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 people may develop symptoms 2 to 7 days after being bitten by a mosquito.

The most common symptoms of CSG virus infection are:

  • Headaches
  • Fever and chills
  • Fatigue
  • Muscle pain

These symptoms may be accompanied by:

  • Gastrointestinal symptoms (vomiting, diarrhea, abdominal pain)
  • Numbness or muscle weakness
  • A rash
  • Hypersensitivity of the eyes to light
  • Visual disturbances
  • A stiff neck
  • Agitation, disorientation or confusion

When to consult

You must see a doctor promptly if you have:

  • Severe or unusual headaches
  • A high fever
  • A stiff neck
  • Confusion or disorientation
  • Muscle weakness


There is no treatment or vaccine for 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.


In some cases, the disease may be severe and result in neurological disorders such as:

  • Meningitis (an infection of the membranes surrounding the brain)
  • Encephalitis (inflammation of the brain)
  • Paralysis of the lower limbs

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 spread to humans through the bite of an infected mosquito.

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 spread through contact with an infected animal or between 2 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, only the severe form of CSG virus infection that affects the central nervous system (e.g., meningitis, encephalitis, meningoencephalitis, myelitis) is reportable. In this case, doctors are required to inform the public health authorities.