Canadian officials are closely monitoring what has become the worst mumps epidemic in the U.S. since 1988 -- with nine Midwestern states now reporting cases of the disease.

A travel advisory is in effect for Canadians who may be planning a trip to the areas most affected.

The virus has spread rapidly, starting in Iowa and moving into nine states and affecting about 1,100 people. No deaths, however, have been reported in connection to the outbreak.

The U.S. Center for Disease Control is investigating cases in several other states.

Iowa, the state where the outbreak began, has now logged 815 cases of mumps, up 200 over the past week. A backlog of paperwork, however, may attribute for the jump, Iowa officials said.

Nebraska has reported 110 cases, and officials in Kansas, Michigan, Illinois, Indiana, Wisconsin, Minnesota and Missouri have all reported seeing cases of the virus.

In early April, doctors in Hamilton, Ont., west of Toronto, discovered a patient who developed the mumps after visiting Iowa.

But the Public Health Agency of Canada says there is currently no outbreak of the mumps in this country.

The agency says they are monitoring the outbreak in the U.S. closely, and is still maintaining a warning that was issued on April 13 to travellers planning to visit the U.S.

"We have communicated information to the provinces and territories and the ministries of health, to inform them of the situation and to ask them to remain vigilant for cases occurring in their jurisdictions," Dr. Shelley Deeks of the Public Health Agency told CTV Newsnet.

Anomaly

What worries experts, however, is that an unusually high proportion of those affected have previously received the two-dose vaccine that covers measles, mumps and rubella.

Investigators are trying to determine the cause of the anomaly.

"We used to believe the vaccine was 95 per cent effective," Dr. Neil Rau, an infectious disease specialist, told CTV Newsnet.

"There may be something novel about this strain of virus that indicates this vaccine is not as effective as we once thought."

"It could also be that the vaccine works very well when you keep having the disease in your midst," Rao said.

However, he added, it's possible that people lose their immunity to the virus once the risks have been minimized through vaccination, and when it reappears, people aren't prepared to deal with it and the effect is amplified.

The Public Health Agency of Canada has reported that college-age students between 18 and 22, have been most affected by the U.S. outbreak.

Deeks said Canada has a high immunization coverage rate, with approximately 80 per cent of children having received the two-dose vaccine.

Possible link

The source of the Iowa epidemic is not known. However, Britain experienced a mumps epidemic that peaked last year with about 56,000 cases.

The Iowa mumps virus is the same variety -- but health officials are still evaluating whether there's a connection.

"It may be related to the fact that people in the U.K. have been avoiding mumps vaccinations because of links to autism," said Rau.

"We know this vaccine does not cause autism, but we may now have a spillover effect from exportation from the U.K. to the United States from this strain."

How it spreads

Mumps is most commonly spread by coughing and sneezing, and symptoms typically include fever, fatigue, headaches and swollen glands under the jaw.

More serious cases can lead to deafness, meningitis, encephalitis, and can cause infertility in both men and women.

The Public Health Agency of Canada is monitoring the outbreak, and is still maintaining a warning that was issued on April 13 to travellers planning to visit the U.S.

The agency is advising Canadians to ensure their children are immunized for diphtheria, whooping cough, tetanus, polio, measles, mumps and rubella -- an advisory that generally stands for all international travel.

Iowa state health officials have released a public health bulletin warning people that mass gatherings will pose a "slight, but real risk" of exposure to the virus.

Officials have not recommended the cancellation of large events or gatherings, but have encouraged organizers to provide mumps education to those in attendance.

Advice

People should cover their mouths when they cough, wash their hands regularly, and avoid sharing utensils in order to minimize the risk of contracting mumps, the state recommended.

The U.S. Centers for Disease Control and Prevention has identified the current strain as similar to the strain of mumps that was identified in the 2004-2005 outbreak in the United Kingdom.

The Public Health Agency of Canada advisory pointed out that the National Advisory Committee on Immunizations recommends all children receive a vaccination against measles, mumps and rubella at 12 months of age.

With reports from CTV's Scott Laurie and The Associated Press