A positive result on a tick-borne disease screen can look alarming. Your dog seems fine, but the test says Lyme positive or Anaplasma positive. What does that actually mean — and what should you and your veterinarian do next?

This article walks through the three most common canine tick-borne diseases detected on annual screening: Lyme disease, anaplasmosis, and ehrlichiosis. It covers how each disease is transmitted, what signs to watch for, how diagnostic testing works, what a positive result does and does not tell you, and how treatment and prevention decisions are made.

How dogs get tick-borne diseases

Three tick species cause most of the trouble in the United States:

Ticks pick up bacteria (Borrelia burgdorferi, Anaplasma phagocytophilum, Ehrlichia canis) when they feed on infected wildlife — mostly rodents and deer. They transmit the bacteria to dogs during a blood meal that typically lasts hours to days. The companion animal parasites council (CAPC) emphasizes that transmission risk is no longer seasonal in much of the country, because tick activity now stretches across most of the calendar year, and the brown dog tick can survive indoors in any climate.

CAPC 2026 forecast: risk is expanding

The Companion Animal Parasite Council has published annual forecasts since 2012. Each forecast is built from over 10 million diagnostic test results per disease per year, submitted by veterinary clinics nationwide, and the forecasts have historically exceeded 94% accuracy.

For 2026, CAPC projects continued geographic expansion of all three tick-borne diseases:

• Lyme disease: persistent high risk in the Northeast and Upper Midwest, with expanding risk into the mid-Atlantic and portions of the Southeast. The blacklegged tick continues to establish new populations in suburban and peri-urban environments.
• Anaplasmosis: high seroprevalence in the Northeast and upper Midwest, with expansion into the Pacific Northwest. Anaplasma platys (transmitted by the brown dog tick) adds risk in the Southwest.
• Ehrlichiosis: sustained high risk across the Southeast and South-Central states, with northward expansion of lone star tick populations into the Upper Midwest and New England. Brown dog tick–driven ehrlichiosis is increasing in the Mountain West.

The practical takeaway: "low-risk area" is no longer a safe assumption. CAPC now recommends year-round tick prevention and annual testing for all dogs, regardless of geography.

Lyme disease in dogs

Pathogen. Borrelia burgdorferi, a spirochete bacterium.

Vector. Blacklegged tick (Ixodes scapularis in the East and Midwest; I. pacificus in the West).

Clinical signs. Most Lyme-positive dogs never develop clinical illness. When signs appear, they typically include fever, shifting-leg lameness, swollen joints, lethargy, and enlarged lymph nodes. These usually develop 2–5 months after infection. A small subset of infected dogs develop Lyme nephritis, a serious immune-mediated kidney disease that can cause protein loss through the urine, progressive renal failure, and death. This form is less common but far more dangerous.

Diagnosis. Point-of-care screening tests (SNAP 4Dx, SNAP 4Dx Plus, and similar assays) detect antibodies to the C6 peptide of B. burgdorferi. This target is highly specific — it does not cross-react with the Lyme vaccine, meaning vaccinated dogs will not produce false-positive results unless they have also been naturally exposed.

A positive C6 antibody test confirms exposure, not necessarily active disease. Your veterinarian will use the following to decide whether treatment is warranted:

• Physical examination findings (lameness, fever, joint swelling)
• Urine protein-to-creatinine ratio (to screen for Lyme nephritis)
• Complete blood count and chemistry panel
• Quantitative C6 titer (to gauge antibody level and track response to treatment)

Treatment. When treatment is indicated, doxycycline is the standard choice, typically for 30 days. Most dogs with clinical Lyme disease respond quickly, with fever and lameness improving within 24–48 hours. Dogs with Lyme nephritis require more intensive management, including proteinuria control, blood pressure monitoring, and often referral to an internal medicine specialist.

Anaplasmosis in dogs

Pathogen. Two species matter clinically: Anaplasma phagocytophilum (infects white blood cells, transmitted by the blacklegged tick) and A. platys (infects platelets, transmitted by the brown dog tick).

Clinical signs. Many infected dogs are asymptomatic. When signs develop, they are often nonspecific: fever, lethargy, reduced appetite, stiff gait or lameness, vomiting, and diarrhea. Platelet-related forms (A. platys) can cause bleeding tendencies, bruising, or nosebleeds.

Diagnosis. SNAP tests detect antibodies to Anaplasma species. Blood smear examination may reveal characteristic morulae (clusters of organisms) inside neutrophils or platelets during peak phases of infection. However, a negative blood smear does not rule out infection. PCR testing on whole blood can confirm active infection, and paired serology (IFA) showing rising titers over 2–4 weeks supports the diagnosis.

Because the SNAP test detects antibodies, a positive result can indicate past or current exposure. Your veterinarian will interpret it alongside clinical signs, CBC findings (especially thrombocytopenia and leukopenia), and the geographic risk profile.

Treatment. Doxycycline for 2–4 weeks is the standard regimen. Clinical improvement is usually rapid — most dogs show noticeable improvement within 24–48 hours of starting treatment. The prognosis for clinical recovery is excellent.

One nuance: treating clinically healthy, antibody-positive dogs is controversial. Current evidence suggests that many dogs clear the organism naturally or remain asymptomatic carriers, and overtreatment with antibiotics has stewardship implications. This is a conversation to have with your veterinarian, guided by clinical context and follow-up testing.

Ehrlichiosis in dogs

Pathogen. Multiple Ehrlichia species infect dogs. E. canis is the most significant in North America, transmitted by the brown dog tick. E. chaffeensis and E. ewingii are transmitted by the lone star tick and can cause similar illness.

Clinical signs. Ehrlichiosis can present in acute, subclinical, or chronic phases. The acute phase (1–3 weeks after infection) typically brings fever, lethargy, enlarged lymph nodes, and sometimes bleeding abnormalities (petechiae, epistaxis) due to thrombocytopenia. The subclinical phase may last months to years with no visible signs. Chronic ehrlichiosis can cause persistent bone marrow suppression, severe anemia, bleeding disorders, weight loss, and neurologic signs.

Diagnosis. SNAP tests detect antibodies to Ehrlichia species. As with anaplasmosis, PCR on whole blood can identify active infection during the acute phase, but sensitivity drops as the organism becomes sequestered in tissues. Paired serology with rising titers is confirmatory. CBC often shows thrombocytopenia, anemia, and sometimes pancytopenia in chronic cases.

Treatment. Doxycycline for 28–30 days is the standard recommendation. Dogs in the acute phase usually respond well. Chronic ehrlichiosis carries a more guarded prognosis and may require longer treatment courses, blood transfusions, and supportive care.

Co-infection is common

Dogs in tick-endemic areas are frequently exposed to multiple tick-borne pathogens simultaneously. Co-infection with B. burgdorferi and A. phagocytophilum is well documented, particularly in the Northeast and Upper Midwest, because both organisms share the blacklegged tick vector. A dog that tests positive for more than one agent on a SNAP panel is not unusual — and the clinical picture can be more complex when multiple pathogens are involved.

Co-infection may produce more severe or atypical signs than either organism alone, and it can complicate the decision about whether to treat. If your dog has multiple positive results, your veterinarian will evaluate the combination of clinical signs, laboratory findings, and geographic risk to build a treatment plan.

How annual tick-borne disease screening works

Most veterinary clinics use a combination antigen-antibody SNAP test — the SNAP 4Dx Plus or its equivalent — as part of annual wellness screening. These tests detect:

• Heartworm antigen (active infection)
• Borrelia burgdorferi antibodies (Lyme exposure)
• Anaplasma species antibodies (anaplasmosis exposure)
• Ehrlichia species antibodies (ehrlichiosis exposure)

This is the same testing platform covered in our 4DX test for dogs article. A single blood draw gives your veterinarian a snapshot of your dog's exposure history and active heartworm status.

Understanding antibody tests. The tick-borne disease results on these screens detect antibodies, not the organism itself. Antibodies mean the dog's immune system has seen the pathogen. They do not confirm active disease, and they do not tell you when exposure occurred. A dog with a positive antibody result may have cleared the infection months ago, may be currently infected and asymptomatic, or may be infected and about to develop clinical signs.

This is why a positive screening test always requires follow-up evaluation by your veterinarian — not automatic antibiotics.

Do all positive dogs need treatment?

No. Treatment decisions depend on:

1. Clinical signs. A dog with fever, lameness, or thrombocytopenia and a positive test warrants treatment. A clinically normal dog with a positive test may just need monitoring.
2. Laboratory findings. Proteinuria in a Lyme-positive dog triggers a workup for Lyme nephritis. Thrombocytopenia in an Anaplasma- or Ehrlichia-positive dog supports active infection.
3. Risk profile. A dog in a high-prevalence area with known tick exposure and a rising quantitative C6 titer may be treated even without overt clinical signs.
4. Antibiotic stewardship. Treating every positive test with doxycycline contributes to antibiotic resistance. Veterinarians weigh the benefit-risk balance for each patient.

The key question to ask your veterinarian is not just "is the test positive?" but "does my dog have evidence of active disease that warrants treatment?"

Prevention: the strategy that matters most

Prevention is more effective and less costly than treating tick-borne disease. The current standard of care includes:

Year-round tick prevention

CAPC now recommends year-round tick prevention for all dogs, everywhere in the United States. This aligns with the geographic expansion documented in the 2026 forecast. Options include oral isoxazoline products (NexGard, Bravecto, Simparica, Credelio), topical products, and collars. Our isoxazoline safety guide covers the safety profile of the oral class.

Lyme vaccination

A Lyme vaccine (e.g., Nobivac Lyme) is available and recommended for dogs in or traveling to high-risk areas. The vaccine does not replace tick prevention, but it adds a layer of protection, particularly against Lyme nephritis. Vaccinated dogs can still test positive on the C6-based SNAP test if they have been naturally exposed — the C6 test does not cross-react with the vaccine.

Annual screening

Annual SNAP testing identifies exposed dogs early, before clinical disease develops. This is especially important for Lyme disease, where early identification of proteinuria can change the outcome.

Tick checks and habitat management

Removing ticks promptly reduces transmission risk. For Lyme disease, the blacklegged tick generally needs to be attached for 24–48 hours before transmitting B. burgdorferi. Checking your dog after outdoor activity — especially in wooded or brushy areas — remains a practical and effective measure.

What to ask your veterinarian

• "Is my dog's positive test result from a screening panel, and what follow-up tests do you recommend?"
• "Does my dog have clinical signs or lab abnormalities that suggest active disease?"
• "Should we run a quantitative C6 titer, a urinalysis, or a CBC to help decide on treatment?"
• "Is the Lyme vaccine appropriate for my dog based on where we live and travel?"
• "Which tick prevention product fits my dog's health profile and lifestyle?"

These questions give your veterinarian the context they need to move from a positive test result to an individualized plan. A positive screen is a starting point, not a diagnosis.

Sources

• Companion Animal Parasite Council. 2026 Annual Pet Parasite Forecasts. https://capcvet.org/articles/2026-annual-pet-parasite-forecasts
• CAPC. Understanding the CAPC Forecast Maps. https://capcvet.org/articles/understanding-the-capc-forecast-maps
• dvm360. CAPC Forecasts Expanding Parasite Risk in 2026. https://www.dvm360.com/view/capc-forecasts-expanding-parasite-risk-in-2026-including-lyme-disease-heartworm-ehrlichiosis-and-anaplasmosis
• AVMA. Growing Risk of Pet Parasites Means Year-Round Prevention Essential, CAPC Says. https://www.avma.org/news/growing-risk-pet-parasites-means-year-round-prevention-essential-capc-says
• FDA. Ticks and Lyme Disease: Symptoms, Treatment, and Prevention. https://www.fda.gov/consumers/consumer-updates/ticks-and-lyme-disease-symptoms-treatment-and-prevention
• VCA Animal Hospitals. Anaplasmosis in Dogs. https://vcahospitals.com/know-your-pet/anaplasmosis-in-dogs
• dvm360. Tick-borne Diseases: Lyme Borreliosis and Anaplasmosis (Proceedings). https://www.dvm360.com/view/tick-borne-diseases-lyme-borreliosis-and-anaplasmosis-proceedings
• CAPC. Tick-Borne Diseases Reported in Most States. https://capcvet.org/articles/tick-borne-diseases-reported-in-most-states-expert-says
• Elanco / Your Pet and You. Dog Tick Diseases on the Rise: Ehrlichiosis and Anaplasmosis. https://yourpetandyou.elanco.com/us/parasites/ehrlichiosis-and-anaplasmosis-in-dogs
• Bowman D, et al. Burden of Tick-borne Infections on American Companion Animals. Parasites & Vectors, 2009. https://pmc.ncbi.nlm.nih.gov/articles/PMC2802828