Insect stings and spider bites can be a cause for concern in horses, sometimes leading to skin irritations, poisoning, allergies and infections.

Several species of pests and external parasites can cause severe health problems in horses. Flies are the most common and persistent pests for horses. Blood-sucking species like horse flies and mosquitoes are even more serious as some are vectors of severe illnesses that can affect horses and humans.

There are also many types of non-parasitic insects and arachnids that can cause irritation, poisoning, and allergies in horses.

The good news is there are steps you can take to protect your horses from all of the world’s most annoying pests. Read on to learn everything you need to know about the most common bugs, flies, spiders, and other nuisances to keep away from your equine friends.

Common Equine Pests

Horses are susceptible to a variety of external pests and parasites. Most equine pests are either insects or arachnids, some of which can cause irritation, discomfort, and even life-threatening reactions.

Insects and arachnids are members of the largest animal phylum on Earth, the Arthropods, which comprise 85% of all species in the animal kingdom. [1]

While insects are important for ecological functions, such as pollination and decomposition, some species of insects can cause issues for horses. [2] The four largest orders of insects are: [3]

  • Coleoptera (beetles)
  • Diptera (flies and mosquitoes)
  • Hymenoptera (ants, bees, and wasps)
  • Lepidoptera (moths and butterflies)

Arachnids belong to the class Arachnida; they include spiders, mites, and ticks. As adults, these pests have a two-part segmented body, eight legs, and do not have wings and antennae. [4]

By understanding the threats posed by these insects and arachnids, horse owners can better protect their animals from stings, bites, and the potential diseases they carry.

Beetles

Coleoptera, generally referred to as beetles, make up 40% of known insect species; they can be found in almost every habitat on Earth. [5]

Beetles vary widely in size, ranging from one millimeter to ten centimeters in length.

In equine toxicology, the most concerning insect belonging to the Coleoptera order is the blister beetle (Epicauta spp. and Pyrota spp.). [4]

Blister beetles

blister-beetle

Blister beetles, or oil beetles, are pollen- and flower-feeding beetles that consume alfalfa blossoms. Over 2,500 known species of blister beetles are named for their cantharidin secretion, a potent blistering agent. [4]

In the United States, the most common types of blister beetles belong to the Epicauta spp. Adult beetles belonging to these species vary in size from 0.16 to 0.70 in (4 to 18 mm). [4]

Adult blister beetles feed on leaves and flowers. In North America, Epicauta spp. have a propensity for consuming flowering alfalfa legumes that are commonly used to produce hay. [6]

Blister beetles are attracted in great numbers to feed on the pollen and nectar of alfalfa blossoms. When horses consume hay infested by the beetle, the toxin enters their system through the gastrointestinal tract, where it is rapidly absorbed. The resulting condition is called cantharidin toxicosis or blister beetle poisoning. [6]

While rare, cantharidin toxicosis is a very severe condition as the ingestion of as little as five grams of beetles is enough to cause the death of a 1,100 lb (500 kg) horse. [6]

Horses that ingest sufficient quantities of the toxin can experience blistering of the entire gastrointestinal tract (from mouth to anus). Additionally, horses with blister beetle poisoning present with rapid heart rate, stiff gait, appetite loss, frequent urination (pollakiuria), renal (kidney) failure, and fever. [6]

No antidote is available for blister beetle poisoning. Treatment focuses on detoxification and supportive care. Horses with blister beetle toxicosis carry a guarded prognosis. [6]

Prevention

Given the absence of a cure, prevention is the key to ensuring the well-being of horses at risk of cantharidin toxicosis.

Due to the secretion of cantharidin, blister beetles have few natural predators, making biological control (i.e. introducing the pest’s natural predator to reduce their population) a non-viable option.

Chemical control (i.e. using pesticide) is also not advised, as the application of pesticides directly to their food can harm horses. With these limitations for pest control, the most effective way of preventing cantharidin toxicosis is hay management. [4][6]

If alfalfa hay is produced on-farm, flowering plants should be checked for the presence of beetles while the alfalfa is flowering. Whether alfalfa is offered in pasture or processed into hay on-site, it is important to confirm there is not a blister beetle infestation before providing home-grown alfalfa to your horses.

