Causes & Risk Factor for TB and Epidemiology

Causes

Tuberculosis is caused by an organism called Mycobacterium tuberculosis. The bacteria spread from person to person through microscopic droplets released into the air. This can happen when someone with the untreated, active form of tuberculosis coughs, speaks, sneezes, spits, laughs or sings. Rarely, a pregnant woman with active TB may pass the bacteria to her unborn child.

Although tuberculosis is contagious, it's not especially easy to catch. You're much more likely to get tuberculosis from a family member or close co-worker than from a stranger. Most people with active TB who've had appropriate drug treatment for at least two weeks are no longer contagious.

Risk Factors:

• Lowered immunity. A healthy immune system can often successfully fight TB bacteria, but your body can't mount an effective defense if your resistance is low. A number of factors can weaken your immune system. Having a disease that suppresses immunity, such as HIV/AIDS, diabetes, end-stage kidney disease, certain cancers or the lung disease silicosis, can reduce your body's ability to protect itself. Your risk is also higher if you take corticosteroids, certain arthritis medications, chemotherapy drugs or other drugs that suppress the immune system.
• Close contact with someone with infectious TB. In general, you must spend an extended period of time with someone with untreated, active TB to become infected yourself. You're more likely to catch the disease from a family member, roommate, friend or close co-worker.
• Country of origin. People from regions with high rates of TB — especially sub-Saharan Africa, India, China, the islands of Southeast Asia and Micronesia, and parts of the former Soviet Union — are more likely to develop TB. In the United States, more than half the people with TB were born in a different country. Among these, the most common countries of origin were Mexico, the Philippines, India and Vietnam.
• Age. Older adults are at greater risk of TB because normal aging or illness may weaken their immune systems. They're also more likely to live in nursing homes, where outbreaks of TB can occur.
• Substance abuse. Long-term drug or alcohol use weakens your immune system and makes you more vulnerable to TB.
• Malnutrition. A poor diet or one too low in calories puts you at greater risk of TB.
• Lack of medical care. If you are on a low or fixed income, live in a remote area, have recently immigrated to the United States or are homeless, you may lack access to the medical care needed to diagnose and treat TB.
• Living or working in a residential care facility. People who live or work in prisons, immigration centers or nursing homes are all at risk of TB. That's because the risk of the disease is higher anywhere there is overcrowding and poor ventilation.
• Living in a refugee camp or shelter. Weakened by poor nutrition and ill health and living in crowded, unsanitary conditions, refugees are at especially high risk of TB infection.
• Health care work. Regular contact with people who are ill increases your chances of exposure to TB bacteria. Wearing a mask and frequent hand washing greatly reduce your risk.
• International travel. As people migrate and travel widely, they may expose others or be exposed to TB bacteria.

Epidemiology

More than 2 billion people (about one-third of the world population) are estimated to be infected with tuberculosis [1].

The global incidence of TB peaked around 2003 and now appears to be declining slowly [2].

In 2006 the World Health Organization (WHO) issued the following estimates [2]:

-The prevalence of active infection was 14.4 million, corresponding to a prevalence rate of 219/100,000 persons.
-The incidence of new cases was estimated to be 9.2 million, corresponding to an incidence rate of 139/100,000.
-Twelve of the 15 countries with the highest estimated TB incidence are in Africa, where the TB incidence rate was 363/100,000 (figure 1).
-In 2006 there were 1.7 million deaths from TB worldwide, a death rate of 25/100,000.
-The epidemiology of tuberculosis varies substantially around the world (figure 2). The highest rates (100/100,000 or higher) are observed in sub-Saharan Africa, India, China, and the islands of Southeast Asia and Micronesia.

-Intermediate rates of tuberculosis (26 to 100 cases/100,000) occur in Central and South America, Eastern Europe, and northern Africa. Low rates (less than 25 cases per 100,000 inhabitants) occur in the United States, Western Europe, Canada, Japan, and Australia.

Poverty, HIV and drug resistance are major contributors to the resurging global TB epidemic [3,4].

Approximately 95 percent of TB cases occur in developing countries.

Approximately 1 in 14 new TB cases occur in individuals who are infected with HIV; 85 percent of these cases occur in Africa [2]. An estimated half million cases of multidrug resistant (MDR)-TB also occur annual in Africans; even higher rates of drug resistant infections occur in Eastern Europe.

It is estimated

that, in 2007, there were 1.37 million incident cases of HIV-positive TB (14.8%

of total incident cases) and 456 000 deaths from TB among HIV-positive people

(equivalent to 26% of deaths from TB in HIV-positive and HIV-negative people,

and 23% of an estimated 2 million HIV-related deaths) (from WHO)