How Chagas Disease Is Diagnosed

The diagnosis of Chagas disease depends on whether the infection is acute, chronic, or congenital.

The optimal time to diagnose Chagas disease is during the acute phase of the illness, when the chance of eradicating the Trypanosoma cruzi (T. cruzi) infection with antitrypanosomal drugs is the highest.

Unfortunately, this opportunity is all too often missed. This is because the symptoms caused by acute Chagas disease are usually mild and not particularly alarming, so people with acute Chagas typically do not seek medical help.

People who live in endemic areas should pay attention to potential symptoms of acute Chagas disease, especially if they have noticed insect bites that are particularly prominent or long-lasting, or if they are aware of an outbreak of Chagas disease in their area. If they are suspicious they should see a doctor.

As for the doctors, it is important that they too remain suspicious that Chagas disease might be present and then do the necessary diagnostic testing. In actual practice, this generally occurs only during recognized local outbreaks, when community-wide screening is instituted.

Making the Diagnosis

During the acute phase of Chagas disease, the number of T. cruzi parasites in the bloodstream is usually quite high. This allows the diagnosis of Chagas to be made by examining specially-prepared blood samples under a microscope. However, the number of T. cruzi in the bloodstream falls off rapidly after the first 90 days, even if no treatment is given. The microscopic examination of blood is no longer a reliable means of diagnosing Chagas disease after that time. The microscope test is almost never useful during the chronic phase of Chagas.

In addition to the microscopic exam, laboratory blood testing also can be very accurate in diagnosing acute Chagas disease. This is done with a polymerase chain reaction (PCR) test, which detects the DNA of T. cruzi in the blood sample.  A positive PCR test, like the microscope test, indicates that T. cruzi organisms are present in the bloodstream.

The symptoms associated with acute Chagas disease—such as weakness, fever, sore throat, rash, and muscle pains—can be easily confused with the symptoms of infectious mononucleosis, or of acute HIV infection. So when a person living in an area endemic for Chagas disease is being tested for either of these conditions, it is usually a good idea to test for T. cruzi infection as well.

In chronic Chagas disease, the T. cruzi organism is usually no longer present in the bloodstream, so microscopic testing of a blood sample is almost always negative, as is the PCR test.

Diagnosing chronic Chagas disease usually relies on detecting antibodies made by the body to fight off the infection. A number of tests have been developed to look for antibodies to T. cruzi, including an enzyme-linked immunosorbent assay (ELISA) and an immunofluorescent antibody assay (IFA).

None of these antibody tests is accurate enough to be used by itself, so to diagnose chronic Chagas disease, at least two different antibody tests are usually done—and if the results are different between them, a third test is then done to serve as a tie-breaker.

At the same time, testing should also be performed to look for any of the other potential causes for the kinds of cardiac and gastrointestinal problems associated with chronic Chagas disease. The list of conditions that can do this is unfortunately quite long, and doctors have to use a lot of clinical judgment in deciding what testing to do, and in what order.

Up to 10 percent of babies born to mothers infected with T. cruzi will develop acute Chagas disease—a condition called congenital Chagas disease. It is important for a baby with congenital Chagas disease to be treated with antitrypanosomal therapy to prevent chronic complications.

The possibility of congenital Chagas disease should be considered in any newborn whose mother is from an area where the disease is endemic. Prenatal testing is often done in pregnant women living in such areas, and babies of the mothers who test positive can then be screened for the disease. 

Screening of the babies themselves for congenital Chagas disease is usually done at birth with PCR testing of the cord blood, or on a blood sample obtained during the first few days after birth. If the mother is known to be positive for Chagas disease and the initial screening of the infant is negative, repeat testing of the baby should be done after a month or two.