Horses serve as hosts for several types of ticks, including those that cause Lyme disease

Horses are the host animals of several types of ticks, including the larger dog ticks (wood ticks) and the smaller deer ticks, and these parasites are both nuisances and carriers of disease.

How to remove a tick

When removing a tick from your horse or yourself, be careful to extract the head along with the body. It is best to use a tool rather than pull on the tick with your fingers.

When you grasp the tick with pliers, forceps, or tweezers, grasp it as close to its head as possible and work it loose gently so as not to leave the head or mouth parts embedded in the flesh. After removing the tick, apply antiseptic to the entry point.

Do not squash the tick because you might get its fluids on your hands and thus expose yourself to a disease the tick may carry.

Checking a horse (and yourself) for ticks should be part of the daily grooming session if a horse is kept in pasture or ridden. Horses at pasture in spring may pick up numerous ticks daily.

Many ticks are round, reddish brown or light brown, with gray-white speckles on the back and near the head. They are quite small and flat when they first attach to a host, and then become plump and round (some get as large as a cherry) when filled with blood. Some grow 20 times their normal size and can drink up to two milliliters of blood. A heavy tick infestation can irritate the host horse enough to interfere with grazing and can lead to loss of body condition or anemia.

Depending on the species, ticks live for two years or more. Adult females feed on warm-blooded animals before they produce eggs, which they lay from late spring to midsummer. The resulting larvae do not feed on warm-blooded hosts until the following spring, when they feed, molt, develop into nymphs, feed again, and then become adults, ready to mate and lay eggs.

Ticks climb onto vegetation to wait for a host to pass by so they can grab on. They crawl to a preferred location (usually a thin-skinned area or a protected area such as the horse's belly, along the sheath or udder, the root of tail or mane, or in the ears-depending on preference of their species) and pierce the skin to start feeding on blood. Once solidly attached, with mouth parts and head buried in the skin, they can be hard to dislodge.

Ticks are the main carriers of many protozoan diseases, including piroplasmosis. The protozoa in most cases survive from one generation of ticks to successive generations by infecting their eggs. Some ticks also spread diseases caused by bacteria, viruses, and rickettsia. A viral disease sometimes spread by ticks is the Western stain of equine encephalomyelitis.

Lyme disease

Tiny deer ticks can spread Lyme disease, a bacterial infection. Deer ticks are about half the size of the American dog tick. The black-legged deer tick looks completely black to the naked eye but under magnification is orange-brown. Lyme disease is a growing problem in several areas of the country, affecting humans, horses, dogs, cattle, cats, and other mammals. It was first recognized in the United States in 1975 after a mysterious outbreak of a unique form of arthritis in people.

The disease was named after one of the towns in Connecticut in which the illness appeared. Research efforts isolated the pathogen in 1982-a spirochete (cork-screw shaped) bacterium subsequently named Borrelia burgdorferi.

Stephen W. Barthold, Ph.D., director of the Center for Comparative Medicine at the University of California-Davis, has been studying these bacteria for a number of years.

"Though there are several genetic variations of this bacterium, it causes pretty much the same disease in whatever host it infects," he said. "For the most part, it is transmitted by the Ixodes ticks, and these ticks are all related; they are similar vectors (disease carriers) around the world."

These ticks and the bacteria they carry are not selective about the hosts they latch onto, Barthold said, and they infect mice, birds, and people, among others.

Two types of Ixodes ticks generally spread Lyme disease in the U.S. In the northeastern and north central regions, the carrier is the deer tick (sometimes called a bear tick), which normally feeds on white-footed mice, white-tail deer, other mammals, and birds.

In those regions, this tick, Ixodes scapularis, also spreads piroplasmosis, a disease that affects many wild and domestic animals and in humans causes symptoms similar to malaria (fever, chills, sweating, vomiting).

On the Pacific Coast and in the Northwest (and possibly the Southeast), Lyme disease is transmitted by the western black-legged tick.

Nymph stage

In the eastern U.S., ticks transmit Lyme disease primarily during the nymph stage, probably because they are smaller than adult ticks and less likely to be noticed. Ticks must feed on the host for at least 12 to 24 hours before they can transmit bacteria. Nymphs are less than two millimeters in size. They are most likely to transmit bacteria after two or more days of feeding.

