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Global Occurrence of Zoonotic Tuberculosis: Ongoing Efforts…

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By: Ashley Parker, Ph.D.

photo credit: via photopin (license)

Tuberculosis (TB) is a disease transmitted via droplets from the throat and lungs of infected individuals, and is caused mainly by the bacterium Mycobacterium tuberculosis. According to the 2015 Global TB report published by the World Health Organization (WHO), TB affects millions of people every year and ranks alongside the human immunodeficiency virus (HIV) as the leading cause of death worldwide; in addition, 9.6 million new cases of TB were estimated in 2014. We typically think of TB as a disease that is spread from person-to-person; however Meera Senthilingam, writer for CNN, recently published an article about the transmission of TB from infected animals to humans, highlighting the spread of tuberculosis via a contaminated food source that originated in a cow infected with Mycobacterium bovis.

Although M. bovis causes less than 2% of the total number of TB cases in the United States (less than 230 cases per year), according to the Centers for Disease Control and Prevention (CDC), the organism is also found in cattle and other animals such as bison, elk, and deer. “An increase in the number of cattle-associated cases in the U.S. was found to be near the Mexico border and among the Hispanic communities,” and was reported to be associated with the ingestion of unpasteurized milk and dairy products such as raw cheese, according to a speaker at the 46th World Conference on Lung Health. In California, the incidence of tuberculosis caused by M. bovis has increased recently. Of the approximately 19,000 patients who were enrolled in the states’ tuberculosis registry between 2003 and 2011, 3.4% of 2,384 cases were infected with M. bovis in 2003, increasing to 5.4% of 1,808 cases in 2011, with six cases having an association with at least one parent or guardian born in Mexico (Gallivan M, et al.). In an effort to manage the incidence and spread of bovine TB, the state of California is implementing strategies to limit the demand and distribution of unpasteurized milk and dairy products.

In other countries such as the United Kingdom, bovine TB is a major challenge for cattle farming industries, particularly in the west and southwest regions of England. In an effort to eradicate bovine TB, the UK government has developed actions outlined in their Bovine TB Strategy for England and UK Bovine TB Eradication Program, which include testing cattle herds for bovine TB and controlling TB in herds when detected, controlling the disease in badgers, improving biosecurity and husbandry on farms, developing TB vaccines for cattle, vaccinating badgers against TB, helping other industry sectors to deal with TB in non-bovine species, and developing the comprehensive bovine TB research program. According to a report by the CDC, many countries, particularly developing countries with limited resources, lack the ability to report all TB cases because of the difficulties with identifying suspected cases and establishing a diagnosis, and issues with recording and reporting cases. Another study presented at the 46th Union World Conference on Lung Health suggested that improper diagnosis and inadequate treatment was occurring in developing regions. In this study, “3,595 cattle and 266 livestock workers in Nigeria were screened for bovine TB; 10.4% of individual cattle, 42.9% of herds, 4.6% of butchers, and 6.1% of marketers were positive for tuberculosis.” There were major concerns about bacterial resistance, considering M. bovis is naturally resistant to pyrazinamide, a first line drug used as a standard treatment. Unfortunately, the CDC reports, of the 55 African countries, only 7 apply disease control measures as part of a test-and-slaughter policy and consider bovine TB to be a notifiable disease, while the remaining 48 apply inadequate control strategies or fail to control the disease. This leaves 85% of cattle and 82% of the human population exposed to bovine TB that is inadequately controlled or not controlled at all. In general, evaluation of M. bovis is not a routine part of laboratory testing for tuberculosis complex isolates, therefore most patients with the disease are not recognized. According to an article by Elizabeth Talbot, MD, the initial diagnostic approach for M. tuberculosis is also standard for M. bovis, including acid-fast staining, mycobacterial culture of relevant specimens, molecular testing for mycobacterial DNA, tuberculin skin test, and interferon gamma release assays. However additional tests should be requested in settings where M. bovis has high incidence rates or may be suspected, in an effort to properly identify the species. These tests include susceptibility testing, biochemical assays, and genomic analysis.

The spread of human M. bovis is a recognizable public health issue that is capturing attention in the United States and abroad. Although this organism primarily poses challenges in developing countries, the WHO recognized the importance and potential threat of human M. bovis in 1950 in the report of the Expert Committee on Tuberculosis. Recently, collaborative efforts have been made to eliminate bovine TB, involving WHO’s Division of Emerging and other Communicable Diseases Surveillance and Control, Veterinary Public Health program of the WHO Regional Office for the Americas, Pan American Health Organization, and additional working groups including the Food and Agriculture Organization of the United Nations, Office International des Epizooties Consultation on Animal Tuberculosis Vaccines. Successful vaccinations for cattle (developed in 1998 by WHO and organizations listed above) are currently being used in some countries. Plans to eradicate the disease are underway in specific countries, and are also part of a global 10-year plan, but will require better epidemiological surveillance to identify high risk areas, and properly implement control and elimination programs. Also, educating and notifying the public about the potential risk factors will help in the prevention of bovine TB.


Written by sciencepolicyforall

January 6, 2016 at 9:00 am

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  1. […] Source: Global Occurrence of Zoonotic Tuberculosis: Ongoing Efforts… […]

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