GMHC Treatment Issues, No. 43; Volume 4 Number 3 -- May 22, 1990
Karen Brudney, M. D.

Exposure, Transmission, and Latency

TB is caused by the bacterium Mycobacterium tuberculosis. As with HIV, infection is lifelong. Although the organism can cause disease in every organ system in the body, the most common site is the lungs. A person must have active, untreated TB in his/her lungs in order to infect others. Transmission occurs when infectious particles of respitory secretions become airborne by coughing, sneezing, or talking. A healthy person becomes infected inhaling these particles, which multiply upon reaching the air passages of the lungs. The most important transmission factors are closeness of contact and the infectiousness of the source. The degree of infectiousness varies broadly among individuals.

As with HIV, there is a distinction between infection and disease. Most infected people (85-90%) are able to contain the infection and never develop disease. This ability depends on the state of the immune system. Infection begins in macrophages in the lungs, where the organism multiplies and eventually causes cell rupture. Infected macrophages then end up in the lymph nodes. Eventually, invasion of the bloodstream occurs, and other body organs become infected.

TB organisms persist in a latent stage indefinitely. The HIV-infected individual, therefore, is at greater risk of developing TB in two ways. If he/she was previously infected in childhood, as often the case for individuals from areas where TB is endemic, such as Haiti, African countries, or the urban ghettos of the U. S., then his/her chance of reactivating a latent infection is high, once the immune system has been attacked by a HIV. An HIV-infected patient exposed to active TB for the first time will be far more likely to progress rapidly to active disease than someone with an intact immune system.

Factors Contributing to the Spread of Tuberculosis

The current TB crisis in New York City probably reflects both of these patterns of disease development. The poorer areas of the city have been hardest hit, in part because of dramatic curtailment of services by the Bureau of Tuberculosis when budgets were slashed during the fiscal crisis of the 1970s. The incidence of active TB had already begun to increase in New York City in 1979, before the appearance of HIV disease in the population most affected by TB, intravenous drug users. The rapid escalation of the inner city housing crisis during the 1980s led to the creation of mass shelters where inhabitants are crowded together, creating conditions in which TB spreads best. There is persuasive evidence that TB does spread in those shelters, although this is difficult to prove. The transmission of HIV among intravenous drug users, many of whom inhabit shelters, poured oil on and already smoldering fire.

Diagnosis and Prevention

Infection can be detected with a simple skin test, the PPD. This can and should be done on anyone infected with HIV, regardless of age, country of origin, or socioeconomic status. Although 10 mm is considered a positive result in the general population, 5 mm is recommended as the cutoff point by the New York City Bureau of Tuberculosis, among others. People with a positive PPD should then be evaluated to determine whether their infection is latent or active. Physicians should take a medical history, do a physical examination, chest x-ray, and possibly other lab tests. If there is no evidence of active disease, isoniazid (INH) should be taken for one year.

It may be difficult to diagnose TB infection in patients whose T4 cell count is under 200. Once T4 cells have decreased below 200, the body loses its ability to react to the PPD, and these patients may appear as falsely negative when tested. For T4 levels between 200 and 400, the results can be considered to be about 50% accurate.

TB infection can be effectively stopped from progressing to active disease with the use of medication. INH has been shown to be effective in preventing progression to disease in up to 70% of patients with intact immune systems. Due to liver toxicity, INH is not recommended as preventive treatment for patients over 35 years of age with an intact immune system since only about 15% of this population will develop active disease. However, due to HIV's presumed ability to spur TB infection on to disease, prophylaxis is of greater concern than possible side effects. No studies have been done to calculate the risk that HIV-infected persons will develop active TB, but it is undoubtedly significantly higher in this group than in the general population.

Treatment of Active Tuberculosis

If the medical history, physical examination, or chest x-ray suggests the presence of active disease, an aggressive medical workup should be done. It is important to remember that the symptoms of TB can and have been easily confused with non- specific, HIV-related symptoms (fever, malaise, weight loss), leading to failure to diagnose the former. It has also been well established that the radiological picture of TB in HIV-infected patients is quite different from the classic chest x-ray in patients with HIV disease. HIV-infected patients with TB sometimes even having normal chest x-rays.

The diagnosis of TB rests on finding the organism in body secretions (urine, sputum, pleural or peritoneal fluid), or tissue by microsocopy (acid-fast bacillus), or culture. Culture is particularly important in the HIV population as Mycobacterium avium appears identical to Mycobacterium tuberculosis on smear but requires quite different infection control and treatment implications.

TB disease in the HIV-infected individual responds quite well to standard drug treatment which has been used for years. The current recommendations of the CDC and the American Thoracic Society are to extend the normal treatment schedule by three months for HIV-infected patients. Drug regimens that have been used successfully include INH and rifampin for twelve months, or INH and rifampin for nine months with pyrazinamide for the first two months. Continued use of INH after treatment is an open question. Many physicians feel that INH should be given for life after completion of treatment of active TB. No study has been done to suggest the efficacy of lifetime INH, and there is no evidence that relapse of TB occurs more frequently in the HIV- infected population than it does otherwise. Many clinicians caring for HIV-infected patients do, in fact, treat their patients with INH indefinitely after completion of treatment of active TB.

In summary, the prevalence of TB infection in the HIV population is quite high in these groups previously exposed during childhood. Coinfection with HIV can cause reactivation of latent infection Prophylaxis when dually infected is essential and treatment of active disease is effective.

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