Colorado Tick Fever in the United States, 2013-2022.

Colorado tick fever (CTF) virus is an arbovirus maintained in an enzootic cycle between Rocky Mountain wood ticks (Dermacentor andersoni) and rodent species in the western United States. Individuals with CTF typically present with symptoms including fever, headache, myalgia, and lethargy, with a biphasic illness frequently occurring. We reviewed data on CTF cases reported to the national U.S. arboviral disease surveillance system and identified through testing at the CDC to characterize the epidemiology of CTF from 2013-2022. During this period, 148 CTF cases were identified, all likely infected in an endemic area in one of six states (Montana, Wyoming, Oregon, Colorado, Utah, Idaho). A median of 11 cases (range: 5-37) were identified per year, with an average annual national incidence of 0.04 cases per million population. The median age of cases was 55 years (range: 1-84 years), and 96 (65%) were male. Most (n = 145; 98%) cases acquired infection from April through July. The hospitalization rate was 16% (16 of 102 cases with data), and no deaths were reported. These findings substantiate the continued circulation of CTF virus in the western United States, highlighting the importance of implementing approaches to ensure CTF awareness for medical providers and providing education on tick bite prevention strategies for residents and visitors to risk areas.

Colorado tick fever virus: a review of historical literature and research emphasis for a modern era.

Colorado tick fever virus is an understudied tick-borne virus of medical importance that is primarily transmitted in the western United States and southwestern Canada. The virus is the type species of the genus Coltivirus (Spinareoviridae) and consists of 12 segments that remain largely uncharacterized. Patterns of viral distribution are driven by the presence of the primary vector, the Rocky Mountain wood tick, Dermacentor andersoni. Infection prevalence in D. andersoni can range from 3% to 58% across the geographic distribution of the tick. Infection in humans can be severe and often presents with fever relapses but is rarely fatal. Here, we review the literature from primary characterizations in the early 20th century to current virus/vector research being conducted and identify vacancies in current research.

Increase in Colorado Tick Fever Virus Disease Cases and Effect of COVID-19 Pandemic on Behaviors and Testing Practices, Montana, 2020.

In 2020, Montana, USA, reported a large increase in Colorado tick fever (CTF) cases. To investigate potential causes of the increase, we conducted a case-control study of Montana residents who tested positive or negative for CTF during 2020, assessed healthcare providers' CTF awareness and testing practices, and reviewed CTF testing methods. Case-patients reported more time recreating outdoors on weekends, and all reported finding a tick on themselves before illness. No consistent changes were identified in provider practices. Previously, only CTF serologic testing was used in Montana. In 2020, because of SARS-CoV-2 testing needs, the state laboratory sent specimens for CTF testing to the Centers for Disease Control and Prevention, where more sensitive molecular methods are used. This change in testing probably increased the number of CTF cases detected. Molecular testing is optimal for CTF diagnosis during acute illness. Tick bite prevention measures should continue to be advised for persons doing outdoor activities.

Colorado Tick Fever Virus in the Far West: Forgotten, but Not Gone.

In the past few decades, reported human cases of Colorado tick fever in the western United States have decreased dramatically. The goal of this study was to conduct surveillance for Colorado tick fever virus (CTFV) in Dermacentor ticks in recreational sites in Colorado, Wyoming, and California to determine whether the virus is still present in Dermacentor ticks from these states. Surveillance focused on regions where surveys had been conducted in the 1950s, 1960s, and 1970s. Adult Rocky Mountain wood ticks (Dermacentor andersoni), Pacific Coast ticks (Dermacentor occidentalis), and winter ticks (Dermacentor albipictus) were tested by PCR. A subset of PCR-positive D. andersoni ticks (n = 7) were cultured in Vero cells. CTFV-positive Rocky Mountain wood ticks were found in all states: Colorado (58% prevalence), Wyoming (21%), and California (4%). Although no winter ticks tested positive, Pacific Coast ticks tested positive in one county (Siskiyou County, 15% prevalence) and were positive only in a location that also maintained Rocky Mountain wood ticks and golden mantled ground squirrels, a known CTFV host. In summary, CTFV is prevalent in D. andersoni and D. occidentalis in regions where they are sympatric in California and in D. andersoni in Colorado and Wyoming. Although the number of human CTFV cases has declined dramatically, this decrease in reported disease does not appear to be due to the disappearance or even the decline in prevalence of this virus in ticks in historically endemic regions of the country.

