Tuberculosis: the Leicester school outbreak. Spring 2001

Peter D O Davies,
Director,
Tuberculosis Research Unit,
Cardiothoracic Centre,
Liverpool L14 3 PE

Events

In August 2000 a student at Crown Hills Community College, Leicester was diagnosed to have tuberculosis. He was thought to be non-infectious while at school so that only his close friends were screened for TB. This revealed one further case, also thought to be non-infectious. In February another student in the same year at the school was found to have tuberculosis. In March a teacher at the College was found to have TB.
Screening of all students in the same year as the three student cases and two years below took place. A further 19 cases were found. In addition 3 relatives of the original student were found to have TB. As of 6th April 26 cases have therefore been identified in association with the school.

Tuberculosis outbreaks in the UK

Outbreaks of tuberculosis in the UK are uncommon. The literature suggests there is one about every two years. Numbers of cases are usually between 6 and 12. It is unusual for more than a dozen or so cases to be reported in these outbreaks. This outbreak, 26 cases to date is large by recent standards. The last outbreak as big probably took place in Rutland (now Leicestershire) about 20 years ago. Nearly 30 pupils were diagnosed as having TB around a primary school. There were no fatalities.1. Other outbreaks have been reported around pubs and social clubs.2

Infectiousness of tuberculosis

Though tuberculosis is transmitted by airborne spread (like the common cold) it is not nearly so infectious. Transmission usually takes place after prolonged close association such as usually occurs within households. This is why larger outbreaks are relatively unusual. Such outbreaks are sometimes attributed to a more virulent bacteria but it is more likely that an outbreak occurs because the index case, ( the first to develop disease though not necessarily the first to be diagnosed) has been undiagnosed for so long that he or she passed infection to a considerable number of contacts.

The way tuberculosis affects children is different from adults. The former get "primary" disease which is not usually infectious. Adults develop "post-primary" disease, which is infectious in about half of cases and sometimes very infectious.
The number of bacteria found in the sputum is used to assess infectivity for contact tracing investigation. If these are profuse enough to be seen on a direct smear by microscopy the patient is deemed to be infectious. If the bacteria are too few to be seen under the microscope but are identified by culture techniques only then the patient is said to be probably not infectious. However, these definitions are arbitrary and infection of a contact may have occurred despite a low bacterial count.
Infection may take place in the absence of symptoms. The only evidence may be the presence of a positive skin test. Only one infected person in ten goes on to develop disease but this ratio may be altered if there is immune suppression such as occurs in AIDS.

The time interval between infection and the development of disease, (the incubation period) can be anywhere between six weeks and a life time.

Once a patient has been on treatment for over two weeks they are regarded as non-infectious.

Screening of cases

National guidelines for screening contacts of cases to determine whether infection or disease is present have recently been published 3. By combining an interview, tuberculin skin test and chest x-ray as appropriate infected or diseased individuals can be identified. Those who are found to have disease are treated with a six-month course of antibiotics. Children who have a strongly positive skin test but no evidence of disease are given a three-month course of preventive therapy to prevent infection from developing into disease.

BCG

BCG is a weak vaccine but does give some protection against tuberculosis infection leading on to disease. Trials have shown it to provide 75% protection for 15 years when given to 13-year-old children.4 Further studies have suggested it gives 50% protection to infants 5. National policy is to vaccinate all children aged 13 except for those at special risk such as ethnic minority groups who are given the vaccine at birth.
The few studies done to determine the efficacy of a second or third vaccination show them to have no protective effect, so that there is nothing to be gained by vaccinating those who were vaccinated at birth for a second time in their teens.

Tuberculosis in the UK

A recent study has shown that case rates in subjects of South Asian origin are 28 times more and in Black Africans 50 times more than in the White population.6 These individuals are at especial risk, because of the very high rates of disease in their countries of origin. [It should be remembered that tuberculosis was exported to what is now the developing world from the newly industrialised Europe, especially Britain, in the early 19th Century.7]

Implications for the Leicester outbreak

Many of the students at Crown Hills College are of Asian origin and therefore have a higher risk of developing tuberculosis than the rest of the community. Most will have had BCG at birth so that in the young teenage years any protective effect is likely to be reduced. Now that so many have been found to have tuberculosis, screening is being extended to the rest of the school so it is probable that more cases will come to light. There is no evidence of a drug resistant strain of bacterium.

Pupils with disease will be given six months of treatment and those with a positive skin test but no evidence of disease will be given a course of preventive therapy lasting three months. It is very probable that they will all make a full recovery and be left with no long-term ill effects.

References

1. Wales JM, Buchan AR, Cookson JB. Tuberculosis in a primary school: the Uppingham outbreak Brit Med J 1985;291:1039-40.

2. Q Syed, M A Bellis, N J Beeching, K Tocque, C S D Williams, S Jamieson, S Steele, P D O Davies. Tuberculin testing in two Liverpool Social Clubs: The effects of tuberculosis outbreak on background positivity. Thorax 1996;51:624-627.

3. Control and prevention of tuberculosis in the United Kingdom: Code of practice 2000.
Joint Tuberculosis Committee of the British Thoracic Society.
Peter Ormerod, Craig Skinner, Johhn Moore-Gillon, Peter Davies, Mary Connolly, Virginia Gleissberg,John Watson, Anton Pozniak, Ruth Gelletlie, Ann Cockroft, Francis Drobniewski,
Jane leese. Thorax 2000;55:887-901

4. Hart PD and Sutherland I BCG and the vole bacillus vaccines in the prevention of tuberculosis in adolescence and early adult life. Brit Med J 1977;2:293-5.

5. Packe GE and Innes JA. Ptotective effect of BCG in infant Asians: a case-control study. Arch Dis. Childh 1988;63:277-81.

6.Tuberculosis at the end of the 20th Century in England and Wales: results of a national survey in 1998. AMC Rose, JMWatson, C Graham, AJ Nunn, et al. Thorax 2001;56:167-172.

7. P D O Davies.
Tuberculosis and migration. J Roy Col Phys Lond. 1995;29:113-118.

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