A strong correlation between the length of time an infected tick is allowed to remain attached to a host and the risk of host infection has been documented for several deer tick-borne pathogens including A. phagocytophilum and B. burgdorferi.  If an attached tick is removed within 24 hours the chance of a horse contracting one of these tick-borne diseases is greatly reduced.  This means that horses living in a tick-friendly environment should be checked daily to ensure prompt tick removal.  It is also best practice to thoroughly examine horses for ticks after every trail ride.  Areas on the horse’s body where ticks are commonly found include the lower limbs, the chest, in and around the ears, under the jaw, in the folds of skin at the elbow, on the inner thighs, sheath, udder or under the tail, and at the base of mane and tail hair.  Because deer ticks are very small, they may be difficult to see but the firm bump formed in reaction to their bit can often be easily felt. 

To optimize the removal of the mouth parts and not just the body, it is important to remove an attached tick using the proper technique.  A basic set of fine-tip tweezers works as well for tick removal as any of the tick removal products on the market.  The tick should be grasped with tweezers as close as possible to the skin’s surface.  Pull up with steady pressure.  Do not crush or twist the tick as this causes it to regurgitate blood back into your horses, increases the chance of disease transmission.  Additionally, avoid jerking or twisting the tick as this can cause the mouth parts to break off and remain in the horse’s skin. If the tick separates and the mouth parts remain in the horse’s skin, remove the mouth parts from the horse’s skin if possible.  Once the tick is removed cleanse both the bite area and your hands with rubbing alcohol and water.  Whether attached or not, ticks should also be correctly disposed of by drowning them in alcohol, flushing them down a toilet, placing them in a sealable plastic bag or securely wrapping them up in tape.  Ticks should never be pinched or crushed between one’s fingers or allowed to return to the environment. 

Generally speaking, ticks are known to be more active early in the morning and late in the afternoon. During inactive periods, ticks look for shelter under vegetation and leaf litter.  Keeping overgrown pastures mowed, and brush and shrubs trimmed back out of pastures reduce tick habitats.  Going a step further, maintaining a minimum three-foot wide shortly mowed grass border between pastures and wooded area may also reduce ticks in a pasture.  Limiting free access to wooded areas is another obvious way to reduce a horse’s exposure to ticks. 

Biological control of ticks, rodents, birds, and reservoir hosts is another important means of reducing tick populations on horse farms.  Free-range chickens or guinea fowl are a great tool to reduce tick exposure as they are voracious tick eaters.  Because tick larvae and nymphs feed on rodents, a resident rodent population increases the chances of having ticks. Removing preferred habitats for rodents such as piles of debris, stacks of old hay and other prime mouse nesting areas should have a desirable impact on tick numbers.  Careful storage of grain is also important in reducing mice around the barn along with an abundant use of mouse traps.  Reducing the bird population around the stable is also recommended as immature stages of the deer tick may feed on birds.  Barn cats may be a great option to control both mice and birds if the cats are good hunters and not overfed.

Using a topical insecticide that includes a label claim for repelling ticks on horses allowed access to pasture and/or wooded areas is another sound strategy for reducing tick exposure in horses.  These products may include spray-on, pour-on, or wipe-on products containing cypermethrin, permethrin, pyrethrin, or piperonyl butoxide, which can provide some protection.  Many common fly sprays will contain one of these chemical tick deterrents.  According to the CDC, a product with 0.5% permethrin is sufficient to repel ticks.  Applying these products to a horse’s legs, chest, belly, groin area as well as mane and tail before pasturing or riding them in long grass, brush, or wooded areas can prevent tick bites. 