Colorado tick fever virus induces apoptosis in human endothelial cells to facilitate viral replication.

Colorado tick fever virus (CTFV) belongs to the genus Coltivirus of the Reoviridae family, and it is the causative agent of Colorado tick fever. Symptoms of the infection are characterized by sudden biphasic fever, headache, and petechial rash, while severe forms of the disease can include meningoencephalitis, hemorrhagic fever, and death in children. However, the mechanisms underlying CTFV induced pathology and severe complications remain unknown. As CTFV is spread by tick bites and disseminates systemically via hematogenous routes, we performed in vitro analysis examining the interactions between endothelial cells (ECs) and CTFV. Our findings indicate that dermal microvascular ECs, HMEC-1, are susceptible and permissive to CTFV infection. To investigate the role of CTFV infection on endothelial barrier function, we assessed transendothelial electrical resistance (TEER) by xCELLigence and observed a dose-dependent decrease in cell index, indicating increased vascular permeability starting at approximately hour 18 (MOI=1) and hour 26 (MOI=0.1). Since CTFV induced cytopathic effect and increased vascular permeability in HMEC-1 cells, we hypothesized that CTFV causes apoptotic cell death. Our results showed that HMEC-1 cells infected with CTFV at 48 h caused a significant increase in Annexin V staining with reduced viability compared to uninfected cells suggesting CTFV induces apoptotic cell death in human ECs. Electron microscopy also was consistent with apoptotic features, including chromatin condensation and cell blebbing. Furthermore, CTFV induced caspase-3/7 activation at 24 and 48 h post-infection (hpi). The inhibition of caspase activity using Z-VAD-FMK reduced CTFV induced cell death and significantly reduced viral titer. These results indicated that CTFV can infect ECs, exerting direct adverse effects, leading to vascular permeability and cell death. Overall, our data suggest that caspase-mediated apoptosis is a critical mechanism by which CTFV induces disease in the host and enhances viral replication. Future studies will examine the viral and cellular determinants involved in CTFV induced apoptosis in human ECs.

Prevalence and Strains of Colorado Tick Fever Virus in Rocky Mountain Wood Ticks in the Bitterroot Valley, Montana.

The Rocky Mountain wood tick, Dermacentor andersoni, has long been known to transmit human pathogens. Within the Bitterroot Valley, Ravalli County, Montana, these agents include Rickettsia rickettsii, Francisella tularensis, and Colorado tick fever virus (CTFV). Found in the western United States where wood ticks occur, CTFV causes a biphasic, febrile illness in humans and persists in enzootic cycles involving the ticks and small mammals. CTFV belongs to the genus Coltivirus, family Reoviridae, whose genome consists of 12 double-stranded RNA segments. Previous studies revealed the presence of CTFV-infected ticks and rodents in select locations within the valley in the 1960s and 1970s, using animal and cell culture methods for detection. We aimed to determine the range and prevalence of the virus in adult questing ticks throughout the valley using molecular tools and to examine the genomic variation between virus strains. Adult D. andersoni ticks were collected during 2002-2003 and 2009-2013. RNA extractions and reverse transcription-polymerase chain reaction were performed on 921 ticks, of which 61 ticks were positive for CTFV, resulting in a 6.6% prevalence of infection. Four genetic loci, one from each of the segments 9, 10, 11, and 12, within the viral genome were sequenced. Reassortment was detected between CTFV sequence strains within the valley. This study confirmed the prevalence of CTFV in D. andersoni ticks within the Bitterroot Valley, which has remained at levels found in the 1950s and 60s. Additional CTFV sequences were obtained and evidence of reassortment was observed between strains within the valley.

Case Report: A Case of Colorado Tick Fever Acquired in Southwestern Saskatchewan.

Colorado tick fever virus is transmitted by Dermacentor andersoni ticks. In Canada, these ticks are found in the southern regions of British Columbia (Rocky Mountains) and Alberta, as well as southwestern Saskatchewan. Colorado tick fever should be clinically suspected in patients presenting with a biphasic febrile illness and leukopenia following tick exposure in the appropriate geographic area.

A system for coordinated analysis of translational readthrough and nonsense-mediated mRNA decay.

