Equine Veterinary Journal ISSN 0425-1644 DOI: 10.1111/evj.12122

Is Lyme disease overdiagnosed in horses?

Lyme disease, caused by the bacterial spirochete Borrelia burgdorferi and first recognised in humans in Lyme, Connecticut in 1975 [1], is the most common tick-borne disease in the northern hemisphere [2]. Borrelia infection of horses was first identified by positive serological testing in 1985 [3]. This pathogen can infect humans, dogs, cattle, horses and wild mammals without obvious clinical signs. There are at least 3 strains of the organism that cause infection, including several different species and multiple variants of each species [4]. Borrelia organisms are transmitted to mammals via exposure to ixodid ticks, including Ixodes scapularis and Ixodes pacificus in the eastern and western USA, respectively, and Ixodes ricinicus in Europe. In the USA, the white-footed mouse is responsible for harbouring the spirochete and the white-tailed deer keeps the population of ixodid ticks plentiful. The highest seroprevalence in humans in the USA is in the northeast, with the highest incidence of Lyme disease in the eastern and midwestern states, while seroprevalences in some areas of North America are very low [4]. Although the seroprevalence of Lyme disease in horses in the USA is not known, it is likely to be similar or higher than in humans due to increased exposure to ticks and prolonged tick attachment. In 2000, an article in JAVMA reported that 40% or more of horses in the northeastern USA were positive for exposure to B. burgdorferi based on serological testing [5]. Infection in horses is common, but determining the extent of clinical disease is challenging because most seropositive horses display few clinical signs, thus making equine Lyme disease controversial. Although the nymph stage of the tick is responsible for transmission of Borrelia species in humans, it is the adults that are likely to cause infection in horses and other mammals. The Borrelia organism lives in the gastrointestinal tract of the tick. Ticks must be attached and feeding for 24–48 h in order to transmit the organism into the host. The amount of time for which the tick is attached and feeding, as well as a complicated series of events, determines successful infection of the host. Research has shown that to ensure the successful transmission of the organism changes must occur in surface proteins, including a downregulation of OspA (outer surface protein A) and upregulation of OspC and OspF [4]. This functions to protect it from the host immune system and ensure its survival. Hiding out in connective tissue and synovial membranes may also facilitate survival in the host, allowing for the lack of need of iron for its survival [4]. Clinical signs attributed to Lyme disease in horses are typically nonspecific and have included arthritis, uveitis, encephalitis, lameness with synovitis (swollen joints, joint pain), limb oedema (more likely to be due to Anaplasma rather than Borrelia infection), dermatitis, muscle tenderness, fever, fatigue and chronic weight loss [4]. The most common clinical signs include multilimb or shifting limb lameness, stiffness, muscle soreness, hyperaesthesia, lethargy and behavioural changes. Joint effusion was once thought to be a component of Lyme disease in horses but it is rarely seen now, although it is common in the human form of the disease [4]. Polyarthritis associated with Borrelia infection in horses was first reported in 1986 [6]. It was diagnosed based on high indirect fluorescent antibody titres for B. burgdorferi in serum and synovial fluid in a 12-year-old Shetland pony with a history of chronic lameness and bilateral carpal joint effusion [6]. Previously, there were 2 reported cases of equine uveitis caused by Borrelia based on clinical signs, serology, culture and PCR and/or silver staining of biopsied tissue [6,7]. More recently, bilateral uveitis was reported in 2 horses with Borrelia infection of the eye, in which the diagnosis was made by identification of the spirochete within ocular fluids and then confirmed by positive PCR [7]. It is becoming a common practice to test for Borrelia as well as Leptospira in acute cases of uveitis in horses located in Lyme endemic areas. The first case of Borrelia-associated cutaneous pseudolymphoma was reported in a horse in 2011 [2]. Although dermatological disease is commonly reported in humans with Lyme disease, skin disease has not been reported in horses until this case [2]. A 10-year-old Warmblood mare developed dermal papules on the ventrolateral aspect of the right Equine Veterinary Journal 45 (2013) 529–530 © 2013 EVJ Ltd

