Tuberculosis in Canada

Tuberculosis
Tuberculosis is a severe bacterial infection of the lungs and other organs.

One in three people in the world are infected with Tb.

Two million deaths occur each year from infection.

Contagiousness is related to the length of exposure to and infectivity of the contact person.

Infection is contracted by droplet nuclei from coughing.

The bacteria are then ingested by alveolar macrophages in the lungs causing the primary infection.

This then spreads through the blood (hematogenously) and becomes a latent Tb infection.

Active Tb disease will develop in 10% of these latent infections (usually this occurs within 2 yrs in 80% of those who do develop disease).

The lungs are most commonly infected (about 50%).

On x-ray cavitations, caseation, and fibrosis can be seen.

Only pulmonary TB is infectious to others.

A direct stain for acid-fast bacilli diagnoses active TB and this is confirmed by culture.

Chest X-ray will show lung infection.

Two of the worst types of Tb infection occur more in children under 5 yrs and are miliary (generalized infection) and Tb meningitis.

Latent Tb by definition is symptomless and is diagnosed with the tuberculin skin test (the Mantoux test) where tuberculin protein is injected intradermally on the forearm to see if that person has been sensitized to tuberculin in the past (either with a prior exposure to the disease or vaccination with BCG).

Immunodiagnosis techniques may be used in the future.

Treatment of latent Tb can be months on anti-tuberculosis drugs.

Risk Factors:
Tb is poverty related.

As soon as living conditions improve the Tb incidence goes down.

HIV and AIDS can accelerate the symptoms and progression of Tb.

HIV increases the progression of Tb from 10% of latent infections towards active per life to 10% per year of those affected.

With the rise of HIV Tb has risen and is expected to rise further.

There is an under appreciated risk to the traveler of Tb infection but little hard data on this.

There are reports of Tb outbreaks among airplane passengers who had sat next to heavily infected individuals who were actively coughing during long flights.

Travelers are at risk to exposure.

There are outbreaks of TB, especially among air travel although this is felt to be very low.

Risk to travelers of Tb exposure can be expressed from the rate of Mantoux skin test conversions from negative to positive indicating that exposure has occurred.

A Peace Corps study showed 15 per 1000 traveler years skin test conversions.

In another study health care volunteers had a conversion rate 7.9 while tourists had a rate of 3.5.

Expatriates and long term tourists have a risk of Tb that becomes similar to the host country (1-3 %).

The risk of catching the disease is relative. In order of frequency diseases are; HEPATITIS A (0.3%-2%) > LATENT Tb INFECTION > HEPATITIS B > ACTIVE Tb > TYPHOID (~0.003%)> MENINGITIS> CHOLERA (1 in 500,000) per travel month

TB PREVENTION FOCUSES ON:
1) Avoiding exposure.
2) BCG vaccination.
3) Identifying and treating latent Tb infections.

BCG
BCG (Bacille Calmette-Guerin) vaccination is a live attenuated vaccine of Mycobacterium Bovis (cow tuberculosis) which protects against disease but not infection.

It will induce Tb sensitivity but not infection.

Studies compare different effectiveness and opinions vary from country to country from 0-80%.

In better - designed studies it appears to have a more proven benefit.

Closer to the equator studies have shown less protection.

BCG does protect against the military and meningeal Tb infectionsin children under 5yrs..

Duration is thought to be about 10-15 yrs.

Although used in the past with high- risk population (native children and military personnel) it is not presently given routinely in Manitoba except for high risk Aboriginal children.

In some countries where it is given it is administered at least 6 wks before travel.

Contraindications to its use are: immune suppression, any HIV infection regardless of their CD4 counts, and a positive Mantoux test.

Complications include having an abscess formation at inoculum site.

BCG may be considered in:
1. If patient is under 5 years and at high risk.

(these are more likely to develop meningeal or military Tb - 2 conditions that are prevented by the vaccine.)
2. Poor compliance for follow up i.e. They never return to get their Mantoux done properly or would be noncompliant with Tb medications.
3. Individual would be at risk for severe side effects from anti-Tb drugs.

Still the BCG is infrequently administer outside of Northern Communities.

Diagnosing Tb

The Mantoux test consists of injecting 0.1 ml of purified tuberculin protein intradermally.

The injection site must be read by an experienced doctor or nurse 48-72 hrs later.

This protein is not infectious and there is no risk of acquiring Tb from this test.

The immune system will recognize the tuberculin protein if that person has been sensitized (either be exposure to Tb or with the BCG vaccine) and mount an immune response at the injection site (which becomes red and indurated).

Local guidelines should for interpretation should be followed, as tuberculin doses are not universally standardized.

In Manitoba 10mm of induration is felt to be a positive test.

Sensitivity of this test may be affected by insulin dependant diabetes and pregnancy although these people should still be tested if indicated.

There also exists a booster effect from having a recent Mantoux test if given recently.

Doing a 2 step Mantoux test can detect this.

This test checks to see if a boosting effect occurs from the first injection of tuberculin rather than because of true exposure to tuberculosis.

The manitoux test is done once and then again 1-3 weeks later to see if a boosting effect takes place

If a boosting effect does take place then this can be noted and compared with that patients Mantoux when checked again after their trip.

By doing this 2- step test there is increased sensitivity for catching new cases of Tb and treating them soon to prevent further contagion.

Individuals who had been vaccinated with the BCG vaccine will have a positive Mantoux although if vaccinated in their first year of life they may be Mantoux negative.

If a BCG has been done in the past it is recommended to do the 2 step Mantoux.

Individuals with weakened immune response may show anergy or the inability to mount an immune response against the Mantoux test and will be false negative. (This ironicall includes HIV infected people who are also at higher risk for Tb).

TREATMENT OF POSITIVE CONVERSIONS (New Latent Infections)

WHEN TO TREAT?
All new suspected conversions should be referred to a local Tuberculosis Treatment Program for further assessment and follow-up. Anti-tuberculosis drugs need to be taken for months and patients need to be followed.



TUBERCULOSIS CONCLUSIONS:
1. Long- term travelers have a risk for Tb
2. BCG may be recommended for high- risk travelers (although not in Canada).
3. Serology may replace skin tests in future, which could aid in detection much easier.
4. Newer vaccines unlikely to come out in near future.
At present the best way to control Tb is to promptly treat new cases and have people educated in high risk situations.

Source: 8th ISTM Scientific Assembly 2001,Yellow Book 2001, The CDC Pink Book 2000.

CDC http://www.cdc.gov/nchstp/tb/faqs/qa.htm

tuberculosis.net http://www.tuberculosis.net/

who   http://www.who.int/gtb/

Stop TB http://www.who.int/gtb