Microbiology and Immunology

Leprosy WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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nia, severe cases of Legionnaires’ disease may cause scarring

in the lung tissue as a result of the infection. Renal failure, if

it occurs, is reversible and renal function returns as the

patient’s health improves. Occasionally, fatigue and weakness

may linger for several months after the infection has been suc-

cessfully treated.

Because the bacteria thrive in warm stagnant water, reg-

ularly disinfecting ductwork, pipes, and other areas that may

serve as breeding areas is the best method for preventing out-

breaks of Legionnaires’ disease. Most outbreaks of

Legionnaires’ disease can be traced to specific points of expo-

sure, such as hospitals, hotels, and other places where people

gather. Sporadic cases are harder to determine and there is

insufficient evidence to point to exposure in individual homes.

See alsoPneumonia, bacterial and viral

LLeprosyEPROSY

Leprosy, also called Hansen’s disease, affects 10 –12 million

people worldwide. Caused by an unusual bacterium called

Mycobacterium leprae, leprosy primarily affects humans.

Leprosy is found in tropical areas, such as Africa, South and

Southeast Asia, and Central and South America. In the United

States, cases of leprosy have been reported in areas of Texas,

California, Louisiana, Florida, and Hawaii. Leprosy can take

many forms, but the most familiar form is characterized by

skin lesions and nerve damage. Although leprosy is curable

with various antibiotics, it remains a devastating illness

because of its potential to cause deformity, especially in the

facial features. Fortunately, antibiotic regimens are available

to treat and eventually cure leprosy.

Mycobacterium lepraeis an unusual bacterium for sev-

eral reasons. The bacterium divides slowly; in some tests,

researchers have noted a dividing time of once every twelve

days. This differs from the dividing time of most bacteria,

which is about once every few hours. M. lepraecannot be

grown on culturemedia, and is notoriously difficult to culture

within living animals. Because of these culturing difficulties,

researchers have not been able to investigate these bacteria as

closely as they have other, more easily cultured, bacteria.

Questions remain unanswered about M. leprae;for instance,

researchers are still unclear about how the bacteria are trans-

mitted from one person to another, and are not sure about the

role an individual’s genetic make up plays in the progression

of the disease.

Because M. lepraealmost exclusively infects humans,

animal models for studying leprosy are few. Surprisingly, a

few species of armadillo can also be infected with M. leprae.

Recently, however, wild armadillos have been appearing with

a naturally occurring form of leprosy. If the disease spreads in

the armadillo population, researchers will not be able to use

these animals for leprosy studies, since study animals must be

completely free of the disease as well as the bacteria that cause

it. Mice have also been used to study leprosy, but laboratory

conditions, such as temperature, must be carefully controlled

in order to sustain the infection in mice.

M. lepraeis temperature-sensitive; it favors tempera-

tures slightly below normal human body temperature. Because

of this predilection, M. lepraeinfects superficial body tissues

such as the skin, bones, and cartilage, and does not usually

penetrate to deeper organs and tissues. M. lepraeis an intra-

cellular pathogen; it crosses host cell membranes and lives

within these cells. Once inside the host cell, the bacterium

reproduces. The time required by these slow-growing bacteria

to reproduce themselves inside host cells can be anywhere

from a few weeks to as much as 40 years. Eventually, the bac-

teria lyse (burst open) the host cell, and new bacteria are

released that can infect other host cells.

Researchers assume that the bacteria are transmitted via

the respiratory tract. M. lepraeexists in the nasal secretions

and in the material secreted by skin lesions of infected indi-

viduals. M. lepraehas also been found in the breast milk of

infected nursing mothers. M. lepraemay be transmitted by

breathing in the bacteria, through breaks in the skin, or per-

haps through breast-feeding.

Leprosy exists in several different forms, although the

infectious agent for all of these forms is M. leprae.Host factors

such as genetic make up, individual immunity, geography, eth-

nicity, and socioeconomic circumstances determine which

form of leprosy is contracted by a person exposed to M. leprae.

Interestingly, most people who come in contact with the bac-

terium, about three-fourths, never develop leprosy, or develop

only a small lesion on the trunk or extremity that heals sponta-

neously. Most people, then, are not susceptible to M. leprae,

and their immune systems function effectively to neutralize the

bacteria. But one-fourth of those exposed to M. lepraecontract

the disease, which may manifest itself in various ways.

Five forms of leprosy are recognized, and a person may

progress from one form to another. The least serious form is

tuberculoid leprosy. In this form, the skin lesions and nerve

damage are minor. Tuberculoid leprosy is evidence that the

body’s cellular immune response—the part of the immune

systemthat seeks out and destroys infected cells—is working

at a high level of efficiency. Tuberculoid leprosy is easily

cured with antibiotics.

If tuberculoid leprosy is not treated promptly, or if a per-

son has a less vigorous cellular immune response to the M.

lepraebacteria, the disease may progress to a borderline lep-

rosy, which is characterized by more numerous skins lesions

and more serious nerve damage. The most severe form of lep-

rosy is lepromatous leprosy. In this form of leprosy, the skin

lesions are numerous and cause the skin to fold, especially the

skin on the face. This folding of facial skin leads to the leonine

(lion-like) features typical of lepromatous leprosy. Nerve dam-

age is extensive, and people with lepromatous leprosy may

lose the feeling in their extremities, such as the fingers and

toes. Contrary to popular belief, the fingers and toes of people

with this form of leprosy do not spontaneously drop off.

Rather, because patients cannot feel pain because of nerve

damage, the extremities can become easily injured.

Lepromatous leprosy occurs in people who exhibit an

efficient antibodyresponse to M. lepraebut an inefficient cel-

lular immune response. The antibody arm of the immune sys-

tem is not useful in neutralizing intracellular pathogens such as

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