Lyme Disease

by Lorie A. Valentino, DVM, MS, Diplomate American College of Veterinary Internal Medicine

Diagnosed 30 years ago in Old Lyme and Lyme Connecticut.

Caused by Borrelia burgdorferi, a spirochete bacterium.

B. burgdorferi is hosted by Ixodes species of ticks (also known as deer ticks) which transmit the infection to humans, horses, and other mammals.

Ixodes ticks have a two year life cycle from egg to adult requiring a blood meal at each life stage:

Egg laid in decaying leaves in the spring. Adult female tick dies after laying eggs.
Larval form is hatched and feeds on small mammals such as mice or birds.
Once fed, the larvae molt into nymphs and remain dormant until the following spring.
Next spring, the nymphs feed on mice and other mammals.
The ticks molt again and develop into adults.
The adult ticks feed on large mammals, especially deer, then they mate and lay eggs.

The ticks become infected with B. burgdorferi after the first or second blood meal as larvae or nymphs.

Ticks often acquire B. burgdorferi from white-footed mice.

Ticks must be involved for transmission to mammals.

Ticks must be attached for at least 24 hours to transmit disease.

The nymph stage is most likely to transmit disease because they are small and less likely to be seen.

Clinical signs in horses:

Low-grade fever
Stiffness or multi limb lameness
Swollen Joints
Lethargy and behavioral changes

Diagnosis if the horse has clinical signs of Lyme disease:

Rule out other diseases
Blood test for antibodies (>300 Kela units) and a positive Western Blot
PCR (polymerase chain reaction) testing on joint synovial tissue - INVASIVE

Reasons for a low titer:

Not infected
Infected in past but cleared infection
Infected but hasn’t mounted an immune response yet (takes 40-60 day)

We don’t know how long it takes for a horse to develop clinical signs after being bitten by an infected tick. Could be days to weeks – every horse is different.

Treatment:

Prolonged – one month or more
Tetracycline – must be given intravenously once daily
Doxycycline – Must be given twice daily orally (not absorbed well)