Brock West Nile model viewed with caution

A mathematical model created by Brock University scientists predicting a West Nile virus epidemic can be a useful public health tool, but it isn’t gospel, says a Public Health Ontario entomologist.

Curtis Russell says the model producedby Brock medical entomologist Fiona Hunter and graduate student Bryan Giordano is certainly in line with what current mosquito monitoring in Ontario shows — higher than normal populations of West Nile virus carrying mosquitos and the emergence of human infections in Ontario.

Predictive modelling is difficult, however, when an outcome is dependent on several, constantly shifting factors, he said.

“In the paper, they say the weather is a significant driver of this,” Russell said in a Wednesday interview. “So if you look at the forecasts for the next two weeks, we could be seeing cooler than normal temperatures, which could have a significant impact on mosquito populations.”

On Tuesday, Hunter and Giordano published a paper in the open-access, peer-reviewed journal PLOS ONE, predicting about 340 people will be infected by the West Nile virus by early fall.

The prediction is made by an equation Giordano created using the past 15 years of data on mosquitos carrying the virus and human infections.

As the number of mosquito populations, or pools, carrying the virus rise, the number of human infections grows at a predictable rate.

Hunter said this year’s hot, wet summer is an ideal breeding environment for the species of mosquitos that carry the virus. She said 250 infected mosquito pools have been found in Ontario this year and the model predicts that number will reach about 500 over the next six to eight weeks.

Based on that number, the Brock model predicts about 340 people in Ontario will be infected, a number equivalent to the 2012 West Nile outbreak in Ontario that killed two people. The 2012 outbreak was the most significant since the 2002 West Nile virus epidemic which saw 394 people infected.

Russell said Public Health Ontario carefully tracks mosquito monitoring by public health units, including Niagara Public health department, and all signs are pointing toward a higher number of human infections this year.

While the Brock model is a useful tool pointing toward possible trouble, several factors including weather and preventative measures by the public and governments may ultimately decide how many people are infected, he said.

Nevertheless, Russell said the public needs to be aware that Ontario is entering the season when virus-carrying mosquitos are active.

They tend to bite in the hours between dusk to dawn, and the public is advised to wear long sleeves and lighter coloured clothing and use insect repellant that contains DEET while outside during those hours.

Dr. Must a fa Hirji, associate medical officer of health at the Niagara public health department, agreed the Brock model is useful but not definitive and said monitoring will continue.

No human cases have been found in Niagara so far in 2017.

In the meantime, he said the health department may ramp up public awareness campaigns to reinforce the need to take preventative measures.

Right now the health department’s public awareness efforts have focused on the opioid crisis, but given the number of infected mosquito pools — eight have been found in Niagara this month, according to public health data — more awareness about West Nile virus is likely.

Part of the public health message is a recommendation to remove standing pools of water around homes because that is where mosquitos like to breed.

“Of course, there are many standing pools that are not on private property, and those are our responsibility,” Hirji said.

The health department is using larvicide on pools found on public property as a way to curb the mosquito population, he said.

The virus is spread to humans by mosquitoes that have fed on infected birds.

Most people do not show symptoms, but about one in five infected people will suffer symptoms that may include headaches and fever, nausea, body aches and a rash.

One in 150 people will suffer a high fever, muscle weakness and a sudden sensitivity to light. In very rare cases, people can die from an infection.