Lyme Disease in Dogs

Lyme disease (also called Lyme borreliosis) is a vector-borne infection caused by Borrelia burgdorferi bacteria, which are transmitted through the bite of a tick. The disease affects dogs, humans, horses, and cattle.¹ The disease name comes from Old Lyme, Connecticut, where the disease was recognized in children in 1976.¹ The causative bacterium is named after the biologist who discovered it in 1982, Wilhelm Burgdorfer.^1,2

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Lyme disease is the most common tick-borne disease in the United States; it has been found in all 50 states and in the District of Columbia.² The geographic distribution is determined by the vectors, ticks of the genus Ixodes. In the United States, 2 species of Ixodes ticks are capable of spreading the disease: Ixodes scapularis ticks (eastern black-legged ticks), which are found in the upper eastern and midwestern United States, and Ixodes pacificus ticks (western black-legged ticks), which are found in the western United States.^1,3 The ranges of these 2 species of ticks, however, have been expanding due to bird migration (which helps transport the ticks), suburban sprawl encroaching onto tick territory, and climate changes leading to warmer winters farther north.⁴ 

Lyme disease incidence in the individual U.S. states is shown at capcvet.org/maps/#2020/all/lyme-disease/dog/united-states. Geographic distribution in Canada is similar to that in the United States. I scapularis ticks are found in the eastern provinces of Canada (e.g., Ontario, Quebec, New Brunswick, and Nova Scotia), and I pacificus ticks are found in British Columbia.⁵ Borrelia species have also been found in ticks in Europe and Asia.¹ 

Causative Organism for Lyme Disease

B burgdorferi are spirochetes. These gram-negative, motile, cork-screw shaped, microaerophilic bacteria grow at body temperature in broth media.¹ They are fastidious and do not grow well in standard growth media; growth in a special broth medium can take 6 weeks or longer.² Being microaerophilic, B burgdorferi prefer an environment with little oxygen, but they are not strictly anaerobic. These spirochetes cannot be viewed by standard microscopy methods; visualization requires dark-field microscopy.² Given all of these features, B burgdorferi cannot be cultured in a clinic setting. 

Transmission of Lyme Disease

Although B burgdorferi are transmitted through the bite of infected ticks, ticks do not, themselves, cause the disease.⁶ Ticks are vectors only. The infection cycle usually starts when a tick larva obtains its first blood meal from an infected reservoir host, typically a white-footed mouse in the east or a western gray squirrel in the west.¹ Other reservoir hosts for B burgdorferi include shrews, voles, chipmunks, and deer.^1,6 However, if the tick did not become infected while in the larval stage, it can still become infected through subsequent blood meals as a nymph or adult.² Ticks of all 3 life stages—larvae, nymphs, and adults— feed on multiple species of animals, including squirrels, deer (and other medium to large mammals), reptiles, amphibians, and birds. The ticks need to have a new host at each stage of their life before they can transmit B burgdorferi to a dog. If one host is reduced or missing (i.e., fewer squirrels available for larval ticks to feed on), the infection rate goes down.³ Conversely, in years when reservoir species are abundant, the infection rate goes up. 

After infection with B burgdorferi, Ixodes species ticks of all 3 life stages are capable of transmitting the bacteria. After the bacteria are inside the ticks, regardless of the tick life stage, the bacteria persist through each molt of the tick (called transstadial transmission).^1,3,4 Within Lyme disease–endemic areas, the incidence of B burgdorferi infection in ticks can reach as high as 50% of nymphs and adult ticks.^4,6

B burgdorferi transmission to a dog occurs when an infected tick in any 1 of its 3 life stages bites the dog and remains attached long enough. B burgdorferi transmission requires 24 to 52 hours of tick attachment; the peak time for transmission is 48 to 52 hours of attachment. If the tick is carrying causative organisms for other diseases (e.g., anaplasmosis), these organisms can also be transmitted during this feeding period.² After B burgdorferi have been transmitted, they begin to replicate and will spread through the connective tissue over the course of days to weeks.² This spread pattern explains why the first clinical signs appear in the joints closest to the area of the bite.¹

Rates of transmission are highest in the late spring, when nymphs and adult ticks are feeding, and in the fall, when adult ticks are feeding.^1,2,6 During late fall and winter, B burgdorferi remain dormant inside the immature tick. 