Modern farming practices such as crimping during baling are to be avoided as they do not allow blister beetles and other potential pests to flee the pasture. [6]

Utilizing hay cut outside of blister beetle season, whether produced in-farm or bought, can significantly decrease the risk of cantharidin toxicosis. Leaving hay to dry before baling is considered the best practice, as it allows the beetles to leave the legume and can increase the chances of predators eating the beetle carcasses instead of domestic animals in the pasture. [6]

Flies

Diptera, also referred to as true flies, contains over 100,000 described species, including mosquitoes, gnats, flies, and midges. [4]

These species can be divided into two subcategories: biting and non-biting flies. [4] Both biting and non-biting flies can cause distress and are vectors for a number of diseases in horses.

Mosquitoes

mosquito

There are over 300 species of mosquitoes (Culicidae) worldwide, and over 150 in North America. Mosquitoes are small (3-6 mm; 0.12-0.23 in) and slender biting flies with elongated mouthparts and long legs. [4]

Both male and female adult mosquitoes feed on nectar, but female mosquitos need to feed on blood to produce eggs. [4]

Bites occur more frequently in the early morning and evening hours and during the warmer months. Mosquito bites pose a significant health hazard for horses, as they can spread all forms of the equine encephalitis viruses and West Nile virus. [4]

Equine Encephalitis and West Nile Viruses

Equine Encephalitis and West Nile Virus are mosquito-borne viruses that affect the brain and central nervous system (CNS) in horses. The severe infections caused by these viruses lead to neurological symptoms and have a high mortality rate. [6][7]

Encephalitis viruses are divided into three subgroups: [7][8][9]

  • Eastern equine encephalitis (EEE) is found in eastern Canada and in States west of the Mississippi River, as well as in the Caribbean Islands. It is the most common form of Equine Encephalitis.
  • Western equine encephalitis (WEE) is present in Central and South America, in US States west of the Mississippi River, and in western Canada. Horses infected with WEE show symptoms of muscle contraction.
  • Venezuelan equine encephalitis (VEE) is found in Central and Southern America and is very rare compared to other forms of encephalitis.

All three types of encephalitis and West Nile Virus present with similar symptoms, including: [6][8][9]

  • Fever
  • Head pressing
  • Ataxia (incoordination)
  • Initial hyperexcitability
  • Lethargy
  • Recumbency
  • Circling
  • Blindness

No specific treatment options are currently available for equine encephalitis viruses, and mortality rates are high. [4][8]

Prevention

The majority of horses that survive equine encephalitis and West Nile viruses suffer long-term neurological deficits. Given the high mortality rate and the absence of a specific cure, prevention is of utmost importance to ensure your horse’s welfare. [6][8]

Prevention strategies include annual vaccination for West Nile and encephalitis viruses, and a combination of environmental, cultural, biological, physical, and mechanical control to limit exposure to mosquitoes. [4][8]

Environmental controls

Environmental controls to reduce mosquito populations on your horse farm may include: [4][8]

  • Removing sources of standing water
  • Implementing mud control measures
  • Limiting turnout time during peak mosquito activity
  • Use of fly sheets and face masks
  • Spray-on insecticides
  • Refreshing water troughs frequently
  • Eliminating unnecessary vegetation from pastures and trails
  • Using yellow lights around the property, which are less appealing to mosquitoes

Cultural controls

Cultural pest control refers to human behaviors and routines that help control pest populations.

