Case–Control Study

Testing for CTF virus infection at CDC’s Arbovirus Diagnostic Laboratory (Fort Collins, CO, USA; Division of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases) is performed at the request of state health departments. Testing uses reverse transcription PCR (RT-PCR) on serum or cerebrospinal fluid (CSF) samples collected <14 days after symptom onset and plaque-reduction neutralization test (PRNT) on specimens collected >14 days after symptom onset, as previously described (11,12). During the period under investigation, we conducted RT-PCR testing using primers targeting segment 3 of the viral genome (12). We based neutralizing antibody titers on the highest dilution of serum that reduced viral plaque formation by >90% (we considered a titer >10 to be positive). For specimens collected 8–21 days after symptom onset, we tested using both RT-PCR and PRNT on a case-by-case basis. We defined laboratory-confirmed recent infection as detection of CTF viral nucleic acid in a specimen or a seroconversion with >4-fold change in virus-specific neutralizing antibody titers between paired acute and convalescent serum specimens. We defined a probable infection as detection of virus-specific neutralizing antibodies in a single specimen because the timing of infection cannot be determined. IgM testing typically is not performed because the sensitivity for detection of IgM is lower than for neutralizing antibodies (11).

We identified case-patients and controls through a query of the Arbovirus Diagnostic Laboratory database. We defined a case-patient as a Montana resident who had symptom onset and a confirmed or probable CTF virus infection in 2020. We defined a control as a Montana resident who had symptom onset but negative testing for CTF virus infection in 2020. We contacted persons by phone to describe the investigation and offer participation in a survey collecting data on demographics, clinical symptoms, outdoor recreational and occupational exposures, tick exposures and prevention measures, and changes in behavior related to the COVID-19 pandemic. We collected specific location data for recreational activities and tick exposures whenever possible. Survey questions focused on the potential incubation or 2-week period before symptom onset in 2020 and the same period in 2019.

Healthcare Provider Survey

We developed a survey for healthcare providers and distributed them to staff in 13 hospitals in 10 public health jurisdictions in Montana where either a CTF case was reported in the previous 10 years or CTF virus testing was requested in 2020. The survey collected information on healthcare provider type and demographics, awareness and testing practices for CTF virus and other tickborne diseases, patient encounters for tick bites, and healthcare providers’ interest in educational resources for CTF.

Diagnostic Testing Evaluation

We reviewed diagnostic laboratory methods used for CTF testing during 2020 and in previous years and calculated the proportion of specimens testing positive. To assess whether other tickborne diseases transmitted by Dermacentor andersoni ticks increased in Montana during 2020, we also examined trends in reported cases of Rocky Mountain spotted fever (RMSF) and tularemia.

Data Analysis

We managed data from participant interviews and the healthcare provider survey in Research Electronic Data Capture (Vanderbilt University, https://www.project-redcap.org) (13) and analyzed data using SAS (SAS Institute Inc., https://www.sas.com). For categorical variables, we calculated 95% CIs of the odds ratios (ORs) between case-patients and controls by using the score interval (14); given the rarity of CTF in the population, we expected OR estimates to provide reliable estimates of relative risk. For quantitative variables, we calculated 95% CIs of the differences in means by using the Welch-Satterthwaite approximation for the Student t interval (15). We determined statistical significance from the reported 95% CIs by noting whether they contained the null values of 1 for the ORs and 0 for the mean differences, enabling interpretation in a hypothesis-testing context at a significance level of α = 0.05. We performed geospatial mapping of tick exposure locations by using ArcGIS 10.7.1 (Environmental Systems Research Institute, https://www.esri.com).

Case–Control Study

Of 107 potential participants identified, including 21 case-patients and 86 controls, we were able to contact 36 (14 [67%] case-patients and 22 [26%] controls) who agreed to participate. Both groups consisted of predominantly non-Hispanic White men and had similar age distributions. Most participants from both groups reported having no underlying medical conditions. About one third of participants reported being tested for SARS-CoV-2 when they sought healthcare for their symptoms, and all reported a negative test result (Table 1).

Figure 1

Figure 1. Percentage of Colorado tick fever case-patients (n = 21) and controls (n = 86), by week of illness onset, Montana, USA, 2020.

Symptom onset dates for case-patients occurred during April 1–July 31, 2020; for controls, symptoms occurred during April 1–August 22, 2020. Case-patients were more likely to become symptomatic earlier in the year, having a peak in late April, compared with controls (Figure 1). Symptoms reported by over half of the participants were similar between groups and included fever, fatigue, muscle aches, headache, chills, weakness, and joint pain and swelling (Table 2). Case-patients (11 [79%]) were statistically significantly more likely to report a biphasic illness than controls (3 [14%]).

