Lyme Disease in Horses: What You Should Know

Lyme disease in horses is a tick-borne infection caused by spirochete bacteria of the Borrelia burgdorferi sensu lato complex. It is well documented across northern Europe and North America, and the United Kingdom sits inside the endemic range for the tick that carries it. What makes the condition so difficult in practice is not the bacterium — it is the gap between exposure and disease. Plenty of horses carry antibodies and never take a lame step because of them.

That gap is where most of the confusion, and most of the unnecessary antibiotic use, comes from. The reference point for this article is the 2018 ACVIM consensus statement on Borrelia burgdorferi infection and Lyme disease in North American horses (Divers et al., Journal of Veterinary Internal Medicine 32(2):617–632), which remains the most rigorous published review of what is genuinely established and what is merely repeated. Where UK practice differs — different tick, different Borrelia species, different licensed products — that is flagged explicitly.

This guide is intended to help you have a better-informed conversation with your vet. It is not a substitute for one.

What causes Lyme disease in horses

The Borrelia burgdorferi sensu lato complex

Borrelia burgdorferi sensu lato is an umbrella term for a group of spirochete bacteria that cause Lyme borreliosis. The species mix differs by continent, which matters more than most UK-facing articles admit. In North America, B. burgdorferi sensu stricto dominates almost exclusively. In Europe, including the UK, several genospecies circulate — B. burgdorferi sensu stricto, B. garinii and B. afzelii being the most clinically relevant.

Those species show different tissue preferences in human medicine: B. garinii is more strongly associated with neurological disease and B. afzelii with skin involvement. The practical consequence is that North American case series may not map perfectly onto UK horses. Almost all the equine evidence base — including the consensus statement above — is North American. That is a real limitation, and it is worth knowing when someone quotes a statistic at you.

How transmission happens

Ticks acquire Borrelia by feeding on infected small mammals and birds, which act as the reservoir. Horses are what is called a dead-end host: an infected horse does not go on to infect other ticks in any meaningful way, and Lyme disease is not contagious between horses, or from horse to human.

Transmission is not instantaneous. The spirochetes sit in the tick’s midgut expressing outer surface protein A (OspA), and only migrate to the salivary glands and switch to OspC expression once feeding is underway. Infection risk climbs the longer a tick stays attached, so prompt removal genuinely matters. But there is no safe window, and the widely repeated claim that a tick must feed for 24 to 48 hours before it can transmit is a North American figure, drawn from Ixodes scapularis carrying B. burgdorferi sensu stricto. It does not transfer cleanly to Britain. With the UK vector Ixodes ricinus, transmission begins earlier: in one experimental study roughly half of hosts had acquired a transmissible infection from nymphs removed at about 17 hours, and all were infected by 47 hours (Kahl et al., Zentralblatt für Bakteriologie, 1998). B. afzelii, one of the main genospecies circulating in Europe, is transmitted by I. ricinus earlier still than the North American species (Crippa, Rais and Gern, Vector Borne and Zoonotic Diseases, 2002). The ACVIM consensus statement itself says only that “several hours” are required. A daily check is still the highest-yield thing you can do, because it removes most ticks before the risk peaks; treat it as risk reduction rather than immunity, and take a tick you find this morning as seriously as one you find on day two. Our guide to preventing and treating ticks on horses covers the practical routine.

Where Borrelia goes once inside

After inoculation, spirochetes migrate through connective tissue rather than travelling far in the bloodstream. This is why blood-based direct detection performs so poorly, and why early infection is so hard to confirm. They preferentially localise in collagen-rich tissue — joint capsules, tendon sheaths, fascia, the nervous system and the eye — which explains why two horses on the same field, exposed to the same ticks, can present completely differently or not at all.

Which ticks carry Lyme disease in the UK

Ixodes ricinus — the UK vector

The principal UK vector is Ixodes ricinus, commonly called the sheep tick or castor bean tick. It is not fussy about hosts and will feed on deer, sheep, small mammals, dogs, horses and people. It needs humidity, so it is found in woodland, moorland, bracken, rough grazing, woodland edges and unmown field margins rather than short, dry, well-grazed turf.

The UK Health Security Agency runs a national Tick Surveillance Scheme that accepts submitted tick specimens from the public and vets, and publishes distribution data. If you are trying to establish whether your own area carries real risk, that scheme is a better source than anecdote — tick distribution in Britain has been changing, and reputations lag behind reality in both directions.

When UK tick season peaks

Tick questing activity in Britain is strongly seasonal and typically shows two peaks — a larger one in spring as temperatures rise, and a second in early autumn. Activity broadly runs from March to October, but mild winters increasingly produce questing ticks outside that window.