Cultural controls against mosquitoes may include: [4]

  • Adjusting turnout schedule: Mosquito activity varies by species. Adjusting turnout schedules in accordance with the species of mosquito to avoid horses being outside during times of high mosquito activity can minimize bites
  • Monitoring for sources of standing water: Mosquitoes breed in stagnant water. Regularly checking and eliminating stagnant water sources reduces the mosquitos’ chances of reproducing
  • Controlling mud patches: Mud can harbor mosquitos. Preventing overgrazing on pasture reduces the formation of mud; in high-traffic areas, applying crushed rock, sand, or wood chips can improve drainage, reducing mud formation
  • Ensuring sufficient drainage: Installing gutters and downspouts coupled with landscaping around stables can improve drainage and reduce standing water

Biological Controls

Biological control measures refer to introducing a pest’s natural predators to the habitat. Predators can directly reduce populations by consuming some individuals, and they can indirectly drive out the rest of the population once their presence is known.

Predators of mosquitoes include:[4]

  • Natural predators: Dragonflies, bats, and birds can help control mosquito populations
  • Mosquito-eating fish: Introducing mosquito-eating fish species like Koi or Gambusia into ponds can reduce mosquito larvae
  • Mosquito-killing bacteria: Dissolvable blocks containing mosquito-killing bacteria can be added to large water sources

Physical controls

Physical and mechanical control measures describe creating physical barriers so pests can’t access horses. Examples of physical controls against mosquitoes include: [4]

  • Fly Sheets, Boots, and Masks: Covering the horse with apparel creates a physical barrier between their skin and mosquitoes
  • Flight Impediments: Mosquitoes are weak fliers, therefore installing fans in stalls and improving ventilation in barns can impede their flight
  • Trapping: Carbon dioxide traps can be useful against mosquitoes, provided they are located at a distance from humans and animals

Chemical controls

Chemical control measures describe using poisons or pesticides to eliminate a pest. Pesticides against mosquitoes that are used in equine facilities include: [4]

  • Topical Repellents: Several repellents are available, but their effectiveness is brief (less than a day)
  • Insect Growth Regulators: Can be used in small water sources
Always inform your veterinarian of any animals who may be exposed to pesticides before treating your horse. Some animals are very sensitive to insecticides, particularly cats.

 

Midges

Adult Biting Midge

Biting midges, also known as no-see-ums or punkies, are small flies belonging to the genus Culicoides. They feed on blood and can transmit several diseases to horses. [4][10]

Adult biting midges are typically 1-3 mm in length, making them challenging to see with the naked eye. They have slender bodies and long, delicate legs, with wings that often feature distinctive patterns.

Only female midges feed on blood, which they require for egg production. They primarily bite at dawn and dusk, but they may also be active throughout the day in shaded, humid environments.

Midges can cause intense itching and discomfort for horses due to their bites, often resulting in “sweet itch” or equine dermatitis.

Sweet Itch

The most common health concern for horses linked to biting midges is sweet itch, also known as Culicoides Hypersensitivity, which is a common skin condition caused by an allergic reaction to midge bites. [8]

Horses with sweet itch present a number of symptoms, including: [8]

  • Itching
  • Rubbing
  • Biting
  • Hair loss, including the tail and mane
  • Skin lesions

In severe cases, horses may have behavioral changes such as irritability and agitation. Horses that do not receive adequate topical wound care are at risk of developing secondary infections. [6][8]

Treatment of sweet itch is mainly aimed at reducing discomfort and topical treatment of skin lesions. As with most allergic conditions, treatment is lifelong, and the best way of minimizing symtpoms is prevention. [8]

Prevention

Taking steps to avoid exposure to midges can help improve quality of life for horses with sweet itch.

Turnout management during midge season is an effective prevention strategy for sweet itch. Keeping horses indoors during dusk when midge activity is highest helps minimize exposure. [10]

Other prevention strategies include: [10]

  • Use of fans in stables
  • Installation of fine screens
  • Topical preventives

Bot flies

bot-fly

Bot flies (Gasterophilidae) are parasitic flies that infest the digestive tract in horses. [8]

Different species of bots prefer different sites for egg attachment. [8][11] The most common species of botfly in the United States are: [11]

  • Horse bot fly(Gasterophilus intestinalis): Lays its eggs on the horse’s front legs, abdomen, and flanks
  • Nose bot (Gasterophilus haemorrhoidalis): Lays eggs in the hair of lips. Once hatched, the larvae migrate through the digestive system.
  • Throat bot (Gasterophilus nasalis): Deposits eggs under or near the horse’s jaw. Unlike the nose and common botflies, the eggs do not need to be ingested to hatch.

