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 rash and, when severe, organ damage. The name is misleading (most US cases come from the southeast, not the Rockies), but the disease is the same wherever the dog tick and wood tick live.

Transmission generally requires several hours of attachment. The dog tick is Canada’s most widespread tick and bites people far more often than the blacklegged tick, but the odds that any one bite leads to RMSF are very low.

Symptoms come on suddenly, 2–14 days after the bite:

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

Few rashes start at the wrists and ankles, which makes that pattern a strong clue. About 10% of cases never produce a rash at all, and those are easy to miss.

Emergency room, same day. Any combination of sudden high fever, severe headache, and a rash on the wrists and ankles within 2 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. Treatment must start before lab confirmation. Waiting for results can be fatal.

Doxycycline, started immediately on clinical suspicion. That includes young children. The old worry about doxycycline staining children’s teeth doesn’t apply to the short course used here, and withholding it is far riskier. Blood tests confirm the diagnosis later, but treatment doesn’t wait.

Confirmed Canadian cases are very rare and tend to be travel-acquired in the United States. Both ticks that carry it, 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 ticks.

Even so, treat a sudden fever and a wrists-and-ankles rash after a dog tick or wood tick bite as serious, wherever you are.

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