ISSN 2398-2977      



Synonym(s): Lyme Disease, Borrelia burgdorferi


  • Cause: tick-borne infection with Borrelia burgdorferi  Borrelia burgdorferi, commonly known as Lyme disease. Horses are end-stage incidental hosts.
  • Proliferation at tick attachment site within connective tissue → bacteremia and localization elsewhere in body, especially synovial membranes and tendons.
  • Signs: highly variable and not pathognomonic: pyrexia, lymphadenopathy (local to attachment or widespread), swollen joints, shifting lameness, hyperesthesia, behavioral changes.
  • Diagnosis: difficult as many asymptomatic horses are seropositive. Diagnosis based on combination of geographical area, clinical signs, serologic testing and response to treatment.
  • Treatment: oxytetracycline Oxytetracycline or doxycycline + NSAIDs Therapeutics: anti-inflammatory drugs as first line. If non-responsive minocycline, ceftiofur Ceftiofur or even metronidazole Metronidazole can be used.
  • Prognosis: guarded for neuroborreliosis, variable for non-specific clinical signs.



  • Spirochete: Borrelia burgdoferi Borrelia burgdorferi.
  • Virulence can vary depending on certain variable detectable antigens including outer surface proteins (OSP) A and B.

Predisposing factors


  • 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. 


  • Sustained feeding by a tick infected with Borrelia burgdorferi over a number of hours.


  • 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 occurring most commonly.
  • Localization in joints results in arthritis (weeks or years after the acute phase of the disease).


  • Local infection occurs within hours of tick attachment.
  • Due to a paucity of information the period until clinical signs occur is not known.
  • Arthritis may not occur until weeks or years after the acute phase of the disease.


  • Risk of infection from ticks is highest in the spring, early summer and autumn, in areas of woodland and forest where the vector and host most commonly live.
  • Larval ticks hatch and feed on infected rodents, then transmit the infection to a new host. Deer are the predominant host for adult ticks, with horses acting as incidental hosts.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Divers T J et al (2018) Borrelia burgdorferi infection and Lyme disease in North American horses: a consensus statement.  J Vet Int Med 32 (2), 617-632 PubMed.
  • Wagner B, Goodman L B, Rollins A & Freer H S (2013) Antibodies to OspC, OspF and C6 antigens as indicators for infection with Borrelia burgdorferi in horses. Equine Vet J 45 (5), 533-537 PubMed.
  • Priest H et al (2012) Diagnosis of Borrelia-associated uveitis in two horses. Vet Ophthal 15 (6), 398-405 PubMed.
  • Durrani A Z, Goyal S M & Nadeem Kamal (2011) Retrospective study on seroprevalence of Borrelia burgdorferi antibodies in horses in Minnesota. J Eq Vet Sci 31 (8), 427-429 VetMedResource.
  • Metcalfe K B et al (2008) The prevalence of antibodies against Borrelia burgdorferi found in horses residing in the Northwestern United States. J Eq Vet Sci 28 (10), 587-589 VetMedResource.
  • Divers T J et al (2001) Lyme disease in horses. Comp Cont Educ 23 (4), 375-381 VetMedResource.
  • Manion T B et al (1999) Viable Borrelia burgdorferi in the urine of two clinically normal horses. J Vet Diagn Invest 10 (2), 196-199 PubMed.
  • Chambers M A et al (1996) Novel indirect fluorescent antibody test for Lyme disease. J Vet Diagn Invest (2), 196-201 PubMed.
  • Hahn C N et al (1996) A possible case of Lyme borreliosis in a horse in the UK. Equine Vet J 28 (1), 84-88 PubMed.
  • Carter S D et al (1994) Borrelia burgdoferi infection in UK horses. Equine Vet J 26 (3), 187-190 PubMed.
  • Rees D H et al (1994) Lyme disease - a rare but clinically important disease in the UK. Equine Vet J 26 (3), 175-177 PubMed.
  • Madigan J E (1993) Lyme disease (Lyme borreliosis) in horses. Vet Clin North Am Equine Pract (2), 429 PubMed.
  • Browning A et al (1993) Lameness associated with Borrelia burgdorferi infection in the horse. Vet Rec 132 (24), 610-611 PubMed.
  • Parker J L et al (1992) Lyme borreliosis in cattle and horses - a review of the literature. Cornell Vet 82 (3), 153-174 PubMed.
  • Lindenmayer J et al (1989) Borrelia burgdorferi infection in horses. JAVMA 194 (10), 1384 PubMed.

Other sources of information

  • Wagner et al (2011) A New Sensitive Lyme Multiplex Assay to Confirm Neuroborreliosis in Horses: A Case Report. In: Proc 57th AAEP Convention. pp 70-75.
  • Johnson A C (2010) Lyme Disease in Horses. In: Proc North Am Vet Conference. pp 170-171.
  • Divers T J (2004) Lyme Disease in the Horse: Experimental Studies. In: Proc 43rd BEVA Congress. Equine Vet J Ltd, UK. pp 170.
  • Divers T J, Chang Y F & McDonough S P (2003) Equine Lyme Disease: A Review of Experimental Disease Production, Treatment Efficacy and Vaccine Protection. In: Proc 49th AAEP Convention. pp 391-393.

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