The larvae of the horse bot fly enter the host’s mouth during self-grooming. Once in the mouth, the larvae burrow into the gums and tongue, where they remain for a month before passing to the stomach. After nine months, they detach from the stomach lining are excreted in the feces. [4][11]

Bot flies cause damage to the stomach lining which can lead to: [4][11]

Affected horses generally carry a good prognosis. [8]

Prevention

While bot flies tend not to cause severe conditions in horses, implementing prevention strategies is the most effective way to keep horses comfortable.

Pest control measures against bot flies include: [4]

  • Regularly checking for the presence of bot fly eggs in horses’ manes, and in other areas where bot flies are known to lay eggs, such as legs, abdomen, and face
  • Diligent manure management to prevent pupae accumulation
  • Use of fly masks, sheets, and boots during peak insect activity
  • Removing eggs, if present. This can be performed by either clipping egg-infested hair or stimulating egg hatching with warm water (45°C/113°F) and subsequently washing off the hatched larvae

Chemical control options are limited due to bot fly eggs’ highly resistant outer shell and the short time adult flies spend on horses. Repellents can be used against the pest in its larval stage. [4]

Ants, Bees and Wasps

Hymenoptera is a large insect order with over 100,000 species. 80% of the order is made up of the ant (Formicidae) family and six separate wasp families (Aphelinidae, Encyrtidae, Eulophidae, Braconidae, Torymidae, and Pteromalidae). [11]

Fire Ants

fire-ants

Fire ants are an invasive species in North America, belonging to the genus Solenopsis, which includes over 200 species. [4]

In North America, fire ants are primarily found in the southern United States and Mexico. These ants are carnivorous, feeding mainly on dead animals and plants, although they also have the ability to hunt small animals. [4]

While fire ants can be aggressive, they generally do not pose a significant threat to healthy adult horses. Newborn foals, however, can die as a result of a large fire ant attack. [4][12]

Fire ants immobilize and kill their prey with venom. Although adult horses are not likely to suffer serious consequences from fire ant envenomation, the stings can be painful and irritating.

Horses with fire ant stings can develop a number of clinical signs including: [4]

  • Localized skin necrosis (tissue death)
  • Pustules
  • Pain
  • Itching

Excessive scratching can lead to secondary skin infections, which may require wound care and/or veterinary attention.

Treatment is symptomatic and focused on reducing itchiness and pain. Veterinarians may administer corticosteroids, NSAIDs, and/or antihistamines. [4]

Prognosis for adult horses is excellent. Foals with severe fire ant envenomation have a guarded prognosis. [4]

Prevention

Controlling the fire ant population in pastures is paramount in protecting horses, especially newborn foals. Prevention strategies include: [4]

  • Waste management: Robust waste management, particularly of human food, to minimize attracting fire ants and reducing available food sources that sustain their population. Proper disposal and regular cleaning can help deter ants from nesting and spreading in horse pastures.
  • Mound flattening: Mowing pastures can reduce ant mound size. While this does not decrease the size of the ant populations, and they may even spread over a larger portion of the pasture, the reduction in mound size reduces the probability of horses tripping on mounds, triggering massive fire ant attacks
  • Mound treatment: Pouring boiling water on fire ant mounds has been proven to be 60% effective at reducing ant population. However, the treatment needs to be periodically repeated as the majority of ants do not die
Pouring toxic chemicals (e.g., gasoline and ammonia) on mounds does not reduce ant populations and can cause soil contamination.

