Midge study helps understanding of African horse sickness

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The two kinds of midge traps used in the study in Senegal. The horse in the trap was no more exposed to midge bites than horses in private stables in the area, the researchers said. The trap at right used light to draw the midges. Photos: Parasites and Vectors journal

Researchers who captured more than 250,000 midges in Senegal say one species in particular should be considered a vector for the deadly disease, African horse sickness, in the country.

African horse sickness is endemic to Senegal, transmitted to equines by midges of the Culicoides Latreille genus.

During the last major outbreak in Senegal, in 2007, 1169 horses died from the disease, at an estimated cost of €1.4 million.

The French and Senegalese researchers said that, despite the serious animal health and economic implications of the disease, little is known about factors involved in transmission, including the Culicoides species suspected of being its vectors.

The researchers focused their attention on the Niayes area, which was severely affected by the 2007 outbreak.

They used traps baited with a live horse and two suction light traps at each of five riding centres for three consecutive nights every month for a year.  The horses in the traps were fed and watered as normal, with the researchers stressing that they were no more exposed to Culicoides bites than other animals in private stables in the area.

In all, 254,338 Culicoides midges were collected. They found that 82.4 percent of them – that’s 209,543 midges – were female and 17.6 percent – 44,795 – were male.

Nineteen of the 41 species collected were formally identified as new to Senegal. This increased the number of described Culicoides species found in Senegal to 53.

African Horse Sickness is spread by Culicoides species midges.
African Horse Sickness is spread by Culicoides species midges.

Only 19 of the 41 species found in light traps were collected in the horse-baited traps, amounting to 23,669 specimens.

They found that 94.2 percent of them (22,300) were Culicoides oxystoma, followed by 482 specimens of Culicoides imicola (2 percent) and 446 specimens of Culicoides kingi (1.9 percent).

Culicoides oxystoma should be considered as a potential vector of African horse sickness virus in the Niayes area of Senegal due to its abundance on horses and its role in the transmission of other Culicoides-borne viruses,” Moussa Fall and colleagues reported in the journal, Parasites and Vectors.

The researchers noted that Culicoides oxystoma was already known to be involved in the transmission of bovine arboviruses such as Akabane in Japan. It also is a suspected vector of epizootic haemorrhagic disease virus in Israel and is a potential vector of the bluetongue virus in India.

Culicoides imicola is a proven vector of African horse sickness virus in South Africa and probably in the Maghreb (Morocco) and the Iberian peninsula (Spain and Portugal). It is also the main vector of the related bluetongue virus in South Africa, in the Maghreb and in Southern Europe.

The role of C. kingi as a vector of African horse sickness virus is not clearly established, they said.

The authors said more research was needed to gain a better understanding of the role of the midges in the spread of the disease in Senegal.

Culicoides (Diptera: Ceratopogonidae) midges, the vectors of African horse sickness virus – a host/vector contact study in the Niayes area of Senegal.
Moussa Fall et al.
Parasites & Vectors 2015, 8:39 doi:10.1186/s13071-014-0624-1

The full study can be read here

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