311Chapter 33 Lyme Disease

33 

Lyme DiseaseThomas J. Divers

C H A P T E R 

Etiology

Lyme disease is caused by at least three strains of the spirochete Borrelia burgdorferi sensu lato complex,1,2 which includes several worldwide species and multiple variants of each species. The North American strain is B. burgdorferi sensu stricto,3 which may have several strain variations.4 In Europe, B. afzelii and B. garinii are responsible for most cases of Lyme disease.5 In Asia, B. garinii is most common. Borrelia burgdorferi organisms are helical-shaped, gram-negative, unicellular spirochetes with fla-gellar projections.6 Borrelia burgdorferi bacteria are not free-living organisms and quickly die outside a host. They are maintained in a 2-year enzootic life cycle that involves ixodid ticks (Ixodes scapularis in the eastern United States and I. paci-ficus on the west coast of North America) and mammals.6 The white-footed mouse, which provides a continual source of the spirochete, and deer, which maintain the tick vector, are the most common mammals involved in maintaining the life cycle of this spirochete (Fig. 33-1).

Epidemiology

The seroprevalence of B. burgdorferi in horses in the United States is not known but is likely much higher than in humans but with

similar geographic distribution (Fig. 33-2). The mid-Atlantic and northeastern states have a high seroprevalence, as do areas of Minnesota and Wisconsin extending into southern Canada. Sero-positive horses are rare in the Rocky Mountain states, the Dakotas, Nebraska, and some other areas of the United States. In one New England survey, 45% of horses had Borrelia antibod-ies.7 In a Wisconsin study, 118 of 190 horses were serologically positive.8 In central Europe, there is a high level of antibody positive horses.9-11 Lyme disease in humans seems to be more common in the northern hemisphere but does occur in the southern hemisphere, including Brazil and Australia.

Infection in horses is caused by attachment and prolonged (>24 hours) feeding of infected adult Ixodes spp. ticks. Female ticks are likely the competent vector for horses and can be identified by the complete arch over the anus (see Fig. 33-1). Adult males rarely feed. Nymphs are responsible for a high percentage of infections in humans because they are small and often escape visual inspection.5 It is not known if the immature stages transmit the spirochete to horses. Ixodes scapularis adults are most active in the fall and throughout winter when tem-peratures are above freezing. Once feeding begins, the organism begins its complicated up-and-down regulation of genes to enhance survival in the host. Exact pathogenesis of Borrelia in the horse is not known. After experimental infection of ponies, the organism appears to reside mostly in skin near the tick bite, as well as in connective tissue and muscle and around nerves and blood vessels near synovial membranes.13 A lymphocytic

312 Section 3 Bacterial and Rickettsial Diseases

The organism lives in the tick gut and is transferred to animals during blood meals. Generally, 24 to 48 hours of attach-ment is required to transfer the organism successfully from the tick to the mammalian host.12 This time may be needed for the organism to downregulate an outer membrane protein (OspA), which may be important to maintain survival in the mammalian host.15 Conversely, other surface proteins (e.g., OspC, OspE, OspF) that are in low concentration in the tick gut are

plasmacytic reaction may occur within these tissues, and in experimentally infected ponies, this reaction was associated with the highest concentration of the Borrelia organism.13,14

The species of Borrelia may have predilection for causing disease in certain body sites. In human borreliosis, Borrelia afzelii may be mostly associated with skin disease, whereas B. garinii is more neurotropic and B. burgdorferi is the species to most commonly cause joint disease.

Figure 33-1 Two-year enzootic cycle of Borrelia burgdorferi and distinguishing features of Ixodes species ticks. Inset, Female ventral abdomen.

Eggs deposited

Larva feeds

Infectedhost

Cycle maintainingspirochetes in nature

Uninfectedhost

Nymph feeds

Adultfeeds

AnusAnalgroove

Spring

Winter

Winter

Summer

Summer

Spring

Fall

Fall

Figure 33-2 Risk of Lyme disease in the United States. (Courtesy Centers for Disease Control and Prevention, Atlanta.)

