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California serogroup (CSG) viruses


California serogroup (CSG) viruses include many viruses, such as the Jamestown Canyon and Snowshoe hare viruses, which are the most common in Québec. These viruses are also found throughout Canada and the United States. They are spread through the bite of an infected mosquito.


In most cases, people infected with a CSG virus do not develop any symptoms.

However, some individuals may develop symptoms 2 to 14 days after the mosquito bite. The nature of the symptoms of CSG infection varies. The most frequent symptoms are:

  • Fever and chills
  • Headaches
  • Fatigue
  • Muscle or joint pain
  • Nausea, sometimes accompanied by vomiting, abdominal pain and diarrhea

In rare cases, CSG may cause a severe neurological infection, such as meningitis (infection of the lining of the brain), encephalitis (brain inflammation) or myelitis (spinal cord inflammation), which can be manifested by the following symptoms:

  • Numbness or muscle weakness
  • Hypersensitivity of the eyes to light
  • Visual disorders
  • Stiff neck
  • Agitation, disorientation or confusion

When to consult

You must consult a doctor promptly if you experience one of the symptoms associated with a severe form of CSG infection listed above.


There is no specific treatment or vaccine against CSG viruses. However, most people who are infected recover without treatment.

People who are seriously ill may have to be hospitalized. They will then be treated to stabilize and improve their condition.

Complications and aftereffects

Following an infection with neurological involvement, neurological deficits that may be permanent have been reported.

In rare cases, CSG viruses can be fatal.


CSG viruses are transmitted to humans through the bite of a mosquito carrying the virus.

Mosquitoes generally become carriers of the Snowshoe hare virus by biting small infected mammals such as hares, squirrels, chipmunks and various rodents. For the Jamestown Canyon virus, mosquitoes generally bite larger mammals such as deer and wapiti.

CSG viruses are not transmitted through contact with an infected animal or between two people.

Protection and prevention

The best way to prevent CSG virus infection is to avoid being bitten by mosquitoes. Simple measures can be taken. For more information, consult the page Protecting yourself from mosquito and tick bites and Reducing the number of mosquitoes in your surroundings.

People at risk

Anyone can contract a CSG virus.

People who work outside or participate in outdoor activities are most at risk of infection, especially if they spend a lot of time outdoors without taking any measures of protection against mosquito bites. The risk of CSG virus infection is particularly high if the activity takes place in woods and forests.

Snowshoe hare virus infections are more common in children, while Jamestown Canyon virus infections are more common in adults.

Additional information

In Québec, the CSG virus infection is a reportable disease. Doctors are required to inform the public health authorities.

See also

Last update: July 5, 2023


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