CPD Article | EQUINE
Age predisposition
• All ages can be affected.
Sex predisposition
• None known.
• Breed/Species predisposition
• All horses can be affected.
• Signs more commonly reported in eventing/
performance/sports horses due to performancelimiting
effects.
Public health considerations
• No evidence of being a communicable disease to
humans, but risk of direct infection from infected
ticks in that geographical area.
Cost considerations
• Loss of use.
• Treatment.
Pathogenesis
Etiology
• Spirochete: Borrelia burgdoferi.
• Virulence can vary depending on certain variable
detectable antigens including outer surface
proteins (OSP) A and B.
Predisposing factors
General
• Presence of tick vectors and wild animal reservoir
hosts in endemic areas.
• Reservoir hosts vary according to geographical
regions, but include rodents, squirrels, birds, whilst
deer act as a host to maintain the tick population.
• Flies, mosquitoes and fleas have been shown to
contain the spirochetes but only speculated to
transmit the bacteria.
Specific
• Sustained feeding by a tick infected with Borrelia
burgdorferi over a number of hours.
Pathophysiology
• The horse is an incidental host for Lyme borreliosis.
• Although frequently considered the host for the
spirochete, deer are a dead end host, showing no
apparent clinical signs and not acting as a reservoir
for further infection of ticks. They do act as a
reservoir for the tick themselves though.
• The intermediate host for the larval nymph stage
are rodents including the White-footed mouse
(Peromysius leukopus), grey squirrel and various
other birds and rodents.
• The infectious pathway involves the ixodid group
of ticks, especially Ixodes ricinus in the UK. Very
occasionally other species of tick or biting flies are
involved.
• The tick life cycle occurs over a 2 year period and
includes three stages.
• Other animals, eg horses, dogs, rabbits, birds, bats
and humans, become infected when bitten by a
stage of tick carrying the organism.
• The ticks often become infected during the larval
stages when feeding on rodents, the adult stages
usually feeding on the larger species, such as the
horse.
• Transfer of the organism requires a sustained period
of feeding. During this time, surface proteins on
the spirochete adapt to the vertebrate host. The
length of time for this lag period depends on the
genospecies of Borrelia burgdorferi.
• Infection of the horse occurs via tick bites → local
multiplication of the bacteria leads to a secondary
bacteremia and colonization of bacteria throughout
the body. Preferred migration of bacteria is through
connective, perineural and perivascular tissue in
skin, fasica, muscle and synovial membranes, with
localization in synovial structures and tendons
• Volume 22 Issue 02 | June 2020 •
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