Purpose
Introduction
The CDC has no official guidance or recommendations for screening asymptomatic international travelers in the absence of specific risk factors for infectious diseases. Nevertheless, screening returning travelers, especially from low- and middle-income countries, represents a substantial portion of the activity of many travel health and tropical medicine clinics. The recommendations outlined in this chapter are based on review of current literature and have been formulated by the authors.
The scientific literature on the clinical utility and cost-effectiveness of screening asymptomatic travelers is sparse. Asymptomatic travelers can harbor many infections acquired during travel, some of which have the potential to cause serious sequelae or have public health implications. In some cases, these will include pathogens rarely found in the traveler's country of origin. U.S. medical practitioners might have little familiarity with these travel-associated diseases, and specific diagnostic tests might not be readily available or will require expertise in their proper interpretation.
Deciding to screen
The decision to screen an asymptomatic person for travel-acquired pathogens depends on their exposure history, itinerary, type of travel, immune status, and the public health implications of identifying infection. Screening healthy short-term travelers for infectious diseases, especially people who do not report a particular exposure, is not routinely necessary. On the other hand, healthcare professionals may consider performing screening tests for long-term travelers (e.g., adventure travelers, expatriates, humanitarian aid workers, missionaries, travelers visiting friends and relatives) who might have prolonged or heavy exposure to epidemiologically relevant pathogens with potential for long-term consequences. Those who are, or are expected to become, immunosuppressed could be screened based on exposure to pathogens linked to infections known to be exacerbated by the specific type of immunosuppression.
Obtaining a comprehensive travel history, including details about a traveler's dietary adventures, outdoor exposures, and sexual activity, is essential in guiding screening decisions, even when the reliability of the exposure history may be questionable in certain situations. Although the type of travel is commonly linked to risk in the travel medicine literature, it is of limited predictive value when constructing a practical assessment of risk in the individual case. Given these uncertainties, the screening recommendations presented here are focused on broad categories of travelers and emphasize capturing the infections of greatest clinical significance, with the understanding that other "exotic" infections will not always be detected.
For the long-term traveler on hiatus from a continuing assignment abroad, the periodic travel health consultation offers the healthcare professionals a chance to screen for infectious diseases, to conduct a general health evaluation, and to review health behaviors, malaria prophylaxis, and vaccination status (see Long-Term Travelers and Expatriates chapter). It is an opportunity to promote and reinforce primary prevention by discussing behavioral or other risk factors that could predispose the traveler to ill health (e.g., exposures to contaminated food and drink, arthropods, and freshwater sources; drug use; high-risk sexual behaviors). It is important to underscore that intermittent travel health consultations should not be regarded as a substitute for routine periodic health examinations. These regular health assessments, which may involve screening for cardiovascular disorders, diabetes, hypertension, and malignancies, have their own unique goals and should be recognized as such.
Benefit and risk of screening asymptomatic travelers
Before performing screening tests for asymptomatic returned travelers, evaluate the sensitivity and specificity of each test as well as the risk and cost to the patient. The low prevalence of certain infections in asymptomatic travelers will influence the positive predictive value of a given screening test and increase the likelihood of a false-positive result. As a result, the asymptomatic traveler could be subjected to further investigations, generating costs, anxiety, and other possible harms related to diagnostic follow-up. Thus, screening asymptomatic travelers entails complex considerations of benefit versus risk.
Screening traditionally has been viewed as a secondary prevention intervention, an attempt to identify occult illnesses or health risks. Cost-effectiveness of screening depends on the disease of interest, potential outcomes associated with the disease both for the individual traveler and the public's health, and whether an early intervention could reduce morbidity or mortality. Different considerations apply to asymptomatic screening of newly arrived immigrants and refugees; for recommendations regarding these individuals, see the Post-Arrival Medical Screening for Newly Arrived Refugees, Immigrants, and Migrants chapter.