Biological control is aimed at reducing pest populations by exploiting natural predators. Natural predators of fire ants include: [4]

  • Ant-decapitating flies
  • Entomopathogenic fungi

Chemical control for fire ants involves insect growth regulators; these chemicals do not kill the ants but rather prevent young ants from developing into adults and adult ants from reproducing. When implementing chemical control, choosing products that are safe for horses is essential. [4]

Killer Bees

africanized-bee

Africanized bees, commonly referred to as killer bees, are a hybrid between the East African lowland honeybee (Apis mellifera scutellata) and the European honeybee (Apis mellifera) and are found in both South and North America. [4]

Killer bee venom is no more toxic than that of regular honeybees. However, killer bees are more dangerous as they attack in greater numbers, defending their nests with over one hundred individuals. In comparison, the European honey bee typically defends with around 20 to 30 bees. [4][13]

Africanized bee attacks can cause several symptoms in horses, including: [4]

  • Severe swelling around the sting sites
  • Painful muscle contraction (rhabdomyolysis)
  • Colic
  • Kidney damage
  • Acute renal (kidney) failure

While bee envenomation is uncommon in horses, Africanized bee attacks have resulted in several deaths. [14][15]

Treatment of bee stings is symptomatic, options include: [4]

  • Hot and cold compresses
  • IV fluids
  • NAIDs
  • Antihistamines
  • Corticosteroids
  • Antibiotics

Prognosis depends on how much venom is in the horse’s body relative to body weight, and how quickly medical intervention occurs. Most horses do not require medical attention for a small number of bee stings and are expected to make a full recovery. [4]

Prevention

Preventing Africanized bee-related injury relies on: [4]

  • Minimizing potential hive sites by discarding empty containers and other objects that could provide nesting sites for Africanized honeybees
  • Sealing structural gaps larger than 1.2 in (30 mm) in diameter, reducing the probability of bees nesting in building cavities

Removal of hives should only be carried out by licensed specialists. Currently, no recommended chemical control methods are available. [4]

Moths and Butterflies

Lepidoptera is the third largest order of insects and includes butterflies and moths. The order is made up of over 10,000 species and 100 families worldwide. [4]

Lepidoptera undergoes a complete metamorphosis during its life cycle. The four stages of development are: egg, larva (caterpillar), chrysalis or pupa, and winged adult. [4]

There are 14 families of insects in this class that can cause health issues in horses. In North America, one species is the tent caterpillar (Malacosma spp.), which can cause abortions and fetal abnormalities in pregnant mares. [4]

Tent caterpillars

eastern tent caterpillar

There are multiple species of moth whose larvae are known as “tent caterpillars.” In particular, the Eastern tent caterpillar and the Forest tent caterpillar are associated with life-threatening conditions in horses. [4]

Eastern tent caterpillars most commonly live on wild black cherry trees and pose a major health concern as, when ingested, they cause mare reproductive loss syndrome (MRLS).

MRLS is a condition that causes both early and late abortions in horses. It was identified for the first time in the spring of 2001 when more than 1,500 abortions occurred in Kentucky. Mechanisms of action are not currently understood, but ingestion of tent caterpillars has been confirmed as the root cause in this devastating regional occurrence. [4]

Forest tent caterpillars are found throughout hardwood forests in the United States and can cause pericarditis in horses. [4]

Prevention

Due to the absence of specific treatment and the significant threat risk that eastern tent caterpillars can pose to pregnant mares and unborn foals, prevention is the best way to ensure safety for horses in at-risk areas.

In addition to regular property inspection and manual removal of caterpillar tents from the property, pest control strategies against tent caterpillars include: [4]

  • Turnout management: If caterpillars are present horses should not pasture in the area
  • Proper storage of feed: Keeping feed covered is useful in preventing caterpillar contamination
  • Light management: Minimizing light pollution around forested areas attracts fewer moths
  • Deforestation of relevant species: Removing or avoiding planting trees preferred by the tent caterpillar (Crataegus, Malus, Prunus spp.)
  • Tree bands: Wrapping trees with slippery or sticky bands to restrict caterpillar movement

Taking advantage of tent caterpillars’ natural predators is an environmental friendly approach; predators include: [4]

  • Parasitic wasps
  • Birds: encouraging local bird populations by installing birdhouses can help manage caterpillar populations.

