Project Spotlight: Rabies in Wildlife and Livestock in Mongolia

Project Spotlight: Rabies in Wildlife and Livestock in Mongolia

 

Problem

Rabies is a zoonotic disease, which is almost always fatal among individuals who develop symptoms. The disease is caused by the rabies virus, which is typically transmitted through saliva, this occurs often through animal bites. Globally, 50,000–60,000 people die each year from rabies, with deaths attributed to ineffective reservoir animal control and inadequate access to pre-and post-exposure prophylaxis (PEP). While rabies reservoir species differ geographically, 90% of human cases worldwide can be attributed to domestic or stray dogs. The global economic impact of this disease is estimated at $8.6 billion annually. Six percent result in livestock losses. 

With an estimated 37% of the population of Mongolia owning livestock and 26% living in a pastoralist lifestyle, Mongolia is particularly vulnerable to this disease. Red foxes were responsible for most of the reported wildlife cases (317) followed by wolves (151). Most rabid animals were reported in the Khuvsgul, Uvurkhangai and Govi-Altai aimags, and a positive correlation was found between livestock numbers per soum and the number of rabies cases reported.

Study

Data was acquired from the General Authority for Veterinary Services and the National Center for Zoonotic Diseases (NCZD). The NCZD actively investigates and confirms reports of rabies across Mongolia. The General Authority for Veterinary Services receives data from both the NCZD and veterinarians that report positive rabies cases. All rabies testing in Mongolia relies upon direct fluorescent antibody assays, which is the standard for routine rabies determination. Testing would occur when rabies was suspected due to clinical signs or suspected exposure. Each report included the animal’s geographic coordinates, aimag (province) of origin, year of report and type of animal. Cases reported between 2012–2018 from all 21 aimags (Figure 1) and the independent provincial municipality Ulaanbaatar were included. Population and livestock density data were obtained from the Mongolian National Statistics Office (https://opendata.1212.mn/en/doc). The use of positive-only passive surveillance can allow for an assessment of the locations from which most reports of rabies are coming from as well as the relative contribution from each animal group. It is important to note that the quality of positive-only data can be influenced by various factors, such as whether clinical signs are observable in the affected animal, awareness of rabies by the persons reporting the disease and diagnostic test sensitivities.

Spatial analyses were conducted using ArcGIS (ESRI) and R 3.6.1 (www.r-project.org). The coordinate location of each animal was projected in GIS. Two spatial analyses were conducted: kernel density and linear regression. Kernel density uses a 2-dimensional scanning radius to compute the density of rabies case distribution. 

The goal of the regression analysis was to evaluate how the human population and livestock abundance contributed to the number of rabies cases reported in animals. The units of analysis were the 339 soums (counties) in Mongolia. The outcome variable was the total count of rabies cases per soum (including both wild and domestic animals), linear predictor variables were the number of humans and the number of livestock per soum, and we used a Poisson likelihood function. To account for spatially correlated error, a neighbor matrix was conducted of the 339 soums in Mongolia. Model comparison strongly favored this correlation structure.

Impacts of Rabies

Two thousand three hundred fifty-nine rabies cases were reported in domestic animals (1,872 cases) and wildlife (487 cases) within Mongolia between 2012–2018.

Rabies virus was most commonly reported in cattle (861 cases), followed by red foxes (317 cases), goats (268 cases) and sheep (251 cases). The aimag with the most rabies case reports was Khuvsgul (328 cases), followed by Uvurkhangai (272 cases) and Govi-Altai (266 cases).

There was a positive correlation between the number of livestock per soum and the number of reported rabies cases. No association was seen between human population density and the number of reported rabies cases in livestock or wildlife.