What is Rocky Mountain Spotted Fever?

Rocky Mountain Spotted Fever (RMSF) is a bacterial disease (Rickettsia rickettsii) transmitted by ticks of the genus Dermacentor. These ticks are most common during spring and summer. Rocky Mountain Spotted Fever is very rare in San Mateo County.

How is Rocky Mountain Spotted Fever transmitted?

Rocky Mountain Spotted Fever is transmitted through the bite of an infected tick.

If I am bitten by a tick, will I get Rocky Mountain Spotted Fever?

There have been cases of this disease in San Mateo County, but it is extremely rare.

What are the signs and symptoms of Rocky Mountain Spotted Fever?

• Sudden onset of fever
• Nausea
• Vomiting
• A red, spotted, non-itchy rash covering much of the body including the palms of hands and soles of feet
• Lack of appetite
• Severe headache

The rash first appears six days after the onset of fever and is often not present or may be very subtle when the patient is initially seen by a physician. Younger patients usually develop the rash earlier than older patients. Most often it begins as small, flat, pink, non-itchy spots (maculae) on the wrists, forearms, and ankles. These spots turn pale when pressure is applied and eventually become raised on the skin. This type of rash occurs in only 35% to 60% of patients with Rocky Mountain Spotted Fever. The rash involves the palms or soles in as many as 50% to 80% of patients; however, this distribution may not occur until later in the course of the disease. As many as 10% to 15% of patients may never develop a rash.

Three key diagnostic signals are 1) a previous tick bite 2) a rash covering the trunk 3) severe illness with a sudden onset.

Visit the CDC website to learn more about Rocky Mountain Spotted Fever

Page last reviewed: February 20, 2025