In addition, general use of pesticides and insecticides on the property is helpful in controlling populations of tent caterpillars. [4]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants

Spiders: Brown Recluse and Black Widow

black-widow-spider

In North America the two most common venomous spiders are black widow spiders (Latrodectus spp.) and brown recluse spiders (Loxosceles spp.). Bites vary in severity depending on a number of factors including the species of spider, size of the horse, and location of the bite on the body. [4]

Black widow venom is one of the most deadly toxins by volume in the world. It contains a number of different proteins and enzymes which disrupt nerve function. [6]

In horses, the onset of symptoms of black widow spider envenomation occur 30 minutes to two hours from the time of the bite and include: [6][8]

  • Muscle tremors, cramping, and rigidity
  • Ataxia (incoordination)
  • Facial paralysis
  • Ascending paralysis
  • Dyspnea (difficulty breathing)
  • Agitation
  • Colic

Rarely, black widow envenomation can cause heart damage in horses. [6]

Brown Recluse Spider

Brown recluse venom contains a number of enzymes that produce cytotoxic and hemolytic effects in the horse, which can lead to abnormal bleeding. [16]

Symptoms of brown recluse spider envenomation in horses include: [6]

  • Swelling around the bite site
  • Skin necrosis (death)
  • Secondary infections

Less commonly, clinical findings in cases of brown recluse envenomation show: [6]

In most cases of spider bites, envenomation is not witnessed, and diagnosis is presumptive based on symptoms and possible history of exposure. [6] There is no specific diagnostic test to definitively confirm spider envenomation. [6]

Regardless of the species of spider involved, horses require supportive care, including: [6]

  • Wound care
  • NSAIDs
  • Symptomatic treatment
  • Intravenous medications

Currently, the only antivenom available against spider bites is for black widow envenomation. Horses suffering from brown recluse spider envenomation may benefit from the application of cold compresses and may require antibiotics for secondary infections. [6]

The prognosis for spider envenomation varies depending on the severity of symptoms and the species of spider involved. The majority of horses bitten by black widow spiders recover completely within ten days from the bite. [6]

In the case of brown recluse spider envenomation, it may take several weeks for the bite wound to heal completely. [6][8]

Prevention

Prevention strategies against black widow and brown recluse spider bites involve several different control measures aimed at habitat reduction. Recommended practices including: [17][18]

  • Decluttering
  • Sealing entry points
  • Placing sticky traps

In addition, general use of insecticides helps control spider populations. In cases where a large spider population is present or suspected, using a profession pest control service is recommended.

Mites

scabies mite

Mites are microscopic arachnids, measuring less than 0.4 inches in length. The majority of mite species are harmless, but a number of parasitic mites infest the skin of mammals, including horses.

In addition to general irritation and discomfort, some types of parasitic mite infestation can lead to secondary skin infection, hair loss, and mange. [4]

There are four main parasitic mites that feed on horses: [4]

  • Itchy leg mite (Chorioptes equi): Feeds on horses’ leg skin and fat and is associated with progressive lymphedema
  • Horse follicle mite (Demodex equi): Generally do not cause any significant problems for horses. Rarely, infected horses can develop a pustular reaction and abscesses in the neck area
  • Scab mite (Psoroptes equi): This species feeds on horse skin; their feces cause an allergic reaction that leads to hair loss and scabs on the mane and tail
  • Common mite (Sarcoptes scabiei): This species burrows in the skin; females lay eggs in the burrows. Some horses have allergic reactions to this type of mite, causing skin lesions frequently located on the ears, neck, and head

Prevention

While mites do not pose severe health concerns to horses, controlling horse mite populations can increase general well-being, as severe cases of mange can cause skin lesions due to excessive scratching that, in turn, can lead to secondary skin infections.

In addition to maintaining good health and hygiene, control strategies against mites include: [4]

  • Regular grooming to detect mites early
  • Ensuring appropriate biosecurity and quarantine protocols are in place on the premises and when going to shows or competitions
  • Clipping and proper disposal of clipped hair
  • Hot lime sulfur washes
  • Permethrin sprays
  • Organophosphate pesticides
Consult your veterinarian before using permethrin or organophosphate pesticides. Some animals are highly sensitive to these formulations, especially cats.