The nonsense-mediated mRNA decay (NMD) pathway degrades mRNAs containing premature termination codons, limiting the expression of potentially deleterious truncated proteins. This activity positions the pathway as a regulator of the severity of genetic diseases caused by nonsense mutations. Because many genetic diseases result from nonsense alleles, therapeutics inducing readthrough of premature termination codons and/or inhibition of NMD have been of great interest. Several means of enhancing translational readthrough have been reported to concomitantly inhibit NMD efficiency, but tools for systematic analysis of mammalian NMD inhibition by translational readthrough are lacking. Here, we introduce a system that allows concurrent analysis of translational readthrough and mRNA decay. We use this system to show that diverse readthrough-promoting RNA elements have similar capacities to inhibit NMD. Further, we provide evidence that the level of translational readthrough required for protection from NMD depends on the distance of the suppressed termination codon from the end of the mRNA.

Colorado tick fever in the United States, 2002-2012.

BACKGROUND: Colorado tick fever (CTF) is an acute systemic febrile illness caused by the CTF virus (CTFV). The last national summary of CTF cases in the United States included cases reported through 2001. This study summarizes national surveillance data for CTF from 2002 through 2012 and examines trends in the epidemiology and testing of identified CTF cases. METHODS: Because CTF is not nationally notifiable, we identified CTF cases through solicited reports from state health departments and diagnostic laboratory records. For all cases, we collected data on age, sex, county of residence, travel history, symptom onset date, laboratory testing, and clinical outcome. Poisson regression was used to examine trends over time in case counts, and simple linear regression and logistic regression were used to examine trends in case characteristics. RESULTS: From 2002 through 2012, 75 CTF cases were identified with a median of five cases per year (range 3-14). Forty-seven (63%) cases occurred in males and 49 (65%) occurred in people aged ≥40 years. The majority (80%) of cases had onset of illness during May through July. Cases occurred in residents of 14 states but the infections were acquired in six western states. Wyoming had the highest annual incidence of CTF among residents (3.4 cases per million population), followed by Montana (1.5 per million), and Utah (0.5 per million). Over the 11 years, there was an increase in the proportion of cases diagnosed by RT-PCR testing and in the proportion of cases among travelers to another state. CONCLUSIONS: CTF cases continue to occur annually among residents and visitors to the western United States. Public health prevention messages about decreasing tick exposure should be targeted to residents and travelers who will spend time outdoors in an endemic region during the spring and summer months.

Infection with Colorado tick fever virus among humans and ticks in a national park and forest, Wyoming, 2010.

BACKGROUND: Colorado tick fever (CTF) is an underreported tick-borne viral disease occurring in the western United States. CTF illness includes fever, headache, and severe myalgia lasting for weeks. Wyoming has one of the highest CTF incidence rates with approximately 30% of infected persons reporting tick exposure in a Wyoming National Park or Forest before symptom onset. We assessed CTF virus infections among humans and Dermacentor andersoni ticks in Grand Teton National Park (GRTE) and Bridger-Teton National Forest (BTNF). METHODS: In June of 2010, 526 eligible employees were approached to participate in a baseline and 3-month follow-up serosurvey and risk behavior survey. Seropositivity was defined as antibody titers against CTF virus ≥10, as measured by the plaque reduction neutralization test. Ticks were collected at 27 sites within GRTE/BTNF and tested by RT-PCR for the CTF virus. RESULTS: A total of 126 (24%) employees participated in the baseline and follow-up study visits. Three (2%) employees were seropositive for CTF virus infection at baseline. During the study, 47 (37%) participants found unattached ticks on themselves, and 12 (10%) found attached ticks; however, no participants seroconverted against CTF virus. Walking through sagebrush (p=0.04) and spending time at ≥7000 feet elevation (p<0.01) were significantly associated with tick exposure. Ninety-nine percent (174/176) of ticks were D. andersoni, and all were found at ≥7000 feet elevation in sagebrush areas; 37 (21%) ticks tested positive for CTF virus and were found at 10 (38%) of 26 sites sampled. CONCLUSIONS: Although no GRTE or BTNF employees were infected with CTF virus during the study period, high rates of infected ticks were identified in areas with sagebrush at ≥7000 feet. CTF education and personal protection measures against tick exposure should be targeted to visitors and employees traveling to the high-risk environs identified in this study.

Characterization of the stop codon readthrough signal of Colorado tick fever virus segment 9 RNA.

