Major study explores West Nile disease risk among US horses

West Nile Virus detection probability in the contiguous US.
West Nile Virus detection probability in the contiguous US. Image: Humphreys et al.

The highest risk areas in the United States for West Nile infection in horses have been described by researchers in a just-published scientific paper.

The researchers examined the occurrence of the mosquito-borne virus across the contiguous US, which covers more than 9.8 million square kilometers.

As expected, the risk among horses of being infected was not uniform across the US, nor was it constant throughout the year.

Nationwide risk patterns generally indicated that few locations were free of any disease risk from July to October, which was identified as the period of highest risk.

However, John Humphreys and his fellow researchers with the US Department of Agriculture noted that several multi-county regions showed particularly elevated risk (see the graphic below) during this period.

High-risk clusters were identified in central Pennsylvania, eastern Iowa, western Texas, central Montana, the coastal South Atlantic states, northwest Minnesota, eastern Washington, the Idaho-Oregon border, and along the central Gulf coast in a region centered on Lower Louisiana.

Interestingly, the Louisiana and Pennsylvania clusters underlie migratory flight paths linking the Gulf Coast to the northeastern US, which were previously identified as routes of West Nile Virus transport by non-waterfowl bird species.

Spatiotemporal distribution of equine West Nile Disease relative risk.
Spatiotemporal distribution of equine West Nile Disease relative risk. Maps depict the estimated relative risk by US county for the months July to October. Column aligned at center displays the contiguous US with lateral columns providing closer views of locations demarcated on US map at top center. Darker tones signify increased risk and lighter tones relatively lower risk. A relative risk value of 1 indicates that model predicted cases were comparable to the expectation given the number of horses in the county, values below 1 highlight counties with relatively low risk, and values above 1 suggest increased risk (higher than expected given the horse population). Image: Humphreys et al.

In addition to high-risk clusters, several relatively isolated areas showed a disproportionately high relative risk based on the associated horse population.

For example, individual counties in Colorado, North Dakota, and California displayed risk twice as high as expected.

The variability seen in risk distribution suggests that, although the virus has been detected throughout the US, environmental conditions at some times and locations are more conducive to disease spread than conditions at other times and locations.

West Nile Virus, first identified in Uganda in 1937, causes disease in humans, horses, and some bird species.

Since its introduction to the US in 1999, with the first detection in New York, about 30,000 horses have been impacted by West Nile neurologic disease, and hundreds of additional horses are infected each year.

The study team, writing in the journal Viruses, said research describing the drivers of West Nile disease in horses is greatly needed to better anticipate disease risk, improve disease surveillance, and reduce future economic impacts on the horse industry and owners.

The researchers used an integrated approach to examining West Nile, looking at horse abundance and virus exposure, vector and host distributions, and a variety of climatic, socio-economic, and environmental risk factors.

Birds are the reservoir hosts for the virus, so the researchers quantified avian host community dynamics across the continental US to aid their analysis.

Perhaps the most intuitive indicator of disease was the detection of the virus itself, they said.

“When mapped as a time-series, virus detection probability suggested a general shift from south-to-north during the onset of summer (May–June), a majority coverage of the US during the height of summer (June–September), and a north-to-south recession associated with the beginning of winter (October–November).”

During the coldest months (November–April), the highest virus occurrence probabilities were predominantly restricted to southern portions of the US and coastal areas.

Virus detection likelihood increased along a latitudinal gradient as temperatures warmed. Exceptions to this pattern were identified along the Atlantic and Pacific Coasts, where relatively high virus detection probabilities persisted throughout the winter months. This aberration, they said, was likely linked to thermal buffering of coastal areas by oceans.

“Thermal buffering moderates low-temperature extremes in coastal areas, among other effects, to provide winter refuge for birds (potential reservoirs) that might otherwise migrate. However, it is also possible that managed water and sewage systems in some coastal locations facilitate mosquito overwintering.”

Drought showed a dynamic relationship to West Nile disease. Non-drought conditions were associated with increased risk, but abnormal dryness and moderate drought suppressed the disease.

“As drought intensity increased beyond moderate levels to reach the severe and extreme stages, disease risk sharply increased before once again diminishing to have a negative disease influence during periods of exceptional drought,” they reported.

“Although moderate drought conditions may reduce host and vector water access, severe drought levels may exacerbate the situation sufficiently to instigate vector and host aggregation at the few remaining water sources. As drying conditions further intensify to be classified at the exceptional level, water becomes so rare as to be a limiting factor for virus transmission.”

The bird communities present also had a bearing on West Nile risk, they found, with opportunities for virus sharing, host switching, and spillover into horses.

The authors said they suspected that vaccination practices may explain some of the West Nile variation revealed in the study, but factoring this in proved difficult in the absence of detailed vaccination information.

“We did choose to assess the influence of household income as a potential proxy of vaccination, under the hypothesis that horses located in relatively high-income areas might be more protected from West Nile disease due to owners being able to afford more consistent vaccination.

“Although we found that West Nile disease risk decreased in areas with increased income, the linkage between household income and horse vaccination rates remains speculative in the absence of additional data.”

The researchers said their findings can be used to prioritize vaccination programs and optimize virus surveillance and monitoring.

The study team comprised Humphreys, Angela Pelzel-McCluskey, Lee Cohnstaedt, Bethany McGregor, Kathryn Hanley, Amy Hudson, Dannele Peck, Luis Rodriguez and Debra Peters, all affiliated with the US Department of Agriculture; and Katherine Young, with New Mexico State University.

Humphreys, J.M.; Pelzel-McCluskey, A.M.; Cohnstaedt, L.W.; McGregor, B.L.; Hanley, K.A.; Hudson, A.R.; Young, K.I.; Peck, D.; Rodriguez, L.L.; Peters, D.P.C. Integrating Spatiotemporal Epidemiology, Eco-Phylogenetics, and Distributional Ecology to Assess West Nile Disease Risk in Horses. Viruses 2021, 13, 1811.

The study, published under a Creative Commons License, can be read here

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