Spread of African horse sickness: Is the answer blowin’ in the wind?

A negatively stained bluetongue virus–like particle. Photo: Public domain via Centers for Disease Control and Prevention, via Wikimedia Commons
A negatively stained bluetongue virus-like particle. Photo: Public domain via Centers for Disease Control and Prevention, via Wikimedia Commons

Researchers have carried out a big-data assessment to learn more about the potential for African horse sickness and bluetongue to be spread long distances by wind dispersing the biting midges that carry the viruses.

African horse sickness is a deadly infection of equids that is endemic to the equatorial, eastern, and southern regions of Africa. Bluetongue is an infection that affects livestock, mostly sheep.

The disease syndromes are distinct but share much in common, including a high death rate in susceptible animals and being transmitted by blood-feeding Culicoides midges.

Outbreaks of both diseases must be reported to the World Organisation for Animal Health (OIE).

Bluetongue is considered a serious re-emerging disease, having caused a number of major epidemics in Europe since 1998.

African horse sickness has been less of a problem in recent years due to vaccine development, but movement restrictions remain for equines within and from endemically infected countries.

Three researchers from the Australian science agency CSIRO have revisited a body of work published by R.F. Sellers between 1977 and 1991 exploring the possibility that outbreaks of both diseases might occur via long-distance wind dispersion of the midges.

Sellers investigated the role of wind dispersal by looking at wind directions on weather charts around the time of outbreaks.

Peter Durr, Kerryne Graham and Rieks van Klinken reanalyzed six of the outbreak scenarios described in Sellers’ papers by using a custom-built Big Data application which coupled a user-friendly web-interface with an established atmospheric dispersal model.

The software enabled more sophisticated modeling than was possible when Sellers undertook his work.

The trio, writing in the journal Frontiers of Veterinary Science, said there was strong support from their reanalysis of the role of long-distance wind dispersion of midges for the African horse sickness outbreak in Spain in 1966 and, to a lesser degree, for the outbreak in Cyprus in 1960.

However, for the bluetongue outbreaks, the reassessments were more complex, and for one of them (western Turkey in 1977) the study team was able to discount long-distance wind dispersion as the means of direct introduction of the virus.

By contrast, while the outbreak in Cyprus in 1977 showed wind dispersal was a possible means of introduction, there was an apparent inconsistency in that the outbreaks were localized while the dispersion events covered much of the island.

For Portugal in 1956, long-distance wind dispersion from Morocco on the dates suggested by Sellers was very unlikely to have been the pathway for introduction, they reported.

And, for the detection of serotype 2 in Florida in 1982, long-distance wind dispersion from Cuba would require an assumption of a lengthy survival time of the midges in the air column.

Except for western Turkey, the bluetongue reanalyses showed the limitation of long-distance wind dispersion modeling when used by itself, pointing to the need for additional data in the modeling process.

“A further refinement, which will become increasingly important to assess long-distance wind dispersion, will be the use of virus and vector genome sequence data collected from potential source and the incursion sites.”

Discussing their findings, the study team noted that there was no experimental support when Sellers wrote his papers suggesting long-distance wind dispersal of Culicoides midges might be a mechanism for the spread of the two viruses.

“The inference was based purely on ‘ruling out’ other pathways, especially the movement of animals.

“Combining this with retrospective observations of wind direction and meteorological conditions, he was able to conclude that wind-dispersed infected Culicoides was a possible source of introduction for most of his case studies.”

Since then, there has been a steady accumulation of experimental evidence and field observations to support long-distance wind dispersion of Culicoides midges as a plausible pathway of introduction of viruses.

The researchers said the challenge was now to develop validated systems which integrated long-distance wind dispersion modeling with infection network reconstruction.

“We suggest that this can be assisted by borrowing insights and techniques from Big Data science where the competing objectives of managing data quantity while maintaining data quality are fully recognized and solutions are actively being developed.”

Sellers’ Revisited: A Big Data Reassessment of Historical Outbreaks of Bluetongue and African Horse Sickness due to the Long-Distance Wind Dispersion of Culicoides Midges
Peter A. Durr, Kerryne Graham and Rieks D. van Klinken
Front. Vet. Sci., 20 July 2017 https://doi.org/10.3389/fvets.2017.00098

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


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