Screening for nonparasitic infections
Arboviruses
Chikungunya and dengue
Screening rarely recommended
Screening for chikungunya or dengue in asymptomatic travelers is not typically recommended because there are no specific treatments for infection once identified (see Chikungunya and Dengue chapters). Travelers concerned about the risk for complications after a secondary dengue virus infection sometimes request screening. Cases of severe dengue in travelers are rare, however, and generally there is no specific intervention if an asymptomatic traveler is found to be seropositive. The important exception would be those considering vaccination with one of the currently available dengue vaccines. Laboratory confirmation of a previous dengue virus infection is required prior to vaccination.
Dengvaxia is approved in the United States for use in children 6–16 years of age with proven prior dengue virus infection and living in dengue-endemic territories. However, in 2024, the manufacturer announced that Dengvaxia will be discontinued due to a lack of demand in the global market. Qdenga is currently available in many jurisdictions, including most European countries, with few restrictions, but some countries advise confirmation of prior infection before giving this vaccine to travelers. There are no data on the safety and efficacy of these vaccines in travelers at this time.
Zika
Screening rarely recommended
The prevalence of Zika virus infection in many countries has decreased dramatically since 2017; as a result, the likelihood of a false-positive test result has increased (see Zika chapter). Moreover, Zika virus IgM antibody persists months after infection, making it difficult to determine the date of infection, which is crucial information for judging the risk in a pregnant woman. Due to these considerations, it is not recommended to conduct serologic testing for asymptomatic pregnant women who have recently traveled to an area with risk of Zika. Nucleic acid amplification testing (NAAT) may be considered during pregnancy within a 12-week period after travel but is not routinely recommended in asymptomatic pregnant women due to very low yield. Nonetheless, travel medicine professionals should remain vigilant for the potential re-emergence of Zika in the region of travel and review screening guidelines for travelers, including pregnant women and their partners (see CDC's Zika for Healthcare Providers website.
Coronavirus disease 2019
Screening rarely recommended
The COVID-19 pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus, has waned, and asymptomatic screening is no longer mandated or recommended in most jurisdictions (see COVID-19 chapter). Infection is commonly acquired during travel, at least partly related to the crowded and poorly ventilated conditions frequently encountered during travel, and diagnosis of COVID-19 can help prevent further spread to contacts after travel. Unfortunately, commercially available rapid tests perform poorly in those without symptoms, even when individuals are infected and contagious, making asymptomatic screening of limited value. Nevertheless, the emergence of variants makes the evolution of this pandemic unpredictable. Guidance for populations and travelers may continue to evolve.
Sexually transmitted infections and blood-borne pathogens
Screening often recommended
High rates of sexual activity with new partners, including sex workers, have been documented in overseas backpackers, military personnel, expatriate workers, and people doing volunteer work (see Sex and Travel chapter). Low rates of condom use have been reported in some of those studies. Moreover, travelers might engage in other high-risk activities (e.g., getting a tattoo or piercing, using injection or intranasal drugs, receiving medical or dental care). Returning travelers with asymptomatic hepatitis B, hepatitis C, HIV, mpox, or syphilis infection pose public health risks and might be hesitant to volunteer a relevant exposure history.
A detailed clinical history on risk factors for sexually transmitted infections and blood-borne pathogens is recommended for all travelers; always pursue screening according to published guidelines. Screening for people with relevant exposures should include testing for HIV, syphilis, chlamydia, and gonorrhea. For travelers with an identified specific risk factor (e.g., blood exposure, condomless sex) who have not been previously vaccinated against hepatitis B virus (HBV), perform HBV testing; hepatitis C virus (HCV) testing also is indicated. Careful evaluation for signs and symptoms compatible with mpox infection is advised, but asymptomatic screening is not recommended at this time.
Tuberculosis
Screening recommended for high-risk travel only
The incidence of tuberculosis (TB) infection related to travel is difficult to estimate. Rates of infection probably parallel those in the local population. Those with a history of work in high-prevalence settings (e.g., healthcare institutions, refugee camps) merit screening. Pre-travel and post-travel tuberculin skin testing (TST) can require as many as 6 visits to a healthcare professional—2 pre-travel visits (or 4 visits for a 2-step test) and 2 post-travel visits after potential exposure. The TB screening process can be simplified by using the interferon-gamma release assay (IGRA), which is more expensive but less likely to yield false-positive results in people who received a previous bacillus Calmette-Guérin vaccination.
