Can You Get Lyme Disease from Horses?

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

Ask the experts

On a recent visit with our daughter in Rhode Island she spent some time discussing that many of her horse loving friends have developed Lyme disease without any evidence of tick bites, a condition she defended emphatically. She suspects flies, of which there are many around the barns, or perhaps mosquitos.

Is there any chance that another vector exists for the passing of the disease?

Doctor's response

Your daughter and her horse loving friends are half-right (or, if you prefer, half-wrong). They are wrong about the flies and mosquitos but right about the horses.

Flies and mosquitoes?: The only proven transmitters of Lyme disease in the United States are ticks, namely:

  • The deer (or black-legged) tick in the Eastern U.S.; and
  • The related western black-legged tick in the Western U.S.

These are hard-bodied ticks. Like all ticks, they need a blood meal to progress from one stage to the next in their life cycle: from larva to nymph to adult. It takes about two years for the tick to go through all three of these stages, reproduce, and die.

Horses: Horses can carry the ticks that carry the spirochete (a form of bacteria) that causes Lyme disease. To tell the story in a more orderly way, in the Northeastern (and Upper Mid- Western) U.S., the life cycle of the vector, the tick, is like this:

Larva: Eggs laid by adult female ticks in the spring hatch into larvae later in the summer that reach their peak activity in August. No bigger than the dot in MedicineNet.com, the larva waits on the ground until a small mammal or bird brushes up against it. The larva then attaches itself to its host (the small mammal or bird), begins feeding, and over a few days swells up with blood.

If the host (the small mammal or bird) is already infected with the Lyme disease spirochete (from a previous tick bite), the larva of the tick is likely to become infected, too. In this way, infected hosts in the wild serve as spirochete reservoirs, infecting ticks that feed upon them. The preferred reservoirs are white-footed mice which live in large numbers in Lyme-endemic areas of the Northeast and Upper Mid-West in the U.S. Other mammals and ground-feeding birds can serve as alternative reservoirs.

Nymph: After feeding, the larvae drop off their hosts and molt (transform) into nymphs in the fall. The nymphs remain inactive throughout the winter and early spring and in May (in the U.S.) become active again. Nymphs on vegetation near the ground attach onto a small mammal or bird and feed for 4-5 days, swelling with blood. If infected during its larval stage, the tick nymph can transmit the Lyme disease bacteria to its host. If not already infected, the nymph may become infected if its host has the Lyme disease spirochete. In highly endemic areas of the Northeastern U.S., about a quarter of nymphs harbor the Lyme disease spirochete.

Humans come into contact with infected nymphs during the nymphs' peak spring activity (late May through July). Although the nymphs' preferred hosts are small mammals and birds, humans and their pets are suitable substitutes. Nymphs are responsible for most human Lyme disease.

Adult: Engorged, the nymph drops off its host into the leaf litter and molts into an adult. The adults actively seek new hosts throughout the fall, waiting up to 3 feet above the ground on stalks of grass or leaf tips to attach onto deer (its preferred host) or other larger mammals including humans, dogs, cats, horses, and other domestic animals. Peak activity for adult deer ticks occurs in late October and early November.

In winter the adult ticks who lack a host take cover under leaf litter or other surface vegetation and become inactive. When temperatures begin to rise, they resume the quest for hosts in a last- ditch effort to obtain a blood meal allowing them to mate and reproduce. This second peak of activity is in the spring (usually in March and April).

Adult female ticks that attach to deer feed for about a week. (Males feed only intermittently.) Mating may take place on or off the host and is required for the female's successful completion of the blood meal. The females then drop off the host and become gravid (pregnant). They lay their eggs beneath leaf litter in the spring and die. The eggs hatch in the summer and the two-year cycle begins again.

To sum up matters, flies and mosquitoes have never been shown to serve as vectors for the spirochete that causes Lyme disease. The only proven vector in the U.S. is the tick. However, horses can serve as hosts for ticks, as many horse people know.

How could your daughter's "horse loving friends have developed Lyme disease without any evidence of tick bites"? They must have been bitten (by ticks, not horses) without knowing it. The ticks may well have been in the horses' coat.

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Reviewed on 1/11/2018
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