It is hard to come up with good things to say about flies, but I’ll give it a go anyway.
They are an ancient group of creatures, having evolved more than 200 million years ago. There are more than 150,000 known species of flies, with likely many more thousands of species remaining undescribed.
Flies occupy a tremendous range of ecological niches on land masses throughout the world, with the exception of Antarctica. From an evolutionary perspective, they are highly successful insects.
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On the flip side, if you’d like to purposely ruin your lunch, look up the details of how these miniscule critters go about eating and defecating.
Among this ubiquitous and diverse group of insects are those that pose a challenge to animal health.
Flies are a harassing nuisance in all livestock animals, causing stress and contributing to production losses as they seek relief. More time spent moving from place to place, swishing tails, quivering skin, stamping feet and flicking ears translates to less energy available for growth and production.
Animals that walk (or run!) to escape incessant fly burdens spend considerably less time eating.
And if an animal develops a skin wound, flies are rapidly attracted to the decaying flesh and fluid secretions from the damaged skin. The females quickly lay eggs, which hatch into the larval form we commonly call maggots.
Spreading quickly in damaged tissues, maggots further exacerbate wounds and lead to poor outcomes if not treated promptly.
Sheep are particularly prone to fly strike, in which larvae infest the skin around the tail, especially if the wool is caked with fecal material. Fly strike in sheep has important implications for welfare and production in this species.
While maggots can be used therapeutically to treat skin wounds in animals (and people) under specific clinical conditions, maggots in wounds are generally a cause for immediate concern.
In the mid to late summer, we expect to find those sticky yellow eggs adhered to the hair on horse’s legs. These bot fly eggs hatch when horses groom themselves and the larvae enter the mouth.
When larvae worm their way through the tongue and gums, horses can salivate, shake their heads and have chewing issues.
The larvae then move to the stomach, where they can live happily over the winter. Eventually they detach and exit via the feces.
Larvae develop into adult flies, which go on to lay more eggs and perpetuate the life cycle.
Thankfully, these particular flies cause only minor inflammation in the mouth and stomach, and the larval form is easily eliminated with ivermectin-type deworming medication. Owners can also remove eggs from the hair coat during grooming to decrease the burden of the larval form.
Horseflies, those large biting ones that emerge in the mid-summer, make large enough wounds in the skin for the tissue to bleed. These flies feed on the blood, and the open wounds also attract other fly species.
Horses become understandably agitated by these aggressive bites, but the flies can also transmit a number of blood-borne infectious diseases such as equine infectious anemia.
Skin nodules, called warbles,, are among the significant issues with flies and cattle.
Although well-controlled now with anti-parasitic medications, the encysted larvae of the Hypoderma fly can lead to production losses and decreased hide quality. Flies are implicated in the spread of many infectious cattle diseases, including pink eye.
While mosquitoes may get more attention for their ability to transmit viral diseases such as West Nile Virus, large and common fly species have wide-ranging impacts on animal health. In addition to topical anti-fly medications, environmental controls such as fly traps and proper manure management are important undertakings to reduce the impact of flies on animals.
Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. X: @JRothenburger