Adult ticks can also transmit the disease, but because they are larger and more likely to be seen and removed within a few hours, they rarely have time to transmit Lyme disease to humans. They are more likely to spread the disease to dogs, horses, and other animals that are not checked as often for ticks. In the eastern U.S., adult ticks are most active during the cooler months of the year when fewer humans are outdoors.

Kerwin E. Hyland, Ph.D., professor emeritus of biological sciences at the University of Rhode Island, said deer ticks position themselves on the tips of grasses and shrubs and latch onto people or animals that brush against the vegetation. The ticks usually attach to the legs of a horse or human (unless the horse picks them up on its head or neck while grazing) and then crawl to various parts of the body.

"In the process of taking a blood meal, there is a certain amount of passage of fluids back and forth between the tick and the host," Hyland said. "You can call it regurgitation or salivation-some of the fluids from the tick are passed back into the host-and if there are spirochetes in the fluid, that's how the infection is passed from the tick to the host."

He said that in the fall "we see almost exclusively the adult ticks. If they don't find a host, they may stay around all winter and survive by getting down into the leaf litter on the ground. Then if there's a nice warm day in January or February, they come out and become active again-as long as they are still seeking a host. Once they find a host and take a blood meal, they fall to the ground. The females lie there in the leaf litter and lay their eggs in the spring.

Find small host

"The larvae hatch in July or August and have to find a small host, such as a mouse or chipmunk," Hyland continued. "People frequently see a lot of these very small seed ticks, but these larvae are not usually positive for Lyme disease because they have not yet bitten an infected host.

"When they've just hatched, they rarely have the spirochetes in their digestive tracts yet. The larvae latch onto a small host, primarily the white-footed mouse, which has the spirochetes in its blood. When the larvae suck the mouse's blood they pick up the bacteria. Then the larvae drop off in five to seven days."

Larvae then become inactive until the next spring when they become nymphs. "The nymph is hot with Lyme disease, and if a nymph gets on you or a horse or dog, it can pass the spirochete to the new host," Hyland said. The nymphs molt into adults in the fall-after taking a blood meal-completing their two-year cycle.

Barthold added that the ticks are rather fragile and do not live in dry climates. "They need a moist environment, such as a forest floor or damp ground, plus the right types of host," he said. "If you live in southern California or in any very arid region, you're not apt to see Lyme disease."

Western cycle

The cycle is slightly different in the West. "Instead of being carried by mice, it is carried by pack rats," Barthold said. "They have their own little tick, Ixodes neotoma, and it has a pack rat cycle, spreading Lyme disease from pack rat to pack rat, rarely being transmitted to people or other animals. But we also have another tick, Ixodes pacificus, that is less selective. In its early stages it often feeds on reptiles (which do not carry the bacteria), but when it inadvertently feeds on pack rats it becomes infected-and if it also inadvertently feeds on horses, dogs, or people, it can transmit the disease.

"But this is a complex and ineffective process, having to get the disease first from the pack rat," he added. "The Ixodes pacificus tick that bites a horse or human may not even carry B. burgdorferi unless it first fed on a pack rat."

Signs of Lyme disease in horses include lameness, arthritis, eye disease, dermatitis, and neurological problems. Tests can be performed to try to isolate the bacteria, but these are not always conclusive. A positive test does not mean the horse has Lyme disease; many horses have come into contact with the bacteria previously but are not ill. If other diseases can be ruled out and your veterinarian suspects Lyme disease, it can be successfully treated in early stages with penicillin.

A vaccine against Lyme disease has been developed for dogs and humans, but researchers are still working on an effective vaccine for horses. For horse owners in areas with ticks, the best prevention is to check horses daily and remove any ticks or nymphs when they are found. They may attach anywhere on the body but often locate along the base of the mane and around the rectum.

Because the tick must feed for a relatively long time to transmit Lyme disease bacteria, checking horses daily for ticks and removing them promptly will minimize the risk of contracting the disorder.

If horses have heavy infestations, they can be sprayed with an insecticide solution. Check with your veterinarian or agricultural extension agent for advice on insecticides, and be sure the product is safe for use on horses.

Another way to reduce tick problems is to deworm with ivermectin every six weeks during spring and summer. This drug paralyzes any blood-sucking parasites and they soon die-including any tick that is attached to the horse at that time.