host factors for susceptibility by adebayo daniel afekhide

7/28/2019 Host Factors for Susceptibility by Adebayo Daniel Afekhide

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HOST FACTORS FOR SUSCEPTIBILITY

It is not every individual exposed to the Mycobacterium tuberculosis that gets

infected with the infection. Susceptibility to disease after infection by

Mycobacterium tuberculosis is influenced by environmental and host factors

(nature and nurture)

A primary infection may heal, the host acquiring immunity in the process while in

other cases, the primary infection may progress to produce extensive disease

locally, or infection may be promulgated or disseminated to produce metastatic or

miliary tuberculosis. In others, primary lesions that are apparently healed may

subsequently deteriorate with reactivation of the disease. Those vital, influential

and important host factors include:

Age

The role of age in the susceptibility to tuberculosis is unequivocal, since

epidemiological data has shown that the population at the extreme of age are more

affected (the children and the elderly), this is attributable to the fact that the

immune system of the children is not fully developed and also to the progressive

deterioration, degradation and retrogression of the immune system of the elderly

due to senescence.

Malnutrition

It is not surprising to see that majority of patient developing the clinical features of pulmonary tuberculosis are people that are undernourished (malnourished) since

malnutrition can have adverse, even devastating effects upon the antigen-specific

arms of the immune system, as well as on many of the more generalized

mechanisms used for host defense.

Viral Infections

Host defend is often crippled and incapacitated by viral infections like Human

immune deficiency virus, which increases susceptibility to other infections like

tuberculosis, this is further buttressed by the fact that most tuberculosis patientsare HIV positive.

Other factors that tend to incapacitate the immune system (innate and acquired),

thereby increasing the susceptibility of an individual to the infection or culminating

in the reactivation of a previously dormant infection include:



Patients receiving chronic corticosteroid therapy for autoimmune diseases

Chronic illnesses like diabetes mellitus

Those on immunosuppressive therapy because of organ transplant and stem

cell transplant

Individuals receiving cancer chemotherapy Chronic lung disease is another significant risk factor – with silicosis

increasing the risk about 30-fold

Those who smoke cigarettes and drink alcohol have nearly thrice the risk of

TB than nonsmokers

There is also a genetic susceptibility, for which overall importance remains

undefined.

Community factors

The community factors also play a role in the epidemiology of tuberculosis. Those

at high risk thus include: people who inject illicit drugs, inhabitants and employees

of locales where vulnerable people gather (e.g. prisons and homeless shelters),

medically underprivileged and resource-poor communities, high-risk ethnic

minorities, children in close contact with high-risk category patients, and health

care providers serving these patients.

In conclusion both nature and nurture must be contributing to the risk of

tuberculosis. If undue emphasis is put on inheritable factors, the humanitarian need

to improve the lot of the poorest of the world who run the greatest risk of disease

will be overlooked. On the other hand, an emphasis on nurture may well have

contributed to the initial failure to detect and acknowledge the resurgence of the

disease that has occurred in the developed countries in recent years and its threat to

all levels of society.

Source of infectionHumans are the reservoir of the human strain (Mycobacterium tuberculosis) and

patients with tuberculosis constitute the main source of infection. The reservoir of

the bovine strain which can also affect humans is cattle, with infected milk and

meat being the main source. With pasteurization of milk the incidence of the

bovine tuberculosis has reduced drastically. It is only common in the northern part



of Nigeria where they ingest or consume raw milk from their cattle without

pasteurization.

Transmission

When people with active pulmonary TB cough, sneeze, speak, sing, or spit, theyexpel infectious aerosol droplets 0.5 to 5.0 µm in diameter. A single sneeze can

release up to 40,000 droplets. Each one of these droplets may transmit the disease,since the infectious dose of tuberculosis is very low (the inhalation of fewer than10 bacteria may cause an infection).

People with prolonged, frequent, or close contact with people with TB are at particularly high risk of becoming infected, with an estimated 22% infection rate.

A person with active but untreated tuberculosis may infect 10 – 15 (or more) other

people per year. Transmission should only occur from people with active TB -

those with latent infection are not thought to be contagious. The probability of transmission from one person to another depends upon several factors, including

the number of infectious droplets expelled by the carrier, the effectiveness of ventilation, the duration of exposure, the virulence of the M. tuberculosis strain,

the level of immunity in the uninfected person, and others. The cascade of person-

to-person spread can be circumvented by effectively segregating those with active

("overt") TB and putting them on anti-TB drug regimens. After about two weeks of effective treatment, subjects with nonresistant active infections generally do not

remain contagious to others. If someone does become infected, it typically takes

three to four weeks before the newly infected person becomes infectious enough totransmit the disease to others.

host factors for susceptibility by adebayo daniel afekhide

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