Zika virus infections in Canadian travellers returning from the Caribbean and south and central America were both more common and their effects more severe than researchers had expected, a new study has found.

The study published Monday in the Canadian Medical Association Journal, looked at Canadian travellers who had visited Canadian Travel Medicine Network (CanTravNet) clinics for suspected travel-related illnesses after trips down south.

Of the 1,118 travellers studied between October 2015 and September 2016, 3.7 per cent, or 41 travellers, had a Zika infection. That was equal to the number diagnosed with dengue, the fastest-growing mosquito-borne illness.

But unlike the dengue patients, many of the Zika patients developed serious problems from their infections. Ten per cent of the 41 patients with Zika developed severe complications. They ranged in age from 13 to 70.

“That was a bit surprising to us because we didn’t see that level of complication in other illnesses that can have a complicating course as well, such as dengue virus,” study author Dr. Andrea Boggild, clinical director in the Tropical Disease Unit at the Toronto General Hospital, told CTV News Channel Monday.

Two travellers developed the neurological disorder Guillain–Barré Syndrome, or GBS-like syndrome, a condition that has been linked Zika virus by previous research.

Also, among three pregnant women infected with Zika, two transmitted the virus to their unborn babies.

Among the other travellers with Zika infections, 88 per cent developed a rash and 80 per cent had fever. About half complained of muscle or joint pain or headaches, while one in six developed pink eye, or conjunctivitis.

Little is known about the Zika virus among Canadian travellers, who tend to be highly mobile, says Dr. Andrea Boggild, clinical director in the Tropical Disease Unit at the Toronto General Hospital, and assistant professor in the department of Medicine at the University of Toronto.

“We didn’t really know what to expect because there is no synthesized data for Canadians,” Boggild said.

“We were a little surprised to find a five per cent prevalence of neurological complications and congenital complications in another five per cent.”

It’s generally thought that Zika presents relatively mild symptoms -- and that is true for most people, says Boggild, whose clinical practice is dedicated to returned travellers and migrants who are ill. Many people return to Canada or come to Canada not knowing they have the Zika virus because they have no symptoms or only mild ones that disappear.

“But in others, the complications can be severe, including serious neurological effects and severe fetal abnormalities.”

Jennifer, a 35-year-old Toronto-area resident who did not want to be identified by her full name, contracted Zika while on vacation in Nicaragua last August. Despite taking precautions, including using plenty of bug spray, she was bitten by mosquitos and developed a serious rash on her legs.

She then experienced a pins-and-needles sensation in her legs and had trouble walking. When doctors in Nicaragua suspected Zika, Jennifer flew back to Toronto. She went to one hospital where she was told she didn’t have the virus and sent home.

But after she collapsed and was taken to another hospital by ambulance, Jennifer was correctly diagnosed with Zika.

Jennifer developed Guillain-Barre syndrome and had to re-learn how to walk, first with a walker and then a cane. She wanted to warn Canadians that Zika is not only a threat to pregnant women or those planning to conceive.

"It has turned my life upside down for six months," she told CTV News.

Boggild also has concerns about the effects of Zika on developing brains. Doctors know that non-mature brain cells, such as those of fetuses, are “almost entirely permissive to Zika infection” and that mature brain cells are almost fully resistant. What isn’t known yet is where the line between non-mature and mature is.

“From my perspective, a young infant is not that different from a fetus. But we don’t know where the risk for children subsides.”

Since there is no vaccine or other preventative drugs for Zika, the researchers urge women who are pregnant or planning to get pregnant to defer travel to affected areas. Those who do travel should protect against mosquito bites with clothing and repellents containing DEET or picaridin, and use condoms during sex.

Barrier protection during sex should continue for six months after travel to affected areas for men and for two months for women, says Boggild.

Zika was introduced to the Americas in 2013, and was declared an international public health emergency by the World Health Organization in February 2016. Endemic infection has now been confirmed in more than 50 countries, including parts of the United States.

Among the 41 patients diagnosed with Zika, almost 60 per cent were female, including 19 of child-bearing age. Except for one case of infection through sexual intercourse, researchers believe all cases were likely transmitted by mosquitoes.

Referral bias could have played a role in the high incidence of severe complications, the study authors note. Travel clinics get patient referrals from family doctors, emergency rooms and walk-in clinics.

“It’s important to take the findings with a bit of a grain of salt because our cohort was comprised only of ill, returned travellers who were sick enough to seek care either from their family doctor or an emergency room and then be referred on to our specific centres,” Boggild told CTV News Channel.

But if that were a significant bias, researchers would have expected to see the same results in patients with other mosquito-borne viruses, such as dengue or chikungunya, but they didn’t.

The researchers say the continued gathering of large-scale data on imported Zika is needed to understand the true prevalence of complications among travelling Canadians.

With files from CTV News’ medical specialist Avis Favaro and producer Elizabeth St. Philip