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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 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.

Heartland virus (HRTV), the first pathogenic Phlebovirus (Family: Bunyaviridae) discovered in the United States, was recently described from two Missouri farmers. In 2012, we collected 56,428 ticks representing three species at 12 sites including both patients' farms. Amblyomma americanum and Dermacentor variabilis accounted for nearly all ticks collected. Ten pools composed of deplete nymphs of A. americanum collected at a patient farm and a nearby conservation area were reverse transcription-polymerase chain reaction positive, and eight pools yielded viable viruses. Sequence data from the nonstructural protein of the Small segment indicates that tick strains and human strains are very similar, ≥ 97.6% sequence identity. This is the first study to isolate HRTV from field-collected arthropods and to implicate ticks as potential vectors. Amblyomma americanum likely becomes infected by feeding on viremic hosts during the larval stage, and transmission to humans occurs during the spring and early summer when nymphs are abundant and actively host seeking.

Deer tick virus (DTV), a variant of Powassan virus (POWV), appears to be maintained in nature in an enzootic cycle between Ixodes scapularis ticks and small mammals. Although POWV infection of human beings is rare, a recent report suggests increasing incidence and the possibility that POWV may be an emerging tick-borne zoonosis. Therefore, we assessed the long-term stability of the POWV transmission cycle in northwestern Wisconsin. Adult I. scapularis and Dermacentor variabilis were collected from Hayward and Spooner, Wisconsin, screened for infection by reverse transcriptase polymerase chain reaction (RT-PCR), and virus was isolated. Seventeen of 1,335 (1.3%) of I. scapularis and 0 of 222 (0%) of D. variabilis ticks were infected. All isolated virus belonged to the DTV genotype of POWV. These findings suggest stable transmission of POWV in this focus over ten years and highlight the potential for this agent to emerge as a public health concern.