Recognising tick attachment on a horse

Ticks head for thin, warm, sheltered skin. On a horse the usual places are:

• Under the jaw and throatlatch
• Around and inside the ears
• The chest between the forelegs, and the inside of the elbows
• The inside of the thighs and the groin
• The udder or sheath
• Under the tail head and around the dock
• The mane and forelock, where they are easily missed

They feel like small, hard, smooth lumps. Size varies enormously with life stage and how long they have fed — a nymph is poppy-seed sized and genuinely easy to miss, while an engorged adult can be pea-sized or larger. A hand run over these areas at the end of a hack takes under a minute. Nodules that turn out not to be ticks are worth reading up on in our guide to the most common horse skin diseases, and biting-insect lesions are covered in our pieces on lice and mange.

Correct tick removal

1. Use fine-tipped tweezers or a purpose-made tick hook. Both are cheap; keep one in the grooming kit and one in the lorry.
2. Grasp the tick as close to the skin as possible, at the mouthparts, not the swollen body.
3. Pull steadily upward, or twist gently if using a hook designed for it. The aim is to lift the mouthparts out intact.
4. Clean the site and note the date. Watch it for a fortnight.
5. If your vet may want the tick identified, keep it in a sealed container.

Do not apply petroleum jelly, alcohol, nail varnish, or a lit match. Those methods stress the tick and can prompt it to regurgitate gut contents into the wound, which is precisely the thing you are trying to avoid. A retained mouthpart is a minor local irritation; a stressed, regurgitating tick is a genuine infection risk. If the site later becomes hot, swollen and painful rather than settling, that may be secondary bacterial infection — see cellulitis in horses.

Ixodes ricinus also transmits Anaplasma phagocytophilum. Equine anaplasmosis produces a much more clear-cut acute illness — fever, limb oedema, lethargy — and can occur alongside or instead of Lyme. UK laboratories including Liphook Equine Hospital commonly offer testing for both together, and that is sensible.

Signs of Lyme disease in horses: what is actually documented

This is the section where most online content goes wrong, so it is worth being precise about the difference between documented and reported.

The three well-documented syndromes

The ACVIM consensus statement is unambiguous. The best-documented naturally occurring syndromes attributed to B. burgdorferi infection in horses are:

1. Neuroborreliosis — neurological Lyme disease
2. Uveitis — inflammation inside the eye
3. Cutaneous pseudolymphoma — a nodular skin lesion at the bite site

Three, not four. Joint disease, shifting lameness and generalised stiffness are frequently listed elsewhere as established signs of equine Lyme disease; the consensus statement explicitly says there is little research and no overt clinical data to document generalised lameness and stiffness as an equine Lyme syndrome. That distinction matters clinically, because a stiff horse with a positive titre is far more likely to be stiff for another reason entirely.

Neuroborreliosis: neurological symptoms of Lyme disease in horses

This is the most serious documented presentation and the one that justifies aggressive treatment. Reported signs across published cases include:

• Atrophy of the epaxial (spinous) muscles
• Dysphagia — difficulty swallowing
• Laryngeal dysfunction, occasionally severe enough to cause respiratory distress
• Facial paresis and other cranial nerve deficits
• Spinal cord ataxia and paresis
• Behavioural change and dullness
• Hyperaesthesia — marked sensitivity to touch
• Muscle fasciculations
• Neck and back stiffness with pain

Two details from the published case series are worth holding onto. Fever is inconsistent and most often absent — so a normal temperature does not rule neuroborreliosis out. And the duration of neurological dysfunction before death in the reported fatal cases ranged from 2 to 730 days, with a median of 120 days. This is not usually a disease that declares itself overnight; it more often grumbles.

The clinical picture overlaps heavily with other neurological conditions, notably equine herpesvirus myeloencephalopathy and — in imported horses or those with travel history to the Americas — equine protozoal myeloencephalitis. Definitive diagnosis usually requires cerebrospinal fluid sampling.

Uveitis and the equine eye

Borrelia is one recognised trigger for uveitis in horses. Affected eyes show pain, tearing, photophobia, squinting, a constricted pupil and corneal cloudiness; chronic cases may develop cataract or retinal detachment. Notably, in published equine reports, 8 of 9 horses with Lyme-associated uveitis had neuro-ocular borreliosis — the eye and the nervous system tend to travel together.

Any painful, cloudy or weeping eye is an emergency. Call your vet the same day, regardless of what you think the cause is. Delay costs vision. See our guides to equine recurrent uveitis and to distinguishing it from conjunctivitis and pink eye.

