Mycobacterial infections (tuberculosis)

Notification for tuberculosis

Tuberculosis (TB) is a ‘routine’ notifiable condition and must be notified by medical practitioners and pathology services in writing within 5 days of diagnosis or clinical suspicion.

This is a Victorian statutory requirement.

Primary school and children’s services centres exclusion for tuberculosis

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

Asymptomatic contacts are not excluded.

Infectious agent of tuberculosis

Mycobacterium tuberculosis (human TB) and M. bovis (cattle TB) are the usual infective agents. M. africanum and M. canetti are both rare causes of TB in Africa. Several animal pathogens have been reported to cause zoonotic TB in humans: M. caprae (cattle), M. microti (rodents) and M. pinnipedii (seals).

Identification of tuberculosis

Clinical features

TB is an acute or chronic infection caused by the tubercle bacillus M. tuberculosis, and rarely by M. bovis or M. africanum. Active disease may occur soon after the initial infection (primary progressive disease). More usually, the initial pulmonary infection goes unnoticed, with lesions healing, sometimes leaving traces of calcified scar tissue. Viable organisms may remain in these lesions (latent TB), allowing reactivation at a later stage, often when the case is immunosuppressed through other disease processes, medication or old age.

Such reactivation may occur locally in the lungs (typically apical zones), or through haematogenous or lymphatic spread producing miliary, meningeal or other extrapulmonary involvement. The pattern of disease may vary, depending on the age of the individual case and their immune status. HIV-infected cases are at increased risk at any CD4 count; however, those with CD4 counts below 200 are more likely to present with atypical or disseminated TB infection.

Common symptoms include:

• a chronic cough, sometimes accompanied by haemoptysis
• fevers and night sweats
• loss of weight
• malaise and fatigue.

Clinical suspicion of active disease should be high in those who have a newly positive tuberculin skin reaction or interferon-gamma release assay (IGRA), juveniles with positive tuberculin reactions, and those with a history of inadequately treated active TB. Positive tuberculin reactors with inactive TB on chest X-ray and no prior diagnosis of active TB remain at some risk. The risk of developing the disease is highest in children under 3 years of age, lowest in later childhood, and rises again for adolescents, young adults and the very old.

If immunocompetent, approximately 3–4 per cent of infected individuals progress to active disease in the first year after tuberculin conversion, and an additional approximately 5 per cent reactivate during the remainder of their lifetime. For the immunocompromised, the risk of progressive disease and reactivation is much higher. For example, HIV-positive patients have a risk of reactivation of 5–10 per cent per year. This should influence decision making in treatment of latent TB.

Diagnosis

TB is diagnosed by consideration of:

• clinical presentation
• tuberculin skin test using the Mantoux procedure or IGRA
• radiographic examination, sometimes including computed tomography (CT) scans
• microscopy, direct staining and culture of sputum or other specimens for the presence of M. tuberculosis
• molecular amplification (by polymerase chain reaction – PCR) and gene probes, which may assist in rapid diagnosis.

Definitive diagnosis of TB rests on isolation of M. tuberculosis (or M. bovis) from sputum, urine, biopsy material, cerebrospinal fluid or other clinical specimens. A negative sputum, PCR, IGRA or Mantoux 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 the Victorian Infectious Diseases Reference Laboratory.

Incubation period of tuberculosis

The period from infection to development of the primary lesion or significant tuberculin reaction is about 3–9 weeks.

Public health significance and occurrence of tuberculosis

TB occurs worldwide, with one-third of the global population thought to be infected. The World Health Organization (WHO) collects extensive data on TB notifications, treatment and outcomes for most countries around the world. The 2012 global report showed that there were 8.7 million new cases in 2011, and 1.4 million deaths, 95 per cent of which occurred in low- and middle-income countries. Sixty per cent of global cases of TB are in South-East Asia and western Pacific regions. On the whole, Asia has the largest absolute number of new cases; however, sub-Saharan Africa has the highest proportional incidence, at 260 cases per 100,000 population. TB remains the leading killer for HIV-infected cases (25 per cent), and, worldwide, TB is second only to HIV/AIDS as the greatest killer due to a single infectious agent.

Incidence had been decreasing steadily over past decades in developed countries. However, this pattern reversed, with increasing rates of TB seen in the United States from the mid 1980s. A combination of factors is thought to be responsible, including high rates of HIV infection, overcrowding, falling living standards, waning allocation of healthcare resources to TB programs, and the subsequent development and spread of resistant strains. In the United States, large outbreaks of TB have occurred in institutions, particularly prisons and hospitals. These outbreaks have predominantly affected HIV-infected people.

The increasing trend in the United States has now been reversed, and the incidence rate is once again declining.

Notified cases of TB in Victoria dropped dramatically from 1,000 cases in 1954 to 292 in 2000, with the notification rate falling from 47 to 6 per 100,000. However, this was followed by a gradual increase in the number of notifications from 2003 to 2012, reaching its peak in 2010 with 435 cases. This corresponded with an average notification rate of 7 per 100,000 over the 10 years from 2003 to 2012 (range from 6.5 per 100,000 in 2004, 2010 and 2011, to 8.0 per 100,000 in 2010).