Studies assessing IGRA use for serial testing demonstrated large variations in the rate of conversion and reversion. Fully investigate any positive TST or IGRA result, assess symptoms suggestive of active TB disease, and obtain a chest x-ray (for more information, see the Tuberculosis chapter).
Screening for parasitic infections
Travelers often are most concerned about the possibility of an occult parasitic infection (see Post-Travel Parasitic Disease Including Evaluation of Eosinophilia chapter). Unfortunately, the literature shows that patient questionnaires and common laboratory testing used to screen for parasitic diseases have poor sensitivity and specificity. Studies have shown that even an exhaustive risk-factor history in asymptomatic patients is unable to reliably detect those who would or would not have evidence of parasitic infection. Physical examination is equally unrevealing.
Most commonly, a stool examination is performed, typically microscopy. Several molecular assays are commercially available to detect a panel of bacterial, viral, and parasitic pathogens. In some cases, these panels are more sensitive than traditional testing methods, and even asymptomatic people often are found to harbor pathogens. The clinical implications of asymptomatic carriage, sometimes at a low level, are unknown for most of these agents, and the risks and benefits of treatment are not well studied. Serologic tests typically are more sensitive for parasitic infections; some have performance limitations related to specificity but are often preferred for screening asymptomatic travelers.
For questions about parasites and screening for parasitic infections, see the Centers for Disease Control and Prevention (CDC) parasites website or contact the CDC.
Helminths
Travelers often are concerned about "worms," by which they usually mean intestinal helminths (see Post-Travel Parasitic Disease Including Evaluation of Eosinophilia chapter). Infections of travelers with large burdens of the common nematodes (e.g., Ascaris, hookworm, Trichuris) are rare. Questioning returning expatriates infected with intestinal helminths has disclosed no attributable symptoms compared with uninfected controls. The life cycles of almost all helminths preclude any real risk of ongoing person-to-person transmission from asymptomatic hosts in high-income countries; helminths generally have a natural lifespan of months to a few years, which ensures eventual spontaneous clearance. In addition, low-intensity infections are of limited clinical importance, although in rare cases aberrant migration of Ascaris spp. can result in clinical disease. The exception to this is Strongyloides stercoralis.
Strongyloidiasis
Screening recommended for high-risk exposure and immunosuppressed travelers
For Strongyloides infections, non-specific symptoms can be easily overlooked, but most importantly, the duration of carriage after infection is unlimited due to auto-infection. In the setting of immune dysregulation (e.g., corticosteroid therapy, hematologic malignancy, hematopoietic stem cell transplant, solid organ transplant) or co-infection with human T-lymphotropic virus type 1, serious complications due to disseminated disease is well described. The initiation of corticosteroids or other immunosuppressive medication at any time after travel should prompt consideration of Strongyloides and potential empiric treatment.
Consider screening for strongyloidiasis in select high-risk travelers with potential skin exposure to human feces, usually a result of walking barefoot in areas without proper sanitation facilities. Unfortunately, the sensitivity of stool-based biomolecular and parasitological methods is low. Molecular detection of helminths is more sensitive and specific compared to microscopy, but sensitivity is still insufficient for screening purposes. Moreover, molecular techniques are not widely available outside the reference laboratory and research settings. Serologic methods are often required, as discussed elsewhere in this chapter. To increase testing sensitivity, a combination of a parasitological methods and serology is recommended in immunocompromised high-risk individuals. Empiric treatment in high-risk individuals should be considered when testing is not readily available.
Schistosomiasis
Screening recommended for high-risk exposures
There is no evidence to demonstrate that the low-burden Schistosoma infections typically found in travelers lead to the types of complications found in endemic areas (e.g., liver fibrosis, malignancy). Nevertheless, the possibility of complications cannot be entirely ruled out, particularly in people who have more intense exposures (see Schistosomiasis chapter). Even brief exposures to freshwater lakes and rivers in known endemic areas in Africa are associated with substantial seroconversion rates. In addition, rare complications due to ectopic egg migration occasionally can occur in light infections and without warning. Ectopic migration to the central nervous system or spine can result in catastrophic sequelae.