Ticks

Closeup of a tick on a plant straw

Ticks are small parasitic arachnids (0.12 to 0.20 in); there are over ten thousand species of ticks worldwide, which can be divided into two separate categories:

  • Hard ticks (Ixodidae)
  • Soft ticks (Argasidae)

While spread worldwide, ticks are more likely to live in humid areas. [19]

Lyme disease

Lyme disease is caused by the bacteria Borrelia burgdorferi carried by the black-legged tick (Codes scapularis), commonly found in southeastern and south-central regions of Canada and central and eastern states in the US. [19]

Horses infected with the bacteria can develop neuroborreliosis (the neurological manifestation of Lyme disease). Symptoms include: [4][19][20]

  • Atrophy of spinal muscles
  • Muscle loss
  • Facial paralysis
  • Ataxia (incoordination)
  • Intermittent shifting
  • Lameness
  • Stiffness

In addition to neuroborreliosis, affected horses can develop uveitis that causes a number of clinical signs, including: [20]

  • Blepharospasm (squinting)
  • Miosis (constricted pupils)
  • Corneal edema (cloudy eye)

Diagnosis of Lyme disease is complex and is generally based on the exclusion of other possible causes of the symptoms. Treatment is often presumptive and includes administration of tetracycline, an antibiotic. [4]

Horses that develop neuroborreliosis carry a poor prognosis; horses with cases of Lyme disease that do not progress to the neurological manifestations generally carry a good prognosis. To date, there is no FDA-approved vaccine for Lyme disease in horses. [4]

Anaplasmosis

Anaplasmosis is caused by the Anaplasma phagocytophilum bacteria which is transmitted to horses through the bite of an infected tick (I. scapularis or I. pacificus). Anaplasmosis is a seasonal condition, with the majority of cases being reposted in winter and spring. [21]

Infected horses develop a number of symptoms within three to ten days from exposure, including: [4][21]

  • Fever
  • Lethargy
  • Loss of appetite
  • Elevated heart rate
  • Jaundice (yellow gums or eyes)
  • Ataxia (incoordination)

Diagnosis is based on the presence of clinical signs, blood work, and regional and seasonal tick activity. Most affected animals are able to recover without treatment within three weeks. Severe cases may require the administration of antibiotics and supportive care. [21]

Prevention

Tick prevention strategies encompass a number of approaches. As ticks tend to prefer moist and shady environments, landscape management measures include: [4]

  • Trimming vegetation
  • Removing dead foliage and vegetation
  • Removing potential rodent nesting sites (e.g., fallen logs and rocks) near pastures
  • Keeping potentially infected wildlife (e.g., deer) out of pastures
  • Keeping grass cut as short as possible
  • Inspecting horses for ticks immediately after returning from trail rides

Daily tick inspections are key to controlling tick-borne illness in domesticated animals. Ticks tend to migrate to the warmest part of the body, and they are usually picked up when walking through long grasses or shrubbery.

Therefore, particular areas of emphasis for tick inspection include around the ears and eyes, under the jaws and axillae (armpits), under the chest, neck and belly, and in the groin. [4] If any ticks are found embedded in a horse’s skin, remove them promptly with tweezers or pliers. Do not twist or crush ticks during removal.

Chemical preventives against ticks include: [4]

  • Topical pour-ons and sprays
  • Treated fly sheets
  • Landscape pesticides (requires professional pest control)

Biological control strategies are not recommended, as tick predators such as fire ants pose health risks to horses and other pasture animals.

Pest Control and Equine Management

Insect stings and spider bites represent a serious health concern for horses. Several prevention and management strategies can be employed to minimize the pest populations.

