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By Lin H. Chen, MD
Clinical Instructor, Harvard Medical School
A number of epidemiology and outbreak reports were presented at the 7th Conference of the International Society of Travel Medicine, held in Innsbruck, Austria, in May. A plenary session titled "Under-Appreciated Infectious Risks in Travel Medicine" included discussions on TB.
Here is a summary of the proceedings:
• Cobelens F, Van Deutekom H. Tuberculosis.
The epidemiology of TB was reviewed by Frank Cobelens, MD. One-third of the world’s population has been infected with TB. Approximately 8.4 million infections occur per year, and
2 million deaths occur each year. World Health Organization studies of TB in 54 settings showed that up to 37% of infected people are resistant to at least one drug, and up to 14% have multidrug-resistant tuberculosis (MDR-TB).
In some of the settings studied in Russia, Iran, China, and Estonia, 75% of cases were shown to be MDR-TB. The risk of TB in long-term travelers to areas of high incidence has been estimated to be three in 1,000 per month. Therefore, the risk of latent TB infection (LTBI) in travelers is greater than the risk of hepatitis B, typhoid fever, or meningococcal disease.
The two approaches to prevention of TB in travelers are vaccination with Bacille Calmette-Guerin (BCG) and identification of LTBI using a tuberculin skin test (TST) followed by prophylactic treatment. The protective efficacy of BCG against TB has been difficult to predict for the individual traveler. The duration of BCG protection is unclear (10-15 years), and there are limited data on the efficacy of repeated BCG vaccinations. TST, on the other hand, is dependent upon good technique, proper interpretation, and the tuberculin used.
The sensitivity of TST is decreased in individuals with cellular immune suppression. The specificity of TST may be complicated by the booster effect, which may result from LTBI in individuals with waning immunity as well as atypical mycobacterial infections and past BCG vaccinations. In order to reconcile the effect of BCG on TST, two-step testing at a one-week interval is recommended in travelers who have had BCG vaccination in the remote past, looking for a booster response.
For physicians who treat patients diagnosed with LTBI, there is an increasing choice of regimens. Isoniazid for six to nine months is currently the most common therapy but is associated with hepatotoxicity in 1% of patients. If resistance were suspected, the combination of rifampin and pyrazinamide for two months becomes the regimen of choice. If intolerance to pyrazinamide were to develop, rifampin alone would be used for four months. Patients need to be monitored clinically for hepatotoxicity, and baseline liver function tests should be considered in patients with increased risk for hepatotoxicity.
GeoSentinel, a global surveillance network of travel and tropical medicine clinics established to track disease trends in travelers, has been collecting data on confirmed TB cases in travelers, long-term immigrants, and short-term immigrants. Long-term immigrants are those who immigrated five or more years prior to the diagnosis. The network has registered 10,785 travelers and long-term immigrants and 2,786 short-term immigrants from January 1997 through November 2000. There were 95 TB cases among the short-term immigrants (rate = 3.41%) and 44 TB cases among travelers and long-term immigrants (rate = 0.45%). Seven of these cases occurred in travelers/expatriates. This information should lead to some clarification of TB risk in travelers.