African horse sickness: Where are midges that can carry the disease likely to spread?

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The distribution of Culicoides imicola, with the dots indicating confirmed occurences. The graphics provides a legend of land classification. The cross-hatched areas are zones of inhospitable habitat - the Adriatic Sea, Sahara Desert and Red Sea which separating the three model-fitting zones.
The distribution of Culicoides imicola, with the dots indicating confirmed occurences. The graphics provides a legend of land classification. The cross-hatched areas are zones of inhospitable habitat – the Adriatic Sea, Sahara Desert and Red Sea which separating the three model-fitting zones.

The potential for a midge known to spread African horse sickness and bluetongue virus to expand its range into the Americas and Australasia has been highlight by an international team of researchers.

The midge, Culicoides imicola, could even spread to the milder northern parts of New Zealand, according to the researchers, whose findings have been published in the open-access journal, PLoS ONE.

The scientists used climate data to identify the midge’s potential to spread globally and also looked at its potential distribution under future climatic scenarios out to 2070 (see the graphics below).

“There appears to be substantial opportunity for range expansion of C. imicola into the Americas and Australasia should it be translocated to these regions,” the scientists reported.

“The climate appears moderate to highly suitable over large areas,” they said, noting that horses and also domestic farm animals, which can be infected by the bluetongue virus, abound.

“Should C. imicola spread southwards into the Indonesian archipelago or Papua New Guinea, it is entirely feasible that it could spread to Australia, potentially increasing the risk of transmission of bluetongue and African horse sickness in Australia, and possibly the northern parts of New Zealand with a milder climate.

In Australia, C. imicola would be adding to existing community of insects which they described as competent vectors. “So the extent of the added risk is unknown,” they said.

Climate change would likely see a northern expansion of suitable climates for the midge, especially in Europe and China.

Bluetongue and African horse sickness are both midge-borne viruses, causing significant economic losses and animal deaths. African horse sickness has a high mortality rate and there are fears it could seriously damage the horse industry if it reached the likes of Britain.

The French, American and Australian researchers said their modelling highlighted the role of irrigation in supporting the occurrence of C. imicola in dryer regions.

In Europe, the modeled potential distribution of C. imicola extended further west than its current reported distribution.

Under the climate change scenarios considered, its modeled potential distribution could expand northward in the northern hemisphere, whereas in Africa its range may actually contract in the future.

“The biosecurity risks from bluetongue and African horse sickness viruses need to be re-evaluated in regions where the vector’s niche is suitable,” they wrote.

“Under a warmer climate, the risk of vector-borne epizootic pathogens such as bluetongue and African horse sickness viruses are likely to increase as the climate suitability for C. imicola shifts poleward, especially in Western Europe.

C. imicola is an afro-oriental species of midge. It is widespread and has been reported from South Africa to southern Europe, and from western Africa to southern China.

Transmission of bluetongue and African horse sickness was first demonstrated in this species, whereas it has been suspected for other Afrotropical Culicoides midges, such as C. bolitinos, among others.

It is considered a key vector of both viruses in Africa, in the Middle East and the southern part of Europe due to the huge abundances it can attain.

African horse sickness is endemic in areas of Sub-Saharan Africa. It has sporadically caused devastating outbreaks outside Africa, such as in 1959–61 in the Middle East and in Asia, where more than 300,000 equids died. There was also an outbreak in 1987–90 on the Iberian Peninsula.

Less than 10 years later, several serotypes of bluetongue virus were transmitted intensively, leading to the deaths of hundreds of thousands sheep in the Mediterranean basin, in areas previously unaffected by the virus.

Big populations of C. imicola were recorded in most of these areas, where it was thought to be absent.

“Some authors have suggested that the distribution of C. imicola has expanded its range northwards in the last decades in Mediterranean Europe as climatic changes reduced the frequency and intensity of cold temperatures that limit overwintering potential, whereas other studies challenged this hypothesis,” they said.

The geographical range of C. imicola appeared primarily limited by cold stress and dry stress, and to a lesser extent wet stress, the researchers wrote. They cited the probable role of irrigation in extending the potential range of C. imicola into dry regions.

“Under the future climate scenario, parts of Namibia, Botswana, Kenya, Somalia and Ethiopia could become unsuitable for C. imicola in the future, whereas the model projects a northern expansion of suitable climate, especially in Europe and China.

“However, presence of the vector is only one of the required conditions for vector-borne diseases to spread,” they stressed. “Appropriate hosts and presence of the pathogen are also required along with climatic conditions enabling vector activity, in particular vector-host contact, and pathogen replication.

“A complete risk assessment should also consider host densities and their relationship with vector densities and account for transmission routes from infected areas,” they said.

“Under a warmer climate, the risk of vector-borne epizootic pathogens such as bluetongue and African horse sickness viruses are likely to increase as the climate suitability for C. imicola shifts poleward, especially in Western Europe.”

The research was carried out by Sylvain Guichard, Hélène Guis, Annelise Tran, Claire Garros, Thomas Balenghien and Darren Kriticos.

Guichard S, Guis H, Tran A, Garros C, Balenghien T, et al. (2014) Worldwide Niche and Future Potential Distribution of Culicoides imicola, a Major Vector of Bluetongue and African Horse Sickness Viruses. PLoS ONE 9(11): e112491. doi:10.1371/journal.pone.0112491
The full study can be read here

Global climate suitability for Culicoides imicola under historical conditions as represented by the CLIMEX Ecoclimatic Index for the completed model. The cross-hatching indicates suitable areas when irrigation is integrated in the model.  Highly suitable areas are the darkest.
Global climate suitability for Culicoides imicola under historical conditions as represented by the CLIMEX Ecoclimatic Index for the completed model. The cross-hatching indicates suitable areas when irrigation is integrated in the model. Highly suitable areas are the darkest.
The researchers forecast variation of ecoclimatic suitability for the midges based on existing climate date and projected for 2070.
The researchers forecast variation of ecoclimatic suitability for the midges based on existing climate date and projected for 2070.

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