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Mycobacterial infections (tuberculosis)

Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures | International measures | Additional sources of information

Victorian statutory requirement

Tuberculosis (Group B disease) must be notified in writing within five days of diagnosis including clinical suspicion.

School exclusion: exclude until receipt of a medical certificate from the treating physician stating that the child is not considered to be infectious.

Contacts are not excluded.

Infectious agent

Mycobacterium tuberculosis (human tuberculosis) and Mycobacterium bovis (cattle tuberculosis) are the infective agents.

Identification

Clinical features
Tuberculosis (TB) is an acute or chronic infection caused by the tubercle bacillus Mycobacterium tuberculosis, and rarely by M. bovis or M. africanum. The initial pulmonary infection usually goes unnoticed with lesions healing, sometimes leaving traces of calcified scar tissue. The infection may however progress to pulmonary tuberculosis, or through blood or lymphatic spread produce miliary, meningeal or other extrapulmonary involvement.

Common symptoms include:

  • a chronic cough sometimes accompanied by haemoptysis
  • fevers and night sweats
  • loss of weight
  • feeling generally unwell.

Clinical suspicion of active disease should be high in those who have a newly positive tuberculin reaction, juveniles with positive tuberculin reactions, and those with a history of inadequately treated active tuberculosis. Positive tuberculin reactors with inactive tuberculosis on chest X-ray without a previous diagnosis of active tuberculosis remain at some risk. The risk of developing the disease is highest in children under three years of age, lowest in later childhood, but rises again for adolescents, young adults and the very old.

Method of diagnosis
TB is diagnosed by a consideration of the following:

  • clinical presentation
  • tuberculin skin test using the Mantoux procedure
  • radiographic examination, sometimes including CT scans
  • bacteriology, direct staining and culture of sputum or other specimens for the presence of M. tuberculosis
  • molecular amplification (PCR) and gene probes assist in rapid diagnosis.

Definitive diagnosis of TB rests on isolation of M. tuberculosis (or
M. bovis) from sputum, urine, biopsy material, CSF or other clinical specimens. A negative sputum test does not rule out a diagnosis of TB. Recovery and identification of mycobacteria from specimens has become more rapid with test procedures such as liquid medium systems and DNA probes. Further information on these tests can be obtained from the Mycobacterium Reference Laboratory at Victorian Infectious Diseases Reference Laboratory.

Incubation period

Infection to the primary lesion or significant tuberculin reaction is about four to twelve weeks.

Public health significance & occurrence

Tuberculosis occurs worldwide and had been decreasing steadily over past decades in developed countries. This pattern was reversed with the arrival of HIV and increased mobility of the world’s population. Tuberculosis in the USA was on the increase in the early 1990s.

A combination of factors is thought to be responsible for this increase including the high rate of HIV infection, overcrowding, limited health care resources and falling living standards. In the USA, large outbreaks of TB have occurred in institutions, particularly prisons and hospitals. These outbreaks have predominantly affected HIV-infected persons. The increasing trend in the US has now been reversed and the incidence rate is once again declining.

The World Health Organization (WHO) estimated that a third of the world’s population is infected and tuberculosis accounts for three million deaths annually. One-fifth of all deaths in adults in developing countries relate to TB. Two-thirds of the world’s tuberculosis-infected people reside in Asia and this will have a significant impact on the control of TB in Australia as a result of increased immigration.

Notified cases of TB in Victoria have dropped dramatically from 1000 cases in 1954 to 292 in 2000. However, the rate of decline in the incidence of TB has reached a plateau with an average incidence rate of 6.2/100 000 over the last five years (range 5.1/100 000 in 1998 to 7.0/100 000 in 1999).

The proportion of notified cases that were overseas-born has also increased from 37% in 1970 to 86% in 2000. In Victoria the highest country-specific incidence rates are in the Vietnamese, Indian, Filipino and African born populations. This reflects the pattern of disease in their countries of birth. Of the overseas-born patients, almost 50% present with disease within five years and 30% present within two years of their arrival in Australia.

Reservoir

Humans are the primary reservoir. Diseased cattle rarely act as reservoirs.

Mode of transmission

TB is transmitted mainly by inhalation of infectious droplets produced by persons with pulmonary or laryngeal tuberculosis during coughing, laughing, shouting or sneezing.

Invasion may occur through mucous membranes or damaged skin.

Extrapulmonary tuberculosis, other than laryngeal infection, is generally not communicable. Urine is infectious in cases of renal tuberculosis. Bovine tuberculosis results mainly from ingestion of unpasteurised milk and dairy products. Aerosol transmission has been reported among abattoir workers.

Period of communicability

In theory, the patient is infectious as long as viable bacilli are being discharged from the sputum. In practice, the greatest risk of transmitting infection is in the period prior to diagnosis of an open case. A sputum smear positive case is more infectious than a case only positive on culture. The risk of transmitting the infection is significantly reduced within days to two weeks after commencing appropriate chemotherapy.

Susceptibility & resistance

Everyone is susceptible to infection, however some groups are more susceptible to infection and progression to active disease than others. Special groups at risk are:

  • recent immigrants and refugees from countries with a high incidence of tuberculosis including Vietnam, India, China, Africa and the Philippines
  • those in close contact with a case of active TB
  • Aboriginal people and Torres Strait Islanders in some parts of Australia
  • immunosuppressed patients
  • those with HIV infection and AIDS
  • the elderly
  • diabetics
  • drug and alcohol-dependent people
  • people living in substandard, overcrowded conditions
  • institutionalised people including prisoners
  • health professionals.

The disease does not always confer protective immunity as reinfection can occur.