Cutaneous pseudolymphoma

Firm, sometimes ulcerated dermal nodules can develop at the site of a tick bite. On biopsy these show a lymphocyte-rich infiltrate that looks suggestive of lymphoma, but immunohistochemistry reveals mixed lymphoid hyperplasia rather than neoplasia, and Borrelia PCR on the tissue can be positive.

Be aware how rare this is: the consensus statement records it in one naturally infected horse, plus experimentally infected ponies. It is real, it is diagnostically important because it is mistaken for cancer, but it is not something you should expect to see. The differential that matters is equine cutaneous lymphosarcoma.

Early signs of Lyme disease in horses

Owners frequently ask what the earliest signs are. The honest answer is that horses do not reliably produce the classic early marker seen in people. The expanding “bullseye” rash of erythema migrans, which makes early human Lyme so recognisable, is not a documented feature of equine infection — and a horse’s coat would hide it anyway.

In practice, the meaningful early event is not a sign at all; it is the exposure. What should raise your index of suspicion is:

• A known attached tick, particularly one attached for more than 24 hours
• Grazing or regular hacking in known tick habitat
• A change that persists — dullness, reluctance to work, unexplained weight loss, altered behaviour — rather than a single off day
• Any of the documented syndromes above appearing within weeks to months of tick season

Record the date you removed a tick. If a horse becomes unwell weeks later, that note turns a vague history into useful clinical information.

Why shifting lameness and stiffness are not proof

A great deal of vague, low-grade discomfort gets labelled “Lyme” on the strength of a positive antibody test. The signs usually cited — shifting lameness, muscle soreness, girthiness, irritability, reluctance to go forward, mild stiffness — are genuinely reported by owners and vets, but they are also the presenting signs of a long list of much more common problems:

• Equine gastric ulcer syndrome
• Kissing spine and other back pathology
• Sacroiliac disease
• Hindlimb lameness and sub-clinical lameness generally
• Low-grade laminitis
• Dental pain, poor saddle fit and rider-related asymmetry

None of that means a horse with vague signs cannot have Lyme disease. It means a positive titre is not, on its own, evidence that Borrelia is the cause — and that working through the common differentials first is not your vet being obstructive. It is the correct order of operations.

How vets diagnose Lyme disease in horses

Why a positive blood test is not a diagnosis

This is the single most misunderstood part of equine Lyme management, so it is worth stating plainly: a positive antibody test tells you the horse has been exposed to Borrelia. It does not tell you the horse is ill because of it.

The seroprevalence figures make the point. In North American surveys of horses that were not selected for showing clinical signs, reported seropositivity ranged from 0.2% in Texas to 14.8% in the Pacific Northwest, 33% of presumed healthy horses in south-west Virginia and 45% of normal horses in the Northeast. In clinically suspect populations the numbers run much higher — 58.7% in one Minnesota series, and 94% in one Maryland study of suspected cases.

In other words, in a high-prevalence region you could test a field of entirely sound horses and turn up a substantial number of positives. There is also no known correlation between the magnitude of the antibody titre and the likelihood of disease — a high number is not a sicker horse.

Comparable population-level seroprevalence data for UK horses is scarce. Anyone quoting you a confident British percentage should be asked where it comes from.

The Cornell Lyme Multiplex assay explained

The Animal Health Diagnostic Center at Cornell University developed a Lyme Multiplex assay that measures antibodies against three different outer surface proteins simultaneously. Because the bacterium switches which protein it expresses as infection progresses, the pattern of the three results carries staging information that a single ELISA cannot.

Read together, OspC and OspF help separate a recent infection from long-past exposure. They still need clinical context: the assay is an aid to a vet’s judgement, not a replacement for it. UK samples are typically submitted through a referral practice or a laboratory offering a comparable panel.

PCR and direct detection

PCR looks for Borrelia DNA itself rather than the immune response to it, so a positive is far more meaningful. The catch is sampling: because spirochetes migrate through connective tissue rather than circulating, blood PCR performs poorly and is generally not recommended. PCR on synovial fluid, cerebrospinal fluid, aqueous humour or a skin biopsy from an affected site is far more useful, and a negative result on any of them does not fully exclude infection elsewhere.

What else it could be: differential diagnosis

A working diagnosis of equine Lyme disease generally requires four things together: plausible tick exposure, a compatible clinical picture, supportive laboratory results, and reasonable exclusion of other causes. Your vet will be working through something like this:

Treatment of Lyme disease in horses

Antibiotic options compared

Treatment is antibiotic, and the tetracyclines are the mainstay. Doses below are those cited in the ACVIM consensus statement on Lyme disease in North American horses (Divers et al., Journal of Veterinary Internal Medicine 2018;32:617-632). They are given here so you can follow the reasoning, not so you can medicate a horse. Antibiotic choice, dose and duration are veterinary decisions, and in the UK these are prescription-only medicines.

On duration: human late-stage Lyme is commonly treated for around four weeks, and equine practice broadly follows that, with longer courses where neurological involvement is suspected. Some practitioners have advocated a month-long intravenous tetracycline course for suspected equine cases on the strength of the experimental data below. Practice varies, and your vet is balancing efficacy against the real risks of prolonged antibiotic use.

What the pony study actually showed

The reason tetracyclines are favoured is a specific experimental result, and it is worth quoting properly because it is usually paraphrased into mush. In experimentally infected ponies assessed by post-mortem tissue culture and PCR, infection was eliminated in:

• 4 of 4 tetracycline-treated ponies
• 2 of 4 ceftiofur-treated ponies
• 1 of 4 doxycycline-treated ponies
• 0 of 4 untreated ponies

Four ponies per group is a very small study, and there is a second problem that usually gets left out. The doses were not the ones in clinical use: the ponies given doxycycline received 10 mg/kg once daily, half the daily dose listed in the table above, and the ceftiofur group received 2.2 mg/kg once daily. The gap between 4 of 4 and 1 of 4 is therefore confounded by dosing and cannot be read as a clean comparison between the drugs. None of the ponies were clinically ill either. The consensus statement is explicit on the point: available evidence does not clearly support the use of one drug over the others. Notably, vets and owners still report clinical improvement on doxycycline and minocycline with less risk of adverse events — which may reflect genuine benefit, or may reflect that many treated horses were never made ill by Borrelia in the first place.

Supportive care

Non-steroidal anti-inflammatories such as phenylbutazone or flunixin help with pain and inflammation but do nothing about the infection. Corticosteroids are generally avoided early on because they suppress the immune response and muddy the diagnostic picture; their use is reserved for severe uveitis or acute neurological inflammation, under specialist supervision. Horses with dysphagia need their feeding and hydration managed carefully.

Monitoring the response to treatment

Response is judged clinically, not serologically. Antibody titres — OspF in particular — can remain elevated for months or years after successful treatment, so repeat blood testing is generally unhelpful for confirming cure and can be actively misleading.

Keep a written record instead. Note willingness to go forward, comfort under saddle, gait quality, appetite, demeanour and any specific deficit your vet identified, and score them the same way each week. A horse improving slowly over six weeks is easy to miss day to day and obvious in a notebook.

Prognosis: is Lyme disease in horses curable?

The short answer: infection is treatable, and many horses do well, but “curable” is a stronger word than the evidence supports in every case — particularly once neurological or intra-ocular damage is established.

Favourable outcomes

Horses with cutaneous pseudolymphoma, or with mild non-specific signs that genuinely were attributable to Borrelia, generally respond well to prompt treatment. Skin lesions resolving on antibiotic therapy is in fact part of how the diagnosis is confirmed.

Guarded outcomes: neuroborreliosis

Neuroborreliosis carries a materially worse prognosis, and the driver is how much nerve damage exists before treatment starts. Be realistic about the published record. The consensus statement describes the prognosis for treated neuroborreliosis as generally poor, with only a single case report of successful treatment in the literature, and notes that horses which died had often been treated for months beforehand with drugs that should in theory have worked. Some are left with residual deficits, a proportion deteriorate despite appropriate therapy, and the published case series include fatalities. For Lyme-associated uveitis the consensus is equally blunt: the prognosis for restoring vision is poor. Equine recurrent uveitis can also become self-perpetuating once triggered, continuing independently of the original infection.

What happens if Lyme disease goes untreated

Most exposed horses never develop clinical disease and need no treatment at all — that is the usual outcome, and it is important not to lose sight of it. For the minority that do develop one of the documented syndromes, leaving it untreated is a poor idea. In the published fatal neuroborreliosis cases, neurological dysfunction ran for a median of 120 days before death. Untreated uveitis threatens sight.

The practical position is not “treat every positive test” and it is not “wait and see”. It is: investigate a horse that is genuinely unwell, and treat on the strength of the whole picture.

Chronic Lyme, relapse and reinfection

Whether persistently positive serology in treated horses reflects ongoing infection or simply durable antibody is not settled — the consensus statement raises the question without resolving it. That uncertainty is why “chronic Lyme” is contested territory in equine practice as much as in human medicine, and why open-ended antibiotic courses on the basis of a persistent titre are hard to justify.

Reinfection is a separate and more certain risk. Previous exposure confers little useful protective immunity, so a horse that has had Lyme disease can acquire it again the following season. Prevention is not a one-off task.

Preventing Lyme disease in horses

Daily tick checks: the highest-yield measure

Because infection risk rises with the length of time a tick is attached, a genuine daily check during the active season removes most ticks before that risk peaks and prevents a large share of infections. As above, it reduces risk rather than eliminating it. It costs nothing and takes a minute. Work the high-risk sites listed above with your hands, not your eyes — you will feel a nymph you would never spot. Do it after hacking through woodland, bracken or rough grazing in particular.

Repellents and acaricides

Synthetic pyrethroids — permethrin, cypermethrin and deltamethrin — are the active ingredients with meaningful acaricidal effect. In the UK, what is actually licensed for horses is more limited than the general availability of these compounds suggests, and products licensed for cattle or sheep should not be assumed safe or legal to use on a horse. Ask your vet or an SQP which specific product is appropriate, and follow the reapplication interval on the label rather than a rule of thumb.

Plant-based repellents containing citronella or geraniol have some deterrent effect but are short-acting and unreliable as a sole strategy in genuine tick country.

Pasture and habitat management

Ticks need humidity and shelter at ground level. Practical measures:

• Keep grazing topped short; ticks quest from longer vegetation
• Cut back bracken, brambles and rank margins along fence lines
• Create a mown buffer strip between paddocks and woodland edge
• Fence horses out of the wettest, most sheltered woodland corners during peak season
• Reduce deer access where that is practical — deer are the key maintenance host

Our complete guide to paddock care covers the wider grazing management picture, and the British Horse Society’s horse health resources are a sound general reference.

Vaccination

There is no licensed equine Lyme vaccine, in the UK or elsewhere. The consensus statement notes there is strong evidence that appropriate vaccination could prevent infection in horses — but no approved equine product exists to deliver it.

Some vets use a canine Lyme vaccine off-label in heavily exposed horses. Evidence for safety and efficacy in the horse is limited, this is an off-label decision your vet takes responsibility for, and it should be based on a genuine individual risk assessment rather than general anxiety. One practical consequence: vaccination raises OspA antibody, which affects how future multiplex results are interpreted. Make sure any vaccination is recorded.

Lyme disease in South African horses

South African owners should treat Lyme disease as a low prior probability, not a routine differential. Ixodes ricinus is a European tick and is not the dominant tick fauna of southern Africa, and there is limited evidence for endemic Borrelia burgdorferi infection in South African horses.

A South African horse with unexplained lameness, neurological signs, fever or uveitis should be worked up for the locally relevant conditions first — above all equine piroplasmosis (Theileria equi and Babesia caballi), which is genuinely endemic and genuinely tick-borne, and African horse sickness, which is transmitted by Culicoides midges rather than ticks. Lyme belongs well down that list unless something specific puts it there.

Two situations do warrant considering it. Horses imported from Europe or North America may carry infection acquired before travel, which can become clinically apparent under the stress of relocation. And horses with a recent travel history to an endemic region have a plausible exposure route. For high-value horses moving between continents, screening at import is a reasonable precaution — interpreted, as always, alongside clinical signs rather than in isolation.

When to call your vet

Call your vet the same day if your horse has:

• Any painful, weeping, squinting or cloudy eye — this is always urgent
• Difficulty swallowing, choking, or noisy or laboured breathing
• Incoordination, weakness, stumbling or a change in gait quality
• Facial asymmetry, a drooping lip or ear, or an inability to blink
• Marked sensitivity to touch, or a sudden and out-of-character behavioural change

Arrange a non-urgent visit if you are seeing persistent dullness, unexplained weight loss, muscle wastage along the topline, or a performance change that has not resolved in a fortnight. Bring your tick-removal dates with you — and expect, quite properly, to work through the common causes before Lyme disease is landed on.

Frequently Asked Questions

The bottom line

Lyme disease in horses is real, it is present in the UK, and it deserves to be taken seriously when a horse shows one of the documented syndromes — particularly anything neurological or ocular. It is also over-diagnosed, on the strength of antibody tests that were never designed to answer the question being asked of them.

The two things worth actually doing are unglamorous. Check your horse for ticks every day through the season and remove them properly, which prevents most infection outright. And when something is wrong, investigate it properly rather than reaching for a titre — because the commonest reason a horse is stiff, sour or reluctant is almost never Borrelia.

This article is general information for horse owners and is not veterinary advice. Always consult your own vet about a specific horse. Clinical detail is drawn from the 2018 ACVIM consensus statement, Cornell AHDC, Liphook Equine Hospital, the MSD Veterinary Manual and Cornell University College of Veterinary Medicine.