Acute phase (direct and concentration methods)

Several easy methods are available to search for parasites when they are present in large numbers, as in the acute phase. They could be divided in direct tests and concentration methods.

Direct tests

The simplest and cheapest direct test is the fresh blood smear. A drop of peripheral blood from the patient is collected from the ear, fingertip, foot, or from a vein through a syringe. 10 ^L of blood are immediately deposited on a smear and a cover slip (22 X 22 mm) covers the drop. The amount of 10 is ideal for a nice preparation (i.e., a very thin smear that allows seeing red blood cells separated from each other). The preparation should be mounted in a microscope with an objective of 40 X and ocular of 10 X (i.e., 400X). If T. cruzi is present, it will be seen as a refringent body with very quick movements, disturbing the quiet red blood cells. It is advisable to have some previous training, which may be just an observation for some minutes of such a preparation. As an exercise, this could be prepared from infected mouse blood in those laboratories that work with T. cruzi. This parasite easily contaminates humans, so all measures to avoid a laboratory accident should be taken, like the use of gloves and facial mask (personal protection equipment).49,50 More than 100 laboratory accidents have been documented and published and probably a similar number of cases have not been recorded.49

In cases of vector transmission, with less than 20 days from the start of symptoms, it is common to find one parasite every 10—50 fields. The smear should be looked at for 100—400 fields before being informed as negative. The test may be performed in several smears or on different days. If a single examination is negative, concentration methods may be applicable if the clinical suspicion persists. In some transfusional cases27 or immunosuppression, several parasites may be seen in one field.

If a motile, refringent flagellate is found, the diagnosis is made. No further analysis is necessary. Check if clinical data are available (i.e., fever of unknown origin, recent transplantation or transfusion). The preparation may be dried and stained, but for better visualization, a proper smear as for differential count of leukocytes is preferable.

 
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