Changing livestock vaccination policy alters the epidemiology of human anthrax, Georgia, 2000–2013
Introduction
Anthrax is a zoonotic disease that is found on nearly every continent (except Antarctica) [1], [2]. The causative agent, Bacillus anthracis, is a soil-borne Gram-positive bacterium with the remarkable ability to survive in the environment for long periods of time, perhaps years, and infect a wide range of hosts [2], [3]. Herbivorous animals are most often infected [2], [3], [4]. Human infections are typically a result of contact with infected animals or their by-products (e.g. meat or hides) during activities such as livestock slaughtering [1], [4].
Targeting livestock with annual vaccination is the most effective method to control anthrax in both humans and animals in endemic regions [1], [5]. The most widely used vaccine is the live attenuated Sterne strain (34F2) [5]. In the former Soviet Union (FSU), livestock anthrax vaccination combined with improvements in occupational safety produced a nearly 10-fold reduction in animal cases with a concomitant decline in human incidence [6]. Similar decreases were observed in Europe and the United States following mass vaccination of livestock [7]. However, despite the effectiveness of vaccination, anthrax persists in areas with weakened health infrastructures and long-term vaccination strategies may be needed in endemic areas [1], [8]. Countries of the FSU, sub-Saharan Africa, and southeast Asia have (re)emerged as foci for transmission [9].
The country of Georgia has experienced repeated outbreaks of human anthrax with a recent increase in human incidence (2010–2012) [10]. Reports of an anthrax-like disease in humans dates to the 17th century [11] and >500 locations have been registered as foci (permanent locations of anthrax risk) in Georgia since 1881 [12]. During Soviet governance, anthrax was a mandatory reportable infectious disease in both humans and livestock. Following the dissolution of the Soviet Union in 1991, and Georgian independence in December of the same year, anthrax reporting remained mandatory. To combat the spread of anthrax, from 1995 through 2006, the government carried out annual compulsory livestock vaccination. In 2007, Georgia ended this policy, placing the responsibility of vaccination on private livestock owners [13].
Yet, little is known about how the alteration in livestock immunization policy and the concomitant decline in the number of anthrax vaccine doses administered affected the epidemiology of human anthrax. In the context of >90% private livestock ownership in Georgia, and the high risk associated with agricultural production, identifying changes in anthrax epidemiology are crucial for implementing control strategies and limiting its spread. Our objective was to assess how the change in livestock vaccination policy impacted the epidemiologic characteristics of human anthrax in Georgia from 2000–2013 by identifying changes in risk factors and rates of self-reported sources of infection.
Section snippets
Methods
We obtained passive surveillance data on epidemiological surveys of human anthrax case patients and the annual number of livestock vaccine doses administered from the National Centers for Disease Control and Public Health (NCDC) during 2000–2013. We estimated national incidence rates per 100,000, using population data from the Georgian national census (Georgian State Statistical Committee, GeoStat). To describe the trend in human anthrax incidence and identify trend breakpoints if present, we
Temporal trends
From 2000 to 2013, 736 human anthrax cases (annual range: 15–143) were reported in Georgia (Fig. 1). During this 14-year period, the trend in rates was characterized by a breakpoint in the regression line in the year 2010 (95% CI: 2008, 2011) indicating an increasing rate of reporting post-policy change with an AAPC = 10.2% (95% CI: 9.3, 10.9; p = .02) (Fig. 1). The annual human incidence per 100,000 increased from 0.6 cases (95% CI: 0.4, 0.8) in 2000 to 3.7 cases (95% CI: 3.1, 4.4) in 2013.
Discussion
During Soviet governance (1950–1980), the incidence of human anthrax declined, in part, due to widespread livestock vaccination mirrored by reductions worldwide [5], [6], [7], [9], [19]. However, in endemic areas, long-term livestock vaccination may not be tenable due to costs and there is a paucity of evidence regarding the impact of cessation policies [9]. Our findings show that following changes to livestock vaccination policy in Georgia, which reduced immunizations administered, there was a
Conclusion
The change in livestock vaccination policy has dramatically affected the epidemiology of human anthrax in Georgia. In addition to the increasing rates of transmission, the epidemiology of the disease has shifted such that ethnic enclaves, and in particular, Azerbaijanis, are now at highest risk. Furthermore, the likely spillover of contaminated meat and animal by-products from agriculture into markets has increased the frequency of infections among occupations uncharacteristically at risk for
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