Infestations of horse flies in Texas may be behind a spike in anthrax in the state, with 18 confirmed cases so far this year, including in horses.
Other animals affected this year include exotic antelope, goats, white-tailed deer and cattle. In an average year, the Texas Animal Health Commission diagnoses two or three cases.
Anthrax is naturally found in soil, and spores may remain dormant for decades before emerging. This year’s wet winter followed by a dry, warm summer were the ideal conditions for the bacteria to emerge, said Dr Reid Redden, AgriLife Extension sheep and goat specialist in San Angelo.
“I’ve spoken to ranchers in the affected regions who have had animals die of anthrax or who have animals exhibiting the signs and symptoms of the disease,” Redden said. “However we don’t want people to panic. This disease is fairly well confined to a region in southwest Texas, and animals can be protected in advance by being vaccinated.”
Animals typically contract anthrax through eating contaminated soil or inhaling spores. Once an animal is infected, the disease can be spread through their bodily fluids, hide and meat, and caution should be taken.
The symptoms of anthrax occur three to seven days after the animal has been infected and may include seizures, staggering and difficulty breathing. In later stages of the disease, animals will bleed from their orifices. Typically animals die within 48 hours of symptom onset.
Role of horse flies
Horse flies are known vectors of anthrax, but the extent of their role in spreading the disease is unknown.
Dr Sonja Swiger, a Texas A&M AgriLife Extension entomologist said the reports of horse fly infestations are higher than she’s ever experienced during her 11-year career.
“It’s the worst year I’ve seen,” she said.
“We don’t have much data on seasonal horse fly populations to compare, but we can speculate that all the rain we’ve had the last few years has created the right environmental conditions for higher horse fly numbers.”
Swiger said numbers should begin to wane because temperatures have risen to more inhospitable levels. But the pest can resurge with later summer rains or in the fall.
Horse flies have a maximum range similar to most flies – five to 10 miles – though most fly populations remain relatively close to the breeding ground where they emerged.
“We don’t expect horse flies to spread the disease outside the affected area. It’s just something worth noting because there have been so many cases of anthrax this year,” Swiger said. “We can’t confirm that it’s related to the above-average horse fly populations, but it’s certainly possible and something we want to learn more about.”
Despite the name, horse flies are not host-specific and will feed on opportune human and animal hosts, she said. Horse flies only bite and feed once a day.
Like mosquitoes, only female horse flies bite because they need the host’s blood for egg production. But unlike mosquitoes, horse flies cut the host and consume blood as it drips from the wound.
They typically stay in shaded areas, such as along tree lines. They consume carbohydrates, such as nectars and honeydew, but females will range from cover to hunt hosts once a day.
Horse flies typically lay eggs over winter and in early spring. Eggs are typically laid in shady, semi-aquatic to moist areas, including around the edges of ponds or water tanks.
“They can be in any number of locations, and that makes them hard to treat effectively,” Swiger said. “The larvae are maggots, but they look like maggots on steroids. They are also very predacious and will feed on each other.”
Controlling horse flies is difficult, but a few things can help reduce numbers and relieve animal bites.
There is no available data focused on the effectiveness of pour-on products designed to curtail horse flies. But pyrethroid, especially synthetic pyrethroid-based pours, have been shown to provide temporary relief. And most synthetic pyrethroid products are not labeled for horses.
“There’s just not much labeled for horses, and even on cattle those chemicals don’t do much to deter horse flies,” she said. “Permethrins may work, but again, it’s going to be limited relief.”
The best defence for horses is to move them from the infested area, into a barn or to cover them with lightweight summer sheets designed to stop biting flies and mosquitoes.
Traps specifically designed for horse flies can reduce numbers in infested areas, Swiger said.
“They are visual hunters so many traps look like a black ball, which mimics the belly of an animal,” she said. “We trapped 350 horseflies and 200 deerflies over a 10-week period in two counties for a study. So, they do work. We just don’t have enough data yet to say how effective they are.”
• If anthrax is suspected, a sample must be drawn by a vet and sent to the Texas A&M Veterinary Medical Diagnostic Laboratory for testing. The carcasses of deceased animals must be disposed of by burning.