Kansas State University

–by Cassandra Olds, Livestock and Veterinary Entomology

Horse owners may be noticing the appearance of small yellow eggs laid on the legs of their horses. All four legs can be targeted but higher egg numbers are normally observed on the front legs (Figure 1). These eggs are laid by the horse bot fly, Gasterophilus intestinalis (DeGeer) with each female laying between 150 and 1000 eggs! Eggs are glued to the hair shaft and multiple eggs can be laid on a single hair strand (Figure 2).

Figure 1: Horse bot fly eggs laid in batches attached to hair on the hind (A) and front (B and C) legs of a horse. This horse had well over 700 eggs in total.

Figure 2: Removed horse bot fly eggs still attached to horse hair. Multiple eggs can be laid on a single hair strand. These eggs were removed prior to hatching as the egg casing remains intact and closed.

Eggs hatch within 10 days, stimulated by horse licking and increased humidity. Larvae enter the horse mouth and imbed in the tissue where they remain for roughly 28 days. After molting, second stage larvae exit mouth tissues and are swallowed, thereby moving the stomach. The larva can remain in the stomach for 9-12 months where it molts into the third and final larval stage. After maturing the third instar detaches from the digestive tract and is passed out in the feces. The larva will burrow into the soil and pupate where it will remain for 1-2 months. As horses tend to create manure piles, the number of flies pupating in an area can become significant. Adult flies can cause significant fly worry, adverse reactions to flies can result in horses injuring themselves trying to escape from attack. Under high numbers, reduced grazing can result in weight loss.  Significant mouth irritation can occur when first instar larvae burrow into oral tissue and although horses can tolerate low levels of stomach parasitism, large numbers can cause blockages, colic and reduced nutrient uptake.

Control can be achieved through a combination of sanitation, egg removal and correctly timed oral dewormer. Removing eggs immediately will significantly reduce the risk of a horse consuming larvae and initiating the infestation cycle. A simple and inexpensive ($3-7) bot egg knife (Figure 3) can be used to physically remove eggs from the legs.

Figure 3: A bot fly knife can be used to remove bot fly eggs from horse hair by scraping downwards with the direction of hair growth.

Deworming one month after eggs were noticed will kill any second instar larvae that may have been consumed and now reside in the stomach. A second round of dewormer can be administered 6 months after the first dose to remove any third instar larvae. Removing manure piles from the site can reduce the number of adult flies emerging.

–by Cassandra Olds — Livestock and Veterinary Entomology

Biting flies (horn, stable, horse and deer flies) are a common problem encountered by horses. Animals under attack can exhibit a number of fly worry behaviors including skin twitching, leg stamping, tail swishing and general agitation. Horses trying to escape from flies can stampede and injure themselves so effective fly control is essential! Although pyrethroid based sprays can kill flies on horses, a study carried out this summer determined that their repellant effects wore off within 4 hours. In contrast, physical barrier protection through a full body fly sheet, fly boots and fly mask, reduced fly worry behavior by 80-90% and was sustained throughout the day. Physical barriers inhibit the ability of the fly to reach the host skin and take a blood meal, not only does this reduce fly worry but also reduces the risk of pathogen transmission.

Horse flies (Figure 1 A and B2) and deer flies (Figure 1 B1) both have brightly colored eyes although, the larger black horse fly (Tabanus atratus) (Figure 1 C) can also often be seen. The fly in Figure 1 C was observed trying to feed off a fully covered horse but was unsuccessful and moved onto an uncovered horse nearby where it fed successfully despite attempts from the horse to remove it! Horn flies (Figure 1 D-G) are commonly found on cattle although horses can also be a suitable host. These flies feed 20-30 times a day causing significant fly worry. Horses can often be observed with saliva patches on their back around horn flies (Figure 1 E) indicating repeated attempts to get rid of flies. Horn flies tend to feed together as a group on the neck, withers and belly. Scarring can commonly be seen under the belly in horses with large numbers of flies sustained over a long period. Fly sheets with belly coverings are most effective against horn flies. Stable flies (Figure 1 H) have a strong preference for horse legs, especially front legs. Covering legs though the use of fly boots reduces stamping which can cause stress on the leg and hoof. Best fly protection can be achieved by selecting a well fitted sheet that covers as much of the body as possible (Figure 2). Fly boots come in a variety of types and it is best to select on which can cover as much of the lower leg as possible. Face masks are especially useful in later summer when flies attempt to feed around the horse eye, nose and mouth which can cause irritation and sores. As with all horse gear, check daily for correct fit or signs of rubbing or discomfort!

Figure 2: Example of a well fitted full body fly protection

— by Cassandra Olds — Livestock and Veterinary Entomology

K-State Research and Extension news service  — Released: July 27, 2022

In this video, K-State Research and Extension veterinary entomologist Cassandra Olds outlines the best ways to control flies in livestock herds, a problem that she says “plagues most producers most years.”

While most people associate flies as one breed, there are actually four types that impact livestock herds: houseflies, horn flies, stable flies and face flies.

“Lucky for us, each fly has a preference for where it likes to be on the animal, or not on the animal,” Olds said, noting that horn flies prefer the animal’s back or belly; stable flies are common on the front legs; houseflies hang out around the feed bunk; and face flies are found on the animal’s face.

“Each fly can lay 100 eggs every couple days, so over her lifetime, she may lay 500 eggs,” Olds said. “So getting rid of adults is important, but getting rid of the places where they breed is more important. By knowing which fly pest you have, you can look for areas where these flies may be breeding.”

View the full video, including Olds’ recommendations for control, at https://youtu.be/CruB4fINMKY

MORE INFORMATION:

K-State Veterinary Entomology, https://www.veterinaryentomology.org/vetpestx

K-State Research and Extension video by Dan Donnert
785-532-5804
ddonnert@ksu.edu

For more information:
Cassandra Olds
colds@ksu.edu

–by Dr. Cassandra Olds, Livestock and Veterinary Entomology

Horn flies are most well known for being a cattle pest where high numbers can cause significant animal stress and production losses. Although seen much less frequently and in lower carrying numbers, horn flies can be bothersome to horses especially those grazing together with or near cattle. On horses, as with cattle these flies are found on the back of the animal, sometimes moving up the neck and down to the rib area. Horn flies will cluster together with their heads pointing down towards the ground as they feed. Each horn fly will take a small blood meal 20-30 times a day. When disturbed, horn flies will fly up briefly but settle back onto the animal quickly. If you suspect your horse may be playing host to a population of horn flies you can use fly spray on the affected area to remove them.

Photos by Cassandra Olds —  Livestock and Veterinary Entomology