Control measures

Preventive measures
BCG vaccination has limited application in developed countries where the incidence of TB is low. It is an effective vaccine in reducing TB meningitis and death in babies and children less than five years in countries of high TB prevalence. It is not recommended for general use in the Australian community but should be considered for specific high risk groups such as infants and young children travelling for extended periods to countries with a high incidence of TB. (Refer to The Australian immunisation handbook, National Health and Medical Research Council).

Control of case
With the introduction of potent anti-TB drugs, hospitalisation of tuberculosis patients is no longer mandatory unless social conditions or coexisting medical conditions dictate otherwise.

Patients with pulmonary TB should be isolated either at home or in hospital until they have been on adequate anti-TB therapy for 14 days and sputum smears are negative.

Appropriate education and counseling about minimising the risk of transmission of infection should be provided to all patients, particularly those with pulmonary TB. There is no restriction on the movement of patients with non-pulmonary disease.

Written notification of tuberculosis is required within five days of diagnosis. On receipt of a notification, a public health nurse is allocated to the patient to provide support, assist with treatment compliance and to assess the requirements and extent of contact tracing.

Adequate anti-TB chemotherapy for an appropriate period of time will result in almost 100% cure rate. Short treatment regimens have been in use for some years. These involve the use initially of three or four drugs (isoniazid [INH], rifampicin, pyrazinamide and may include ethambutol) for two months, and continuing with isoniazid and rifampicin for a further four months. Where there is evidence of drug resistance to isoniazid or rifampicin or to both, short course anti-TB chemotherapy is inappropriate.

The success of treatment relies heavily on patient compliance and direct supervision should be the aim of any treatment program. Compliance is important to prevent the development of drug resistance.

Multi-drug resistant TB (MDRTB)
Resistance to at least isoniazid and rifampicin (whether or not it is also resistant to other drugs) is classified as multi-drug resistant. MDRTB is rare in Australia. It has remained at less than two percent per year in the past 15 years. There is however a potential risk of MDRTB in Victoria as most of the patients notified each year are overseas born, many from countries with high rates of drug resistant TB.

Control of contacts
Exclusion of contacts is not necessary, unless they have signs and symptoms consistent with pulmonary TB.

Contact tracing and surveillance are the responsibility of the Department of Health and are managed by the TB Program. Anyone identified by health care workers as a contact of a case of TB should be referred to the TB Program.

Contact investigation consists of:

  • history taking
  • tuberculin testing
  • radiographic examination.

The extent of investigation is governed by the characteristics of the source case. The scope of investigation is extended when the following factors in the source case are present:

  • acid fast bacilli (AFB) in sputum smear
  • cavitation on chest X-ray laryngeal TB
  • cough, particularly if productive of sputum, or
  • evidence of tuberculin conversion in any of the contacts.

Note: Tuberculosis testing should never be omitted for child contacts.

Following tuberculin testing contacts can be grouped as:

  • Negative reactors
    • Tuberculin conversion takes a few weeks and may not have occurred yet in these contacts.
    • Testing should be repeated in eight to 12 weeks after a break of contact or in some cases initial testing may be delayed for eight weeks.
    • Chest X-rays may be considered on an individual basis.
  • Positive reactors
    • Initial positive reactors should be evaluated to exclude active disease. The positive tuberculin test may signify recent tuberculin conversion or an incidental finding.
    • Contacts identified by the TB Program as requiring further assessment are referred to specialist physicians for exclusion of active disease or consideration for treatment of latent infection.
    • When X-ray and physical examination are normal, contacts with positive reaction may be offered isoniazid treatment of latent infection, given once daily at a dosage of 5 mg/kg body weight to a maximum of 300 mg daily. Treatment should be for a minimum period of nine months with appropriate monitoring for liver toxicity.
    • Contacts with positive reactions, who do not undertake treatment of latent infection, should be kept under surveillance and followed up with chest X-rays taken at six months and 12 months.

Control of environment
There are no specific environmental controls as the greatest risk of transmission of infection is prior to diagnosis. However, a patient with pulmonary tuberculosis should be isolated from any new contacts and young children (either in hospital or at home) until at least 14 days after commencing appropriate anti-tuberculosis treatment.

Fresh air, sunlight and covering the mouth and nose when coughing are all appropriate patient education and environmental control measures.

Outbreak measures

It is unusual for an outbreak of TB to occur due to the chronic nature of the disease and the extended incubation period. In the event of two or more cases occurring concurrently in a single setting, contact tracing and investigation would be extended to identify a possible unknown source case.

Special settings
Nosocomial transmission of tuberculosis does occur, particularly in cases where diagnosis is delayed. It is important that a high index of suspicion for tuberculosis is maintained, particularly in patients with respiratory symptoms and belonging to a high risk group for TB such as overseas born from high prevalence countries, immunosuppressed patients and the elderly (both Australian and overseas born).

All suspected and active cases of tuberculosis must be placed in respiratory isolation and appropriate infection control measures implemented, including use of submicron or particulate filter masks for health care workers and surgical masks for patients during transport within the hospital. In the event of a health care worker being exposed to an undiagnosed case of tuberculosis, appropriate contact tracing and screening measures must be implemented. Investigation and management will be as for contacts (above).

Health care facilities are required to have protocols and guidelines for tuberculosis prevention and management in place, including a tuberculin skin test screening policy (refer to Management, control and prevention of tuberculosis guidelines for health care providers, Department of Human Services 2002).

International measures

All countries are required to report TB surveillance data to the World Health Organization. This data informs policies and strategies aimed at the global control of TB. Migrants and long term visitors to Australia are screened for evidence of TB prior to being granted a visa.

Additional sources of information

 


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