Diagnosis of Lyme Disease

Diagnosis of Lyme disease is often based on a history of tick exposure, clinical signs, and a positive antibody test result.^1-3 

Clinical Signs of Lyme Disease

In Lyme disease–endemic areas, seroprevalence rates of up to 90% indicate high levels of exposure.¹ However, for most dogs exposed to B burgdorferi–infected ticks, the developing disease is subclinical; few, if any, signs are noticed.³ Unlike humans, a skin rash in the typical bull’s-eye pattern does not develop when dogs are bitten by an infected tick.¹ 

If clinical signs do appear, they can include abrupt onset of lethargy, fever, lameness, and warm and swollen joints. The lameness may be confined to 1 joint, or it may shift from 1 joint to another.^1,2 Physical examination may reveal peripheral lymphadenopathy, thin body condition (low body condition score), dehydration, peripheral edema, pleural effusion, or ascites.¹ 

Dogs with advanced Lyme disease may experience chronic intermittent arthritis, renal disease, heart disease, and/or neurologic disease. The 2 main clinical manifestations of Lyme disease are arthritis and nephritis. The resulting renal changes are called Lyme nephritis and can include glomerulonephritis and protein-losing nephropathy.^1-3 Renal failure resulting from Lyme disease is usually fatal.^4,6 Lyme disease–associated heart conditions may be pericarditis, endocarditis, or conduction disorders. Lyme disease–associated neurologic conditions can produce behavioral changes or seizures. 

Antibody Titers for Lyme Disease

Antibody titers can provide helpful diagnostic information, although antibodies cannot be detected for 4 to 6 weeks after the bacteria have entered the dog.^3,6

Diagnostic tests for antibodies to B burgdorferi are available as immunofluorescence assay, enzyme-linked immunosorbent assay (ELISA), Western blot, and polymerase chain reaction. ELISAs are available for in-house screening.² Western blot testing is the gold standard and can be used if the ELISA result and the clinical presentation are discordant (do not indicate the same diagnosis).¹

Because most infections in dogs are subclinical, infection must be differentiated from previous exposure.^1,3 In the absence of clinical signs, a positive antibody test result does not indicate disease;¹ rather, a positive test result indicates only previous exposure to Borrelia bacteria.

Other Diagnostic Tests for Lyme Disease

For dogs showing active signs of disease, joint cytology can be helpful. Tapping 3 or 4 joints is recommended. In dogs with active disease, nondegenerative neutrophils in joint fluid may be increased. Culturing joint fluid to look for B burgdorferi bacteria is difficult and takes a long time; thus, culture is not recommended.¹

Other diagnostic tests that can be performed on symptomatic dogs include blood chemistry analysis, which may indicate azotemia or markedly decreased albumin. A complete blood count might show mild to marked thrombocytopenia and, if the dog has nephritis, mild to moderate anemia and leukocytosis. For dogs with nephritis, a urinalysis may show isothenuria and proteinuria.¹

The American College of Veterinary Internal Medicine (ACVIM) Consensus group recommends routinely testing dogs in Lyme disease–endemic areas for antibodies by using an in-house screening ELISA, evaluating dogs with a positive ELISA result for signs of joint pain/inflammation, and testing for proteinuria in the urine.^3,4

Treatment for Lyme Disease

Treatment with antibiotics is indicated for dogs with a positive ELISA result and clinical signs of Lyme disease. The drug of choice is doxycycline. Although no optimal dose or duration of treatment has been determined,¹ the Companion Animal Parasite Council recommends a doxycycline dose of 10 mg/kg PO q24h for 30 days,³ and the recommended course of treatment is usually 4 weeks.^1,4 If doxycycline is unavailable or is contraindicated for a given patient, amoxicillin or azithromycin can be prescribed.^1,4 For dogs exhibiting substantial joint pain, nonsteroidal anti-inflammatory drugs or opioids can be used. Glucocorticoids are not recommended.^1,2 Doxycycline is recommended for dogs with Lyme disease–associated nephritis, but these dogs must also receive symptomatic treatment, based on the degree of renal changes. 