Almost half of all participants (44% [16/36]) reported spending more time outside in 2020 because of the COVID-19 pandemic compared with the same period in 2019, and we identified no difference between groups. A statistically significantly greater proportion of case-patients (86%) reported spending >4 hours outdoors recreating on weekends than controls (50%) (Table 3). Participants reported several outdoor activities; hiking or walking on unpaved trails was most common for both groups. Case-patients reported fewer instances of yardwork or gardening in the 2-week period before symptom onset than did controls and were statistically significantly more likely than controls to report finding a tick crawling on themselves (100% vs. 41%) or a tick attached to themselves (93% vs. 36%) (Table 3).

Figure 2

Figure 2. Geographic locations in Montana and Idaho, USA, where case-patients (n = 12) and controls (n = 4) reported tick acquisition during the 2 weeks before symptom onset in a Colorado...

Among patients who reported finding ticks on themselves, 86% (12/14) of case-patients and 44% (4/9) of controls provided specific information about where they acquired the tick or ticks (Figure 2). Tick exposures occurred in national forests, in state parks, and on private land in several areas of Montana; 6/12 case-patients reported acquiring their tick or ticks in the vicinity of the Bitterroot Valley (5 in Ravalli County and 1 in Mineral County). One Montana resident with CTF reported acquiring a tick in Idaho (Figure 2). Except for 1 case-patient, all other case-patients reported that their tick exposures occurred in locations >4,000-ft elevation.

Healthcare Provider Survey

A total of 36 healthcare providers from 10 public health jurisdictions responded to the survey. Most respondents were physicians (22 [61%]), worked in outpatient clinics (24 [67%]), and had a nonpediatric specialty (26 [72%]). The median number of years practicing was 15 years (range 1–41 years), including 10 years (range 0.5–40 years) practicing in Montana. Only 2 providers indicated that they had more patients reporting tick bites in 2020 than in 2019, whereas 5 had fewer patients reporting tick bites in 2020 than in 2019. Three healthcare providers reported ordering more CTF virus tests in 2020 than in 2019, and 2 providers ordered fewer CTF tests. Most providers (31 [86%]) reported no change in their awareness of CTF virus in 2020 compared with 2019. Many providers (26 [72%]) were interested in educational resources for CTF.

Laboratory Testing Evaluation

Before 2020, testing for CTF virus in Montana was conducted by the state public health laboratory using an indirect immunofluorescence assay (IFA) to detect CTF virus–specific IgG antibodies (16,17). Cases were reported as probable if virus-specific IgG was detected in a single serum specimen and confirmed if a 4-fold rise in IgG was detected between paired acute and convalescent serum specimens. During 2011–2019, a median of 156 specimens (range 90–208 specimens) were tested for CTF annually by using IFA, and average positivity was 1.3% (Table 4). We did not have data on the numbers of specimens tested per patient or timing of specimen collection in relation to onset.

In 2020, because of the surge in SARS-CoV-2 testing, Montana began sending specimens for CTF testing to CDC, which primarily used RT-PCR for acute specimens and PRNT for convalescent specimens. A total of 137 CTF tests were performed at CDC on specimens sent on behalf of 107 Montana residents with symptom onset in 2020; of those, 21 tests (15.3%) were positive. Of the RT-PCR tests performed, 18/84 (21.4%) were positive (confirmed cases) on serum specimens collected a median of 3 days (range 0–14 days) after symptom onset. Of the PRNT tests performed, 3/53 (5.7%) were positive (probable cases) on serum specimens collected 17, 23, and 42 days after symptom onset. For 25 patients, RT-PCR and PRNT were performed on the same specimen; of these, 4 had discordant results, 3 (12%) were RT-PCR-positive and PRNT-negative (collected 8, 12, and 14 days after onset), 1 was RT-PCR-negative and PRNT-positive (collected 17 days after onset), and 21 had concordant negative results (Table 4).

During 2020, no changes occurred in testing for RMSF or tularemia. The numbers of cases reported were within the range of those reported during previous years in Montana: 2 cases of RMSF reported in 2020 compared with an average of 5 cases (range 1–11 cases) during 2010–2019, and 1 case of tularemia in 2020 compared with an average of 4 cases (range 1–7 cases) during 2010–2019.