Termination codon readthrough is utilized as a mechanism of expression of a growing number of viral and cellular proteins, but in many cases the mRNA signals that promote readthrough are poorly characterized. Here, we investigated the readthrough signal of Colorado tick fever virus (CTFV) segment 9 RNA (Seg-9). CTFV is the type-species of the genus Coltivirus within the family Reoviridae and is a tick-borne, double-stranded, segmented RNA virus. Seg-9 encodes a 36-kDa protein VP9, and by readthrough of a UGA stop codon, a 65-kDa product, VP9'. Using a reporter system, we defined the minimal sequence requirements for readthrough and confirmed activity in both mammalian and insect cell-free translation systems, and in transfected mammalian cells. Mutational analysis revealed that readthrough was UGA specific, and that the local sequence context around the UGA influenced readthrough efficiency. Readthrough was also dependent upon a stable RNA stem-loop structure beginning eight bases downstream from the UGA codon. Mutational analysis of this stem-loop revealed a requirement for the stem region but not for substructures identified within the loop. Unexpectedly, we were unable to detect a ribosomal pause during translation of the CTFV signal, suggesting that the mechanism of readthrough, at least at this site, is unlikely to be dependent upon RNA secondary-structure induced ribosomal pausing at the recoded stop codon.

Powassan encephalitis and Colorado tick fever.

This article discusses two tick-borne illnesses: Powassan encephalitis, a rare cause of central nervous system infection caused by the Powassan virus, and Colorado tick fever, an acute febrile illness caused by the Colorado tick fever virus common to the Rocky Mountain region of North America.

Detection of Colorado Tick Fever viral RNA in acute human serum samples by a quantitative real-time RT-PCR assay.

A quantitative real-time RT-PCR assay for the detection of Colorado Tick Fever (CTF) viral RNA in human clinical samples is presented. The sensitivity of this assay has been shown to be greater than that of the isolation of virus in Vero cells by standard plaque assay in a direct comparison. The specificity of the CTF quantitative real-time RT-PCR assay was determined by the exclusive detection of CTF viral RNAs when applied to a diverse panel of CTF viral isolates and reference strain agents known to circulate in areas of CTF virus transmission. Lastly, the quantitative real-time RT-PCR assay demonstrated exceptional sensitivity for the detection of CTF viral RNA in acute human serum. The quantitative real-time RT-PCR assay is efficient, sensitive and specific and as such is useful for the detection of CTF viral RNA in the diagnostic or research laboratory.

Termination and read-through proteins encoded by genome segment 9 of Colorado tick fever virus.

Genome segment 9 (Seg-9) of Colorado tick fever virus (CTFV) is 1884 bp long and contains a large open reading frame (ORF; 1845 nt in length overall), although a single in-frame stop codon (at nt 1052-1054) reduces the ORF coding capacity by approximately 40 %. However, analyses of highly conserved RNA sequences in the vicinity of the stop codon indicate that it belongs to a class of 'leaky terminators'. The third nucleotide positions in codons situated both before and after the stop codon, shows the highest variability, suggesting that both regions are translated during virus replication. This also suggests that the stop signal is functionally leaky, allowing read-through translation to occur. Indeed, both the truncated 'termination' protein and the full-length 'read-through' protein (VP9 and VP9', respectively) were detected in CTFV-infected cells, in cells transfected with a plasmid expressing only Seg-9 protein products, and in the in vitro translation products from undenatured Seg-9 ssRNA. The ratios of full-length and truncated proteins generated suggest that read-through may be down-regulated by other viral proteins. Western blot analysis of infected cells and purified CTFV showed that VP9 is a structural component of the virion, while VP9' is a non-structural protein.

Recombinant VP7-based enzyme-linked immunosorbent assay for detection of immunoglobulin G antibodies to Colorado tick fever virus.

