HIV infection

• occupations or environments that damage the lung (mining, dust, smoke).

As well as variation amongst individuals, there are also considerable differences in the susceptibility of populations. This can be measured by the annual tuberculosis infection rate, which compares the tuberculin reaction of non-vaccinated subjects of the same age every 5 years. With BCG vaccination at birth, this cannot be done any longer, but data obtained before this became a universal policy is still valid. Another method of estimating incidence is from tuberculosis notifications as seen in Fig. 13.2.

There are also environmental factors and density is as important as susceptibility of the population. The dose of bacilli that the individual will meet is increased by continued contact over a period of time. This dose/time factor is more likely to be found in conditions of poverty and overcrowding. If the dose is sufficiently large and maintained for long enough, even the defences of the immunologically competent individual may be broken down.

The risk of infection is greatest in the young and rises again in the old, so overcrowding increases the opportunity for infection to be acquired at a younger age. Since the young mix extensively, they will have a greater opportunity for passing on infection. At the other end of life, the elderly often form persistent foci in a community, a potent source of infection to the young.

HIV infection has changed the epidemiology and presentation of tuberculosis, especially in Africa, leading to more lower lobe and extrapulmonary disease. (There are estimated to be more than 20 million persons worldwide with dual tuberculosis and HIV, with the majority of these cases in Africa.) Reduced host response has increased susceptibility and allowed reactivation or reinfection to take place as well as increasing the likelihood of new infection

Fig. 13.2. Tuberculosis notification rates, 2002. Reproduced by permission of the World Health Organization, Geneva.

The designations employed and the presentation of material on this map do not imply the expression of any opinion whatsoever on the part of the World Health Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries, Dashed lines represent approximate border lines for which there may not yet be full agreement.

Fig. 13.2. Tuberculosis notification rates, 2002. Reproduced by permission of the World Health Organization, Geneva.

from a contact or case; however, there is evidence to suggest that HIV/tuberculosis patients are less infectious. Conversely, tuberculosis patients are more likely to rapidly progress to full-blown acquired immunodeficiency syndrome (AIDS) when infected with the HIV virus. Initially HIV-infected tuberculosis patients commonly present with pulmonary infection similar to the HIV negative case, but as the disease progresses, extrapulmonary tuberculosis predominates and other manifestations of HIV disease, such as chronic diarrhoea, generalized lymphadenopathy, oral thrush and Kaposi's sarcoma, are more common. All HIV-positive cases should, therefore, be investigated for tuberculosis and all tuberculosis cases tested for HIV. Despite the increase in extra-pulmonary tuberculosis, it is still the sputum-positive case that is responsible for transmission of infection and this must remain the priority in searching for cases.

Consumption of unpasteurized milk may result in bovine tuberculosis in humans where the disease is present in the animal population. This presents with enlargement and suppuration of the cervical lymph nodes rather than pulmonary disease. It is now less common than before with the testing of cattle and pasteurization of milk, but in developing countries where cattle and their produce are an important part of the diet, such as in Central and South America, bovine tuberculosis is found.

Incubation period The period between infection and development of the primary complex is 4-12 weeks.

Period of communicability A new, untreated case of tuberculosis will normally produce organisms for 12-18 months, but in those that develop a low-grade infection with chronic cough, infection can continue for a considerable period of time (about 5 years). Once treatment has started, the person becomes non-infectious in about 2 weeks.

Occurrence and distribution Tuberculosis is found worldwide (Fig. 13.2) in various levels of severity. Countries of low preva lence are defined as those where less than 10% of children under 15 years have a positive tuberculin test. These are largely the countries of Western Europe and North America. Tuberculosis, however, is increasing in Eastern Europe and the former USSR. Nearly the whole of the tropical world has a high prevalence rate with some countries experiencing over 50% of the under 15-year-olds being tuberculin-positive. In addition, urban areas have higher prevalence rates than rural areas. The rates are high in Africa and parts of South America. Asia, India, Myanmar, Thailand and Indonesia all have high tuberculin-positive rates. In the Americas, the indigenous peoples have a much higher rate than the non-indigenous. There is a high susceptibility in Pacific Islands in which tuberculosis was an unknown disease until the arrival of explorers, who introduced the disease.

Control and prevention There are four main strategies for the control and prevention of tuberculosis in the following order of priority:

• search and contact tracing for new cases;

• adequate treatment of all cases, especially the sputum-positive;

• improvement of social and living conditions;

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