Insects and Parasites: Fighting Fall Foes
Autumn is a transitional time of year for most of the United States. Pastures die out, shorter days and cooling temperatures are a prelude of the winter chills yet to come, and numerous animal and insect species migrate to gentler climes or head into dormancy. However, some horse-troubling insects and parasites become more active during late summer and fall. Unchecked, they can be an annoyance, stress a horse's energy reserves during the winter months, or cause illness or death. It is up to you to protect your horse.
The most common autumn-troubling insects and parasites are bots, small strongyles, mosquitoes, flies, winter ticks, and horse lice.
Bots are among the most familiar and widely distributed equine parasites in the United States. Explains Lee Townsend, PhD, Extension Entomologist at the University of Kentucky, "Horse bots are honey bee-sized flies that dart around and glue their tiny eggs or nits to body hairs of horses, donkeys, and mules. Most of the egg laying is done during August and September, but may continue until the first hard frost."
Eggs of the common horse bot might hatch within five days of incubation. The newly hatched bot larvae then enter or are taken into the mouth when the horse licks or nibbles that spot. "After spending about three weeks in soft tissue of the lips, gums, or tongue, the bots migrate to the stomach or small intestine, where they use sharp mouth hooks to attach to the lining of the organ," Townsend says. "Bots spend about seven months there, where they can damage the lining of the stomach or small intestine, interfere with the passage of food, or cause other gastrointestinal disorders."
Bot eggs can be seen on a horse's coat. Clinical signs of bot infection are vague, states John Cheney, DVM, Associate Professor, and Head of the Parasitology Section and Veterinary Diagnostic Laboratory at Colorado State University. "Horses don't do as well, and they may have a rough, dull hair coat. Death from bot infection is very unusual unless a large number of bot larvae are found where the stomach empties into the small intestine."
Adds Dennis D. French, DVM, Dipl. ABVP, Professor of Veterinary Science at Louisiana State University, "The ulcerative lesions caused by the bot larvae attachments in the stomach are quite similar to those observed via endoscopic examination of horses with ulcers. This may be a source of confusion in diagnosis and eventual treatment for these problems that look similar, but are separate diseases."
Bots definitely should be controlled in the fall, emphasizes Cheney. "You need one treatment in the fall, then a second bot treatment about three to four months later since the eggs on the hair of the horse are viable up to 100 days. In the past, owners were told to wait a month after the first killing frost and treat. But necropsies on some of the horses that have died (from other causes) showed bots in the stomach despite the de-worming. So, two dewormings are almost essential to get good control."
Greatest control is achieved via equine deworming products labeled for horse bots. Ivermectin, the active ingredient in a variety of products, controls bots and other internal parasites, says Townsend.
Moxidectin is also effective against bots. "But the drug must be dosed correctly at the correct weight of the horse to kill all the bot larvae," French notes.
Clipping hairs that harbor eggs is not a practical solution for these pests, warns Townsend. Similarly, "Sponging warm water (110-112ï¿½F) on areas of the forelegs where nits are attached may stimulate some eggs to hatch, and the small larvae can then be washed off; this is of limited value and must be repeated frequently because new eggs are attached daily while flies are most active."
Small strongyles are less pathogenic than large strongyles, but these parasites can cause weight loss, diarrhea, constipation, colic, and occasionally death. "During warm weather, horses can pick up small strongyles larvae from blades of grass," Cheney says. "In the fall as the weather becomes cooler, small strongyles become inhibited (encysted) in their development in the large intestine. When the weather becomes more favorable, all these inhibited stages develop at one time, producing massive damage to the wall of the large intestine."
In the Gulf Coast region, it appears that few small strongyle larvae are transmitted during the hot summer. Transmission usually doesn't occur until the autumn, when temperatures cool and larvae are better able to survive. Therefore, strategic treatments are delayed there until the weather cools later in the fall or winter.
