[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other RNA viruses](https://durgical.com/en/microbiology/groups/other-rna-viruses/) › Reoviruses, orbiviruses and Colorado tick fever virus

# Reoviruses, orbiviruses and Colorado tick fever virus

- Virus
- Non-enveloped (naked)
- For completeness

Relatives of rotavirus with double-stranded RNA. Human reoviruses cause mild colds; Colorado tick fever virus causes a two-phase fever after a tick bite.

Updated Nov 13, 2026

Also called: orthoreovirus, orbivirus, coltivirus, CTF virus

## Quick facts

**Type**
Virus · [Other RNA viruses](https://durgical.com/en/microbiology/groups/other-rna-viruses/)

**Gram / shape**
Icosahedral with a double-layered capsid

**Spread**
Orthoreoviruses · Colorado tick fever · Orbiviruses

**Causes**
Colorado tick fever · Rare meningitis

**Treatment**
There is no specific antiviral

On this page

- The basics Key points
- At a glance
- Morphology
- Transmission
- Memory aid
- What the colours mean

- Clinical Virulence factors
- Diseases it causes
- Clinical picture
- Diagnosis

- Treatment Treatment
- Prevention

- Study Test yourself

- Connections Connections

## Key points

- Reoviruses: naked, segmented double-stranded RNA (rotavirus is the famous relative)
- Colorado tick fever: Dermacentor andersoni tick, western United States and Canada
- Saddle-back (biphasic) fever and leukopenia
- Virus inside red cell precursors: prolonged viraemia and transfusion risk
- Treatment is supportive; no vaccine

## 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 →](https://durgical.com/en/conditions/viral-meningitis/)
- Mild upper respiratory or gastrointestinal illness from orthoreoviruses [Common cold (acute viral rhinopharyngitis) →](https://durgical.com/en/conditions/common-cold/)

## 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](https://durgical.com/en/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

## Memory aid

"The Colorado saddle": fever, a pause, fever again, like a saddle between two Rocky Mountain peaks, with the virus riding inside red cells.

## Test yourself

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

## What type of genome do reoviruses have?

Segmented double-stranded RNA inside a naked double-shelled capsid.

## Which tick spreads Colorado tick fever virus?

The Rocky Mountain wood tick, Dermacentor andersoni.

## Why can Colorado tick fever be passed by transfusion?

The virus infects marrow precursors and persists inside red blood cells, so viraemia lasts for weeks.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/microbiology/reoviruses-orbiviruses-and-colorado-tick-fever-virus/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

## Connections

## Related 2

- [Rotavirus](https://durgical.com/en/microbiology/rotavirus/)
- [West Nile virus](https://durgical.com/en/microbiology/west-nile-virus/)

## Related conditions 2

- [Viral (aseptic) meningitis](https://durgical.com/en/conditions/viral-meningitis/)
- [Common cold (acute viral rhinopharyngitis)](https://durgical.com/en/conditions/common-cold/)

[← Back to the group: Other RNA viruses](https://durgical.com/en/microbiology/groups/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](https://www.cdc.gov/colorado-tick-fever/about/index.html) (CDC, 2024); [Increase in Colorado Tick Fever Virus Disease Cases and Effect of COVID-19 Pandemic on Behaviors and Testing Practices, Montana, 2020](https://wwwnc.cdc.gov/eid/article/29/3/22-1240_article) (CDC, Emerging Infectious Diseases, 2023).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/microbiology/reoviruses-orbiviruses-and-colorado-tick-fever-virus/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/reoviruses-orbiviruses-and-colorado-tick-fever-virus/