Legend

No risk

Low risk

Medium risk

High risk

313Chapter 33 Lyme Disease

with marked muscle wasting, ataxia, and depression. No cere-brospinal fluid (CSF) was evaluated, but, on necropsy, spiro-chetes were identified by Steiner silver impregnation in both cases, predominantly in the affected dura mater of brain and spinal cord. No spirochetes were identified by Silver stain within the neural parenchyma or within neurogenic atrophied muscle. Borrelia burgdorferi sensu stricto was identified by poly-merase chain reaction (PCR) with highest spirochetal burdens in tissues with inflammation, including spinal cord, muscle, and joint capsule. In another report, a horse with severe neck stiff-ness that progressed to ataxia had lymphohistiocytic meningitis and B. burgdorferi deoxyribonucleic acid (DNA) in the CSF.31 That horse had originally responded to doxycycline treatment but relapsed after discontinuing the treatment. Another case in a Thoroughbred hunter that presented for lameness and ataxia had lymphocytic pleocytosis and Borrelia PCR-positive CSF. The horse responded well to doxycycline but had some deterio-ration when treatment was discontinued.33

Two other suspect horses the author examined had ataxia and severe lymphocytic infiltration of the meninges. There are other published reports of neurologic dysfunction in a horse attributed to Lyme disease.25,26 From these accumulated reports, it would appear that ataxia and lumbar muscle wasting caused by lymphohistiocytic meningitis and radiculoneuritis with occa-sional fasciculations and neck stiffness are characteristic of neu-roborreliosis in the horse. Cerebrospinal fluid (CSF) would likely have a lymphocytic pleocytosis and might be PCR posi-tive for Borrelia burgdorferi. In another case report, panuveitis and arthritis was reported in a pony.34 Recently, bilateral uveitis in two horses was found to be the result of Borrelia infection of the eye.35 Borrelia was found on cytologic examination and confirmed by PCR in the vitreous in both horses. No organisms were observed in the aqueous, although one aqueous sample was PCR positive. Borrelia was also observed with silver stain in the inflamed uveal tissue. In addition to the uveitis, one of the horses had a history of hyperesthesia and the other had muscle wasting over the topline. A chronic multifocal lympho-histiocytic ganglioradiculitis and neuritis with presumptive neu-ronal degeneration in the spinal nerves was found in the second horse along with inflammatory lesions in multiple organ systems, including the heart, lung, liver, spleen, kidney, skin, skeletal muscle, and nervous tissue. Inflammatory cells were predomi-nantly lymphocytes though often accompanied by plasma cells, histiocytes, and/or neutrophils. The distribution of the infiltrates was often within connective tissues or centered on blood vessels. Another report describes a horse with multiple lymphohistio-cytic cutaneous nodules over the masseter muscle 3 months following removal of a tick (Fig. 33-4); this horse was confirmed as having Lyme pseudolymphoma. The horse responded com-pletely to doxycycline treatment as might be expected for a cutaneous form of borreliosis.36

The high fever and limb edema typically reported in associa-tion with Borrelia seroconversion are most often the result of Anaplasma phagocytophila infection because many ticks are con-comitantly infected with both Borrelia and A. phagocytophila.37

Experimental infection of ponies caused consistent lesions in the skin and inconsistent lesions in muscle, fascia, nerves, and perisynovial tissues, but clinical signs were not observed. Except for the lymphocytic/histiocytic dermatitis, the pathol-ogy has been mild in most of the experimentally infected ponies. Until clinical signs can be experimentally reproduced, the association between B. burgdorferi infection and clinical disease in many horses will remain speculative and no doubt somewhat controversial.

The diagnosis of Lyme disease is most common in sport horses, but this might be because subtle clinical signs of stiffness and hyperesthesia are most easily recognized in sport horses and may not be related to any genetic predisposition for disease.

upregulated, particularly OspC in the tick salivary gland, to enhance complement resistance and other methods of immune evasion in the mammalian host.16 The changes in expression of surface proteins may be triggered by the blood meal. Other proteins, such as the C6 peptide found in a dominant invariable region of the variable major protein like gene VisE, permit antigenic variation, ensuring survival in the host.17 Many other components of the agent may be important for infection but these are not entirely understood.18 Borrelia burgdorferi may also survive in the host by residing in collagen and connective tissue likely related to low-molecular-weight decorin-binding pro-teins, having no requirement for iron and by possibly forming resistant cysts within the host.19-21 Persistent infection, espe-cially following antibiotic treatments, has been very controver-sial in humans, and most studies do not support any such syndrome of chronic infection/chronic Lyme disease following appropriate antibiotic therapy.22,23 Conversely, a recent article provided evidence that both a cyst (round bodies) and biofilm-like colonies exist which have some potential to cause chronic disease.24