Small strongyle prevention only works during the inhibitive (encysted) state, so treatment should be initiated in the fall when pastures start to die down and the weather becomes cooler. Cheney recommends administering fenbendazole daily for five days to kill encysted stages. Daily dewormers with moxidectin also have action against some of the encysted larvae, says French.
Mosquitoes remain active throughout much of the autumn in temperate and warmer regions. Mosquitoes can cause itchy skin reactions from bites, and carry encephalitis viruses (including West Nile). "In some areas of the country, encephalitis viruses are a big threat in the late summer and early fall where there is increased rainfall," notes French. "The rainfall allows mosquitoes to proliferate." Most horses which contract an encephalitide suffer permanent neurologic damage.
While there is currently no prevention for West Nile virus or equine infectious anemia (EIA), incidence of Eastern (EEE) and Western (WEE) equine encephalitis can be reduced or prevented by vaccinating horses one month prior to the mosquito season, followed by a booster six months later if the horse is residing where mosquitoes are present for much of the year.
To reduce mosquito breeding, eliminate sources of standing water--old tires, bucket lids, sheets of plastic, cans, etc.--where mosquitoes breed and develop. Stock tanks with mosquito fish, which consume mosquito larvae, or place mosquito dunks (with bacteria that kill larvae, but are safe for horses and other animals) in water tanks. You also can apply mosquito repellent to your horse daily. However, French warns that sprays are "very ineffective in protecting horses during periods when ambient temperatures are high and horses sweat. The sprays simply do not stay on the horse."
Also, minimize your horse's exposure to mosquitoes by keeping them stalled at dusk and dawn when mosquitoes are most active.
Flies of certain types remain active through the fall, depending on the region. "Horse fly bites can really annoy horses and cause tremendous reactions in horses that are sensitive to their bites," French says. "Horse flies have also been documented to transfer EIA from an inapparent carrier to naï¿½ve horses." EIA-affected horses become lifelong carriers of the disease; the disease itself is progressive and fatal. (For more information on EIA, see "Equine Infectious Anemia" in the August 1998 issue of The Horse.)
Horn flies are bloodsuckers that cause scaly scabs on a horse's belly. Face flies can feed around the eyes of the horse, causing conjunctivitis. Stable flies can produce summer sores and anemia, and deer flies feed on blood and cause painful bites.
"We don't really have a very good fly product for prevention," says Cheney. "Pyrethroid ear tags for cattle, which release insecticide over a two-month period, can be hung from the halter to repel face and horn flies feeding around the horse's head, or insecticidal strips can be braided into the tail to help repel flies."
Alternatively, topical solutions can be applied per label instructions, but they are not long-lasting and, as noted previously, don't remain viable very long on sweaty horses. French says that keeping horses stalled during the day reduces exposure to flies.
Winter ticks (also known as horse ticks or elk ticks) are very common in the western United States. "They can cause ulcerated lesions, unthriftiness, slow loss of blood, and, in large enough numbers, cause anemia," Cheney says. "They can actually bleed a horse to death."
Engorged female ticks are about the size of a thumbnail. Ticks are large enough to be seen on the horse, and they usually attach in bunches. They are transmitted from host to host. While there are no good tick preventions, reduced stocking density minimizes transmission.
A variety of tropical sprays can be used to kill ticks per label instructions. Repeated treatments are necessary. "If the owner just has a few horses, a hand-pump garden sprayer works fine," Cheney says. "But if you have a lot of infected horses, you'll have to use a power spray or something similar. There is some evidence that ivermectin will kill ticks, but it's a very slow kill, and you have to repeat the treatment often during the tick season."
Horse lice are less common due to the use of ivermectin. However, some lice can still present problems in horses. While blood-sucking lice are controlled by systemic parasiticides, chewing lice are not -- they can be a problem in herds. Lice are "generally not the cause of debilitating conditions, but cause added stress during the winter as an energy drain," Townsend states. Lice also cause itching, rubbing, hair loss and, in sufficient numbers, anemia.
