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Accurate anthrax distribution key to formulating effective control strategies

The predicted future anthrax distribution can be used to guide and prioritise surveillance and control activities and optimise allocation of limited resources. File pic

IN Zimbabwe, anthrax is endemic with outbreaks being reported almost annually in livestock, wildlife and humans over the past 40 years.

Accurate modelling of its spatial distribution is key in formulating effective control strategies.

Under current bioclimatic conditions, eastern and western districts of Zimbabwe were modelled as highly suitable, central districts moderately suitable and southern parts marginally suitable for anthrax occurrence.

Predictions demonstrated that the suitable (8%) and highly suitable (7%) areas for anthrax occurrence would increase under Representative Concentration Pathway (RCP) 4.5 scenario.

In contrast, a respective decrease (11%) and marginal increase (0,6%) of suitable and highly suitable areas for anthrax occurrence were predicted under the RCP8.5 scenario.

The percentage contribution of the predictors varied for the different scenarios.

Areas currently suitable for anthrax should be targeted for surveillance and prevention.

The predicted future anthrax distribution can be used to guide and prioritise surveillance and control activities and optimise allocation of limited resources.

In the marginally to moderately suitable areas, effective disease surveillance systems and awareness need to be put in place for early detection of outbreaks.

Targeted vaccinations and other control measures including collaborative “One Health” strategies need to be implemented in the predicted highly suitable areas.

In the southern part, where a high decrease in suitability was predicted, continued monitoring would be necessary to detect incursions early.

Anthrax is a zoonotic disease of concern that occurs naturally in herbivorous wildlife and livestock, thereby significantly affecting human livelihoods and biodiversity.

The disease is one of the neglected tropical diseases which is caused by the gram-positive spore-forming bacterium Bacillus anthracis.

In terms of geographic distribution and endemism, anthrax is found in several regions across the globe such as Asia, Australia, North and South America, southern parts of Europe, sub-Saharan Africa and Central and South America [2, 3].

The spatial distribution of the disease is attributed to the ability of Bacillus anthracis to form spores that thrive well under diverse environmental conditions.

Although the disease burden of anthrax in herbivores is not fully known, studies have shown that anthrax outbreaks occur almost every year, killing thousands of animals and transmitting the disease to humans upon consumption of the meat.

The disease is of global concern as it results in high animal mortality with subsequent threats to human health.

Despite a decrease in reported livestock anthrax cases globally in the past decade between 20 000 to 100 000 cases of the disease are still being recorded each year, especially in developing countries.

The disease also affects human beings, with 1,83 billion people living within high anthrax-risk areas and Africa recording the highest human incidences of the disease.

In fact, human anthrax cases often associated with animal anthrax epidemics in resource poor communities occur at least every year in African countries such as Zambia, Zimbabwe and Ethiopia.

Thus, there is need to develop or adopt methods that allow for better understanding of current and future spatial distribution of anthrax as a preamble to identifying potential anthrax hotspots.

Zimbabwe has an estimated cattle herd of around 5,5 million, with 90% of the national cattle herd under the smallholder sector.

Over the years, the national herd has declined due to increased mortality from anthrax and tick-borne diseases such as January disease and Heart water.

Previous studies have reported the following cattle deaths emanating from tick-borne diseases in Zimbabwe: 3 430 in 2017; 1 133 in 2018; 1 903 in 2019; 2 772 in 2020 and 1 478 in 2021.

In fact, it has been reported that cattle deaths from tick-borne diseases can be as high as 9% of the national herd.

This is despite an increase in the surveillance and disease control measures to curtail the occurrence and spread of tick-borne diseases by the Department of Veterinary Services.

Typical anthrax outbreaks in the country are usually recorded during the dry (July to October) and wet (November to February) seasons.

Anthrax is transmitted via several modes in livestock and wildlife. Ingestion of spores during grazing in landscapes that previously experienced anthrax outbreaks is the primary mode of transmission in animals.

Scavenging animals, biting flies or poor disposal of infected animal carcasses facilitate disease transmission through exposing vegetative cells to oxygen, thereby resulting in spore formation.

Vaccination and proper carcass disposal are the main methods of control in the event of outbreaks.

The spatial distribution of anthrax is influenced by several factors which include livestock density, soil pH, availability of surface water, rainfall, temperature dynamics and vegetation cover. - BMC Public Health

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