Consider serologic screening in asymptomatic travelers who bathed or swam in freshwater canals, lakes, or rivers in areas endemic for schistosomiasis. Other types of freshwater (e.g., adequately chlorinated swimming pools) carry no exposure risk because they do not support the larval parasitic forms. Screening becomes most sensitive only 8–10 weeks after potential exposure and is useful only in those who have not been infected with a schistosome previously. Schistosoma antigens (e.g., circulating anodic antigen) can be detected in blood and urine in active infection with good sensitivity and can be used to monitor cure after treatment, but these tests are not widely available.
Empiric therapy vs screening
Interpreting traditional tests for the parasites that cause schistosomiasis and strongyloidiasis can be challenging. Urine and stool examination for Schistosoma spp. and stool examination for Strongyloides lack sensitivity, particularly in low-burden infection; thus, serologic testing has been advocated as the best screening tool. Problems inherent to serologic screening include expense, lack of easy availability, and lack of standardization.
Fortunately for patients with schistosomiasis (or strongyloidiasis), treatment is easy and effective. The common antihelminthic agents used for short-course therapy (e.g., albendazole, ivermectin, praziquantel) have excellent safety profiles. Be aware, however, that rare but severe adverse events can occur when using certain antihelminthics in patients who have occult, unsuspected co-infection with other parasites. Of note, albendazole and praziquantel can cause increased intracranial pressure with focal signs, seizures, and retinal damage in people with neurocysticercosis; diethylcarbamazine can provoke ocular damage in people infected with Onchocerca; and ivermectin can cause encephalopathy in people infected with Loa loa.
Filariasis
Screening rarely recommended
Reports of travelers with late complications from asymptomatic filarial infections are virtually nonexistent, and filarial screening (blood or skin snips for microfilaria) is generally not recommended for asymptomatic travelers. Chronic onchocerciasis can sometimes be mistaken for atopic dermatitis, leading to inappropriate and often ineffective chronic treatment.
Other helminthic infections
Screening rarely recommended
Helminth parasitic infections rarely seen in returning travelers include fascioliasis, neurocysticercosis, and paragonimiasis, among others. Screening asymptomatic travelers for these infections is generally not appropriate. Primary care professionals should refer patients to an infectious disease specialist when biological, clinical, or radiologic abnormalities increase suspicion for these infections. Except for Strongyloides, intestinal helminths (e.g., Ascaris, Enterobius, hookworms, Trichuris) rarely cause severe illness in travelers and screening is not recommended.
Protozoa
Blood- and tissue-dwelling
Malaria
Screening rarely recommended
No justification can be made for screening most asymptomatic travelers for malaria, whether by blood film, molecular methods, or serologic tests. No available tests can detect the latent hepatic forms (hypnozoites) of Plasmodium vivax or Plasmodium ovale. Remind travelers to seek evaluation for unexplained fever and to notify practitioners of international travel within the past 12 months.
Immigrants with frequent and regular exposure to malaria might gradually develop partial immunity, which can result in low-level parasitemia with minimal symptoms. Immigrants from malaria-endemic areas might later recrudesce with more severe illness, but this phenomenon is rare in non-immigrant travelers. Of note, in rare cases, travelers compliant with prophylaxis might still acquire malaria; often they will present with low parasitemia infections and distorted parasitic forms on blood films, and their symptoms may manifest after ending prophylaxis. These factors may render diagnosis difficult. However, testing asymptomatic travelers is generally inadequately sensitive and is not recommended. Rather, advise travelers to remain vigilant for symptoms, particularly unexplained fever.
Trypanosomiasis
Screening rarely recommended
Occult trypanosomiasis in asymptomatic travelers (as opposed to immigrants) appears to be extremely rare. Screening tests (e.g., molecular diagnostics, serology) are of unknown value. Consider Trypanosoma cruzi testing for travelers who lived for >6 months in rustic housing (e.g., shelters with mud walls and thatched roofs) in endemic areas of Latin America, especially if they report having seen triatomine bugs inside their dwelling. Also consider testing in people who received blood products in an endemic area or in travelers with clinical manifestations compatible with acute Chagas disease.
East African trypanosomiasis has affected travelers but typically causes rapidly progressive symptoms. West African trypanosomiasis generally is not reported in travelers, although typical changes in mood and cognition can be non-specific and very indolent. Refer patients to an infectious disease specialist when these infections are suspected based on biological, clinical, or radiologic abnormalities.
Intestinal
Screening rarely recommended
Treat symptomatic intestinal protozoa infections, particularly Entamoeba histolytica which can cause severe disease and ectopic infections (e.g., liver abscess). Except for E. histolytica infection (which is only rarely asymptomatic) and Giardia species, the incidental finding of pathogenic protozoa in asymptomatic patients is of questionable significance.
The most common protozoa found in asymptomatic travelers are Blastocystis, Dientamoeba, and Giardia species. Only Giardia has clear pathogenic potential. History of exposure to contaminated food or water has poor predictive value. No evidence suggests that asymptomatic carriers are likely to develop symptoms later, and the medications used to treat these protozoa can have adverse effects. In theory, asymptomatic carriers pose a public health risk, but transmission by asymptomatic travelers appears to be rare. In addition, stool microscopy for protozoa is expensive, not very sensitive, not highly reproducible, and many laboratories have limited expertise; thus, screening for giardiasis is not recommended unless evidence of onward transmission is present.
Microscopy cannot distinguish Entamoeba histolytica from Entamoeba dispar. Differentiation requires further specimen collection and testing. Studies reveal that most travelers with Entamoeba on microscopy are carrying E. dispar. Antigen testing for E. histolytica and Giardia (among others) is fairly reliable but lacks the potential to screen for multiple intestinal parasites with a single test, and only some antigen tests are able to differentiate E. histolytica from E. dispar. Asymptomatic screening for amebae is not recommended.
Commercial molecular methods to screen stool specimens for multiple pathogens simultaneously typically include several protozoa, generally with better sensitivity than microscopy. These assays also can specifically distinguish potentially pathogenic E. histolytica from nonpathogenic amoebae. They offer rapid turnaround times and, although costs remain high, these assays are increasingly being used in returned travelers with non-specific gastrointestinal complaints. Some of these panels detect organisms for which pathogenicity remains controversial (e.g., Blastocystis and Dientamoeba). Identifying these pathogens can lead to patient anxiety and unnecessary treatment; thus, screening asymptomatic travelers for these intestinal protozoa is not recommended.
General guidelines
Eosinophilia
Screening recommended for long-term exposure
Screening for eosinophilia is a common test because it is quick, universally available, and theoretically of value in detecting invasive helminths. Multiple studies have shown, however, that testing for eosinophilia has poor sensitivity for identifying parasitic infections; the low prevalence of infection in asymptomatic travelers means that the positive predictive value is poor, and the finding of eosinophilia can lead to an extensive and often fruitless search for a cause, generating patient anxiety and high costs. Many cases of eosinophilia resolve spontaneously, possibly because of infection with nonpathogenic organisms or a noninfectious cause (e.g., allergy, drug reaction). Repeat eosinophil counts after several weeks or months before embarking on an extensive investigation. Specificity is generally low in those without high-risk and long-term exposures.
A recent study in travelers and migrants showed that those with helminthic infection (as compared to other diagnoses) had much higher eosinophil counts. Counts can be highly variable, though, even within a single day, and are suppressed by endogenous or exogenous steroids. Using absolute eosinophil counts, rather than eosinophils as a percentage of leukocytes, is more reproducible and predictive.
Screening for eosinophilia should be considered only in those at high risk of helminthic infection. This would include those with stays beyond 3–6 months under conditions of poor sanitation and hygiene. The initial workup for sustained eosinophilia should include concentrated stool microscopy and Strongyloides serology. Additional testing for schistosomiasis and filariasis should be performed in individuals with relevant exposures in an endemic region. Failure to identify an etiology after this preliminary workup will often require consultation with a specialist.