Lyme Disease

In the Midwest, Lyme disease (also called equine borreIiosis) is spread by the bite of the tick species Ixodes scapularis, also known as the deer tick or the (Eastern) blacklegged tick. Both the nymph and adult stages of the deer tick can be infected with Borrelia burgdorferi, the bacterium that causes Lyme disease. Nymphs, the third stage of development in the deer tick life cycle, are generally present from early spring until early summer, whereas adult ticks are usually found during the fall. As a result, spring and fall are the seasons which pose a greater risk of contracting Lyme disease for horses and other animals. The size of a poppyseed, deer ticks in the nymph stage may be more of a threat to horses because their very small size makes them difficult to find on the horse’s body. However, the adult stage is more likely to be infected because of the deer tick’s three-host life cycle. It is generally reported that a tick must be attached for 36 to 48 hours or more before the Lyme disease bacterium can be transmitted to a host animal.

A blood test exists to detect the presence of antibodies for B. burgdorferi bacterium. A positive blood test does not necessarily indicate current clinical illness as the presence of antibodies could be in response to a previous infection. Currently there is no known relationship between the magnitude of blood titers and likelihood of disease. As a result, a positive blood test that only generally indicates current or past infection is not reliable for determining the likelihood of current or future actual illness. Because of this lack of direct correlation, Lyme disease morbidity in horses infected with the B. burgdorferi bacterium is unknown. Treatment of Lyme disease in horses is similar to the treatment methods in people and dogs, with doxycycline being the primary medication of choice. 

Equine Granulocytic Anaplasmosis

Equine granulocytic anaplasmosis (EGA), previously called equine ehrlichiosis, is the most common tick-borne disease in horses. Equine Anaplasmosis is a zoonotic tick-transmitted disease cause by the Gram-negative bacterium Anaplasma phagocytophilum (formerly known as Ehrlichia equi). Deer ticks (Ixodes scapularis) are the most common vector of this rickettsial pathogen. The Lone Star tick (Amblyomma americanum), generally found from Texas north to eastern Kansas and then east to the Atlantic, is also a vector for EGA while reservoir hosts include small rodents like the white-footed mouse, chipmunk, and vole. Research in mice published in 2021 showed that 25% of mice were infected with A. phagocytophilum when infected ticks were removed at 24 hours of feeding time. For each of the tick feeding durations of 36, 48 and 60 hours, transmission of the disease was 75%. When infected ticks were allowed to remain attached to the mice and feed for 72 hours, transmission of the disease was 93%.

Disease transmission most commonly occurs in the fall, winter, and spring. Studies report a wide range in prevalence of A. phagocytophilum infection in horses with anywhere from 17 – 67% of healthy horses having a positive serum antibody test. Horses usually show signs of illness 10 to 45 days after infection. EGA disease severity is usually mild with many horses recovering within 14 days. The more common signs of illness in equines include, fever, lethargy, depression, anorexia, jaundice, muscle stiffness, mild ataxia, overt lameness and/or distal limb edema.  In horses, like in humans, anaplasmosis is an acute self-limiting disease, meaning it will typically resolve on its own. The supportive care needs of symptomatic horses can reportedly range from very little care needed to expensive and extensive supportive care. The most effective drug treatments for EGA are doxycycline or oxytetracycline. 

Equine Piroplasmosis

Equine Piroplasmosis (EP, also called babesiosis) is another tick-transmitted, blood-borne disease of equine. At least 14 tick species are believed to be potential natural vectors for spreading the two specific protozoa that cause EP, Theileria equi and Babesia caballi. Found in southern states, the tropical horse tick (Dermacentor nitens) is the only known natural vector for EP in the United States. Infected horses can transmit the disease to other horses through ticks, the sharing of contaminated equipment (e.g. needles, syringes, dental speculums, etc), or the administration of contaminated blood products. Infected broodmares can also infect their foals in utero. To ensure the U.S. remains free of this tick-borne disease, all horses imported to the U.S. must test negative for both protozoa responsible for the disease. In addition, newly confirmed cases of domestic equine piroplasmosis must be reported to both state and USDA veterinarians.

