Rocky Mountain spotted fever

Rocky Mountain spotted fever (RMSF) is caused by a bacterium that invades the cells lining small blood vessels. The damage to those vessels produces the characteristic rash and, when severe, drives organ damage. The name is misleading — most US cases come from the southeast, not the Rockies — but the underlying biology is the same wherever the dog tick and wood tick live.

Transmission generally requires several hours of attachment. The dog tick — Canada’s most widespread tick — is a much more common bite than the blacklegged tick, but the probability that any one bite leads to RMSF is very low.

Symptoms come on suddenly, two to fourteen days after the bite:

• Sudden high fever
• Severe headache
• Deep muscle pain
• Nausea, vomiting, loss of appetite
• A rash starting on the wrists and ankles around day 3–5 — small flat red spots
• The rash may become petechial (tiny purple-red dots that don’t fade under pressure)

The wrists-and-ankles starting point is unusual for most rashes and is one of RMSF’s tell-tale signs. About 10 percent of cases never produce a rash, which makes them dangerously easy to miss.

Emergency room — same day. Any combination of sudden high fever, severe headache, and a rash on the wrists and ankles within two weeks of a tick bite is a medical emergency. RMSF can progress to multi-organ failure within days.

Tell the clinician about the tick bite directly. RMSF treatment must start before lab confirmation — the delay of waiting for results can be fatal.

Doxycycline, started immediately on clinical suspicion. This is true even for young children — the historical concern about doxycycline staining children’s teeth doesn’t apply at the short courses used here, and the risk of withholding it is far greater. Confirmation comes from blood tests, but treatment doesn’t wait.

Confirmed Canadian cases are very rare and tend to be travel-acquired in the United States. The vectors — the American dog tick and the Rocky Mountain wood tick — are common across central and western Canada, but the bacterium itself appears to circulate at very low levels in Canadian tick populations.

That said: a sudden fever and wrists-and-ankles rash after a dog tick or wood tick bite is worth taking seriously, wherever you are.

Public Health Agency of Canada; CDC tick-borne disease reference guide.