Integrated pest management, or IPM, is a preventive approach that aims to increase the efficiency of pest control, causing the least amount of damage to the environment, humans, and animals. [19]

IPM involves the choice of multiple control methods on a situation-to-situation basis, including: [19]

  • Cultural Control is the least impactful on the environment and involves keeping hygienic routines
  • Physical and Mechanical Control involves using physical barriers to prevent bites, such as fly sheets and masks, as well as fans and screens
  • Biological Control utilizes natural predators to control insect populations
  • Chemical Control uses natural and man-made repellants or pesticides to reduce the insect population

No matter the preferred method, pest control is only effective if it is maintained consistently. Rotating different approaches is recommended to prevent pest resistance, especially to chemical preventives.

Summary

Horses face numerous health threats from arthropods, including insects and arachnids, which can cause irritation, discomfort, and potentially life-threatening conditions.

  • Insects are divided into four main orders: Flies (Diptera), beetles (Coleoptera), Hymenoptera (bees, ants, wasps), and moths and butterflies (Lepidoptera)
  • Mosquitoes (Culicidae) can transmit viral encephalitides, such as Eastern, Western, and Venezuelan equine encephalitis, posing severe health risks to horses.
  • Arachnids like ticks and mites can lead to Lyme disease, anaplasmosis, and mange.
  • Venomous spiders, such as black widows and brown recluses, can cause symptoms such as muscle cramps and tissue necrosis.
  • Prevention and management strategies involve pasture management, insect repellents, and protective gear like fly sheets and masks to minimize pest populations.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Natural History Collections: Arthropoda. The University of Edinburgh.
  2. Meyer, H. J. et al. Insect Pests of Horses. NDSU Extension Service. 1991.
  3. Insect Orders: Complete Metamorphosis. University of Kentucky.
  4. Machtinger, E. T. et al. Pests and parasites of horses. Wageningen Academic Publishers. 2022.
  5. Sha’ari, N. A. and Arumugam, N. Beetles Diversity (Order: Coleoptera) of R.E.A.C.H Biodiversity Centre, Cameron Highlands, Pahang, Malaysia. Universiti Malaysia Kelantan. 2019.
  6. Hovda, L. R. Blackwell’s Five-Minute Veterinary Consult Clinical Companion Equine Toxicology. Wiley Blackwell. 2022.
  7. MacKay, R. J. and de Tonnerre, D. Equine Arboviral Encephalomyelitis. MSD Veterinary Manual. 2022.
  8. Wilson, D. A., Ed. Clinical veterinary advisor: The horse. Elsevier Saunders. 2012.
  9. Long, M. T. Equine Viral Encephalomyelitis (Encephalitis). MSD Veterinary Manual. 2019.
  10. Hendrix, C. M. Flies and Mosquitoes of Horses – Horse Owners. MSD Veterinary Manual. 2019.
  11. Arthropod Pests of Equines. University of Arkansas.
  12. Rasplus, J.-Y. et al. Hymenoptera. Chapter 12. BioRisk. 2010.
  13. Africanized Killer Bees: Information About African Bees. PestWorld.
  14. Sherman, R. A. What Physicians Should Know about Africanized Honeybees. West J Med. 1995.
  15. Ribeiro, P. R. et al. Acute Renal Failure in a Horse Following Bee Sting Toxicity. Cienc. Rural. 2020. doi: 10.1590/0103-8478cr20190940.
  16. Mullen, G. R. and Durden, L. A. Medical and veterinary entomology, 3rd ed. Academic Press, Elsevier. 2019.
  17. Integrated Pest Management: Spiders. University of California, San Francisco School of Nursing. 2009.
  18. Vetter, R. S. and Hedges, S. A. Integrated Pest Management of the Brown Recluse Spider. Journal of Integrated Pest Management. 2018.
  19. Lyme disease in horses. Cornell University College of Veterinary Medicine. 2023.
  20. Anoka, I. A. et al. Brown Recluse Spider Toxicity. Treasure Island (FL): StatPearls Publishing. 2024.
  21. Straubinger, R. K. Lyme Disease (Lyme Borreliosis) in Horses. MSD Veterinary Manual. 2019.