masseter, associated with engorged lymphatics at the periphery of the lesions [2]. An engorged adult tick had been removed from this site 3 months before. Histopathology and immunohistochemistry were used to diagnose the lesions as a form of pseudolymphoma, and PCR was positive for Borrelia [2]. Experimental infection of ponies with B. burgdorferi caused consistent microscopic lesions in the skin in the area where the ticks were attached. These microscopic skin lesions were similar to those seen in this case. Complete resolution of the lesions was seen with a 30 day course of doxycycline [2]. Several cases of neuroborreliosis have been documented in horses [4,8]. Signs have included muscle wasting, ataxia, back pain, severe neck stiffness and pain, and cranial nerve deficits [9]. In some of these cases, a diagnosis was made on necropsy examination demonstrating chronic necrosuppurative-to-nonsuppurative, perivascular-to-diffuse meningoradiculoneuritis [9]. Spirochetes were identified by Steiner silver impregnation in the dura mater of the brain and spinal cord [9]. Experimental infection of ponies with Borrelia resulted in sporadic lesions in muscle, fascia, nerves and perisynovial tissues; however, no clinical signs were observed in these ponies despite finding these lesions [4]. In one case of neuroborreliosis in which the neck was the most severely affected site, the horse was depressed and painful and unable to lift or hold its head, so that its muzzle was no more than 0.5 m from the ground. A diagnosis was made by ruling out other possible causes and by comparing antibody levels in the serum and cerebrospinal fluid with the enzyme-linked immunosorbent assay and Western blot techniques and demonstrating the likelihood of antibody production within the central nervous system; however, no organism was identified in the cerebrospinal fluid. Exposure of a horse to the Borrelia organism is easily determined in most cases with serological testing. Trying to determine whether exposure also indicates infection and clinical disease, however, is not so easy in horses. Previously, the indirect fluorescent antibody test and the enzyme-linked immunosorbent assay were favoured tests for diagnosing equine borreliosis, and equivocal results were confirmed by the western blot. Currently, the tests that are available include the IDEXX SNAP 4Dx Plus (canine heartworm antigen-Anaplasma phagocytophilumplatys-Borrelia burgdorferi-Ehrlichia canis-ewingii antibody test kit) test, a qualitative test for dogs that detects serum antibodies to the C6 antigen. It is commonly used among equine practitioners as a screening test for Lyme disease in horses, although it has not been validated for use in horses. As a screening test, if the results are interpreted in horses according to the manufacturer’s instructions and the same way as they are in dogs, any colour change in the B. burgdorferi antibody-indicator spot indicates a positive result. Vaccination should not cause a positive SNAP test result. In one study performed by Johnson and colleagues to test the use of the SNAP test in horses, the test kits had fair sensitivity (63%) and very high specificity (100%) [10]. Wagner and colleagues maintain that although this test has been used successfully in horses, there are still false-negative and false-positive results [11]. Thus, the recommendation for use of the SNAP 4Dx is to confirm a positive result with another test method [11]. It is this author’s diagnostic plan to use the SNAP test as a screening test and to follow-up positive test results with a Lyme multiplex test. The equine Lyme multiplex test is a fluorescent bead antibody test that detects and quantifies antibodies to the surface protein antigens OspA, OspC and OspF [11]. Based on the information obtained from this test, horses are thought to have been vaccinated (high OspA), recently infected (high OspC) and/or suffering from chronic infection (high OspF) [11]. Other specific information gained includes evidence that high OspC levels are found in early infection but followed by a decline several months later, and high OspF levels may indicate chronic infection or simply long-lasting antibodies rather than a true chronic infection [11]. The challenge remains of how to make a clinical diagnosis of Lyme disease in horses. According to Divers and colleagues, Lyme disease should be diagnosed based on the following factors: 1) possible exposure to B. burgdorferi-infected