Clinical Findings

A wide variety of clinical signs have been attributed to Borrelia infection in horses, but demonstration of cause and effect has been difficult to document in most cases. Clinical signs most often attributed to equine Lyme disease include stiffness and lameness in more than one limb, muscle tenderness, hyperes-thesia, lethargy, and behavioral changes.25-30 Unlike human Lyme disease, joint effusion has been minimal in most Lyme-suspect horses. Muscle wasting and pain over the thoracolum-bar area have been present in a few horses with high serum titers (Fig. 33-3).

There have been several cases of confirmed or suspected neuroborreliosis. In one report, two horses were diagnosed with Lyme neuroborreliosis and both had chronic, necrosuppurative-to-nonsuppurative, perivascular-to-diffuse meningoradiculo-neuritis on necropsy examination.32 Hyperesthesia, lumbar pain, and muscle wasting were the initial clinical findings fol-lowed by ataxia of all four limbs, facial nerve paralysis, and finally, head tremors with depression in one horse. The second horse had a 3-year history of asymmetric muscle wasting and facial paralysis that had not responded to equine protozoal myeloencephalitis (EPM) treatment and was even tually diag-nosed as a peripheral neuropathy that responded to dexametha-sone, but, after discontinuing the treatment, the horse relapsed

Figure 33-3 Muscle wasting and pain over the thoracolumbar area in horse with high serum titer.

314 Section 3 Bacterial and Rickettsial Diseases

vaccination or from cross-reacting flagellar antigen exposure. In experimentally infected ponies, the Western blot assay was posi-tive at 8 to 10 weeks after infection, versus 6 to 8 weeks for the ELISA.13 Recently, a multiplex antibody bead test for OspA, OspC, and OspF quantitative antibody detection has been used in the serodiagnosis of equine Lyme disease.42 The concept is that high levels of OspA would suggest vaccination, OspC would indicate recent infection (antibody against this antigen typically increases with early infection and then declines after 3-4 months), and OspF would suggest either chronic infection or more long-lasting antibodies. OspA antibodies have been found in a higher percentage of nonvaccinated horses with the multiplex assay than were detected by Western blot (WB) testing. The clinical significance of this finding and clinical prac-tice value of the multiplex assay in determining time of infec-tion, active versus past infection, and response to treatment could be significant but may require further investigations with a large number of field cases or ideally with experimentally infected and treated horses.

Clinical signs were not obvious in ponies experimentally infected with Borrelia, and a large number of apparently asymp-tomatic horses are seropositive; therefore clinical diagnosis is difficult and should be based on clinical probability of Lyme disease (knowledge of most common anatomic location of the organism in the horse, reported clinical syndromes), ruling out other diseases, and laboratory testing. Polymerase chain reaction and/or histopathology (usually a lymphoplasmacytic or histio-cytic inflammation) on CSF or tissues can be helpful. To com-plicate the relationship between infection and clinical signs further, the most common drugs (oxytetracycline, doxycycline, and now minocycline) used to treat Borrelia in the horse have antiinflammatory properties that may alleviate musculoskeletal pain in horses, thus preventing response to therapy from being used as a diagnostic test.43

Pathologic Findings

Lesions in experimentally infected horses are mostly limited to a lymphohistiocytic reaction in the skin surrounding the site of the tick attachment and lymphoid hyperplasia of regional lymph nodes.13,14,38 One experimental pony had a mononuclear cell reaction in cutaneous muscle and the panniculus, especially surrounding small arteries and nerves. Perivascular, perineural mononuclear inflammation is reported in human patients with Lyme disease.44 One pony had a nonsuppurative synovitis with mononuclear subsynovial perivascular infiltration. Perivascular mononuclear cell aggregates also formed around small arteries adjacent to the perineurium of peripheral nerves, including the ulnar, facial, sciatic, labial, and fibular nerves and the dorsal spinal nerve roots. A perivascular mononuclear inflammation was also present in skeletal muscle in multiple areas.44 All these changes have been reported in human patients with Lyme disease, and the association of these lesions with Lyme disease in experimental ponies is strengthened by the observation that all lesions in the one pony were most severe on the side of the body where the ticks were attached.44 Pathologic findings in case reports include lymphohistiocytic inflammation of synovial membranes, meninges, nerve roots, and uveal tissue; in many cases, this inflammation is around blood vessels or nerves.

