The short answer
Cytauxzoonosis (sometimes called “bobcat fever”) is a fast-moving, frequently fatal tick-borne disease in domestic cats caused by the protozoan parasite Cytauxzoon felis. It kills 40% of treated cats. It is not yet established in Ontario, but the lone star tick that carries it is expanding northward — putting cytauxzoonosis on the watchlist.
What’s actually happening in the disease
Cytauxzoon felis is a protozoan parasite that lives in two phases:
- Schizont phase: the parasite multiplies inside macrophages (a type of white blood cell), causing them to swell and clog small blood vessels in the lungs, liver, spleen, and brain. This is the lethal stage.
- Piroplasm phase: the parasite enters red blood cells, where it can be detected on a blood smear.
Symptoms develop fast — usually 5–10 days after a tick bite — and include:
- Profound lethargy
- High fever (often >41°C / 105°F early, dropping to subnormal late)
- Loss of appetite
- Difficulty breathing
- Pale or jaundiced (yellow) gums
- Dehydration
- Collapse, sometimes coma
A cat that goes from “fine yesterday” to “barely responsive today” with a fever and yellowed gums is a cytauxzoonosis suspect until proven otherwise — but only if you’re in a region where the disease is established.
Where it is — and isn’t
Cytauxzoonosis is endemic in the southeastern and south-central United States, particularly the south Atlantic, Gulf Coast, and lower Midwest. Bobcats are the natural reservoir host, and they typically tolerate the infection. Domestic cats are accidental hosts and tolerate it very poorly.
In Canada, the disease is not established. We have not had documented natural cases in Ontario.
What’s changing is the vectors. The two tick species that transmit Cytauxzoon felis are:
- Lone star tick (Amblyomma americanum) — the more efficient vector. Historically a US southern species, but established populations are now found in southern New York and northern Pennsylvania, with confirmed individual ticks (likely bird-carried) reported in Ontario.
- American dog tick (Dermacentor variabilis) — already established in Ontario; can also transmit Cytauxzoon felis but less efficiently.
Both are expanding their range northward. Active monitoring is in place. The disease isn’t here yet, but the conditions for it to arrive are accumulating.
Why we’re watching closely in Halton
Three reasons to keep cytauxzoonosis on the watchlist for Ontario cat parents:
- The lone star tick is the fastest-moving emerging tick species in eastern North America. Range expansion has accelerated over the past decade in step with milder winters.
- Halton’s tick surveillance is finding new pathogens we haven’t seen before. Anaplasma phagocytophilum turned up in Halton ticks for the first time in 2025. Surveillance catches the next emerging pathogen before it becomes routine clinical reality.
- 40% case fatality with treatment is not a number you want to encounter unprepared. By the time a Halton cat presents with cytauxzoonosis, the treatment window is already narrow.
For the broader picture, see our 2026 field guide on ticks in Halton.
How it’s diagnosed
Diagnosis combines clinical suspicion (a sick cat with appropriate tick exposure history), bloodwork findings (anemia, low platelets, elevated bilirubin), and visualization of piroplasms on a stained blood smear or via PCR. We’d be sending out PCR if we suspected it.
How it’s treated
Treatment of confirmed cytauxzoonosis combines antiparasitic combination therapy with aggressive inpatient supportive care — IV fluids, oxygen, blood transfusion in severe anemia, several days of intensive monitoring. Even with best-case treatment, mortality remains roughly 40%. Treatment is expensive and outcome is uncertain. This is part of why prevention matters so much.
Prevention — what to do now
Even though cytauxzoonosis isn’t yet in Ontario, the prevention story for cats is straightforward:
- Year-round cat-safe tick preventive for outdoor and indoor-outdoor cats. Cat-safe options include NexGard Combo, Revolution Plus, Bravecto Plus, and Credelio Cat.
- Submit any tick found on your cat to eTick.ca. Particularly important — if it’s identified as a lone star tick (Amblyomma americanum) or American dog tick (Dermacentor variabilis), that changes our cytauxzoonosis risk picture for the household.
- Daily tick checks on outdoor cats. Pair with petting or treat sessions.
- Watch for warning signs after a tick bite. Pale gums, jaundice, fever, profound lethargy, breathing difficulty — call us immediately.
What might change our approach
- Confirmed lone star tick population in Ontario. Currently we see occasional individuals (often migratory bird carried). An established breeding population would shift cytauxzoonosis from watchlist to active concern.
- First documented domestic cat case in Canada. Hasn’t happened. If it does, we’d update prevention messaging immediately.
- A vaccine. None currently exists for cytauxzoonosis. Research is ongoing.
Key takeaways
- Cytauxzoonosis is a fast-moving, often-fatal tick-borne disease in cats — 40% case fatality even with treatment.
- Caused by the protozoan Cytauxzoon felis, transmitted by lone star tick (primarily) and American dog tick.
- Not yet established in Ontario, but the vector ticks are expanding northward.
- Bobcats are natural reservoirs; domestic cats are accidental hosts and tolerate the infection very poorly.
- Symptoms develop fast: profound lethargy, high fever, pale or jaundiced gums, breathing difficulty.
- Prevention strategy: year-round cat-safe tick preventive, daily checks, prompt eTick.ca submission of any tick on a cat — particularly to identify lone star or American dog ticks.
References
- VCA Animal Hospitals. “Cytauxzoonosis in Cats.” vcahospitals.com
- Merck Veterinary Manual. “Cytauxzoonosis in Cats.” merckvetmanual.com
- Veterian Key. “Chapter 84: Feline Cytauxzoonosis.” veteriankey.com
- Ontario Animal Health Network. “2024 OAHN Public Health Update.” oahn.ca
- US CDC. “Lone Star Tick (Amblyomma americanum).” cdc.gov