What is included in the parasitological diagnosis of Chagas disease (American trypanosomiasis)?

Updated: Apr 26, 2019
  • Author: Louis V Kirchhoff, MD, MPH; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Answer

One useful method for identification of parasites in the blood is to put 1.5 µL of anticoagulated blood under a 12-mm circular cover slip and examine 200 fields under 400 X magnification. [126] This allows for the examination of 0.44 µL of blood with each round, which should take about 30 minutes of careful looking. The mobile trypomastigotes are translucent; thus, they are usually detected based on the corresponding movement of RBCs they cause. This approach has no set threshold for deciding that the result is negative. Simply stated, the greater the number of fields examined, the greater the probability of detecting a parasite in an acutely infected person.

Stained thin and thick blood smears may also be examined microscopically. The author is not aware of any comparative data that shed light on the relative sensitivity of examining stained smears versus wet preparations, although the movement of the parasites in the latter would seem to be advantageous.

Another method of evaluation involves using heparinized microhematocrit tubes. This method has been used extensively to test for congenital Chagas disease in infants born to chronically infected mothers. [127] The tubes are typically filled directly from the source and spun. The buffy coat at the interface of the plasma and RBCs is then examined microscopically. The curvature of the tube makes this somewhat difficult, but this problem can be resolved by cutting the tube and examining the buffy coat as if it were fresh blood. This latter procedure carries a risk of accidental transmission and thus should be performed only by experienced personnel.

Indirect parasitologic methods include xenodiagnosis and hemoculture.

In xenodiagnosis, 30-40 laboratory-reared insects are allowed to feed directly or indirectly on the blood of a person suspected to have Chagas disease. At least one month later, intestinal contents of the insects are extracted and examined microscopically for the presence of parasites. Xenodiagnosis is tedious, requires a long time to perform, and yields a sensitivity of only 50% in the best of hands.

Hemoculture, which involves a specialized liquid culture medium that is not available commercially, takes roughly the same amount of time as xenodiagnosis and has roughly the same level of sensitivity, but it is less tedious.

Neither xenodiagnosis nor hemoculture has any reasonable role in the diagnosis of acute Chagas disease, since the results are not available in time for short-term treatment decisions. In addition, their role in diagnosing chronic T cruzi infection is largely limited because of their insensitivity. However, these tests may have a role in resolving borderline serologic results or in evaluating treatment failures. Both hemoculture and xenodiagnosis are viewed increasingly as historical oddities.


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