Tick Activity Increases with the Return of Spring

This article is adapted from “Hot topics in Epidemiology, April-2014” produced by the Disease Control and Environmental Epidemiology Division of the Colorado Department of Public Health and Environment

Colorado residents who spend time outdoors this spring and early summer should avoid tick bites to prevent some diseases. The primary vector of tick-borne diseases in our state is the Rocky Mountain wood tick, Dermacentor andersoni. The Rocky Mountain wood tick is generally found in sagebrush, juniper and pine habitats in areas that have moderate amounts of shrubs and grasses from elevations of 4,000 to 10,000 feet. The adult tick readily feeds on human blood, and can transmit Colorado tick fever (CTF) and Rocky Mountain spotted fever (RMSF). Cases of CTF are common in Colorado, whereas cases of RMSF are not. CTF is not a reportable condition, but RMSF is.

Adult Rocky Mountain wood ticks usually come out from their overwintering sites beginning in March and tick activity peaks in April. We expect most tick activity to end in June because of hot, dry conditions though this is weather dependent. Ticks wait on grasses and shrubs for animals (or people) to pass by and brush up against the vegetation, than the tick hitches a ride. The tick crawls up until it finds an area of skin to bite. Ticks are often found behind the knees, at the waistband and groin area, around the armpits and at the nape of the neck.

Both CTF virus and the bacteria that causes RMSF are found in rodents in nature. Immature ticks (larvae and nymphs) feed on infected rodents and become infected themselves. Adult ticks will feed on larger animals such as deer and humans. People are at risk of getting sick when an infected adult Rocky Mountain wood tick bites them. Unlike people, rodents and other animals infected by these disease agents do not become sick.

If you are bitten by a tick carrying CTF virus or the RMSF bacteria you may become ill. The time from the tick bite to when symptoms begin can be as short as one to two days, or as long as two weeks. Symptoms for these two diseases are similar, with patients experiencing fever, chills, headache, muscle aches, and lethargy. Less than 20% of people with CTF will have a rash on their body, whereas up to 90% of RMSF patients will have a rash. Many patients with CTF will become better on their own within a few weeks, and there is no treatment because CTF is caused by a virus. Rocky Mountain spotted fever is a more serious disease than CTF, it is caused by bacteria and should be treated with antibiotics as soon as possible. People should consult their healthcare provider if they develop a fever and become ill after a tick bite or after having been in tick habitat. Many people who become ill do not recall being bitten by a tick, so they should be sure to mention that they’ve been in tick habitat even if they don’t think they were bitten by a tick.

Recommendations for avoiding tick bites include using a tick repellent and taking a shower after being outdoors. A factsheet about ticks and tick-borne diseases in Colorado is available from the Colorado State University Extension Office. Additional information from the Centers for Disease Control and Prevention (CDC) about avoiding tick bites can be found here. Information about different tick-borne diseases found in the United States is available from the CDC here.