The incubation period of equine piroplasmosis is 7 to 22 days before the horse may show signs of illness. It is important to know that horses can be infected with these protozoa in the absence of clinical signs while others may show acute or chronic signs of disease.  When signs of equine piroplasmosis are present they are often vague and may include exercise intolerance, mild weight loss, anemia, jaundice, inappetence, and transient fever. Chronic infection is possible if a positive-testing horse is not treated. Mortality rates for infected horses generally range from 5 – 10% but may be up to 50%, especially in naïve populations. 

In 2021 the USDA reported 36 new cases of EP from 35,493 horses tested, with two positive horses found in Iowa. Thirty-one positive animals were confirmed to have had IV needle sharing and/or blood doping while five horses had been illegally imported from Mexico, an EP endemic region. In 2020, 29,595 domestic horses were tested and yielded 23 new positive horses located in seven states, including Michigan. All positive horses had been exposed through needle-sharing and/or blood doping practices. 

Horses that survive EP are very concerning because they tend to become carriers without any outward symptoms of illness. These healthy-looking horses may still be positive for the bacteria years after infection and successful drug treatment. These carrier horses are indistinguishable from normal healthy horses as they have no clinical signs of the disease, yet they can still transmit the disease to other horses via ticks or contaminated equipment. It is only through blood testing that healthy-appearing carrier animals can be identified.  This creates a biosecurity challenge and is the reason the USDA has such stringent policies regarding management of domestic horses that test positive for equine piroplasmosis. 

The USDA mandates only a few management options when a horse has a positive blood test for EP. Lifetime quarantine is a challenging management option to execute while euthanasia is often a difficult choice to make. A positive-testing horse can also be exported from the country as a measure to protect the U.S.’s domestic equine population. Another more hopeful management option is long-term quarantine with enrollment in the USDA-APHIS EP treatment program. This program has been successful in permanently clearing the infective protozoa from chronically infected horses and eliminating the risk of EP transmission to other equids.  To be released from quarantine, after going through the USDA-APHIS treatment program, animals must test negative on all available EP diagnostic tests. 

Tick Paralysis

In horses a condition called tick paralysis (or tick toxicosis) is a rare but acutely life-threatening emergency.  Female ticks of the Australian paralysis tick (Ixodes holocyclus), the Rocky Mountain wood tick (Dermacentor andersoni) and the American dog tick (Dermacentor variabilis) release paralysis toxins into the circulatory system of their host during feeding. Present in the saliva of these female ticks, these toxins prevent contraction of muscles in the host resulting in widespread paralysis of all muscles. Spring and summer, when female ticks are most active, are when animals have the highest risk of tick toxicosis which is a progressive, symmetrical, ascending paralysis. The severity of the paralysis does not appear to be directly linked to either the number or size of ticks found on an animal. Horses, sheep, cattle, dogs, and humans are susceptible to these salivary toxins with tick paralysis most commonly reported in dogs.

Signs of tick paralysis in horses include ataxia, inability to stand, and difficulty breathing and/or swallowing as the paralysis ascends.  There is no specific test for tick paralysis.  Usually, diagnosis is dependent on finding attached ticks on the horse and/or if the animal improves after administering an antiparasitic treatment or an antitoxin (currently only available in Australia). When signs of tick paralysis in horses are caught early simply removing all ticks generally results in improvement within 24 hours and complete recovery within 72 hours.  The exception to this applies to the more toxic Australian paralysis tick which is currently not found in the U.S. If ticks are not removed, progression to respiratory paralysis may result in death within 1  to 5 days.

Previously only reported in Australia, the first known cases of equine tick paralysis in the U.S. were admitted to Indiana’s Purdue University Veterinary Teaching Hospital in May 2018. Two miniature horses (3 and 4 years of age) were brought in for weakness that progressed to recumbency within about a 12-hour time period. Upon initial exam approximately 150 and 100 engorged American dog ticks were removed from each animal, respectively. With supportive care both miniature horses made a complete recovery with no residual neurologic deficits.