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Chapter: Microbiology and Immunology: Virology, Virus: Paramyxoviruses

Coltiviruses and Colorado Tick Fever Virus

Coltiviruses resemble the orbiviruses in their morphology and in having two capsids.


Coltiviruses resemble the orbiviruses in their morphology and in having two capsids. The genome consists of a 12-segmented double-stranded RNA. The coltiviruses associated with human disease include Colorado tick fever, Salmon River virus, Banna virus, Beijing virus, Gansu virus, and Eyach virus.

Colorado Tick Fever Virus

Colorado tick fever is an acute viral infection transmitted by the bite of wood tick (Dermacentor andersoni) caused by Colorado tick fever virus. Colorado fever was so named because the illness was believed to occur predominantly in Colorado and was used to distinguish this clinical illness from that of Rocky Mountain spotted fever caused by Rickettsia species. The causative agent of this fever was recognized as a virus in 1946.

Colorado tick fever virus is a double-stranded RNA virus surrounded by two capsids. The virus contains 12 RNA seg-ments. The virus infects and replicates in the bone marrow, lymph nodes, spleen, and liver of rhesus monkey, but without producing any histological abnormalities. The virus has been shown to replicate in erythroid precursor cells without severely damaging them and is present in mature red blood cells.

Colorado tick fever occurs almost exclusively in the Western United States and in Canada. The disease is transmitted by the bite of wood tick, D. andersoni. Larval and nymphal stages of D. andersoni ticks usually transmit the virus among all rodents,but only adult ticks transmit the virus to humans. The ticks acquire the infection on feeding an infected viremic host. The infected ticks subsequently transmit the virus through their saliva during act of feeding on a new susceptible host. Squirrels, rabbits, and deer are the natural animal hosts for the virus.

The virus generally causes a nonspecific febrile illness. The incubation period is short and varies from 3 to 6 days. The clinical manifestations of the acute condition are characterized by the sudden onset of fever, chills, headache with retro-orbital pain, malaise, nausea, and occasionally vomiting. A rash is generally absent by which Colorado tick fever is differentiated from the Rocky Mountain spotted fever.

The fever is typically biphasic (i.e., with two episodes of fever), each of which lasts 2–3 days, separated by a remission of approxi-mately equal duration. In most cases, the febrile period is followed by moderate to marked weakness and malaise. Complications are rare, but may include hepatitis, pericarditis, epididymo-orchitis, atypical pneumonitis, encephalitis, and aseptic meningitis.

Viruses are present in the erythrocytes during the first 2 weeks of disease. This is followed by a period during which the viruses infect and replicate within the erythropoietic cells. In the infected red blood cells, the viruses can live for the life of the cells, which is nearly 120 days. A single attack of infection usually produces lifelong immunity.

Laboratory diagnosis of Colorado tick fever is established by isolation of the virus from the erythrocyte fraction of the whole blood. The virus has been shown to persist in erythrocytes for as long as 120 days. The virus has also been found in the cerebro-spinal fluid of patients with no apparent encephalitis or menin-gitis. The virus isolation, however, takes nearly 1–2 weeks.

Neutralization test in suckling mice, tissue culture neutral-ization, immunofluorescence, and enzyme immunoassay are various serological tests used for demonstration of antibodies in the serum for diagnosis of the condition. A fourfold increase in titer of the specimen during the acute phase and convales-cent phase is demonstrated in nearly all patients.

No specific antiviral treatment is available for Colorado tick fever. The condition is usually self-limited and can be prevented by avoiding contact with the wood tick.

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Microbiology and Immunology: Virology, Virus: Paramyxoviruses : Coltiviruses and Colorado Tick Fever Virus |

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