The proportion of notified cases that were overseas born has also increased, from 37 per cent in 1970 to 84 per cent in 1998–2002, and 91 per cent in 2008–12. In Victoria, the highest country-specific incidence rates are in the Indian-, Vietnamese-, Filipino- and African-born populations. This reflects the pattern of disease in their countries of birth. Of the overseas-born patients, almost 50 per cent present with disease within 5 years (30 per cent present within 2 years) of their arrival in Australia.

Reservoir for tuberculosis

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

Mode of transmission of tuberculosis

TB is transmitted mainly by inhalation of infectious droplets produced by people with pulmonary or laryngeal TB during coughing, laughing, shouting or sneezing. Invasion may occur through mucous membranes or damaged skin.

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

Period of communicability of tuberculosis

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 before diagnosis of an active case. A sputum smear–positive case is much more infectious than a case positive only on culture. Laryngeal TB is also highly infectious. The risk of transmitting the infection is significantly reduced within days to 2 weeks after commencing appropriate chemotherapy.

Susceptibility and resistance to tuberculosis

Anyone may become infected; however, some groups are more susceptible than others. Special groups at risk are:

• recent immigrants and refugees from countries with a high incidence of TB, including Vietnam, India, China, African nations 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, including those with HIV infection (any CD4 count), the elderly and diabetics; and infants aged less than 5 years
• drug- and alcohol-dependent people
• people living in substandard, overcrowded conditions or those who are homeless
• institutionalised people, including prisoners
• health professionals.

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

Control measures for tuberculosis

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 5 years of age 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, and for infants in some Aboriginal communities. (Refer to the current edition of The Australian immunisation handbook.)

Control of case

With the introduction of potent anti-TB drugs, hospitalisation of TB 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 counselling about minimising the risk of transmission of infection should be provided to all patients, particularly those with pulmonary TB. There is generally no restriction on the movement of patients with nonpulmonary disease.

Written notification of TB is required within 5 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 assess the requirements and extent of contact tracing.

Adequate anti-TB chemotherapy for an appropriate period will result in an almost 100 per cent cure rate. Short treatment regimens have been in use for some years. These involve the initial use of three or four drugs (isoniazid, rifampicin, pyrazinamide and possibly ethambutol) for 2 months, and continuing with isoniazid and rifampicin for a further 4 months. Where there is evidence of drug resistance to either isoniazid or rifampicin, or extrapulmonary disease, 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.

Multidrug-resistant TB (MDR-TB)

Resistance to at least isoniazid and rifampicin (whether or not the strain is also resistant to other drugs) is classified as multidrug-resistant TB. Worldwide, there are approximately 500,000 cases of MDR-TB, although WHO believes that approximately only one-fifth of MDR-TB cases are reported. Estimates for 2011 show that MDR-TB is responsible for 3.7 per cent of new TB cases and 20 per cent of previously treated cases. MDR-TB remains rare in Australia, with fewer than 10 cases per year in Victoria. There is, however, a potential risk of MDR-TB in Victoria, as most of the patients notified each year are overseas born, many from countries with high rates of drug-resistant TB. India and China have the highest burden of MDR-TB, and these two countries, plus the Russian Federation and South Africa, have almost 60 per cent of the world’s MDR-TB cases.

Extensively resistant TB (XDR-TB)

This is defined as an M. tuberculosis isolate that is resistant to isoniazid, rifampicin, a fluoroquinolone and an aminoglycoside. This resistance pattern first emerged in 2001, in Kwazulu-Natal, South Africa, where all XDR-TB cases were in HIV co-infected individuals and 98 per cent died. XDR-TB has since been reported in most other endemic countries. Currently, WHO estimates that 9 per cent of MDR-TB cases are XDR. As of 2013, four cases of XDR-TB had been reported in Australia.

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 and are managed by the TB Program. Anyone identified by healthcare workers as a contact of a case of TB should be referred to the TB Program.

Tuberculin or IGRA testing may be undertaken as part of contact investigation. IGRA has not been validated for the immunosuppressed or very young. However, one benefit, unlike tuberculin testing, is that it does not react for those who have had previous BCG vaccination.

Contact investigation consists of:

• history taking
• tuberculin or IGRA testing
• radiographic examination.

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

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

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

Following tuberculin testing, contacts can be grouped as follows.

Negative reactors:

• Tuberculin conversion takes a few weeks and may not have occurred yet in these contacts.
• Testing should be repeated in 8–12 weeks after a break of contact; in some cases, initial testing may be delayed for 8 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 bodyweight to a maximum of 300 mg daily. Treatment should be for a minimum of 9 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 6 months and 12 months.

Control of environment

There are no specific environmental controls because the greatest risk of transmission of infection is before diagnosis. However, a patient with pulmonary TB 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-TB treatment.

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

Outbreak measures for tuberculosis

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

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

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

International measures

All countries are required to report TB surveillance data to WHO. These data inform policies and strategies aimed at the global control of TB. Migrants and long-term visitors to Australia are screened for evidence of TB before being granted a visa.

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