Because of the low incidence of clinical Lyme disease, prophylactic treatment when a tick has been found attached to a dog is not recommended.^2,3 Antibody titers are not a helpful guide to treatment because they can remain elevated for years.¹

More specific information about treatment for Lyme disease can be found in the ACVIM Consensus 2018 update.⁴

Prognosis for Lyme Disease

As mentioned earlier, most dogs exposed to infected ticks remain asymptomatic. For those in which clinical signs do develop, recovery is usually quite rapid after antibiotics have been started.¹ Dogs with polyarthritis usually respond to antibiotics within 24 to 48 hours. However, remind clients that it is important that they give the full course of antibiotics, regardless of how good the dog looks or may seem.

For dogs with Lyme nephritis, the prognosis is guarded to poor; these dogs are usually euthanized within days to weeks of diagnosis. For dogs not euthanized, death can be secondary to systemic thrombosis or oliguric/anuric renal failure.¹

Prevention of Lyme Disease

As with many vector-borne diseases, the most effective prevention technique is avoidance of the vector, in this case ticks. 

Protect the Dog from Ticks

The primary ways to protect a dog from infection with B burgdorferi are to keep it out of tick-infested areas, use topical acaricides (products that kill ticks and mites) when living in or visiting areas endemic for B burgdorferi, and inspect the dog daily for ticks and remove any found.^1,2 Acaricides that kill ticks quickly or prevent their attachment and feeding are preferred.^1,2,4

Treat the Environment for Ticks

Immature and mature ticks are most highly concentrated in wooded areas on low brush and leaf litter.² Therefore, environmental control includes cutting back any brush and grass around the yard that could be used by questing ticks looking for a blood meal.^2,4 Chickens and guinea hens may also help decrease tick populations, thereby decreasing potential exposure in the dog’s environment.

Vaccinate Dogs at High Risk

For dogs at high risk, in addition to acaricides and environmental controls, vaccination can be considered. The vaccines work best when administered before dogs are exposed to B burgdorferi–infected ticks.^2,4,6 Consult the individual vaccine manufacturer’s directions with regard to immunity duration.

Although Lyme vaccines are not considered core vaccines, the ACVIM Consensus group states that they can be helpful in Lyme disease–endemic areas, especially for clients whose tick prevention compliance may be less than ideal. In the absence of studies, whether Lyme vaccination needs to be boosted every 6 or 12 months remains uncertain.⁴

The ACVIM Consensus group was split on the recommendation for routine use of these vaccines but did agree that the circulating immune complexes that increase transiently after vaccination have no known negative effect on the vaccinated dog.⁴ Group members felt that the most effective way to prevent infection with B burgdorferi is year-round tick control and that vaccination provides another layer of protection in Lyme disease–endemic areas.⁴

Although preventing Lyme disease can seem overwhelming at first, if taken a step at a time, we can offer clients a good chance of protecting their dogs from infection with B burgdorferi and subsequent development of Lyme disease (BOX 1). 

Zoonotic Risk of Lyme Disease

Dogs cannot directly transmit Lyme disease to people (i.e., no zoonotic risk); however, they can carry unfed infected ticks into a household where they might feed on humans. In addition, people are often in the same Lyme disease–endemic areas as the dogs and can independently become infected.¹ Increased incidence of dogs with Lyme disease can be used as an indicator for human risk in the same area.⁶ 

Take-Home Points

• Lyme disease develops when a B burgdorferi–infected Ixodes species tick feeds on a dog for 24 to 52 hours.
• Risk for Lyme disease varies according to Ixodes species tick prevalence in an area.
• Diagnosis is based on clinical signs and serologic test results.
• Treatment requires antibiotics (usually doxycycline).
• Prevention involves avoiding/reducing exposure to ticks and, for dogs at high risk, vaccination.
• Dogs cannot transmit Lyme disease to humans.