Chagas Disease (American Trypanosomiasis) Workup

Updated: Sep 08, 2017
  • Author: Louis V Kirchhoff, MD, MPH; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
  • Print
Workup

Approach Considerations

The flowchart below depicts the management of risk for congenital Chagas disease.

Footnote 1. Mexico, as well as all nations of Cent Footnote 1. Mexico, as well as all nations of Central and South America, are endemic for Chagas disease. Chagas disease is not endemic in any of the Caribbean Islands. Women who were born in Chagas endemic countries or who have resided therein for a substantial period are at geographic risk for having Chagas disease. In addition, women who are not themselves at geographic risk but whose mothers are at such a risk and are not known to be seronegative are in turn considered to be at risk for Chagas disease. Footnote 2. The two approaches useful in this regard are microscopic examination of anticoagulated blood and polymerase chain reaction (PCR). Serologic testing of newborns is not useful because assays for specific immunoglobulin G (IgG) will yield a positive result, reflecting the mother’s chronic infection, and because immunoglobulin M (IgM) assays lack acceptable levels of sensitivity and specificity. Footnote 3. All children born to at risk women should be tested serologically since several studies indicate that the rate of congenital Chagas disease in babies born to infected mothers is 2%-10%. Footnote 4. The latter should include periodic monitoring for signs and symptoms of chronic cardiac and gastrointestinal Chagas disease followed by appropriate interventions, when indicated. The usefulness of specific drug treatment in adults with chronic Trypanosoma cruzi infections has not been clearly demonstrated and is a matter of ongoing debate. Footnote 5. The parasitologic cure rate in babies with congenital Chagas disease approaches 100% when a full course of treatment is given during the first year of life. Footnote 6. The sensitivities of microscopic examination of anticoagulated blood and PCR are less than 100%. Thus, the occasional baby who tests negative in these approaches immediately after birth may actually be infected with T cruzi. Serologic testing for specific IgG should be delayed until maternal IgG has disappeared.
Next:

Laboratory Studies

The diagnosis of acute Chagas disease, which includes congenital Chagas disease and reactivation of chronic T cruzi infection in immunosuppressed persons, is based on direct detection of the parasites. In contrast, the diagnosis of chronic infection (indeterminate or chronic symptomatic phases) is generally based on serologic testing, since the low level of circulating parasites precludes microscopic detection and limits somewhat the sensitivity of PCR assays. 

Parasitologic diagnosis

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. [124] 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. [125] 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. 

Serologic diagnosis

Tests for anti– T cruzi immunoglobulin M (IgM) are not standardized, are not available commercially, and should play no role in the diagnosis of acute Chagas disease.

Serologic testing for specific antibodies to T cruzi is the cornerstone of diagnosing chronic T cruzi infection.

Several dozen serologic assays are available in the endemic countries for testing clinical and donor specimens for chronic T cruzi infection. Several of these are available in nonendemic areas with substantial at‑risk immigrant populations. The most widely used today are indirect immunofluorescence (IIF), enzyme-linked immunosorbent assay (ELISA), and indirect hemagglutination, but assays based on other formats are also available. Most use lysates of epimastigotes as target antigens, but several are based on recombinant proteins. [89] At least a dozen rapid tests for diagnosing chronic T cruzi infection have also made it to market. [126]

To ensure accurate results, WHO Chagas experts and other authorities recommend that each specimen undergo testing with two types of assays, and this approach is generally followed for blood donor testing in endemic countries, although in Brazil a single‑assay testing protocol is used to screen donors.

In the United States, the Ortho T cruzi ELISA Test System and the Abbott Prism Chagas Assay are both FDA‑approved for donor screening, but not for clinical testing.

From 2007 through 2014, the Chagas Radioimmune Precipitation Assay (Chagas RIPA) was performed by Quest Diagnostics for confirmatory testing of donor specimens that were repeat positive in the Ortho or Abbott screening assays. [88, 89] Currently, the Abbott Enzyme Strip Assay Chagas (ESA Chagas) is the only FDA‑approved assay for confirmatory testing available in the United States. The Chagas RIPA is available for research and limited clinical testing in the author’s laboratory at the University of Iowa.

Currently, two ELISAs are approved by the FDA for clinical testing (Hemagen Chagas Kit, Hemagen Diagnostics, Inc.; Chagatest ELISA Recombinante v. 3.0, Laboratorios Wiener). For initial testing, the author suggests using the latter through the Division of Parasitic Diseases of the Centers for Disease Control and Prevention (CDC). In all instances, positive and indeterminate results should be confirmed with the Chagas RIPA or the Abbott ESA Chagas. 

Diagnosis via polymerase chain reaction assays

The use of PCR tests for detecting T cruzi has been studied extensively over the past 25 years, and dozens of articles have described this approach. [127, 128]

The use of many distinct primer pairs has been described; the accumulated evidence suggests that assays based on TCZ1/TCZ2 (nuclear repetitive sequence) [129] and S35/S35 (kDNA minicircle conserved region) [130] are the most sensitive.

Methodologies for the best PCR assays for T cruzi have been published, [131, 132] but no kits are available commercially.

Although PCR assays generally appear to yield better sensitivity rates than xenodiagnosis or hemoculture, they are not high enough to justify their use for primary or confirmatory testing of blood donors or in clinical settings. The variable sensitivity is likely due to the extremely low parasite burden in chronically infected persons, meaning that the small blood samples taken for DNA extraction may not contain even a single parasite.

Nonetheless, there are several settings in which PCR assays for detecting T cruzi can have important roles. The first of these is in chronically infected patients who have been treated with benznidazole or nifurtimox. In this instance, a positive PCR result indicates treatment failure, but, given the variable sensitivity, a negative result is not meaningful. Secondly, PCR assays for T cruzi can play a role in detecting acute Chagas disease, particularly congenital T cruzi infection, since their sensitivities have been shown to be greater than microscopic examination in several contexts. [133, 134, 135, 136] Finally, PCR assays are the method of choice for detecting T cruzi infection in insect vectors [137, 138] and in food suspected of being contaminated with parasites. [139]

Previous
Next:

Imaging Studies

No imaging studies are specific for Chagas disease. However, the manifestations of Chagas disease (eg, cardiac, esophageal, and colonic dysfunction) should be evaluated with the appropriate imaging studies, as they would be when they result from any disease process. 

Previous
Next:

Other Tests

Electrocardiography and 24-hour continuous monitoring are the cornerstones of assessing possible dysrhythmias in patients with chronic T cruzi infections, as is the case with rhythm disturbances of any cause. Infected persons should undergo electrocardiography every 6-12 months to monitor for the development of ominous arrhythmias that would require management with drugs or pacemaker placement.

Esophageal manometry and endoscopy may be useful in assessing and managing patients with Chagas disease who have symptoms that suggest esophageal dysfunction. Endoscopy can be used to differentiate chagasic megaesophagus from other causes of esophageal dysfunction (eg, cancer). 

Previous