The Heel Fly 

D. E. Larsen, DVM

I watched in amazement as the group of cows ran through the lower field with their tails in the air. They all came to a stop at the edge of the pond and waded out into the shallow edge of the water.

“What makes those cows run like that?” I asked Dad as he worked on the mowing machine.

“They are running from the heel flies,” Dad said. “Those flies bite them on their heels, and that is why they stand in the water. Those flies cause the warbles we see on the back of cows.”

I had seen the warbles on most of the cows and watched as my older brother sometimes squeezed a large grub out on them. It was a bit of a mystery to me how a fly could cause a warble on the back of a cow by biting her on the heels.


My attention was averted from the new leaves sprouting on the trees outside the classroom window as I heard the parasitology professor mention heel flies.

“There are two species of the heel fly, Hypoderma bovis and Hypoderma lineatum,” the professor explained. “These flies lay their eggs on the hind legs, usually on the heels of cattle. After a few days, the larva hatch and penetrates the skin, then they migrate through the body, ending up on the back of cattle, in what is called a warble, some four to six months later.”

“I’ll be damned,” I thought as I concentrated on the remaining lecture.

“The important thing to keep in mind is the timing of treatment for these grubs,” the professor continued. “By fall, they will be massed around the spinal cord or the esophagus. Treatment at this time will have the potential of causing life-threatening reactions as the dead larva cause local inflammation.”

The veterinary school continued to resolve my childhood mysteries. As a boy, there were no treatments available for these grubs. The large warbles set atop some of the most valuable meat on the carcass of slaughtered animals, and there was no thought of treatment in my world.

“These times vary, depending on latitude,” the professor continued. “You need to make sure you read the label of any of the products you use.”


In the late spring of 1975, I took a call to look at a Jersey cow with a problem involving the warbles on her back. Fresh out of school, I was in a dairy practice in Enumclaw, Washington, and I was intrigued to be looking at warbles. Most of the time, in my early experience, they were just there and caused no real problem.

“This is a cow belonging to George and Sue,” Ann explained. “They are that hippie couple who bought the Allen dairy a few years ago. They have some strange ideas, so it is hard to say what you will be looking at.”

It was a short drive out to the old Allen dairy, and Sue had the young Jersey cow waiting in a stanchion when I arrived. 

“What’s going on with your cow?” I asked Sue as we walked to the barn.

“This is Cindy. She had a bunch of warbles on her back, so George treated her with a mixture of rotenone and linseed oil,” Sue said. “Now it looks like the skin is dead over all those warbles.”

“I have never heard of treating warbles once they are on the back,” I said. “We generally treat them with a pour-on product in the early fall. Dairy cows have to be in their dry period before you treat them. But it is important to do the treatment before November.”

“Yes, but rotenone is a natural product,” Sue said. “We don’t use any of those new artificial products.”

I looked at Cindy. Her back was covered with a mass of warbles, and dead skin covered each warble. I pinched at one warble, and the skin and exudate easily pulled off, leaving a hole almost an inch in diameter.

“This is a mess, Sue,” I said. “Just because it is natural doesn’t make it an appropriate treatment option. Any of the new insecticides would have had the same results. Rotenone killed the grubs, but this isn’t the time to be treating these grubs. The dead grubs lay in there and rotted, which set up an infection that killed the surrounding tissue and skin. This is what you have left.”

“What are we supposed to do to treat them?” Sue asked.

“The best time to treat them is with a systemic product in the fall. The cow has to be in her dry period, and she has to be treated before November. If you are not going to use a product that kills them in the migratory phase, then you shouldn’t treat them at all,” I said. “It’s not a big thing if the cow isn’t going to be slaughtered. It might cause some loss of production when there are this many warbles, but it’s not a major loss. In the early summer, the grubs come out and fall to the ground, where they turn into a fly. Then the wound on the back heals, and the cycle starts over again.”

“And what do we do with her back now?” Sue asked.

“I’m going to remove all these patches of dead skin, flush out the exudate and apply some antibiotic ointment,” I said. “Then we will put Cindy on some antibiotics for a week.”

“We don’t like to use antibiotics,” Sue said.

“Well, I guess we could just let her die a horrible death then,” I said. “Not using antibiotics in this situation would be close to animal abuse.”

“What do we need to do with her milk while we have her on antibiotics?” Sue asked.

“You will need to discard it,” I said. “You can use it for feeding the calves or the cats, but otherwise, you should discard it. There will be a withdrawal period to observe after you complete the course of antibiotics. Things are set up for the appropriate use of antibiotics. If you make a mistake and ship her milk, it might get expensive. They test your tank before putting it on the tanker truck. If your tank tests positive for antibiotics, you could end up paying for an entire truckload of milk.”

“George is pretty careful, so we won’t have that to worry about,” Sue said. “Let’s go ahead and treat her.”

I clipped the entire area of her back involved with the warbles and scrubbed it with Betadine Surgical Scrub. Then I removed the necrotic skin and flushed the pockets with hydrogen peroxide to remove all the pus and debris. I applied a thin coat of nitrofuran ointment to the lesions. Her back was covered with a mass of open wounds.

“You’re going to have to keep these wounds clean,” I said. “I am going to give Cindy an injection of Polyflex and leave you a bottle for daily injections.”

“That sounds expensive. Can’t we just use Combiotic?” Sue asked.

“They have changed the withdrawal times on Combiotic to thirty days,” I said. “Polyflex has a five-day withdrawal time. The money you will lose on milk sales with a thirty-day withdrawal will make your Combiotic much more expensive.”

“Okay, I am sure George will want to talk with you about all this stuff,” Sue said. “I am not sure I understand all the life cycle stuff and why there are treatment times and the like.”

“I’m in the office on Saturday this week,” I said. “I am usually not busy this time of the year. It would be a good time for George to drop by and visit. I can make some copies for him on the life cycle and treatment recommendations.”


Cindy healed uneventfully, and George came by and discussed the heel fly’s life cycle and treatment options. I am sure that his decision was to do nothing for treatment.

Photo by Peter Scholten on Unsplash.

Published by d.e.larsen.dvm

Country vet for over 40 years in Sweet Home Oregon. I graduated from Colorado State University in 1975. I practiced in Enumclaw Washington for a year and a half before moving to Sweet Home to start a practice.

One thought on “The Heel Fly 

  1. Thanks for the link. They sound a lot what I remember as being called a bot fly where I grew up. I checked on bot fly, and it gives warble fly and heel fly as another name for it. I don’t remember them biting the horses, but I do remember seeing them lay yellow colored eggs on the lower legs. We would shave them off, and also have the vet tube worm yearly.

    Liked by 2 people

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