Causes and Risk Factors of Tuberculosis

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More than two billion people, a third of the world's population, are infected with tuberculosis (TB). According to the World Health Organization, there were 10.4 million new cases and 1.7 million deaths from the disease in 2016 alone. The United States accounted for 9,272 of those new diagnoses.

While infection with bacteria is the definitive cause of TB, there are many factors that can make you susceptible to the disease. Knowing what these factors are can help you to take action and decrease your risk of infection.

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Bacteria

Tuberculosis is caused by bacteria in the Mycobacterium complex family.

  • Mycobacterium africanum
  • Mycobacteria bovis
  • Mycobacterium tuberculosis

M. africanum, as the name implies, is most common in Africa, while M. tuberculosis is responsible for tuberculosis in most other parts of the world. These two bacteria cause the bulk of TB disease in humans. M. bovis is unique in that it mainly infects cattle. Drinking unpasteurized dairy products or other exposures to infected animals accounts for a small percentage of human tuberculosis cases.

Mycobacteria infect and live inside cells in our immune system called macrophages. Macrophages normally kill microbes and pathogens, but M. tuberculosis has a thick waxy capsule that protects it against the toxic enzymes the macrophages use to attack it. TB is then able to reproduce inside the macrophage.

How TB Bacteria Spreads

To understand how these bacteria cause infection, you need to understand the difference between latent and active TB infection.

Someone with latent infection has TB bacteria in their body but does not have active disease. Without symptoms like fever, chills, coughing, and weight loss, they are not infectious. Instead, the bacteria lie dormant in their body. As many as 5 to 10 percent of people with latent TB will go on to develop active TB in their lifetime. This most commonly happens within the first two years of the infection.

People with active TB, on the other hand, have symptoms like those described above. They are highly infectious and can spread the disease. When they cough, sneeze, spit, or talk, tubercular bacteria is released into water droplets. Anyone who inhales these droplets could develop TB infection in the lungs.

In the United States, TB transmission is more common in the spring with rates lowest in the fall.

Medical Factors

There are medical conditions that can increase your risk of developing tuberculosis.

Immunosuppression

A weakened immune system makes it harder for your body to fight infection and more likely that latent TB becomes active. It also makes it more likely to be infected with TB in the first place. You can be immunosuppressed based on any of the following:

  • Age: The immune systems of the very young and the very old tend to be weak.
  • Chemotherapy: These treatments fight cancer but also weaken your immune system.
  • Corticosteroids: Anyone who takes oral steroids long-term, the equivalent of 15 mg prednisolone for a month or longer, has a weakened immune system.
  • HIV/AIDS: The risk of developing TB is estimated to be at least 16 times greater for people who also have HIV. As HIV disease worsens or progresses to AIDS, the risk for TB increases further.
  • Organ transplants: To prevent the body from rejecting a transplanted organ (heart, kidney, or liver), people typically take life-long immunosuppressant medications.
  • Tumor necrosis factor (TNF) inhibitors: These biologic medications are often used to treat medical conditions like Crohn's disease, psoriatic arthritis, and rheumatoid arthritis.

Chronic Medical Conditions

The following conditions could increase your risk for TB infection:

How these conditions increase your risk of TB is not exactly known, but it may be due to their effects on the immune system and how well your body absorbs nutrients. If you have one of these conditions, take steps to decrease possible TB exposures.

Lifestyle Factors

There are non-medical risk factors of TB that may be more within your control. Unfortunately, poverty, homelessness, and decreased access to health care can make managing some of these factors challenging.

Diet and Nutrition

Poor nutrition plays a role in TB transmission. Severe malnutrition not only weakens the immune system but leads to weight loss. People who are underweight (body mass index of <18.5) are twice as likely to be infected than people with a higher BMI.

When it comes to specific nutrients, look towards iron and Vitamin D. High iron levels in the blood may promote growth of mycobacteria, making people more susceptible to TB. Vitamin D does the opposite, restricting the growth of mycobacteria. In this way, vitamin D deficiency is a risk factor for tuberculosis.

Whenever possible, it is important to eat a nutrient-rich diet and to maintain a healthy weight. Socioeconomic situations do not always make it possible to do that.

Location

People born in areas endemic with TB are clearly at increased risk for exposure to the bacteria. The following countries, from most to least frequent, account for 64 percent of all tuberculosis cases:

  • India
  • Indonesia
  • China
  • Philippines
  • Pakistan
  • Nigeria
  • South Africa

You may not be able to control where you are born but you can control where you go. At the very least, take precautions when you travel to these locations.

Living Conditions

TB can spread quickly when people are in close proximity. Crowded conditions within a community or even within a household increase that risk considerably. This is especially true when there is poor ventilation within a building.

Homeless shelters, in particular, can be overcrowded and are not always properly maintained. If you are fortunate enough to rent an apartment or purchase a home, consider the safety of your living arrangement.

Substance Abuse

Substance abuse is prevalent among people infected with TB. Smoking cigarettes increases your risk as much as two-fold. Illicit drug use, whether injection or non-injection, and drinking 40 grams (one pint of wine, three 12-ounce beers, or 4 ounces of distilled liquor like vodka or whiskey) or more of alcohol per day also increase the odds of TB transmission.

It is in your best interest to avoid smoking and illicit drugs. If you drink alcohol, only do so in moderation.

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