Winnipeg malathion stock from 2009 still OK to use, city says

A batch of insecticide from 2009 designed to kill adult mosquitoes has been deemed OK for use and will be puffed out into the air this weekend as fogging resumes in Winnipeg, according to the city.

"The Insect Control Branch has used a combination of larviciding and residual treatments to control mosquito populations. These measures alone are no longer sufficient to control the current adult nuisance mosquito population, so an adult nuisance mosquito fogging program will be implemented," Ken Nawolsky, superintendent of the city's Insect Control Branch, told media at a news conference Friday night.

Fogging will resume Sunday between 9:30 p.m. and 6:30 a.m. Monday morning, weather permitting.

In July, CBC News reported that Health Canada told the city it had to stop using its stock of 13-year-old malathion because the chemical had been sitting on the shelf for too long. The federal agency added at the time malathion older than a year should only be used in the event of a public health emergency.

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In June, the city fogged using malathion it had purchased from the Saskatchewan government, which had first acquired the stock in 2003. Saskatchewan sold off the rest of its supply to Winnipeg, claiming it no longer wanted it.

The label for the product on that batch indicated that it shouldn't be kept in storage for more than one year.

According to the city, tests done on its 2009 stock revealed that it meets international safety standards.

"The city's inventory of malathion was recently tested and the stock purchased in 2009 has met the World Health Organization standards for isomalathion content and adult mosquito control can now proceed," Nawolsky said.

"We got confirmation after numerous discussions with Health Canada today, that as long as we used an accredited laboratory and a qualified consultant, and we met the guidelines of the World Health Organization, that we could proceed to use malathion."

The threshold for isomalathion, the toxic substance that forms as malathion ages or is exposed to heat, is 0.4 per cent, Nawolsky said, adding the city's stock came in beneath that threshold.

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In order for the city to justify fogging, three criteria first need to be met.

Mosquito trap counts around the city must first reach a minimum threshold of 25 female mosquitoes for two nights in a row, and one or more quadrants in the city need to be near the 100 female mosquito trap count.  

A further measure, known as the Adulticiding Factor Analysis (AFA), also has to reach a "high" rating.

The citywide trap count was 73 Thursday, moving the AFA rating from medium to high and triggering the need to fog, the city said. 

Fogging is cancelled when temperatures drop below 13 C or if conditions are too windy.

The city said buffer zones will be respected.

In March of 2014, a research group of the World Health Organization released a report saying malathion is "probably carcinogenic to humans." The same report noted its findings were based on "limited evidence of carcinogenicity in humans for non-Hodgkin lymphoma and prostate cancer."