Tick-Borne Diseases in Dogs: Lyme, Anaplasmosis & Ehrlichia

Ticks are more than an irritation — they are vectors for some of the most serious infectious diseases dogs can contract. Unlike the tick bite itself, which is usually minor, the pathogens a tick can transmit while feeding may cause multi-system illness that, if left untreated, leads to chronic disease or life-threatening complications. Understanding the key tick-borne diseases, how to recognise them, and how to respond is essential knowledge for any dog owner living in or travelling to endemic regions.

Lyme Disease (Borrelia burgdorferi)

Lyme disease is caused by the spirochete bacterium Borrelia burgdorferi and transmitted primarily by the black-legged tick (Ixodes scapularis in the eastern United States and Ixodes pacificus on the West Coast). In Europe, Ixodes ricinus is the primary vector. The tick must typically be attached for 24 to 48 hours before transmission occurs, which is why prompt tick removal is so important.

Lyme disease is most prevalent in the northeastern and upper midwestern United States, the Pacific coast, and much of northern and central Europe. Rates of infection have expanded significantly over recent decades as tick populations spread into new areas.

Only a minority of infected dogs — approximately 5 to 10% — ever develop clinical illness. When they do, the most common presentation includes:

• Shifting-leg lameness that may affect different joints on different days
• Joint swelling and pain, particularly in the carpal (wrist) joints
• Lethargy and exercise intolerance
• Fever (38.5–40°C / 101.5–104°F)
• Decreased appetite

A serious but less common complication is Lyme nephritis — an immune-mediated glomerulonephropathy that can result in kidney failure. This is seen particularly in Labrador Retrievers and Golden Retrievers. Signs include vomiting, weight loss, increased thirst and urination, and oedema. Lyme nephritis carries a poor prognosis and requires aggressive veterinary management.

Anaplasmosis (Anaplasma phagocytophilum and Anaplasma platys)

Anaplasmosis is caused by two distinct species of obligate intracellular bacteria. Anaplasma phagocytophilum infects white blood cells (granulocytes) and is transmitted by the same Ixodes ticks that carry Lyme disease — meaning co-infection is common. Anaplasma platys infects platelets and is transmitted by the brown dog tick (Rhipicephalus sanguineus), with a broader geographic range including the southern United States, parts of Europe, and tropical regions.

Symptoms of A. phagocytophilum infection typically appear within one to two weeks of the tick bite and include:

• High fever
• Lethargy
• Decreased appetite
• Lameness (in some cases)
• Vomiting and diarrhoea
• Thrombocytopenia (low platelet count) — detected on blood work

A. platys causes cyclic thrombocytopenia — periodic drops in platelet counts at roughly two-week intervals. Many affected dogs show mild or no clinical signs, but some develop petechial haemorrhages (small pinpoint bleeds) or unusual bruising.

Ehrlichiosis (Ehrlichia canis and Ehrlichia ewingii)

Ehrlichiosis is caused by Ehrlichia canis (transmitted by the brown dog tick) and Ehrlichia ewingii (transmitted by the lone star tick, Amblyomma americanum). E. canis infection is one of the most clinically significant tick-borne diseases in dogs worldwide.

The disease progresses through three phases:

Acute phase (1–4 weeks after infection): Fever, lethargy, loss of appetite, lymph node enlargement, and abnormal bruising or bleeding. Platelet and white blood cell counts typically fall on blood work.

Subclinical phase: The dog may appear to recover, but the organism persists in the spleen. This phase can last months to years. During this time, blood work abnormalities may be mild and the dog may appear outwardly well.

Chronic phase: In dogs who fail to clear the infection, severe disease can develop, including profound bone marrow suppression causing pancytopenia (low red cells, white cells, and platelets), severe bleeding, secondary infections, weight loss, and neurological signs. Chronic Ehrlichiosis carries a guarded to poor prognosis.

E. ewingii tends to cause milder disease — predominantly polyarthropathy (joint inflammation in multiple joints) and fever — with a generally better treatment response.

Rocky Mountain Spotted Fever (Rickettsia rickettsii)

Despite its name, Rocky Mountain Spotted Fever (RMSF) is found throughout the Americas, with the highest incidence in the south-central and southeastern United States, as well as parts of Central and South America. It is transmitted primarily by the American dog tick (Dermacentor variabilis), the Rocky Mountain wood tick (Dermacentor andersoni), and the brown dog tick.

RMSF is one of the most severe tick-borne diseases and can be rapidly fatal in both dogs and humans if not treated promptly. Rickettsia rickettsii infects the cells lining blood vessels, causing widespread vasculitis. Clinical signs include:

• High fever (often above 40°C / 104°F)
• Severe lethargy and depression
• Joint pain and oedema (swelling)
• Petechial haemorrhages on the gums and skin
• Neurological signs including ataxia and vestibular disturbance
• Oedema of the face and extremities
• Cardiovascular instability in severe cases

RMSF is a true emergency. Dogs can deteriorate within 24 to 48 hours of symptom onset. Immediate veterinary care and prompt initiation of treatment are critical to survival.

Diagnosis: The 4Dx Test and Blood Work

The IDEXX 4Dx Plus test is the most widely used in-clinic screening panel for tick-borne diseases in dogs. It detects antibodies or antigens for:

• Borrelia burgdorferi (Lyme disease)
• Anaplasma phagocytophilum and Anaplasma platys
• Ehrlichia canis and Ehrlichia ewingii
• Dirofilaria immitis (heartworm — included in the same panel)

The 4Dx is an excellent screening tool but has limitations. It detects antibodies, which take time to develop — testing too soon after exposure (within the first three to four weeks) may yield a false negative. A positive result confirms exposure but does not by itself prove active infection causing the current illness. Your vet will interpret 4Dx results alongside a full blood count, chemistry panel, urinalysis, and clinical signs. For RMSF, specialised PCR testing or paired serology is often required, as the 4Dx panel does not include Rickettsia.

Treatment: Doxycycline as the Cornerstone

For Lyme disease, Anaplasmosis, and Ehrlichiosis, the first-line treatment is doxycycline — a broad-spectrum tetracycline antibiotic that is highly effective against intracellular and tick-borne pathogens. The standard protocol is doxycycline at 5 mg/kg twice daily (or 10 mg/kg once daily) for a minimum of four weeks. For Ehrlichiosis, some veterinarians extend treatment to six or eight weeks depending on disease severity and clinical response.

RMSF also responds rapidly to doxycycline, and treatment should begin on clinical suspicion — before laboratory confirmation — given the speed at which the disease can progress. Most dogs with RMSF who are treated promptly show marked improvement within 24 to 48 hours of starting antibiotics.

Dogs with severe disease may require hospitalisation, intravenous fluids, blood or platelet transfusions, and supportive care. Regular re-check blood work is important to confirm recovery and resolution of haematological abnormalities.

Long-Term Effects and Chronic Disease

Most dogs treated early for Lyme disease, Anaplasmosis, or E. ewingii infection recover fully. However, dogs with Lyme nephritis face a significantly worse prognosis regardless of treatment. Dogs with chronic E. canis infection may experience lasting bone marrow damage that does not fully reverse even after the organism is eliminated. Dogs who have had RMSF may have residual neurological deficits if vascular damage was extensive before treatment began.

For dogs who test positive on the 4Dx but are not currently ill, your veterinarian will guide you on monitoring frequency and when — if ever — to initiate treatment. Annual 4Dx screening is recommended for all dogs in tick-endemic areas.

Prevention: The Most Effective Strategy

Prevention is significantly more effective than treatment, and it begins with reliable tick control. Your veterinarian is your most important resource for selecting the right product for your dog. First-line options include:

• Oral isoxazoline preventatives (Bravecto, NexGard, Simparica, Credelio) — these prescription products kill ticks rapidly after attachment, before most pathogens can be transmitted. They are considered the gold standard for tick prevention.
• Veterinary-approved topical spot-on treatments — products containing fipronil, permethrin (not for use in cats), or afoxolaner
• Seresto collar — provides sustained repellent and killing activity for up to eight months
• Lyme vaccine — recommended by many vets in high-risk areas as an adjunct to tick prevention, particularly for dogs with heavy outdoor exposure. The vaccine does not protect against other tick-borne diseases, so tick prevention remains essential even in vaccinated dogs.

Natural tick repellents — such as rose geranium oil, neem, or citronella products — are sometimes used by owners seeking complementary approaches. These may offer some limited deterrent effect but have no clinical evidence demonstrating meaningful protection against tick-borne disease transmission. They should be used only as adjuncts and never as replacements for prescription-grade prevention.

Written by Sarah Bennett, Certified Animal Nutritionist. This article is for informational purposes only and does not replace professional veterinary advice. Always consult your veterinarian for diagnosis and treatment specific to your pet's condition.