Tuberculosis in Travelers and Immigrants

Transmission and Pathogenesis

TB is caused by bacteria of the Mycobacterium tuberculosis complex , which belong to the family Mycobacteriaceae and the order Actinomycetales. Among the species of the M. tuberculosis complex, M. tuberculosis is the most common in human disease. The M. tuberculosis complex also includes Mycobacterium bovis (see section on Mycobacterium bovis and Nontuberculous Mycobacterial Infection ). M. tuberculosis is characterized by a waxy component of the cell wall, mycolic acid, which is neutral on Gram stain. The designation of M. tuberculosis as the “acid-fast bacillus” derives from its distinctive staining property: resistance to decolorization by acid alcohol after stained with basic fuchsin.

The typical mode of transmission is inhalation and deposition of droplet nuclei containing M. tuberculosis on the respiratory bronchiole or alveolus, located beyond the protective mucociliary blanket of the respiratory tree. When droplet nuclei are deposited in the terminal airway passages, bacilli are asymptomatically engulfed by alveolar macrophages. M. tuberculosis survives within alveolar macrophages and proliferates intracellularly. Thereafter, bacilli are transported to hilar lymph nodes and then, via thoracic duct, spread systemically to other organs. In the majority of hosts, cell-mediated immunity effectively contains bacilli by formation of granulomas in 2-10 weeks after acquisition of M. tuberculosis , and subsequently TB infection becomes latent. Latent TB infection (LTBI) is marked by a positive tuberculin skin test (TST) or a positive interferon-gamma release assay (IGRA). A small proportion of infected individuals show fibrotic or fibronodular lesions in the upper lung fields on chest radiographs (CXRs), presumably as a result of self-limited pulmonary disease that may have been sub-clinical in the past. Persons with these radiologic findings, however, are at greater risk of reactivation and should be encouraged to take treatment for LTBI (see section on Treatment). In addition, a conventional notion was that once someone has LTBI, this person is protected against further acquisition of M. tuberculosis. Studies showed that reinfection can occur in high-incidence settings, especially among human immunodeficiency (HIV)-infected individuals ( ). Reinfection is uncommon in low-incidence settings, especially among immunocompetent hosts ( ).

Most cases of active TB arise from reactivation of dormant foci of infection. All the factors that determine the small proportion of individuals whose TB infections will become reactivated are not known. However, certain conditions are known to increase the risk of reactivation. ( Table 25.1 )

When TB is “reactivated” at a later time, pulmonary TB is the most common form, but TB in extrapulmonary sites can be seen (see section on Extrapulmonary Tuberculosis ).

Those who have spent substantial time indoors with an infectious TB case may become infected with M. tuberculosis . The infection rate among the household contacts of sputum acid-fast bacilli (AFB) smear-positive pulmonary TB cases is around 30-40%. At least 8 hours in a confined indoor space is considered the minimal duration for raising concern about TB transmission.

Persons with LTBI are not infectious and cannot spread TB infection to others. The rate of progression from LTBI to active TB is around 5% within the first 2 years of acquisition and, thereafter, approximately 0.1% per year in immunocompetent adults. Age less than 5 years and immunocompromised status, especially HIV infection, increase the risk of progression to active TB ( ).

Epidemiology

The global epidemiology of TB has been affected by the acquired immunodeficiency syndrome (AIDS) epidemic. Most of the estimated TB cases in 2013 occurred in Asia (56%) and the African region (29%). Incidence rates vary from high in sub-Saharan Africa and in South and Southeast Asia to fewer than 10/100,000 population in the United States, Canada, and most of Western Europe ( Table 25.2 ) ( ).

Conditions Mimicking Tuberculosis

In evaluating patients with symptoms of pulmonary TB, clinicians should be reminded that TB may simulate many other diseases. Pneumonia, lung abscess, neoplasm, and fungal and parasitic infections may be mimicked by TB. The patient who originates from or who has traveled to a foreign country presents an additional diagnostic challenge. For example, as coccidioidomycosis is prevalent in persons from northern Mexico, it should be considered in the differential diagnosis of fibrosing, cavitary pulmonary disease in Mexican immigrants. While deep tissue fungal infections are rare in refugees from Southeast Asia, paragonimiasis is often confused with TB. Paragonimiasis is endemic in Asian countries and should be considered, particularly when raw crawfish consumption is reported. The diagnosis is made by identifying the parasite in sputum or in lung biopsy specimens ( Chapter 48 ).

Mycobacterium bovis and Nontuberculous Mycobacterial Infection

M. bovis and other nontuberculous mycobacterial infections may be seen in immunocompetent patients who have lived or traveled abroad. Generally, M. bovis infection is acquired by consumption of unpasteurized milk from infected cows. Human infections with M. bovis have been essentially eliminated in developed countries as a result of the pasteurization of milk and TB-control programs for cattle. TB caused by M. bovis is almost exclusively recognized in immigrants to the United States from the regions where these two control measures are absent ( ).

Mycobacteria other than M. tuberculosis , or nontuberculous mycobacteria (NTM), can cause pulmonary and extrapulmonary diseases. Most cases of cervical adenitis in refugee or immigrant children from African or Asian countries should be presumed to be M. tuberculosis disease rather than NTM, whereas cervical adenitis caused by Mycobacterium avium complex would be more likely in US-born children.

Tuberculosis in Tourists

Despite the high incidence of TB in many parts of the world, tourists from the United States, Canada, or western European countries to TB-endemic areas do not seem to be at significant risk of exposure when the purpose of their trip is business, tourism, missionary, research, or volunteering ( ; ; ). Epidemiologic studies have shown that casual contact with an infectious TB case usually does not result in transmission of infection. The important determinants of transmission consist of (1) infectiousness of the index case, (2) environment where TB exposure occurs (e.g., a confined, small space with poor ventilation increases the risk of transmission), and (3) cumulative hours of exposure (e.g., at least 6-8 hours in a confined space even if the index case is highly infectious). TB transmission is typically seen among the family members or within a close social network. Although transmission might occur during long air travel exceeding 8 hours, the public health risk from this is considered very low. Therefore, visitors to TB-endemic areas who follow normal tourist routes for a period of less than 2-3 weeks are unlikely to experience sufficient personal contact with infectious TB cases to acquire TB infection. TB in persons with foreign travel histories is more likely to occur in those who have traveled or lived abroad for several months or years, as is the case with students and expatriates.

Other travel scenarios more relevant to concern for TB transmission include health professionals providing medical aid work in high-risk settings and outbound medical tourists who obtain services in hospitals where TB patients may also be cared for.

Clinical Features