention

Recommendations aimed at preventing infections include limiting contact with vectors and wild reservoir hosts. Although the parasite is usually transmitted in the feces of the vector, it is thought that most infections in dogs in the United States are acquired through the ingestion, which releases the organisms into the mouth of the dog. Use of integrated pest management methods, such as changing outside lighting, housing dogs indoors at night, and optimizing pesticide regimens, should be used to reduce contact with vectors.^19,20 Additionally, dogs should not be fed fresh meat from reservoir hosts.^19,20 Removing T. cruzi antibody-positive females from the breeding stock may reduce vertical transmission. Dogs used as blood donors should also be screened to determine their status.

Dogs are more competent definitive hosts than either cats or people are and are considered important reservoir hosts in Central and South America.¹⁹ However, the risk of acquiring infection from an infected dog is thought to be extremely low in the United States at this time.²⁰ Nevertheless, because infections can be passed through blood, veterinarians and their staffs need to be especially careful when handling blood from an infected dog, and any accidental needle sticks should be reported to the Centers for Disease Control and Prevention immediately.²⁰

Lora R. Ballweber, DVM, MS
Department of Microbiology, Immunology, and Pathology
College of Veterinary Medicine and Biomedical Sciences
Colorado State University
Fort Collins, CO 80523