"Horse lice are easily introduced into a herd or picked up where horses are bunched together," Townsend says. "They can be kept out of closed herds, but are easy to acquire with purchased animals or at events where many horses are brought together."
Lice are visible on close inspection and detectable during routine grooming.
To prevent lice, Townsend recommends that newly purchased horses be treated with an insecticide and kept isolated from other horses until the treatment course is completed. Regular deworming with ivermectin also might reduce lice infestations, Cheney believes. "Although ivermectin is not labeled for lice control in horses as it is in cattle, the incidence of lice has dropped considerably since the introduction of ivermectin deworming programs."
For treating lice, topical insecticides, especially those containing pyrethrum, are very effective. "Two applications are usually necessary to eliminate lice from an animal or herd because lice eggs are not killed by the treatment," Townsend says.
Because of weather and climatic variations throughout the United States, the seasonal activities of insect and parasite populations will differ from region to region. Therefore, consult with your veterinarian to identify which pests will threaten your horses each fall, and when you should enact autumn prevention.
New Tick Control Methods Might Reduce Lyme Disease
Lyme disease is the most prevalent vector-borne human disease in the United States, and it also affects horses, dogs, and other animals. Many Lyme disease cases occur in the northeast, but the disease is seen everywhere. Lyme disease is spread by tiny Ixodes ticks that feed on small mammals such as mice during the larval stage, then feed on white-tailed deer later in their life cycle. In many areas, deer populations are increasing in urban and rural settings, and deer spread the disease-bearing ticks to horse and human populations.
One of the most important steps in controlling these ticks is removing their natural hostï¿½the deer. However, deer control isnï¿½t easy, and some people encourage wildlife to populate their areas. Therefore, tick control on deer needed to be an option. It now is.
Mat Pound, BS, MS, PhD, research entomologist with the USDA Agriculture Research Station (ARS) team in Kerrville, Texas, says, "We first came up with an ivermectin-medicated corn. They love it (corn), but they donï¿½t eat much because it fills them up. So it makes a good dosing medium."
After testing this at the Kerr Wildlife Management Area (reducing free-living Lone Star ticks 93% just by feeding the deer ivermectin), the researchers tried it on ranches in southern Texas. "After one season of feeding the deer, the ranches were free of ticksï¿½one of them for the first time ever in the history of the eradication program (nearly a century)." However, ivermectin is not yet labeled by the government for this use in deer.
The next challenge was to find a way to treat deer with a topical pesticide that would not leave a residue in the meat, since ivermectin has a 45-day withdrawal time prior to human consumption of a treated animal. The researchers decided to use Amitraz, since there is no withdrawal period for this topical pesticide and itï¿½s very specific for ticks (Amitraz is not safe for horses). "We came up with a delivery device we call a four-poster, because it looks like a little four-poster bed with a grain bin in the middle," says Pound.
Theyï¿½re still working on licensing the four-poster. "If you donï¿½t have a pesticide licensed for use on deer, you might as well not license the delivery device," he says. "Weï¿½re trying to license both things together, and are hoping they will be available within a few months."
In order to get the corn, the deer puts its head into the four-poster, rubbing against two vertical "paint rollers" that apply pesticide to its head, neck, and ears. "Since about 90% of the adult ticks on a white-tailed deer are on the front end of the body, this works well. The deer then transfer some of the pesticide to their underside (and to each other) when they lick and groom themselves," he explains. (ARS is also working on a machine that puts a pesticide collar on deer.)
"This could pretty much get rid of the ticks around homes and protect kids and pets, or horses," he says. "You find very few ticks out in pastures unless they are brought in by other animals. Most ticks are in the periphery and the population is maintained by the deer, so if you control the population on the deer, you control it on the horses."ï¿½Heather Smith Thomas
About the Author
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.
POLL: University Equine Hospitals