Lyme disease

ticks, 2) clinical signs compatible with Lyme disease, 3) the absence of other causes of disease, 4) high antibody levels to B. burgdorferi [10] and 5) in some cases, identification of the organism or compatible cytology/pathology (Divers, personal communication). Is Lyme disease overdiagnosed in horses? The answer is probably, possibly and who knows? Many clients are convinced that their horses have Lyme disease, and many equine veterinarians treat horses for Lyme disease. One of the problems is that a positive response to treatment cannot always be used as a diagnostic aid, because the most commonly used antibiotics, including doxycycline, oxytetracycline and now minocycline, have anti-inflammatory properties. Clinical signs improve or resolve, so many horses are assumed to have been treated successfully for Lyme disease; hence, the dilemma continues. Divers maintains that until clinical signs can be reproduced experimentally, the association between infection and clinical disease in many horses will remain speculative [4]. J. Bartol New England Equine Medical and Surgical Center, New Hampshire, USA

References 1. Steere, A.C. (2001) Lyme disease. N. Eng. J. Med. 345, 115-125. 2. Sears, K.P., Divers, T.J., Neff, R.T., Miller, W.H. Jr and McDonough, S.P. (2011) A case of Borrelia-associated cutaneous pseudolymphoma in a horse. Vet. Dermatol. 23, 153-156.


J. Bartol

3. Marcus, L.C., Patterson, M.M., Gilfillan, R.E. and Urband, P.H. (1985) Antibodies to Borrelia burgdorferi in New England horses: serologic survey. Am. J. Vet. Res. 46, 2570-2571. 4. Divers, T.J. (2006) Lyme disease. In: Equine Infectious Diseases, 1st edn., Eds: D.C. Sellon and M.T. Long, W.B. Saunders, Philadelphia. pp 310-312. 5. Magnarelli, L.A., Ijdo, J.W., Van Andel, A.E., Wu, C., Padula, S.J. and Fikrig, E. (2000) Serologic conformation of Ehrlichia equi and Borrelia burgdorferi in horses from the northeastern United States. J. Am. Vet. Med. Assoc. 217, 1045-1049. 6. Burgess, E.C., Gilette, D. and Pickett, J.P. (1986) Arthritis and panuveitis as manifestations of Borrelia burgdorferi in a Wisconsin pony. J. Am. Vet. Med. Ass. 189, 1340-1342. 7. Priest, H.L., Irby, N.L., Schlafer, D.H., Divers, T.J., Wagner, B., Glaser, A.L., Chang, Y.F. and Smith, M.C. (2012) Diagnosis of Borrelia-associated uveitis in two horses. Vet. Ophthalmol. 15, 398-405. 8. James, F.M., Engiles, J.B. and Beech, J. (2010) Meningitis, cranial neuritis, and radiculoneuritis associated with Borrelia burgdorferi infection in a horse. J. Am. Vet. Med. Ass. 237, 1180-1185. 9. Imai, D.M., Barr, B.C., Daft, B., Bertone, J.J., Feng, S., Hodzic, E., Johnston, J.M., Olsen, K.J. and Barthold, S.W. (2011) Lyme neuroborreliosis in two horses. Vet. Pathol. 48, 1151-1157. 10. Johnson, A.L., Divers, T.J. and Chang, Y.F. (2008) Validation of an in-clinic enzyme-linked immunosorbent assay kit for diagnosis of Borrelia burgdorferi infection in horses. J. Vet. Diag. Invest. 20, 321-324. 11. Wagner, B., Goodman, L.B., Rollins, A. and Freer, H.S. (2013) Antibodies to OspC, OspF and C6 antigens as indicators for infection with Borrelia burgdorferi in horses. Equine Vet. J. 45, 533-537.

Equine Veterinary Journal 45 (2013) 529–530 © 2013 EVJ Ltd

Is Lyme disease overdiagnosed in horses?

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