Therapy

The three most frequently used drugs for treatment of Lyme disease in horses are intravenous (IV) tetracycline and oral (PO)

Diagnosis

The diagnosis of exposure to Borrelia can usually be determined by serology but to definitively diagnose that a horse is currently infected is more difficult and to determine whether clinical disease is associated with Borrelia is extremely difficult. Enzyme-linked immunosorbent assay (ELISA) or immunofluorescent antibody (IFA) testing have been the preferred screening tests for detection of antibodies indicating exposure. If the ELISA value is not greater than the laboratory control, it suggests no exposure, distant exposure but no current infection, or recent infection (within 1-2 months).13 A repeat sample taken in 2 months that remains negative would help rule out recent infec-tion.13 Experimentally infected ponies successfully treated with antibiotics (no organisms found at euthanasia) had titers that decreased to preinfection ELISA units 4 months after treatment was initiated.38 Antibiotic-treated ponies that remained infected at euthanasia had some decrease in titer during antibiotic treat-ment, but never as low as preinfection, and had a rebound increase in titer after discontinuing antibiotic therapy.17 In field-infected and treated cases, this same magnitude of decline in ELISA antibody level is rarely seen. The value of the C6 SNAP test (IDEXX Laboratories, Westbrook, ME), based on antibody to a peptide that reproduces the sequence of the invariable region 6 (an immunodominant, conserved region), has good correlation with the ELISA in horses; specificity is better than sensitivity.39 Vaccination should not cause the C6 to be positive. In humans the C6 antibody cannot always be used to assess treatment outcome or the presence of active infection but has comparable sensitivity to Western blot for diagnosing Lyme disease.40,41

The principal value of the Western blot assay is both in separating horses that are ELISA positive because of natural infection from horses that are ELISA positive because of

Figure 33-4 Horse with multiple lymphohistiocytic cutaneous nodules.

A

B

315Chapter 33 Lyme Disease

were not as effective in the experimental pony study (e.g., ceftiofur [Naxcel, Pharmacia & Upjohn, Kalamazoo, MI]).38 If IV tetracycline is administered as treatment, renal function should be monitored since tetracycline-induced acute renal failure can develop in horses. Use of a tetracycline class drug in pregnancy could result in abnormal fetal development.

Prevention

The means for prevention of Lyme disease in endemic areas may include (1) the prevention of tick exposure or prolonged (>24 hours) attachment, (2) the provision of early antimicrobial treatment after known Ixodes exposure, or (3) vaccination. Various insecticidal sprays can be used to prevent tick infection, but most are not approved for use in horses and efficacy in the horse is unproven. Currently, no adverse effects are known to result from use of the more common canine tick sprays (e.g., fipronil [Frontline, Merial, Duluth, GA]; Advantix [Bayer, Shawnee Mission, KS]) in the horse. Permethrin-based insecti-cides are approved for use in horses, but efficacy against ticks is not well documented. Any use of insecticides approved for dogs and close observation for ticks should be performed most diligently in late summer, fall, and early winter (after fly season) because this is the most common time for adults to attach. If ticks are found on a horse, they can be examined to determine if they are Ixodes spp. (see Fig. 33-1), the only genus of North American tick known to transmit B. burgdorferi.

Ponies were completely protected against experimental infection by vaccination with a recombinant OspA antigen vaccine when infected ticks were attached 92 days after the third vaccination.14 Duration of protection in the horse, adverse effects, and efficacy of the currently available canine vaccines are unknown. Differentiation of natural exposure antibody from an OspA vaccine antibody response can often be made by either immunoblot testing, multiplex PCR, or C6 SNAP test. Several canine-approved Lyme vaccines are available and are commonly used in horses. All could result in production of protective antibodies, and the author is not aware of adverse effects being reported. The proper dose and frequency of administration are undetermined, but in the experimental vaccine study, neutralizing antibodies declined significantly within 5 months after the last vaccine inoculation. The decision to vaccinate is complicated because of the belief of many that the incidence of clinical disease in exposed horses is low. Clini-cal disease in high level sport horses appears to be more common, and vaccination has therefore been used frequently in that population of horse. Although vaccination following prior infection is more questionable, it is possible that horses can become reinfected if they do not develop or maintain pro-tective antibody (i.e., antibody against OspA).