Duration of travel and other risk factors
Table 10.2.1 and the following traveler classification scheme provide general guidelines for screening asymptomatic returned travelers for imported infections.
Table 10.2.1: Considerations for screening asymptomatic travelers
| Risk Factor or Exposure | Suggested Screening Tests |
|---|---|
|
All travelers Short stay (<3–6 months), no identified risk factor/exposure |
No additional screening |
| Long-stay (>3–6 months), poor sanitation or hygiene | CBC with eosinophil count, liver transaminases, creatinine, CRP |
| Sexual contact | HIV, syphilis, chlamydia, gonorrhea; HBV, if not previously vaccinated (for men who have sex with men, people who have sex with unknown partners); HCV |
| Tattoo, piercing, injection or intranasal drug use, medical/dental care | HIV; HBV, if not previously vaccinated; HCV |
| Pregnant women who traveled in known current Zika virus endemic or epidemic area or sexual contact with a partner who traveled in these areas | Screening asymptomatic travelers of childbearing potential with potential exposure (but without ongoing risk) is not routinely recommended outside an outbreak situation. Testing using NAATs up to 12 weeks after potential exposure in endemic or epidemic regions can be considered in pregnant women. |
| Healthcare worker | TB screening (TST or IGRA, TST preferred for serial testing)) |
| Prolonged residence (>6 months) with population in an area with high TB burden | TB screening (TST or IGRA) |
| Walking barefoot on soil potentially contaminated with human feces or sewage | Strongyloides serology* |
| Exposure to freshwater rivers, lakes, or irrigation canals in endemic regions | Schistosoma serology |
Notes
Abbreviations: CBC, complete blood count; CRP, C-reactive protein; HBV, hepatitis B virus; HCV, hepatitis C virus; IGRA, interferon-γ release assay; NAAT, nucleic acid amplification test; TB, tuberculosis; TST, tuberculin skin test.
*Combination of serology and parasitological methods should be used in immunocompromised individuals.
Short-term travelers
Screening asymptomatic short-term (<3–6 months) travelers is usually low-yield and should be directed by specific risk factors revealed in the history. A history of prolonged (>2 weeks) gastrointestinal symptoms during travel can suggest protozoal infection. Consider serologic testing of travelers who bathed or swam in unchlorinated freshwater sources in regions with known schistosomiasis risk, especially Sub-Saharan Africa.
In addition, consider serology testing for Strongyloides in select high-risk travelers who have skin exposure to soil likely to be contaminated with human feces, usually individuals with a history of frequently walking barefoot outdoors. In immunocompromised individuals, a combination of serology and stool parasitological methods may increase testing sensitivity. Obtain a sexual history; screen for sexually transmitted and blood-borne infections, if warranted. Zika or other arbovirus testing for asymptomatic travelers (including pregnant women) with potential exposure is generally not recommended (see Zika chapter). Consider TB screening for those returning from work in health care or other high-risk settings.
Long-term travelers and expatriates
The overall yield of screening increases for longer-stay (>3 months) travelers. The emphasis should be on those with the longest stays and the most problematic sanitary conditions or other exposures. In some cases, employers require certain tests, partly for liability reasons. Performing stool examinations mostly provides psychological reassurance. Consider obtaining serologic testing for schistosomiasis and strongyloidiasis in people with recent or remote travel histories to endemic areas and who report some level of risk.
A complete blood count with white blood cell differential and eosinophil counts, liver transaminases, creatinine, and C-reactive protein are usually the basic set of tests performed. Interpret results cautiously; abnormalities might trigger further testing. Zika virus testing for asymptomatic travelers with potential exposure, including pregnant women, is generally not recommended outside of a recognized outbreak. Limit TST or IGRA testing to travelers who worked in a healthcare or similar setting or who had intimate and prolonged contact with residents of a highly TB-endemic area for ≥6 months. Only perform other screening based on exceptional exposures or knowledge about local outbreaks.
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