The prevalence of rodent-borne zoonotic viruses in Europe among the small mammals inhabiting farming landscapes

By Dr. Priyom Bose, Ph.D.Reviewed by Danielle Ellis, B.Sc.May 19 2022

In northwest Spain, scientists screened 526 wild small mammals for zoonotic viruses and identified several viruses, such as hantavirus, orthopoxvirus, and arenavirus. Their findings have been documented in a study available in Emerging Infectious Diseases.

Background

Scientists stated that zoonotic viruses threaten human beings because of the risk of spillovers. This is why it is extremely important to regularly screen and identify potential zoonotic viruses that can infect humans. Previous studies have reported that rodents are the main reservoirs of emerging zoonoses. These studies have also revealed that fluctuations in the population of zoonotic reservoirs (e.g., rodents) could modulate the infection risk.

Some factors that increase the risk of rodent invasion and enhance the possibility of pathogen spillover include anthropogenic changes in land use, irrigation, and intensified agricultural practices. Scientists reported that Microtus arvalis (common vole) is a commonly found rodent in farming areas of north-western Spain. In the past, spillover of zoonotic bacteria, e.g., Francisella tularensis and Bartonella spp, have been reported from this area.

A new study

In this study, researchers screened a total of 526 individual animals belonging to four species. Scientists reported the presence of rodent-borne zoonotic viruses, such as hantavirus, arenavirus, and orthopoxvirus, in Europe. These viruses are prevalent among the small mammals inhabiting farmlands. Additionally, researchers observed phase dependency, i.e., natural fluctuations of common vole numbers affect viral prevalence.

This study is based on farmed landscapes of the Tierra de Campos region of Castilla-y-León, north-western Spain. This area is dominated by four small mammals, namely, Apodemus sylvaticus (long-tailed field mouse), Microtus arvalis (common vole), Crocidura russula (greater white-toothed shrew), and Mus spretus (western Mediterranean mouse).

In the current study, researchers captured live small mammals between March 2013–March 2019. They obtained different samples that included blood, liver, spleen, and lungs and were stored at –23°C. Scientists performed molecular analysis of these samples.

Key findings

Researchers conducted an immunofluorescence assay and detected the presence of hantavirus, arenavirus, and orthopoxvirus IgG in serum samples. In this study, scientists utilized fluorescein isothiocyanate (FITC) anti-IgG as a secondary antibody and studied the samples under a fluorescence microscope.

The authors isolated RNA from lung and liver tissues and DNA from the spleen and liver. These samples were used for molecular analysis. Arenavirus was detected in the liver samples, and hantavirus was identified from lung samples using single-step reverse transcription PCR (RT-PCR). However, orthopoxvirus was detected from the mixture of spleen and liver samples using the conventional pan-poxvirus PCR method followed by a specific PCR method.

The current study reported that hantavirus was commonly prevalent in 1.6% of common voles, whereas arenavirus was identified in 5.9% of long-tailed field mice and 2.2% of common voles. Researchers further detected orthopoxvirus in 1.3% of common voles and 48% of western Mediterranean mice. Interestingly, scientists observed the prevalence of arenavirus was significantly more in males than females.

Previous studies related to viral zoonoses in Spain have reported a low prevalence of arenavirus and hantavirus among humans. These studies have also detected the presence of hantavirus antibodies in red fox and arenavirus antibodies in red fox and long-tailed field mice.

Scientists reported that although the prevalence of hantavirus was low and did not vary between the phases of the common vole population cycle, it harbored all three screened viruses, which might enhance the associated risk of virus spillover. During population peak, the density of common vole can reach as high as 1,000 per hectare. Researchers stated that the risk of orthopoxvirus infection is growing in Europe because there is no smallpox vaccination for the human population above 45 years of age. 

Future studies

The authors stated that more molecular analysis must be conducted in the future, and the infectivity of hantavirus and orthopoxvirus must be analyzed. Additionally, circulation pathways must be identified, which will help determine the possible transmission routes as well as the degree of risk of infection in the human population. Scientists believe that this study will help local authorities to improve their virus surveillance strategies to prevent and minimize zoonotic spillovers, especially for individuals residing near farmlands.