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Tuberculosis

Epidemiology of Infectious Diseases: Tuberculosis

Causal agent
An infection caused by the bacterium Mycobacterium tuberculosis complex. This includes M. tuberculosis (most cases),  M. africanum, M. canettii, and  M. bovis (primarily from cattle)1. 

TB primarily affects the lungs (pulmonary TB).  However, tuberculosis can affect any organ or tissue, in the body including; the lymph nodes, pleura, pericardium, kidneys, bones and joints, larynx, middle ear, skin, intestines, peritoneum, and eyes1

About 10% of those initially infected will eventually develop active TB disease (50%  within the first 2 years following infection). 90% of untreated infected individuals will never develop active TB (latent TB infection).

Bacilli survive in latent form which may reactivate in later life. The risk of reactivation increases with age, chronic disease and immunosuppression (e.g. HIV/AIDS)2.

Reactivated TB is often pulmonary and without treatment carries a high mortality.

Common clinical features
TB disease develops slowly in the body, and it may take several months for symptoms to develop (HPA). 

Infection with TB is commonly characterized by:

  • Fever and night sweats
  • Persistent cough lasting 3 weeks or more that may produce discoloured or bloody sputum.
  • Pain with breathing or coughing (pleurisy)
  • Weight loss
  • Symptoms of TB disease in other parts of the body depend on the area affected.

Epidemiology

  • In 1993 the World Health Organization declared TB a 'global emergency'.
  • TB remains one of the world's leading infectious causes of death among adults.
  • According to the WHO, one-third of the world's population is infected with the TB bacillus.
  • There were 9 million new TB cases and approximately 2 million TB deaths in 2004.
  • More than 80% of all TB patients live in sub-Saharan Africa and Asia.
  • Increases in TB have been reported worldwide since the 1980s, notably in Southeast Asia and sub-Saharan Africa.  Important factors in the resurgence of TB include the HIV/AIDS pandemic, neglect of TB control programs, poverty and immigration.
  • TB is the leading cause of death among people who are HIV-positive. In Africa, HIV is the single most important factor determining the increased incidence of TB in the past 10 years.
  • A disease commonly associated with poverty and overcrowding, deaths from TB in Europe fell from approximately 500/100,000 population in 1850 to 50/100,000 population by 1950.
  • In England and Wales notified cases TB (respiratory and non-respiratory) declined from 117,139 cases in 1913 to 5086 cases in 1987 (HPA).
  • However, rates of TB in England and Wales have shown an increase since 1987. Between 1989 and 2003 notifications increased by approximately 30%.
  • Data from the Health Protection Agency Annual Report for TB notifications (2003), show 45% of all cases overall occurred in the London region, where the rate was 41.3/100000 population. All other UK regions reported a rate between 3.3/100,000 in Northern Ireland to 15.2/100,000 in West Midlands3.
  • 60% of all cases in England and Wales were reported in persons aged 15-44 years and 5% among 0-14 year olds.
  • 70% of all cases were born abroad, with TB rates 23 times higher among those born abroad than among those born in the UK.
  • The highest proportion of TB infections are reported in the Indian, Pakistani and Bangladeshi ethnic groups (36% of cases) followed by white (26%) and black (25%) ethnic groups.
  • However the highest rates of infection are reported among the black African ethnic group (283/100,000) followed by Indian, Pakistani and Bangladeshi (124/100,000) ethnic group3.
  • In the UK two-thirds of TB disease is pulmonary.  

Reservoir
Primarily humans.
In some areas diseased cattle, badgers, swine and other mammals are infected.

Mode of transmission
Person to person via inhalation of M. tuberculosis bacilli in droplet nuclei from coughing, sneezing and talking.

However, the risk of transmission depends upon the amount of bacilli in the sputum, the nature of the cough, the closeness and duration of the interaction, the virulence of the organism and the susceptibility of the contact1,2.  

Bovine TB is spread primarily through the ingestion of unpasteurised milk or milk products and in some cases through airborne transmission.

Risk groups for TB infection include the immunosuppressed (including, AIDS, cancer, lymphoma), alcohol and drug users, diabetics and severe malnutrition and recent arrivals to the UK from high prevalence countries.

Incubation period
3-8 weeks, range 3-12 weeks (from infection to reaction to tuberculin test).
Latent infection may be many decades2.

Period of Communicability
As long as there are viable organisms in the sputum.

Most sputum smear positive cases stop being infectious after 2 weeks following appropriate treatment.

Prevention and control

  • Tuberculosis is a statutorily notifiable disease.
  • In England and Wales enhanced TB surveillance started in January 1999.

In September 2005, changes in national BCG vaccination policy in England and Wales came into effect, aimed at providing an improved targeted BCG vaccination programme for;

  • All infants living in areas where the incidence of TB is 40/100,000 population or >.
  • All infants with a parent or grandparent who was born in a country where the incidence of TB is >40/100,000 population.
  • In addition older unvaccinated children with specific risk factors for TB to be identified and tested and vaccinated if appropriate. 

References

  1. Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association; 2004.
     
  2. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.
     
  3. Health Protection Agency, Tuberculosis, available at:  http://www.hpa.org.uk/infections/topics_az
     
  4. World Health Organization. Tuberculosis,  Fact sheet no104. Available at: http://www.who.int/mediacentre/factsheets

© CM Kirwan 2006