VP6, VP7, VP9, VP10, VP11, and VP12 of Colorado tick fever virus (CTF virus), a virus member of the genus Coltivirus, family Reoviridae, were expressed in bacteria with the pGEX-4T-2 vector. A partial sequence of VP7 (designated pVP7) was chosen to elaborate an enzyme-linked immunosorbent assay (ELISA) for detecting anti-CTF virus immunoglobulin G (IgG) antibodies in humans. This was based on two observations: (i) among all expressed proteins, pVP7 showed the highest immunoreactivity to an anti-CTF virus hyperimmune ascitic fluid; (ii) to provide the highest selectivity of antibody detection, the expressed sequence was chosen within a region which is highly divergent (49% amino acid identity) from the homologous sequence of another coltivirus, the Eyach virus. The pVP7 ELISA was evaluated with 368 serum samples from French blood donors and found to provide 98.1% specificity. Assays with the Calisher set of human serum samples, positive for anti-CTF virus antibodies (C. H. Calisher, J. D. Poland, S. B. Calisher, and L. A Warmoth, J. Clin. Microbiol. 22:84-88, 1985), showed that the pVP7 ELISA provided 100% sensitivity for the tested population. After elaboration of recombinant-protein-based ELISAs for diagnosis of infections with members of the viral genera Orbivirus, Orthoreovirus, and Rotavirus, it was shown that a recombinant protein could be used to detect antibodies to the human pathogen Colorado tick fever virus.

Colorado tick fever.

Colorado tick fever, also known as mountain fever and mountain tick fever, is a well-described, viral, tick-borne disease common to the Rocky Mountain region of the United States and Canada. The Rocky Mountain wood tick, Dermacentor andersoni, is the primary vector. The triad of high fever, severe myalgia, and headache is typical, but not specific. Although a self-limited disease in most cases, severe complications may occur. PCR techniques have been developed that allow the diagnosis to be established from the first day of symptoms. Ribavirin may merit consideration in the appropriate clinical setting.

Genus Coltivirus (family Reoviridae): genomic and morphologic characterization of Old World and New World viruses.

We report a genomic and morphologic study of the European Eyach (EYA) virus (genus Coltivirus, family Reoviridae) and a comparative analysis with the American Colorado tick fever (CTF) virus (the type species of the genus). The previously established, but distant, antigenic relationship between these viruses was strengthened by genetic findings (presence of cognate genes, amino acid identity between 55 and 88%, similar conserved terminal motifs, suspected read-through phenomenon in segment 9 of both viruses) and by indistinguishable ultramicroscopic morphologies. Moreover, putative constitutive modifying enzyme activities were suspected to be carried out by homologous viral proteins (RNA-dependent RNA polymerase, methyl/guanylyl transferase, NTPase). These findings, together with the comparative analysis to genomes of southeast Asian isolates, support the recent classification of arboviruses with 12 segments of dsRNA within two distinct genera (genus Coltivirus and genus Seadornavirus) and raise interesting questions about the evolutionary origins of coltiviruses. The previously proposed hypothesis that EYA virus was derived from an ancestral virus introduced in Europe with the migration of lagomorphs from North-America, would imply a divergence date between American and European isolates of over 50 million years ago (MYA). This analysis allows for the first time to propose an evolutionary rate for virus dsRNA genomes which was found to be in the order of 10(-8) to 10(-9) mutations/nt/year, a rate similar to that of dsDNA genomes.

Serologic survey for selected infectious disease agents in swift and kit foxes from the western United States.

A serologic survey of swift fox (Vulpes velox) and kit fox (V. macrotis) from the western USA was conducted for 12 infectious diseases. Samples from swift fox were collected between 1987 and 1992 from Colorado (n = 44), Kansas (n = 10), and Wyoming (n = 9). Samples from kit fox were collected in California (n = 86), New Mexico (n = 18), Utah (n = 9), and Arizona (n = 6). Overall antibody prevalence rates were 33 of 110 (30%) for canine parvovirus (CPV), 9 of 72 (13%) for canine distemper virus (CDV), 23 of 117 (20%) for vesicular stomatitis New Jersey, 16 of 117 (14%) for vesicular stomatitis Indiana, six of 117 (5%) for Cache Valley virus, five of 117 (4%) for Jamestown Canyon virus, one of 97 (1%) for rabies virus, one of 117 (1%) for Colorado tick fever virus, and one of 117 (1%) for western equine encephalitis virus. In addition, antibodies were not found to Yersinia pestis, Francisella tularensis, and Borrelia burgdorferi in serum from 25 Colorado swift fox. Adult swift fox from Colorado had serologic evidence of exposure to CPV more often than juveniles. No juvenile swift fox from Colorado had serum antibodies to CDV. There were season-specific differences in serum antibody prevalence for CPV for swift fox from Colorado. No viruses were isolated from ectoparasites or fox from Colorado.