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Colorado tick fever is also known as mountain tick fever, American tick fever, and Rocky Mountain tick fever.

Colorado tick fever (CTF) is a viral infection that develops after a bite from a Rocky Mountain wood tick.

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Colorado tick fever can be caused by an infected Rocky Mountain wood tick biting a human.
Image credit: CDC / Dr. Christopher Paddock, Public Health Image Library

Formally recognized as a virus in 1946, CTF occurs mostly in people living in or visiting the western part of the United States (U.S.) and western Canada, according to the American Centers for Disease Control and Prevention.

Hundreds of cases of CTF are reported yearly in the U.S. The actual number of cases may be higher, as symptoms often go unrecognized, which means the condition remains undiagnosed.

There are not enough research studies on CTF to know approximately how many cases of CTF there are in the U.S. and Canada each year.

One study from 2010 reported 91 confirmed cases of CTF from 1995 to 2003, indicating a 2.7 percent annual incidence for a population of one million. The annual incidence, however, appeared to decrease during the 9-year study period.

Men were found to have a higher risk than women, and the highest rates of infection were in people above age 51.

Humans can get CTF if bitten by an infected Rocky Mountain wood tick. Small animals, including squirrels and chipmunks, may also carry the virus.

While rare, there have been a few cases where CTF was caused by a blood transfusion.

Risk factors for being bitten by a Rocky Mountain wood tick include:

  • Living or traveling in Rocky Mountain forest areas that are 4,000-10,000 feet above sea level. That includes the following western states of the U.S.: Montana, Idaho, Wyoming, Colorado, Utah, and Oregon.
  • Living in or visiting these areas from April to September, especially if hiking or camping.

Ticks appear to be most active in springtime, as this is the time when health officials see a significant number of reported tick bites.

If a person finds a tick on their body, they should remove it right away before it attaches itself to the skin.

If it has already attached itself, it should be grabbed as close to the skin as possible and pulled straight. Tweezers are the easiest way to do this. If tweezers are not available, it is acceptable to remove a tick by hand.

It is important to remove a tick as quickly as possible. The longer the tick is attached, the more likely it is to pass on the virus.

Once a person has removed the tick, it is important to wash the affected area with soap and water. Anyone who has recently had a tick bite should pay attention over the weeks that follow for CTF symptoms.

A person should seek medical attention if a tick has buried itself deep in the skin or if they have difficulty removing it by themselves. Any sign of muscle weakness or paralysis requires emergency medical attention.

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The first symptom of CTF may be a high fever that subsides and then returns.

The symptoms of CTF start within 3 to 5 days after a tick bite.

The first noticeable symptom is a fever that may last for a couple of days, subside, and then return for another few days. This called a “saddleback fever.”

In some cases, temperatures associated with CTF can be high, reaching 105°F (40.5°C). The fever may be accompanied by:

  • a headache (usually behind the eyes)
  • chills and sweating
  • sensitivity to light
  • skin or muscle pain
  • malaise (a general feeling of being unwell)
  • extreme fatigue

Other symptoms may include:

  • weakness
  • nausea
  • loss of appetite
  • vomiting
  • diarrhea
  • abdominal pain
  • rash

Most often, symptoms will dissipate without treatment, but complications can occur in some people. Potential complications include:

  • meningitis, an infection of the brain and spinal cord membranes
  • encephalitis, or swelling of the brain
  • bleeding that has no known cause

While children usually recover from CTF faster than adults, they also have a higher risk of complications. Adults, however, may have mild symptoms that linger for weeks or months.

Deaths due to complications are rare but have occurred.

Doctors will diagnose CTF after a physical exam, medical history, and blood tests. People who suspect they have CTF should inform their doctors of recent travel to at-risk locations, and of recent camping or hiking trips.

Bites from other ticks produce similar symptoms, and a doctor will need to rule out other tick-borne diseases before making a definitive diagnosis of CTF.

There is no specific treatment for CTF. Symptoms of CTF, such as fever and pain, may be treated with over-the-counter medications, such as acetaminophen and ibuprofen.

Doctors may check for bacterial infections and, if necessary, administer intravenous fluids. In some cases, an infectious disease specialist may be consulted.

If the person still has a tick on their body, the doctor will remove it with tweezers near the point of attachment. They will try to avoid any twisting or squeezing as they remove the tick to prevent leaving parts of the tick behind.

A person diagnosed with CTF should continue treating the fever and pain until they are instructed by a doctor to stop. They should not donate blood or bone marrow for at least 6 months.

People should let their doctors know of any weakness and fatigue lasting several weeks after the tick bite.

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Checking pets for ticks is recommended to minimise tick exposure.

It is a good idea to take precaution against ticks, especially between April and September when ticks are more active.

The best way avoid ticks when camping or hiking is to avoid tall grass and wooded areas and to use hiking trails where possible.

Tick repellents minimize the chance of a tick finding its way to exposed skin. There are also products available for treating clothing and equipment.

Additional ways to minimize tick exposure are:

  • Bathing and showering after coming indoors.
  • Conducting tick checks with a full-length mirror. Parents should check for ticks behind a child’s ears, in the belly button, behind the knees, between the legs, around the waist, and in the hair.
  • Checking pets and equipment for ticks.
  • After a camping or hiking trip, putting clothes in a dryer on a high heat to kill any ticks on dry clothing. Leave wet clothes in the dryer for a longer period.
  • Washing all clothes in hot water in a washing machine.

It is important to note that pets, horses, and livestock are exposed to more ticks than humans. Even if they are exposed to ticks, it may take longer before they are bitten because of their dense fur.

People who spend time in stables and people who keep pets at home are at risk of encountering a tick from another animal. It is important to protect pets and livestock with frequent tick checks, tick control medicines, vaccinations, and by limiting their exposure to high-risk areas.

You can purchase tick repellent online.

Colorado tick fever generally improves on its own without treatment. Medications are available to reduce uncomfortable symptoms, such as fever and pain. Most people recover within a couple of weeks and children generally recover faster than adults.

Complications, including death, are rare. Some sources claim that if a person has had Colorado tick fever once, they are unable to get it again.