Mycobacteria are acid-fast, aerobic bacilli that grow extremely slowly. The cell wall contains complex lipids and glycolipids, one of which-the cord factor-is responsible for producing granulomas. No extracellular enzyme or toxins have been identified. The capacity of mycobacteria to produce disease is therefore attributed to their ability to multiply within phagocytic cells and to withstand intracellular enzymatic digestion.
Tuberculosis is largely due to Mycobacterium tuberculosis.
Tuberculosis is present worldwide with an extremely high prevalence in Asian countries, where 60-80% of children below the age of 14 years are infected. Tuberculosis is spread predominantly by droplet infection. The prevalence of tuberculosis increases with poor social conditions, inadequate nutrition and overcrowding.
The characteristic lesion is a granuloma with central caseation and Langhans’ giant cells. The primary infection usually involves the lungs, but can involve other areas such as the ileocaecal region of the gastrointestinal tract. It is almost always accompanied by lymph node involvement. In most people the primary infection heals leaving some surviving tubercle bacilli. With a lowering of host
resistance these are reactivated producing local spread as well as haematogenous spread to all organs of the body, including the lungs, bones and kidneys. This particularly occurs in the elderly, in alcohol abusers, in patients with diabetes mellitus, lung disease, or after gastrectomy, as well as in patients who are on corticosteroids or are immunosuppressed. There is a high incidence in patients infected with HIV. Occasionally the primary infection progresses locally to a more widespread lesion; haematogenous spread can also occur. Tuberculosis in the adult is therefore usually the result of reactivation of old disease, occasionally a primary infection or, more rarely, reinfection.
Pulmonary tuberculosis is the commonest form; this is described on p. 683, along with the chemotherapeutic egimens. Tuberculosis also affects other organs, including:
ASTROINTESTINAL TRACT-mainly the ileocaecal area, but occasionally the peritoneum is affected, producing ascites.
GENITOURINARY SYSTEM-the kidney is mainly involved, but tuberculosis is also the cause of painless, craggy swellings in the epididymis and salpingitis, tubal abscesses and infertility in females
ENTRAL NERVOUS SYSTEM-tuberculous meningitis and tuberculomas
SKELETAL SYSTEM-arthritis and osteomyelitis with cold abscess formation can occur SKIN-producing lupus vulgaris EYE-producing choroiditis, iridocyclitis or phlyctenular keratoconjunctivitis
PERICARDIUM-producing constrictive pericarditis
ADRENAL GLANDs-causing destruction and producing Addison’s disease
LYMPH NODES-this is a common mode of presentation, especially in young adults and children. Any group of lymph nodes may be involved but hilar and paratracheal lymph nodes are the commonest. Initially the nodes are firm and discrete but later they become matted and can suppurate and form sinuses.
SCROFULA is the term used to describe massive cervical lymph node enlargement with discharging sinuses. It is most often due to M. tuberculosis, and rarely to M. scrofulaceum or M. kansasi. Signs of acute inflammation are absent. Leprosy (Hansen’s disease) The causative organism is the acid-fast bacillus M. leprae. Unlilce other mycobacteria, it does not grow in artificial media or even in tissue culture. While inability to culture the organism obtained from lesions and secretions is suggestive of M. leprae, it is not diagnostic. The following additional properties have been found to be useful in its identification:
• Loss of acid fastness following pyridine extraction.
• The ability of the organism to grow slowly in the footpad of mice. Other mycobacteria also grow in the footpad of mice, but these produce distinct histological changes.
• The ability of M. leprae to oxidize 3,4-dihydroxyphenylalanine to pigmented products.
• The ability of this organism to invade peripheral nerves, a property not demonstrated by other mycobacteria. Leprosy is found primarily in Asia and Africa. Endemic foci are still present in the former USSR and parts of the USA. Of the 15 million people with leprosy worldwide, about two-thirds are in Asia. The precise mode of transmission is still uncertain but it is likely that nasal secretions play an important role. Once an individual has been infected, subsequent progression to clinical disease appears to be dependent on several factors:
SEX-males appear to be more susceptible than females. In India, the ratio of affected males to females is 2 : 1.
GENETIC SUSCEPTIBILITy-studies in twins have shown a concordance in identical but not in nonidentical twins.
IMMUNOLOGICAL RESPONSE of the individual to the bacillus.
Two polar types of leprosy are recognized:
1 Tuberculoid leprosy-a localized disease that occurs in individuals who exhibit a marked immunological resistance to the organism.
2 Lepromatous leprosy-a generalized disease that occurs in individuals with impaired cell-mediated immunity (CMI). Two subdivisions of lepromatous leprosy are included in the classification. The patient is said to have the ‘subpolar’ lepromatous (LL,) form when he has passed through a borderline phase before becoming lepromatous, and the polar lepromatous (LLp) form when the patient is lepromatous throughout. The following types are also recognized:
BORDERLINE LEPROSY has features of both the polar varieties and is subdivided into borderline-tuberculoid, borderline and borderline-lepromatous. Borderline leprosy is an unstable state characterized by increasing numbers of M. leprae bacilli and decreasing numbers of lymphocytes.
INDETERMINATE LEPROSY is characterized by one or more hypopigmented, sometimes erythematous, illdefined macules of variable size. Sensation, sweating and hair growth over the macules are usually normal.
NEURITIC LEPROSY is not associated with skin lesions. The affected nerve is enlarged and firm and sensory loss in the area of the nerve’s distribution is present. Two indices are currently in use to evaluate the response to treatment of patients in whom the skin-smear test for acid -fast bacilli is positive:
1 Bacteriological index (EI). This index is an objective way of evaluating the response to treatment. The skin-smear is graded from 1+ to 6+ depending upon the number of bacilli present per high-power field. The BI is calculated by taking the mean result of four slide examinations. For example, a decrease in BI from 6 to 3.5 on therapy indicates a good response to treatment.
2 Morphological index (MI). This is the percentage of solid staining acid-fast bacilli on smears (solid bacilli represent viable bacteria). A patient with an MI of 0% is not infectious.
The incubation period varies from 2 to 6 years, although it may be as short as a few months or as long as 20 years. Leprosy should be considered in any individual who presents with hypopigmented skin patches associated with loss of sensation, especially to touch or temperature, and evidence of nerve involvement (thickening or tenderness), in whom non-cultivable acid-fast bacilli have been identified in skin smears. The onset of leprosy is generally insidious. However, acute onset is:known to occur and patients may present with a transient rash, with features of an acute febrile illness, with evidence of nerve involvement, or with any combination of these.