Ticks and tick-borne diseases
Family: Argasidae, Ixodidae
Ticks are classified as arachnids, which includes spiders, scorpions, and mites. There are two families of ticks that occur in Colorado, known as hard ticks (Ixodidae) and soft ticks (Argasidae). Ticks are parasites that feed on the blood of animal hosts, and after engorging their bodies become distended and pea shaped. In Colorado, they are most common at higher elevations.
- In Colorado, ticks are most likely to be encountered at higher elevations.
- Several disease-causing pathogens are transmitted to humans through tick bites, including Colorado tick fever, Rocky Mountain spotted fever, tick-borne relapsing fever, and tularemia. In some instances, it is advised to seek medical attention when symptoms occur after a tick is discovered embedded in the skin.
- Lyme disease is not a cause of concern in Colorado since the tick species that transmit the disease-causing pathogen do not inhabit the state.
- It is recommended to avoid tick habitats in the spring and early summer when ticks are the most active. Wearing protective clothing, such as long sleeves and pants, is also advised.
Adults of Rocky Mountain wood tick, a species that is the most frequently encontered Colorado and can transmit several disease-causing pathogens. Image credit: Mat Pound, USDA Agricultural Research Service, Bugwood.org
Adult of American dog tick, which is a species of hard tick. Image credit: Susan Ellis, USDA APHIS PPQ, Bugwood.org
Adults of the brown dog tick, which is a species of hard tick. Image credit: Mat Pound, USDA Agricultural Research Service, Bugwood.org
Adult of winter tick, which is a species of hard tick. Image credit: Mat Pound, USDA Agricultural Research Service, Bugwood.org
Adult of Otobius lagophilus, which is a species of soft tick. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adults of the ear tick (Otobius megnini), which is a species of soft tick. Image credit: Mat Pound, USDA Agricultural Research Service, Bugwood.org
Rash outbreak caused by Rocky Mountain spotted fever. Image credit: Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, Bugwood.org
Life history and habits
All ticks have four life stages: egg, larva, nymph, and adult. Eggs are laid in masses, often in the thousands. Larvae have six legs, and after feeding and developing will molt into an eight-legged nymph. After continuing to feed and develop the nymph molts into an adult, with adult male and female ticks differing in size. Some species can complete their life cycle in one year, while others reach maturity after two or three years. Most tick bites in Colorado occur in spring through summer. Ticks will perch on vegetation when a suitable host is detected nearby, a behavior that facilitates contact with the potential host.
There are 27 species of ticks known to inhabit Colorado, with the most encountered being the Rocky Mountain wood tick (Dermacentor andersoni). Ticks can transmit several pathogens to humans, which can result in Colorado tick fever, Rocky Mountain spotted fever, tick-borne relapsing fever, tularemia, and less commonly, tick paralysis. Fortunately, Lyme disease is not present in wild animal populations in Colorado, nor are the tick species capable of transmitting the pathogen causing the disease. All reported cases of Lyme disease are suspected to have originated from exposure in other states.
Hard ticks (Family: Ixodidae)
Hard ticks are distinguished from soft ticks by the hard plate (scutellum) on the back behind the head. Their mouthparts are positioned in front of the head and easily visible. There are four species of hard tick of concern in Colorado: The Rocky Mountain wood tick (Dermacentor andersoni), the American dog tick (Dermacentor variabilis), the brown dog tick (Rhipicephalus sanguineus), and the winter tick (Dermacentor albipictus).
Soft ticks (Family: Argasidae)
Soft ticks do not have a hard plate (scutellum) on their back and have a less uniformly rounded body. Their mouthparts are positioned beneath the head and are not visible from above. Soft ticks feed more frequently than hard ticks and tend to be associated with their hosts continuously. In Colorado, there are three soft tick species of concern. They are Ornithodorus hermsi, Otobius lagophilus, and the ear tick (Otobius megnini).
Colorado tick fever
Colorado tick fever is caused by a virus and the most common tick-borne disease in Colorado. The virus is transmitted by the Rocky Mountain wood tick, with symptoms typically developing between three and seven days after the tick bite. The flu-like symptoms include headaches, fever, chills, and fatigue. In some cases, individuals experience a second occurrence of symptoms after an initial recovery period. Symptoms typically disappear within a few days or weeks. Although rare, this virus can infect the nervous system and cause serious complications.
Rocky Mountain spotted fever
The Rocky Mountain wood tick is the main vector of the bacterium (Rickettsia rickettsii) that causes Rocky Mountain spotted fever, but it can also be transmitted by the American dog tick and brown tick. While this disease can be life threatening, it is a rare occurrence in Colorado. Headaches and upset stomach are common symptoms, and a rash typically develops a few days after a fever. Since this disease is caused by a bacterium, antibiotics can be prescribed by a physician. It is important to seek medical attention immediately if these symptoms are present, especially if a tick was embedded in the skin prior to the occurrence of symptoms.
Tularemia is caused by a bacterium (Francisella tularensis) that can infect cats and many wild mammals such as rabbits, prairie dogs, and muskrats. It is transmitted by the Rocky Mountain wood tick and American dog tick. Most cases of tularemia occur in Boulder and Larimer counties. In most years there are very few cases of infection in humans.
Tick-borne relapsing fever
Tick-borne relapsing fever is also caused by a bacterium (Borrelia hermsii). The soft tick, O. hermsi, is the vector of this disease and is typically associated with nesting rodents. Contact with humans is most likely to occur in rustic cabins when rodents occupy the space behind walls. While this disease can occur year-round, it is rare in Colorado. Symptoms include fever, headache, and aching muscles and joints.
This potentially serious condition is rare and occurs when certain ticks such as the Rocky Mountain wood tick remains attached for long periods of time. Early symptoms include difficulty walking, limb numbness, and difficulty breathing. Fortunately, symptoms tend to abate after the tick is removed. This disease can also affect other mammals such as dogs and cattle.
Avoiding tick habitat is an effective way to prevent encounters. Tick activity is the greatest in the spring and early summer, and they are the most concentrated near their animal hosts in brushy areas along field and woodland edges or in commonly traveled paths through grasses and shrublands. During tick season, it is recommended to walk in the center of paths and avoid brushing on vegetation which may harbor perching ticks in search of a blood meal.
Wearing protective, light-colored clothing is recommended when contact with ticks is possible, as it will make ticks easier to spot after they have been picked up. Long pants and long-sleeved shirts can prevent ticks from attaching to the skin. Pulling socks over the bottom of pant legs will also help prevent ticks from contacting skin.
The most effective tick repellents are DEET, picaridan, IR3535, permethrin, and lemon eucalyptus oil. These repellents are applied to clothing or on the skin directly and are most effective when applied to areas of the lower body that are likely to be in contact with ticks. It is recommended that formulations with lower concentrations be used on children, and that the repellent be applied to clothing instead of skin. Avoid applying to areas likely to contact the mouth, such as the hands, and avoid applying to wounds or irritated skin. Always wash or bathe affected areas after use, especially on children.
When an embedded tick is discovered, it should be removed as soon as possible through the following steps:
- Using a pair of fine tipped tweezers, grasp the tick as close to the skin as possible. Fingers can be used if tweezers are not available and covered with thin plastic or tissue to prevent transmission of disease-causing organisms harbored by the tick.
- Once firmly grasped, pull the tick steadily, taking care to apply force straight away from the skin. Avoid crushing the tick if possible.
- After removing the tick, disinfect the feeding site and wash your hands when done. Live ticks should be disposed of by placing in alcohol, flushing down the toilet, placing in a sealed bag or container, or wrapping tightly in tape.
Note: Covering embedded ticks with petroleum jelly or touching with a hot match are slow to work, if at all. Tick removal should be done quickly, and it is therefore recommended to physically remove the tick using the steps outlined above. If a rash or fever develops within several weeks after a tick is removed, it is recommended to see a doctor and provide details about the tick bite, such as when it occurred and where the tick was most likely acquired.
Cranshaw, W., F. Pearis, and B. Kondratieff. 2019. Colorado Ticks and Tick Borne Diseases. Colorado State University – Extension. Available https://extension.colostate.edu/topic-areas/insects/colorado-ticks-and-tick-borne-diseases-5-593/