Trypanosoma evansi - Microscopic exam, experimental inoculation and molecular diagnosis (PCR)


Trypanosoma evansi is a protozoan of the genus Trypanosoma, Trypanosomatidae family, and Phylum Euglenozoa kinetoplastida class, with worldwide distribution. This parasite affects a wide range of animals, the most frequent horses, camels, dogs, deer and elephants, which causes a disease called trypanosomiasis (also known as "Surra"), a disease of great economic importance in areas tropical and subtropical Africa, Asia, and Latin America (Central and South America). In animals infected it affects their health and productivity, lack pathognomonic clinical signs, so laboratory tests are required to confirm their diagnosis. In South and Central America the main host is the horse. In Asia, the main hosts are camels, cattle, horses and pigs. In Africa, the camel is the most important host. In people, infection with this organism rarely causes disease.

Trypanosoma evansi is has a size of 14 to 33?m x 1.5 to 2.2 microns, with a free flagellum emerging from the rear end and a subterminal kinetoplast. This microorganism is generally monomorphic, although cases have been described of polymorphism. Morphologically it is indistinguishable at the level of electron microscopy and optical Trypanosoma brucei and Trypasonoma equiperdum. It is a parasite of extracellular and intracellular fluids that affects a wide variety of hosts and has the ability to change its main surface glycoprotein VSG (Variant Surface Glycoprotein), allowing you to generate relapses in infected animals, as by inducing appearance of antibodies specific to it, changes the variant, and the new variant is not blocked by the previous antibodies. The VSGs are highly immunogenic and determine variable antigen type VAT (Variable Antigen Type) of a trypanosome which induce protective antibodies specific to opsonizing, binder and lytic activity.

The life cycle of this microorganism involves an intermediate host vector that acts as a mechanical (non - cyclic), generally species bloodsucking flies (Tabanus, Stomoxys, Atylotus, Chrysops, Lyperosia and Haematobia) that transmit the infection through bites. Vampire bats Desmodus rotundus as in South America, can also transmit the infection. Moreover, infection can also occur by iatrogenic transmission through blood, for example, with needle reuse. Likewise, the transmission of infection orally also can occur through ingestion of contaminated meat or blood. Trypanosoma evansi has a life cycle monoxenically without this protozoan suffer evolution in the time lag in transmission between an infected host and a new host. Once in the body of the mammalian host, Trypanosoma evansi multiplication occurs by longitudinal binary fission, and occurs in blood, lymph or cerebrospinal fluid.

The pathogenesis of infection Trypanosoma evansi varies depending on the virulence of the strain and the susceptibility of different animal species acting as hosts. The disease has no pathognomonic clinical signs and generally manifests as an acute form, with high mortality, or chronic, usually with intermittent fever, progressive anemia and decay. During the course of the disease, recurrent episodes of fever and parasitemia are presented. Edemas can occur frequently, particularly in the limbs and abdomen. In addition, there have been cases in which infection by this organism can cause damage to motor function and abortions. Other less common signs and symptoms include diarrhea, jaundice, mocuporulenta runny nose and shortness of breath. When parasites access the central nervous system and the aqueous humor can be observed neurological symptoms and ocular hemorrhage. The disease can occur acutely in young animals and females in gestation period, which causes death within a few weeks. In endemic areas, the infection usually develops chronically and may persist for years.

Recommended tests for diagnosis:


The absence of pathognomonic clinical signs requires the availability of laboratory tests with sufficient sensitivity and specificity. The diagnosis can be based on four types of tests: 1) parasitological examinations to visualize the parasite; 2) methods for detecting the parasite antigen; 3) methods for detecting antibodies; and 4) molecular diagnostic methods to detect DNA.

Parasitological methods to observe the parasite can be made by observing blood smears stained with Giemsa or equivalent; in observing the buffy coat after a test Hematocrit (MHCT: Microhematocrite Centrifugation Techniques); Miniaturized test centrifugation ion exchange (mAECT: Miniature Anion-Exchange Centrifugation Technique), or inoculation rodents experimentally. The sensitivity of these tests is increased from the lower sensitivity corresponding to the observation of blood smears, the more sensitive that corresponds to experimental inoculation of rodents.

Tests based on antigen detection are most commonly used test passive agglutination of latex particles, and immunosorbent assay (ELISA) to detect antigen capture. These tests are considered complementary tests parasitological observation and have sufficient sensitivity in active infections, but its sensitivity decreases when the parasitemia fluctuates and low number of parasites, such as during chronic infections, or when complexed antigen and antibodies.

Antibody detection allows grouping tests two types of methods. Unspecific methods are used in areas with scarce resources: Formalin test; test mercuric chloride; or proof of thymol turbidity. Specific serological tests detect specific antibodies. Of these the most used in endemic areas are: agglutination test (CATT: Card Agglutination Test Trypanosoma); Tripanolisis immune (TL); latex agglutination test (Latex agglutination test) to detect antibodies; fluorescent antibody test (IFAT: Immunofluorescent Antibody Test); and enzyme immunoassay (ELISA: Enzyme-linked Immunosorbent Assay).

The molecular diagnostic tests (PCR) to detect parasite DNA are superior in sensitivity to observation and detection of antigen, both in the overbearing phases, as in the chronic phase, and is considered to be detected a trypanosome per milliliter of blood .

Serologic tests, and require time to have developed antibodies to identify the genus Trypanosoma to cause infection but not the species. As a result, the molecular diagnosis as the most sensitive for identifying the species Trypanosoma evansi method is recommended.

Tests in IVAMI:


  • Microscopic examination of blood smears.
  • Inoculation experimental rodents (mice and hamsters).
  • Molecular diagnosis (PCR), to detect DNA of Trypanosoma evansi.

Recommended sample:


  • Whole blood collected with EDTA (2 to 5 mL).


Preservation and shipment of sample:


  • Refrigerated (preferred) for less than 2 days.
  • Frozen: over 2 days (for molecular diagnostic tests only).


Delivery term:


  • Microscopic examination: 24 hours.
  • Experimental inoculation: 10 to 15 days.
  • Molecular diagnosis (PCR): 24 to 48 hours.

Cost of the test: