CONDITION

Lyme Disease (Borreliosis)

Lyme disease is an infection caused by bacteria called Borrelia burgdorferi, transmitted when an infected tick feeds on a dog or cat for a prolonged period. The condition is named after a town in Connecticut where it was first recognised in people, and it has become increasingly common in parts of Europe, including the UK, where certain tick species carry the bacteria. In pets, the infection can remain silent for weeks or months, or it may trigger signs that range from intermittent lameness and joint pain to more generalised illness. Many owners arrive at this page because their pet has been found to carry ticks after woodland walks, or because a limp has appeared and disappeared without obvious injury, or because a screening test has returned a positive result during routine health checks. Cats are rarely affected in a way that produces visible signs, but dogs can develop patterns of shifting leg lameness, lethargy, or occasionally signs that reflect kidney or heart involvement. This page explores what Lyme disease can look like in dogs and cats, how the bacteria behave once inside the body, the ways in which infection is detected and distinguished from other causes of similar signs, and the range of approaches used when treatment is considered appropriate.

Why this matters now

Lyme disease is transmitted through the bite of infected Ixodes ticks — in the UK and Europe the principal vector is Ixodes ricinus, the sheep or castor bean tick, while in North America the principal vectors are the black-legged tick (also called the deer tick, Ixodes scapularis) in the northeastern, mid-Atlantic, and north-central United States, and the western black-legged tick (Ixodes pacificus) along the Pacific coast, with the geographic distribution of the disease closely following the range of these tick species. In endemic regions, exposure risk exists throughout the warmer months when ticks are actively questing, though the precise seasonality varies by geography and climate. The bacterium requires a minimum of approximately 24-48 hours of tick attachment to transmit, meaning that prompt tick removal can reduce transmission risk. Dogs in endemic areas may encounter infected ticks frequently, and seroconversion — the development of antibodies indicating exposure — is common in these regions, though only a minority of seropositive dogs develop clinical disease. The delay between tick bite and the appearance of clinical signs, which can span weeks to months, means that by the time symptoms emerge, the original tick exposure may be long forgotten.

When clinical Lyme disease does develop, it most commonly manifests as an acute or episodic polyarthritis — inflammation affecting multiple joints — producing lameness that may shift between limbs and may wax and wane over time. This joint involvement can appear suddenly, sometimes with significant discomfort, and may resolve spontaneously before recurring. A subset of affected dogs may develop Lyme nephritis, a serious and potentially life-threatening form of immune-mediated kidney disease characterised by protein loss through the kidneys. This renal manifestation tends to carry a more guarded prognosis and can progress rapidly once established. The relationship between the duration of infection and the likelihood of developing serious complications is not straightforward, and some dogs may develop nephritis without prior obvious joint disease.

Signals & patterns

Early signals

Shifting-leg lameness

One of the more characteristic presentations of Lyme disease is lameness that appears to move between different legs over time. A dog may be lame on one leg for a day or two, appear to improve, then develop lameness on a different leg. This shifting pattern reflects the migratory nature of the joint inflammation.

Joint swelling and warmth

Affected joints may feel swollen, warm, and tender to the touch. The large joints — elbows, stifles, and hocks — are most commonly involved, and the swelling may be subtle or more obvious depending on the degree of inflammation.

General malaise and reluctance to move

Dogs with early Lyme disease may show a general reduction in activity, stiffness on rising, and reluctance to engage in normal activities such as walks or play. This can be accompanied by a mildly elevated temperature and a subdued demeanour.

Intermittent appetite reduction

Mild, intermittent decreases in appetite may accompany episodes of joint inflammation, reflecting the general malaise associated with the systemic inflammatory response rather than a specific gastrointestinal effect.

Later signals

Persistent or worsening lameness

While early Lyme arthritis may be episodic and self-resolving, some dogs develop more persistent joint inflammation that becomes less responsive to time alone. The lameness may become more consistently present and may affect the dog's willingness to exercise and overall quality of life.

Signs of kidney involvement

Lyme nephritis may manifest through increased thirst and urination, reduced appetite, weight loss, lethargy, vomiting, and peripheral oedema (swelling of the limbs or face). These signs may develop relatively suddenly and can progress rapidly. Lyme nephritis represents the most serious potential complication and carries a significantly more guarded outlook.

Progressive lethargy and weight loss

Dogs that develop systemic complications may show progressive decline in energy levels and body condition, reflecting the cumulative effects of chronic inflammation, discomfort, and in cases of nephritis, protein loss and metabolic disruption.

Click to read about the biological mechanisms

How this is usually investigated

Investigating Lyme disease in dogs involves a combination of serological testing, clinical assessment, and exclusion of other conditions that can produce similar signs. The interpretation of test results requires careful consideration of the dog's geographic location, exposure history, and clinical picture, as positive serology does not always equate to clinical disease.

Point-of-care antibody screening

Purpose: In-clinic SNAP tests can detect antibodies to a specific Borrelia antigen (C6 peptide) within minutes, providing rapid screening for exposure. These tests are often performed as part of routine annual screening in endemic areas.
Considerations: A positive C6 antibody test indicates exposure to Borrelia burgdorferi but does not confirm clinical disease, as many seropositive dogs remain healthy. The test may take several weeks after infection to become positive. A positive result in a symptomatic dog supports but does not prove a diagnosis of Lyme disease, particularly if other causes of the clinical signs have not been excluded.

Quantitative C6 antibody testing

Purpose: Quantitative measurement of C6 antibody levels (Lyme Quantitative C6) can provide additional information about the magnitude of the immune response and can be used to monitor response to treatment, as antibody levels typically decline following successful antibiotic therapy.
Considerations: Higher antibody levels may correlate with active infection, but threshold values for distinguishing active disease from exposure alone remain debated. Serial measurements showing declining titres after treatment can support the diagnosis retrospectively and confirm treatment response.

Urinalysis and urine protein assessment

Purpose: Evaluation of urine for protein loss (proteinuria) is important in any dog diagnosed with or suspected of Lyme disease, as it screens for the potentially serious complication of Lyme nephritis. The urine protein-to-creatinine ratio provides a quantitative measure of protein loss.
Considerations: Significant proteinuria in a Lyme-positive dog warrants further renal assessment, as Lyme nephritis can progress rapidly and has a more guarded prognosis than uncomplicated Lyme arthritis. Regular monitoring of urine protein levels is often recommended in Lyme-positive dogs, even those that appear clinically well.

Joint fluid analysis

Purpose: Aspiration and analysis of fluid from affected joints can reveal inflammatory changes consistent with Lyme arthritis, including elevated white blood cell counts with a predominantly neutrophilic pattern.
Considerations: Joint fluid analysis helps confirm the inflammatory nature of the arthritis and can help exclude other joint conditions such as septic arthritis or immune-mediated polyarthritis from other causes. PCR testing of joint fluid for Borrelia DNA may be performed but has variable sensitivity.

Comprehensive bloodwork

Purpose: Complete blood count and biochemistry panel help assess overall health, screen for renal function changes, and evaluate for concurrent tick-borne infections which may coexist in dogs exposed to Ixodes ticks.
Considerations: Dogs in Lyme-endemic areas may be co-infected with other tick-borne pathogens such as Anaplasma phagocytophilum, which is carried by the same tick species. Testing for concurrent infections is often warranted, as co-infections may influence the clinical picture and treatment approach.

Options & trade-offs

Management of clinical Lyme disease centres on antibiotic therapy to address the bacterial infection, alongside supportive care for any complications. The approach may vary depending on whether the dog has uncomplicated Lyme arthritis or has developed the more serious renal manifestation.

Antibiotic therapy

Doxycycline is the most commonly used antibiotic for treating Lyme disease in dogs, typically administered for a course of four weeks. The antibiotic targets the Borrelia spirochaetes and most dogs with uncomplicated Lyme arthritis show significant improvement within 1-3 days of starting treatment.

Trade-offs: While most dogs respond well to antibiotic therapy, some may experience recurrence of clinical signs after completing treatment. The rapid response to doxycycline can serve as a diagnostic indicator — improvement within a few days supports the diagnosis, while lack of response may prompt reconsideration. Gastrointestinal side effects from doxycycline occur in some dogs and can be managed with dietary adjustments.

Anti-inflammatory and analgesic support

Pain management during active episodes of Lyme arthritis may include anti-inflammatory medications to reduce joint swelling and discomfort while the antibiotic therapy takes effect.

Trade-offs: Anti-inflammatory medications provide symptomatic relief but do not address the underlying infection. Their use alongside antibiotics can help maintain comfort and mobility during the acute phase. As with any anti-inflammatory therapy, potential gastrointestinal and renal effects require consideration, particularly in dogs being monitored for Lyme nephritis.

Renal-directed therapy for Lyme nephritis

Dogs that develop Lyme nephritis require a more intensive management approach that may include immunosuppressive therapy to address the immune-mediated component, medications to reduce proteinuria, dietary modification, and supportive care for declining renal function.

Trade-offs: Lyme nephritis carries a significantly more guarded prognosis than uncomplicated Lyme arthritis, and treatment outcomes are variable. Early detection through routine urinalysis monitoring of Lyme-positive dogs may allow earlier intervention. The decision to pursue aggressive treatment involves considering the severity of renal compromise, response to initial therapy, and quality of life.

Prevention strategies

Tick prevention through topical or oral tick-control products represents the primary strategy for reducing Lyme disease risk. Vaccination against Borrelia burgdorferi is available and may be considered for dogs in endemic areas, though the decision involves weighing individual risk factors.

Trade-offs: No tick prevention method is 100% effective, and regular tick checks after outdoor activity remain valuable even in dogs on preventive products. Lyme vaccination protocols vary among veterinary professionals, with some advocating vaccination in high-risk areas and others relying primarily on tick prevention. The cost-benefit analysis depends on geographic risk, lifestyle factors, and individual health considerations.

Common misconceptions

Misconception:

"A positive Lyme test means the dog has Lyme disease"

Reality:

Antibody tests for Lyme disease detect exposure to the Borrelia organism, not necessarily active clinical disease. In endemic areas, a substantial proportion of dogs test positive for Lyme antibodies without ever developing clinical signs. A positive test indicates that the dog's immune system has encountered the bacterium, but clinical disease only develops in a minority of exposed dogs. Positive serology in the absence of clinical signs is generally not considered an indication for antibiotic treatment, though it may prompt urinalysis screening for subclinical proteinuria.

Misconception:

"Lyme disease always causes a bull's-eye rash in dogs as it does in humans"

Reality:

The characteristic erythema migrans rash — the expanding circular rash seen at the tick bite site in human Lyme disease — is rarely if ever identified in dogs. This is partly because the skin reaction is difficult to see under fur, but also because the cutaneous response to Borrelia infection may differ between species. The absence of a visible skin lesion does not exclude Lyme disease in dogs.

Misconception:

"Lyme disease in dogs is a chronic, lifelong condition"

Reality:

Most dogs with uncomplicated Lyme arthritis respond well to antibiotic therapy, often showing improvement within days of starting treatment. While some dogs may experience recurrence, the acute joint inflammation typically resolves with appropriate treatment. Lyme nephritis, however, can be a more persistent and serious complication. The characterisation of Lyme disease as inevitably chronic does not reflect the experience of the majority of clinically affected dogs.

Related conditions

Lyme Nephritis

Lyme nephritis is an uncommon but serious immune-mediated kidney complication associated with Borrelia infection, and awareness of it shapes monitoring after a Lyme diagnosis.

Anaplasmosis

Anaplasmosis is transmitted by the same Ixodes ticks that carry Borrelia, so co-infection occurs, and the two produce overlapping signs of fever, lethargy, and shifting lameness.

Ehrlichiosis

Ehrlichiosis is another tick-borne bacterial infection producing fever, lethargy, and joint discomfort, and it features alongside Lyme disease in the differential picture for dogs with tick exposure.

Babesiosis

Babesiosis is a tick-transmitted parasitic infection considered alongside Lyme disease in dogs with tick exposure and non-specific illness, particularly where travel or an unusual tick population is involved.

Immune-Mediated Polyarthritis

Immune-mediated polyarthritis is a key differential for the shifting-leg lameness and joint pain seen in Lyme disease, and distinguishing the two changes the treatment approach considerably.

Understanding Lyme disease involves recognising the distinction between exposure and clinical disease, appreciating the role of geographic risk and tick ecology, and being aware of the spectrum of possible manifestations from self-resolving arthritis to serious renal disease. For dogs living in or travelling to endemic areas, awareness of tick prevention strategies, familiarity with the early signs of Lyme arthritis, and understanding the value of routine screening and urinalysis monitoring all contribute to informed management of this tick-borne infection.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS