The short answer
Yes. Anaplasmosis is established and growing in southern Ontario. It became a reportable disease in Ontario in February 2023, and Halton Region Public Health surveillance found the first locally-detected cases in 2025 — two blacklegged ticks tested positive for Anaplasma phagocytophilum, the first such detections in Halton’s active surveillance program.
What is anaplasmosis?
Anaplasmosis is a bacterial infection caused by Anaplasma phagocytophilum (granulocytic anaplasmosis) and, less commonly in this region, A. platys (cyclic thrombocytopenia). It’s transmitted by the same blacklegged tick (Ixodes scapularis) that carries Lyme disease, so wherever Lyme is established, anaplasmosis tends to follow.
In dogs, anaplasmosis presents with:
- Fever
- Lethargy
- Lameness (often shifting between legs)
- Decreased appetite
- Thrombocytopenia (low platelet count) — sometimes with associated bleeding tendencies
- Joint pain or stiffness
Symptoms typically appear within a couple of weeks of a tick bite, although exposure can also be silent. The 4DX SNAP test detects antibodies to A. phagocytophilum and A. platys alongside Lyme, heartworm, and ehrlichiosis.
Why “reportable” matters
Becoming reportable in February 2023 means licensed veterinarians and physicians in Ontario are now required to notify public health authorities when they diagnose a confirmed case. That serves three purposes:
- Better surveillance data. Before reportability, case counts were estimates. Now we have actual numbers.
- Faster public health response. Cluster detection is possible when reporting is mandatory.
- More clinical attention. Clinicians who didn’t routinely consider anaplasmosis in the differential are now prompted to.
The fact that Halton’s surveillance program — which had been finding zero anaplasmosis-positive ticks for years — turned up two positive ticks in 2025 is meaningful. The ticks aren’t new. The pathogen is.
What’s driving the spread
Same drivers as Lyme:
- Mild winters with deep snowpack — near-100% blacklegged tick overwinter survival in Halton since 2020.
- Expanding mouse and deer populations — mice are reservoirs, deer are reproductive hosts for adult ticks.
- Migratory birds — moving infected ticks rapidly along the Lake Ontario flyway.
- Longer warm season — tick activity now spans roughly mid-March through mid-December.
Once A. phagocytophilum arrives in a tick population, it tends to establish quickly because the same biology that supports Borrelia supports anaplasma — same vector, same reservoir hosts, same habitat.
For the broader picture, see our 2026 field guide on ticks in Halton and our breakdown of the drivers behind this year’s tick surge.
What anaplasmosis looks like in clinic
A typical case in our practice might be a dog presenting with sudden lethargy and fever, sometimes with shifting-leg lameness, sometimes with a single swollen joint. Bloodwork shows a low platelet count (thrombocytopenia). The 4DX comes back positive for Anaplasma. Symptoms resolve quickly on doxycycline (typically 4 weeks).
Some dogs are silent positives — antibodies on routine 4DX, no symptoms, no bloodwork abnormalities. These don’t get reflex doxycycline. We monitor and re-check. Asymptomatic anaplasmosis exposure is a household alarm bell, not a treatment trigger.
Can humans get anaplasmosis?
Yes — same tick, same pathogen. Human anaplasmosis is reportable in Ontario as well. Symptoms are similar to Lyme but typically develop faster — fever, headache, muscle aches, fatigue, sometimes nausea. It can be severe in older adults and immunocompromised people.
Tick-borne illness in humans is outside our scope of practice. If you find a tick on yourself or a family member, save the tick, submit it to eTick.ca, and contact your physician or pharmacist. Under Ontario’s Minor Ailments Program, pharmacists can assess high-risk human bites and dispense single-dose doxycycline where appropriate.
What about cats?
Anaplasmosis in cats has been reported but is uncommon and typically mild. There is no routine in-clinic feline screening test for it. If a cat presents with unexplained fever, lethargy, or thrombocytopenia and has tick exposure, we’d send out PCR or specific serology. More on cats and tick-borne disease.
What this means for Halton households
Anaplasmosis isn’t yet at the levels of Lyme in Halton, but it’s on the same trajectory. The practical implications are mostly the same:
- Year-round tick preventive on dogs. Same products that prevent Lyme prevent anaplasmosis.
- Annual 4DX SNAP test. It tests for anaplasmosis alongside Lyme.
- Daily tick checks during peak season.
- Yard management.
- Submit ticks to eTick.ca. Every cat or dog tick contributes to the surveillance dataset that public health uses to track this exact spread.
The Lyme vaccine does not prevent anaplasmosis. So a vaccinated dog still needs a year-round tick preventive — the vaccine isn’t a substitute.
Key takeaways
- Anaplasmosis is established and spreading in Ontario, with the first Halton-detected cases in 2025.
- Reportable in Ontario since February 2023.
- Same vector as Lyme (blacklegged tick) — wherever Lyme spreads, anaplasmosis tends to follow.
- Symptoms in dogs: fever, lethargy, lameness, low platelet count.
- 4DX SNAP test detects anaplasmosis antibodies. Symptomatic dogs treated with doxycycline.
- Same prevention strategy as Lyme — year-round preventive, annual 4DX, daily checks.
- Lyme vaccine does NOT prevent anaplasmosis.
References
- Ontario Animal Health Network. “2024 OAHN Public Health Update.” oahn.ca
- Halton Region Public Health. “Lyme Disease and Other Tick-Borne Diseases.” halton.ca
- Public Health Ontario. “Anaplasmosis.” publichealthontario.ca
- NIH/PMC. “Canine infection with Borrelia burgdorferi, Dirofilaria immitis, Anaplasma spp. and Ehrlichia spp. in Canada, 2013–2014.” pmc.ncbi.nlm.nih.gov
- Companion Animal Parasite Council. “Anaplasmosis in dogs.” capcvet.org