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Chapter 5Post-Travel Evaluation

Screening Asymptomatic Returned Travelers

Michael Libman

CDC has no official guidelines or recommendations for screening those returning from international travel who are asymptomatic (see Chapter 9 for recommendations regarding the screening of newly arrived immigrants and refugees). Nevertheless, screening high-risk travelers, such as those who have spent an extended time in developing countries, can reveal occult infections, some of which can cause serious sequelae or have public health implications.

PARASITIC INFECTION

Asymptomatic travelers should not be screened for malaria. All travelers, especially those who did not take recommended malaria chemoprophylaxis during and after the trip, should be reminded to seek evaluation for unexplained fever and notify practitioners of recent travel.

Travelers are often concerned about “worms,” but high-burden infections with common nematodes such as Ascaris, Trichuris, or hookworm are uncommon in returning travelers. Rarely, complications have been reported among initially asymptomatic travelers, such as migration of Ascaris into the biliary system, but the lifespan of most helminths—months to a few years—generally ensures eventual spontaneous cure and precludes ongoing transmission in industrialized countries. The exception is Strongyloides, which can cause serious complications. The duration of carriage is unlimited, and the original burden of infection is irrelevant. Unfortunately, diagnosis by stool examination is insensitive, and serologic methods are often required.

Testing for pathogenic protozoa in asymptomatic patients is not recommended. No evidence suggests that carriers are likely to develop symptoms at a later time, although minor, unreported symptoms could be alleviated, and transmission (which is rare in asymptomatic people) could be prevented. Stool microscopy for protozoa is expensive, relatively insensitive, poorly reproducible, and strongly influenced by the expertise of laboratory personnel. Entamoeba histolytica (pathogenic) cannot be distinguished from E. dispar (nonpathogenic) by microscopy, and several studies suggest that most travelers with Entamoeba identified on microscopy are carrying E. dispar.

Some extraintestinal helminths, particularly Strongyloides and Schistosoma, are capable of eventually causing subsequent serious illness in travelers who are asymptomatic upon return. Low-burden schistosomal infections, such as those typically found in travelers, are not likely to lead to the “classic” chronic complications, such as liver fibrosis or malignancy. However, complications due to ectopic egg migration can occur even in light infections and without warning, although these complications are unusual. Traditional tests for these infections, including stool examination for Strongyloides and Schistosoma spp. and urine tests for S. haematobium, all lack sensitivity, particularly in low-burden infections. For this reason, serologic testing has been advocated as the best screening tool, although it is expensive, available only in some commercial laboratories, and poorly standardized. Treatments for strongyloidiasis and schistosomiasis are inexpensive, easy, and effective. Serologic testing can be considered for travelers who had a prolonged or high-risk exposure (people with environmental exposures in an area of known high endemicity, for example), and all those found to be positive should be treated.

Reports of travelers with late complications from asymptomatic filarial infections are also very rare. Transmission of filarial infection typically requires longer-term exposure (generally >3 months in an endemic area) but should be considered in returning travelers with eosinophilia who have traveled to endemic countries, if no other pathogen commonly associated with eosinophilia can be identified. Filarial serology usually becomes positive only after adult forms have matured, which means waiting ≥3 months after exposure.

Eosinophil counts are commonly used to screen returning travelers when parasitic infection is a concern. This test can suggest the presence of invasive helminths but not protozoa. However, eosinophilia has poor sensitivity for detecting invasive helminths, and although specificity can be high, the low prevalence of infection in travelers means the positive predictive value is low (positive predictive value increases, however, with high-grade eosinophilia). Many cases of eosinophilia resolve spontaneously, and eosinophilia is also often due to noninfectious causes, such as allergy or drug reactions. Eosinophil counts can be repeated over several weeks or months before an extensive investigation is done. Counts may be highly variable over time, even within a single day, and eosinophilia can be suppressed by endogenous or exogenous steroids. Evaluation of absolute eosinophil counts, rather than by percentage of eosinophils among leukocytes, is more reproducible and predictive.

Asymptomatic trypanosomiasis among US residents who have traveled to endemic areas appears to be extremely rare. Screening asymptomatic travelers for trypanosomiasis is generally not recommended. For people traveling to areas of Latin America where Chagas disease is endemic, testing might be considered in cases of prolonged residence in primitive housing (such as with mud walls or thatched roofs) or when the large reduviid bugs have been seen in the residence. Asymptomatic Chagas disease among people who were born in Central or South America has been a concern in recent years, and these people would be candidates for Chagas disease screening, but the screening decision is not based on recent travel history. East African trypanosomiasis has occasionally affected travelers but is generally symptomatic. West African trypanosomiasis is very rarely reported in travelers.

NONPARASITIC ILLNESS

Casual sex with new partners, including with sex workers, is common among travelers, and the low rates of reported condom use are concerning. Returning travelers with acute HIV or hepatitis B virus infection pose public health risks, so screening for sexually transmitted infections should be considered.

The incidence of tuberculosis related to travel is difficult to assess. A history of work in high-prevalence settings such as health care institutions, prisons, or refugee camps should prompt screening. Doing pre- and post-travel skin tests is cumbersome, requiring as many as 4 pre-travel visits for a 2-step test and 2 visits after potential exposure. An alternative is an interferon-γ release assay (IGRA), which is also less subject to false-positive results related to vaccination. IGRAs are expensive, but the cost may be offset by the reduced number of visits and medical costs. In most patients born and raised in nonendemic countries, the pre-travel test can be omitted.

For many long-term travelers, such as expatriate workers and some missionary and aid workers, a visit for asymptomatic screening may be their only interlude from a continuing assignment abroad and is an opportunity for a general health evaluation. The usual recommendations for the periodic health exam, which may include a routine blood count and chemistries, as well as screening for hypertension, diabetes, and malignancy, also apply.

CONCLUSIONS

Most recommendations for screening asymptomatic returning travelers are based on opinion and common sense, rather than convincing evidence. The following general recommendations apply (Table 5-06).

Short Stay (<3–6 Months)

The yield of screening is low and should be directed by specific risk factors revealed in the history. A history of prolonged (>2 weeks) digestive symptoms suggests protozoal infection. Exposure to freshwater in a region endemic for schistosomiasis, especially in Africa, merits consideration of microscopic stool and urine examination in the case of high-intensity exposure, with the addition of serologic testing to diagnose light infections, when egg shedding may not be consistent in travelers and in others who have not had schistosomiasis previously. Antibody tests do not distinguish between past and current infection. Serologic tests for Strongyloides should be considered in those who have a high risk of exposure, usually those who report that they frequently walked outdoors barefoot. A sexual history should be obtained. Work in a health care setting or other area at high risk for tuberculosis may merit screening.

Longer Stay

Stool examinations are of limited utility, but they can reassure the traveler. However, they may also identify a nonpathogenic protozoan, which could distress the traveler. Serologic tests for schistosomiasis and strongyloidiasis can be considered for those who report engaging in high-risk activities (exposure to fresh water or soil, respectively). An eosinophil count is usually done, although results should be interpreted cautiously, and repeat testing may be needed to assess trends in eosinophil count. Screening for sexually transmitted infections should be offered to all travelers. Tuberculin skin test or IGRA should be offered to those who have worked in a health care or similar setting or who have had intimate and prolonged contact with residents of an endemic area for ≥6 months. Possible exposure to bloodborne pathogens should be assessed. Any other screening should be guided by exceptional exposures or knowledge about local outbreaks.

Table 5-06. Considerations for screening asymptomatic travelers

RISK OR EXPOSURE SCREENING TEST
Stays <3–6 months None
Stays >3–6 months, poor sanitation or hygiene Eosinophil count, consider stool ova and parasites
Walking barefoot on soil potentially contaminated with human feces or sewage Strongyloides serologic tests
Exposure to freshwater rivers, lakes, or irrigation canals Schistosoma serologic tests
Sexual contact Screen for sexually transmitted infections
Work in health care setting, close contact (>6 months) with population in a highly TB-endemic area TB screening (TST or IGRA)

Abbreviations: TB, tuberculosis; TST, tuberculin skin testing; IGRA, interferon-γ release assay.

BIBLIOGRAPHY

  1. Baaten GG, Sonder GJ, van Gool T, Kint JA, van den Hoek A. Travel-related schistosomiasis, strongyloidiasis, filariasis, and toxocariasis: the risk of infection and the diagnostic relevance of blood eosinophilia. BMC Infect Dis. 2011;11:84.
  2. MacLean JD, Libman M. Screening returning travelers. Infect Dis Clin North Am. 1998 Jun;12(2):431–43.
  3. Whitty CJ, Carroll B, Armstrong M, Dow C, Snashall D, Marshall T, et al. Utility of history, examination and laboratory tests in screening those returning to Europe from the tropics for parasitic infection. Trop Med Int Health. 2000 Nov;5(11):818–23.
 
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