The symptoms of ZIka infection are usually described as being a much milder than those of other mosquito borne viruses such as dengue and chikungunya. But a new study of Canadian travellers returning from the Americas and the Caribbean where the disease is rife shows the impact was much more severe than expected.
“The common perception that Zika is associated with a very mild clinical course compared with dengue or chikungunya was not borne out in this small cohort,” says Andrea Boggild, of the University Health Network, University of Toronto and Public Health Ontario.
Zika virus, an infectious mosquito-borne disease introduced in the Americas in 2013, became widespread in 2015 and caused severe birth defects in babies of pregnant women who were infected.
And the Canadian study has this stark warning for travellers: “Even in this small cohort, we observed the full clinical spectrum of acute Zika virus, including adverse foetal and neurologic outcomes.
“Our observations suggest that complications from Zika infection are underestimated by data arising exclusively from populations where Zika is endemic.”
The study advises travellers to avoid being bitten by mosquitoes in endemic areas as well as to practise safe sex.
The study analysed data over one year from the Canadian Travel Medicine Network (CanTravNet) on 1,118 travellers who visited one of seven CanTravNet clinics in large cities in British Columbia, Alberta, Manitoba, Ontario and Quebec for a travel-related illness picked up in Central or South America, or the Caribbean.
Of all travellers examined at the seven CanTravNet clinics, 41 (3.7%) had been infected with Zika virus, 41 (3.7%) had dengue and 23 (2.1%) had chikungunya, all mosquito-borne illnesses transmitted throughout the Americas.
Almost 60% (24) of travellers with Zika were female, of whom 79% (19) were of child-bearing age. Except for one case of infection through sexual intercourse, all cases were most likely transmitted by mosquitoes. Three pregnant women were infected, with two cases of congenital infection, and another two travellers had Guillain-Barré or GBS-like syndrome, making a total of 10% of cases with severe complications of Zika.
Conversely, none of the cases of dengue or chikungunya had a complicated course of infection.
The authors acknowledged that the study may not be perfect.
“Referral bias to our centres may have contributed to the more severe clinical presentations noted for Zika, though we would have expected the same phenomenon to occur with dengue and chikungunya were this a significant contributing bias,” Boggild said. “Due to the structure of CanTravNet, our clinics primarily service an adult population, so paediatric cases are under-represented in the database.”