Colorado Tick Fever: What is it? Symptoms, Diagnosis, Treatment, and Prevention

History

Colorado tick fever was first recognized as a separate clinical entity in 1930. However, the CTFV virus was only isolated in 1943 from a patient with an acute febrile illness in the State of Colorado, USA. 

Before the disease was discovered, it was presumed to be a milder form of the Rocky Mountain spotted fever, a more deadly tick-borne disease. Colorado tick fever is also known by other names like American mountain fever, tick fever, mountain fever, and mountain tick fever [1,2].

Epidemiology

CTF is one of the most common viral tick-borne diseases endemic to the western United States. It is a double-stranded RNA virus belonging to Coltivirus genus [3]. In addition to humans, CTFV can cause infections in rodents and a few other mammals [4].

The primary vector of CTFV is the Rocky Mountain wood tick (Dermacentor andersoni). These ticks are found in the western regions of the US and Canada, 4,000 to 10,000 feet above sea level [2,3]. CTF cases are usually reported during the spring and summer since the D. andersoni ticks are most active during this time [2].

Deaths and other serious complications due to CTF are quite rare [2]. A few deaths have been reported in the past, with most of them in children [5]. Though fatalities are low, around 30% of people infected by the CTF virus require hospitalization [3].

The number of CTF cases has decreased significantly since the 1980s. From 1988 to 199, the CDC received reports of about 65 cases a year. Case numbers declined three-fold during the next decade, with an average of 14 cases per year [6]. And finally, from 2010 to 2019, a total of 59 cases were reported to the CDC — that’s just six cases per year on average [7].

By 2020, CTF cases were reported in the states of Arizona, Colorado, Idaho, Montana, New Mexico, Oregon, South Dakota, Utah, and Wyoming. However, CTF is not a nationally notifiable disease, so states are not required to report CTF cases to the CDC. This could also be a reason for the recent decrease in the number of reported cases [7].

How is it Spread?

Studies show that up to 90% of patients diagnosed with CTF were exposed to ticks before the onset of the disease. Although rare, another way to spread the disease is through blood transfusions [3]. Colorado tick fever is spread by:

• Tick bite: Both male and female Rocky Mountain wood ticks can infect humans and animals. When an uninfected tick feeds on an infected animal, it can get infected with the CTF virus. The virus can remain in the body of the tick for a lifetime — on average, two to three years. Humans and animals can become infected with CTFV when bitten by an infected tick [8].
• Blood transfusion: The CTF virus can be transmitted from one person to another through blood transfusions. While this mode of transmission is quite rare, it has been documented to have occurred in the past. Due to the prolonged presence of the virus in the blood, a person diagnosed with CTF should not donate blood for up to six months after getting infected [3].

Biology of the Disease

The CTF virus can infect human red blood cells for a long time. One potential reason is that the virus enters and replicates within CD34+ stem cells in the bone marrow. While researchers are still trying to figure out exactly what these cells do, the CD34+ cells do play a role in hematopoiesis, the production of all cells in the blood. As a result, when the amount of CTF virus increases in the CD34+ cells, it can decrease the number of red blood cells, white blood cells, and platelets (thrombocytes) and cause bone marrow suppression [4].

Diagnosis

Diagnosing Colorado tick fever can be quite challenging because the symptoms, like fever, chills, and more, are common in many other infections. As a result, laboratory testing is the best way to detect the CTF virus. Testing is usually done at public health laboratories because these labs are most likely to have the specialized testing methods needed to trace the virus [5]. 

Regardless of which lab conducts the test, clinicians need the following information to help with differential diagnosis:

• Clinical signs and symptoms experienced by the patient
• Travel history (epidemiology) – places visited and travel dates
• List of outdoor activities that the patient participated in the recent past
• Any history of potential tick exposure [9].

The methods below help with arriving at a final diagnosis: 

• RT-PCR (Reverse-Transcriptase Polymerase Chain Reaction) – Sensitivity is more during the early stages of the disease [9].
• Serological testing – Sensitivity is greater during the later stages of the disease (at least a week after the onset of symptoms) when the antibodies become detectable [5].
• Nucleic Acid Amplification Test (NAAT) – The test is more sensitive during the first few days following the onset of symptoms, as the nucleic acids will be more detectable at this time [5].

Symptoms

Following the bite of an infected tick, it could take anywhere from 1 to 14 days for symptoms to appear [3]. Colorado tick fever symptoms can last for weeks. Depending on the severity of the disease, people may persistently experience fatigue and weakness for months [6].

Unlike other tick-borne diseases, CTF is a biphasic acute febrile illness (occurs in two phases). In this case, patients will initially experience fever and then a short period of relief followed by another episode of fever [5]. Other clinical symptoms of CTF include:

• Chills 
• Headache 
• Fatigue 
• Skin rashes
• Cytopenia (a reduction in the number of mature blood cells), including:
  • Anemia – a lower number of red blood cells
  • Leukopenia – low white blood cell count
  • Thrombocytopenia – reduction in the number of platelets in the blood [3].

Treatment

Currently, no antiviral treatment is available to treat CTF, so treatment is usually supportive [8]. However, hospitalization is required in case of severe illness. Hospitalized patients are given intravenous fluids and medicines to reduce pain and fever [10].

Prevention

There is no vaccine to help prevent Colorado tick fever infections [8]. The preventive measures to lower your risk of getting Colorado tick fever are: 

• Keep your yard free of leaf litter, tall grass, and trash (old furniture and other unused appliances). Mow the lawn regularly [11].
• Avoid visiting places filled with thick grass and brushy or wooded areas since tick density is usually high in these places. 
• Cover your body entirely and wear closed shoes if you have to spend time outdoors or visit places where ticks are prevalent.
• You can use tick repellents that can be sprayed on clothes and tick repellents containing DEET on the exposed parts of your body for extra protection.
• Sometimes, ticks might cling to your clothes, and if you wear them again, the ticks might come in contact with your body. So, it is better to tumble-dry your clothes at high heat each time you return from any outdoor activity.  
• Make it a practice to examine your body after spending time outdoors and remove any ticks attached to the body. Check and remove ticks early to lower your risk of getting bitten. 
• Shower once you are back from outdoor activity. 

When you’re outdoors during the spring and summer, there are other tick-borne viruses to watch out for, like the most common tick-borne infection in the United States, Lyme disease. Other diseases spread by ticks include ehrlichiosis, babesiosis, Powassan virus disease, anaplasmosis, and more.