Face flies linked to strangles risk in horses

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Flies are the bane of horse owners (and horses) everywhere, and aside from being a nuisance, a recent US study has shown they may also be an indirect transmission source of the contagious disease strangles. And another study has explored the use of sticky fly traps in keeping the pesky insects away from horses.

Caused by Streptococcus equi subspecies equi (S. equi), strangles spreads readily from horse to horse and by indirect transmission on items such as clothes, tack, and buckets.

Dr Nic Pusterla, a professor in the Department of Medicine and Epidemiology, at the School of Veterinary Medicine, University of California, Davis, led a study to see if S. equi could be found in face flies on a farm with a confirmed outbreak of strangles.

Stopping an outbreak requires affected horses to be isolated to prevent spread to susceptible animals. Measures are also needed to prevent indirect transmission, such as using separate equipment and clothing for handling affected and unaffected horses, and disinfecting anything that comes in contact with an infected animal.

The researchers collected 1856 face flies (Musca autumnalis) from horses at a thoroughbred farm using conventional fly traps. The flies were tested using quantitative polymerase chain reaction (qPCR), looking for evidence of S equi. Of the total flies caught, ten (=0.54%) tested qPCR positive for S. equi.

“The results may implicate the presence of face flies as a risk factor for the transmission of S. equi and highlight the need to institute proper husbandry measures, biosecurity protocols and fly control in order to reduce the potential for infection in at-risk horses,” the authors concluded.

Stable fly (Stomoxys calcitran).
Stable fly (Stomoxys calcitran). © Equine Science Update
Do sticky fly traps prevent horses being bitten?

Two US researchers have conducted a study to investigate whether sticky traps could be used to help protect horses from the bites of the stable fly, Stomoxys calcitrans.

Stomoxys calcitran causes irritation and distress to a variety of animals (and humans). Adults of both sexes are vicious blood-feeders and inflict painful bites with their long piercing proboscis. Their preferred feeding sites are on the lower legs of horses and cattle, and around the ankles in humans.

Tracey L Tam  and Saundra Tenbroeck of the Department of Animal Sciences, University of Florida, Gainesville, and Jerome Hogsette of the USDA-ARS Center for Medical, Agricultural and Veterinary Entomology in Gainsesville, set out to see how well commercially available fly traps placed at various distances from a horse provide protection from stable fly bites.

The researchers applied fluorescent dust to the eight chestnut mares used in the study. This allowed them to identify which flies had fed off the horse. Different coloured dust was used on different days.

They found that more than 40% of flies captured on traps placed closest to the horses were marked and had fed on the horse before getting caught.  So, the traps had not stopped the flies visiting the horses.

Some trapped flies were marked with more than one colour, indicating that flies could visit the horses more than once without being trapped.

Some marked and unmarked stable flies showed signs of blood in their guts indicating recent feeding.

The authors conclude that, although the traps caught ample numbers of stable flies, they did not prevent them from feeding on the horses.

They suggest: “More work is needed to determine optimal trap placement and densities required to maximize stable fly management with traps.”

Molecular detection of Streptococcus equi subspecies equi in face flies (Musca autumnalis) collected during a strangles outbreak on a Thoroughbred farm. Pusterla N, Bowers J, Barnum S, Hall JA. Med Vet Entomol. (2019) https://doi.org/10.1111/mve.12394

Can attractive sticky traps be used to protect horses from the bites of Stomoxys calcitrans (L.) (Diptera: Muscidae). Tam TL, Hogsette J, TenBroeck S. J Econ Entomol. 2019 Jun 22. pii: toz134. https://doi.org/10.1093/jee/toz134

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