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. These appear from 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.


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 to protect yourself from mosquito and tick bites.

People at risk

Anyone can be infected with a CSG virus.

People who work outside or participate in outdoor activities are at greater risk of infection, especially if they spend long hours outdoors. 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 severe form of CSG virus infection with neurological impairment (e.g., meningitis, encephalitis, meningoencephalitis, myelitis) is a reportable disease. In this case, doctors are required to inform the public health authorities.