Reoviruses, orbiviruses and Colorado tick fever virus

At a glance

Shape

Icosahedral with a double-layered capsid

Genome

Double-stranded RNA in 10 to 12 segments

Envelope

Non-enveloped (naked)

Family

Formerly Reoviridae, now Spinareoviridae (orthoreovirus, coltivirus) and Sedoreoviridae (orbivirus, rotavirus)

Size

About 60 to 80 nm

Morphology

• Naked double-shelled capsid: the name reo means respiratory enteric orphan
• Segmented dsRNA genome: orthoreovirus and orbivirus have 10 segments, coltivirus 12
• Carry their own RNA polymerase inside the core, since host cells cannot copy dsRNA

Transmission

• Orthoreoviruses: faecal-oral and respiratory; most children are infected early and mildly
• Colorado tick fever: bite of the Rocky Mountain wood tick (Dermacentor andersoni) in the western United States and Canada, at high altitude
• Colorado tick fever virus can also pass by blood transfusion because viraemia lasts weeks
• Orbiviruses (such as bluetongue) are spread mainly among animals by midges; human disease is very rare

Reservoir: Small mammals such as ground squirrels and chipmunks (Colorado tick fever); Humans and many mammals (orthoreoviruses)

Vector: Dermacentor andersoni ticks (Colorado tick fever); Culicoides midges and mosquitoes (orbiviruses)

Isolation precautions

What the care team adds to standard precautions.

• Standard

Virulence factors

• Orthoreoviruses rarely cause more than mild upper respiratory or gut infection
• Colorado tick fever virus infects bone marrow precursor cells, so the virus hides inside maturing red cells and viraemia is prolonged
• Marrow involvement gives leukopenia and sometimes thrombocytopenia

Diseases it causes

• Colorado tick fever: acute febrile illness, often biphasic
• Rare meningitis or encephalitis from Colorado tick fever, mainly in children Viral (aseptic) meningitis →
• Mild upper respiratory or gastrointestinal illness from orthoreoviruses Common cold (acute viral rhinopharyngitis) →

Clinical picture

• Incubation of Colorado tick fever usually 3 to 4 days (1 to 14)
• Fever, chills, headache, myalgia and fatigue; remission and relapse separated by 1 to 4 days (saddle-back fever)
• Most recover fully, but weakness can last weeks; serious complications in under 5%
• Orthoreovirus infection is usually silent or mild in children

Diagnosis

• Colorado tick fever: RT-PCR on blood within 14 days of onset
• Neutralizing antibodies (PRNT) after about 2 weeks; a rising titre confirms
• Leukopenia in a patient with a tick bite in the Rocky Mountains is a classic clue
• Reoviruses are rarely sought in routine practice

Treatment

There is no specific antiviral: care is supportive (rest, fluids, antipyretics); severe cases need hospital care. Antibiotics have no role.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

• Paracetamol

Prevention

• Avoid tick bites with long clothing and repellents in the Rocky Mountain region; check for and remove ticks
• Patients should not donate blood for some months after Colorado tick fever
• No vaccine exists

Test yourself

Think of the answer, then tap the question to see it.

Add to my cards (3)

Connections

← Back to the group: Other RNA viruses

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: About Colorado Tick Fever Virus (CDC, 2024); Increase in Colorado Tick Fever Virus Disease Cases and Effect of COVID-19 Pandemic on Behaviors and Testing Practices, Montana, 2020 (CDC, Emerging Infectious Diseases, 2023).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Send

Representative model, for educational purposes only.