Public Health Considerations

There are minimal public health considerations associated with equine Lyme disease. A significant number of Lyme-infested horses with clinical signs of lameness or behavioral changes are positive for spirochetemia or spirocheturia (53% and 20%, respectively, in one study28), but it is considered unlikely that adult ticks feeding on horses would move to humans. Sero-prevalence in horses in an area might be an important sentinel for risk of human Lyme disease in the same area.

The complete reference list is available online at www. expertconsult.com.

doxycycline or minocycline. In experimental ponies, IV tetra-cycline was more effective than PO doxycycline for eradicating Borrelia.38 Intravenous tetracycline obtains much higher tissue concentrations than PO doxycycline, which has low bioavail-ability after per os administration in the horse.45 If blood con-centrations were equal, one would expect doxycycline to be the preferred drug because of better volume of distribution. Doxy-cycline cannot be administered by the IV route to horses because of potential adverse effects.46 Tetracycline should not be given orally to horses because of low bioavailability and risk of active drug reaching the colon and causing diarrhea (unlike PO doxycycline, where much of the unabsorbed drug is inactive in the colon).47 Minocycline has better oral bioavail-ability than doxycycline in the horse, and because it is less protein bound, it attains higher concentration in CSF and aqueous fluids than does doxycycline in the horse.48 In humans, doxycycline treatment for only a few days for cutaneous Lyme disease or for 2 months for lameness or neuroborreliosis is still considered by many the gold standard for therapy; most reports suggest this treatment eliminates the infection.5 Because of the low bioavailability of doxycycline in the horse and possibility of more long-term infection in horses than humans prior to treatment, this protocol may not be as successful in the horse. Group III cephalosporins are sometimes used for neuroborrelio-sis in humans but may not be better than doxycycline treat-ments.5 Penicillin and amoxicillin are sometimes used for cutaneous Lyme diseases but are probably inferior to doxycy-cline.5 In the one report on a cystic form of Borrelia, doxycycline was not effective against this form in vitro but metronidazole was moderately effective.24 In that same report, the biofilm colonies were resistant to both doxycycline and metronida-zole.24 In both humans and horses the organism is resistant to fluroquinolones.49

The exact dose and the frequency and duration of therapy with tetracycline and doxycycline in the treatment of Borrelia infection are not known. A common intensive treatment sce-nario in clinical practice has been to give IV tetracycline 6.6 mg/kg every 24 hours (q12h) for 7 to 10 days, followed by PO doxycycline 10 mg/kg q12h or PO minocycline 4mg/kg q12 for 1 to 2 months. Although IV tetracycline (5 mg/kg q24h for 28 days) was 100% effective in eradicating Borrelia from experimentally infected ponies (treatment initiated 4 months after infection) and causing ELISA titer to return to baseline,38 this same protocol has not been as effective in causing a decline in ELISA antibody in horses with naturally occurring infec-tions.50 It is unknown if this discrepancy is the result of treat-ment failure caused by prolonged field infections prior to treatment, reinfection, and/or molecular mimicry maintaining a persistently high serum antibody concentration. Even with aggressive antibiotic treatment protocol, ELISA titers may not decline remarkably before treatment is discontinued and may remain high for many months. Sensitivity of the WB, C6 SNAP test, and the Luminex assay in determining successful treatment have not been documented but all would be better than the ELISA. The C6 SNAP test is not quantitative in the horse, which makes changes more subjective. It is possible that cyst forms may be present in some chronically infected human or horse cases, and these are not killed by tetracyclines. Metronidazole may be effective against the cyst forms but is not effective against free-living Borrelia.51 In refractory cases, it may be reasonable (but with unproven efficacy) to treat with both a tetracycline family drug and metronidazole. In humans, a chronic immune synovitis occurs in approximately 20% of chronic infections even after successfully treating the infection.

Other classes of drugs used to treat Borrelia in humans have no practical advantages over tetracyclines (e.g., ampicillin), are toxic when given orally to adult horses (e.g., macrolides), or

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315.e2 Section 3 Bacterial and Rickettsial Diseases