Prevalence of latent TB in English dairy farmers

When a farmer has a TB breakdown in his herd, depending on the severity of the breakdown and whether the infection is confirmed, Public Health England offer to perform tests on the farmer and his family. This includes a skin test as applied to his cattle which checks for latent TB. The skin test checks if the recipient has been infected with the TB bacteria by measuring inflammation of the skin in response to injected TB proteins. When the farmer's cattle test positive to the skin test the farmer's cattle are destroyed. However when a human tests positive to skin⁵ (or blood⁴) tests, the data is not even collected. This is according to replies received in Reference 1 from Public Health England. This suggests that in England there is poor awareness of the proportion of people with latent TB and how this proportion may be growing.

An animal with latent TB infection does not feel sick and does not have any symptoms³. According to Reference 2, 5-10% of humans, who are detected to be latently infected with TB, develop clinical tuberculosis during their lifetime. About half of those people who develop active TB are thought to do so within the first two years of infection³. It would appear that Human Health only collate the number of cases for active TB and this number obviously depends on how well TB is being diagnosed.

How may this be relevant?

From the point of view of the general public, if Human Health were to collate the results of performed latent TB tests and make this data available to the public, with notes on how to interpret this data, the public would be better informed of what the risks actually are. This would reduce speculation and uncertainty.

Regarding managing the disease, a clear picture of trends in latent TB would give an earlier warning than obtained from trends in active TB so that, if necessary, appropriate action could be taken in earlier years.

References

1. Proportion of English dairy farmers who test positive to the TB skin test. Freedom of Information request to Public Health England. May 2013.
2. Bovine TB Special Edition. Government Veterinary Journal. VOLUME 16,NO.1,2006.
3. The Difference Between Latent TB Infection and TB Disease. Centers for Disease Control and Prevention. Division of Tuberculosis Elimination. Page last reviewed: 1st September 1 2012.
4. Interferon Gamma Release Assay (IGRA) testing for tuberculosis (TB) - Questions & Answers (Q&As). For Health Care Workers. HPA Tuberculosis Programme Board. DRAFT FOR CONSULTATION. October 2007.
5. Targeted Tuberculin Testing and Treatment of Latent Tuberculosis Infection. An adaptation of a report published in the American Journal of Respiratory and Critical Care Medicine 2000;161:S221--S247. June 2000.

Other references

1. An update on Dianne Summer's illness - identified as the same spoligotype as her dead alpacas. bovinetb.blogspot.co.uk. 14th April 2013.
2. Public health and bovine tuberculosis:what's all the fuss about?
3. Prevalence of Latent and Active Tuberculosis among Dairy Farm Workers Exposed to Cattle Infected by Mycobacterium bovis. PLoS Negl Trop Dis. 2013 April; 7(4): e2177. Published 25th April 2013.