It was an experiment that was part “Outbreak” and part “Trading Places,” but researchers at Colorado State University hoped it might answer one of the more pressing public health questions facing the state: Could the Zika virus spread here?
The conventional wisdom was that it couldn’t, at least not widely, because the disease’s primary vector — a ninja of a mosquito called Aedes aegypti — had no established population in the state. But no one had proved that conventional wisdom true. What if it was wrong? What if the local mosquitoes — particularly those in the Culex genus that already carry the West Nile virus — were also able to transmit Zika?
The mosquito experts at CSU’s Arthropod-borne and Infectious Diseases Laboratory decided to put the conventional wisdom to the test. In their lab, they infected Culex mosquitoes with Zika. And then, for a week as the virus incubated inside their study subjects, they held their breath.
Finally, the results:
“We could not get that virus to replicate in the mosquito,” CSU professor Brian Foy said of the experiment, the topic of a soon-to-be-published article.
Whew.
When it comes to being disease-carrying menaces, not all mosquitoes are created equal, and that explains why Zika is a public health crisis in Miami and a relative public health trifle in Milliken. There are 54 species of mosquitoes commonly found in Colorado, but none of them are known to be able to transmit Zika — even if they can, like the Culex mosquitoes, transmit other diseases.
“It’s still kind of a mystery,” Foy said. “There’s a very fine-tuning match of virus and mosquito.”
The reason is that a mosquito’s transmission of a disease is not as simple as taking infected blood in from one person and then backwashing it into another. Instead, the virus must be capable of reproducing inside the mosquito and then making its way into the mosquito’s saliva — the real source of contamination since mosquitoes digest the blood they consume as food.
“When a mosquito bites a person, it spits into the bite,” Foy said.
With the local-born bugs apparently incapable of carrying Zika, then, the only chance of a mosquito-spread outbreak in Colorado comes from a moonshot: the one-in-a-billion possibility that a Zika-capable mosquito that hitchhiked to the state from elsewhere bites into a Zika-infected person who picked up the virus out of state.
So far, only 21 people in Colorado have tested positive for Zika, and all of them contracted the virus somewhere else.
The only instance of a wayward Aedes aegypti being found in the state came in 2010, in Pueblo County. Another species of mosquito possibly capable of transmitting Zika, Aedes albopictus, has been found sporadically in Weld County, most recently in 2011 and most likely was brought to the state in waste tires disposed of here. But those species are just summer tourists; they can’t survive the winter.
“Remember we’re in Colorado; we’re not in the tropics,” said Michael “Doc” Weissmann, the chief entomologist at Colorado Mosquito Control, which does monitoring and eradication for local governments.
Weissmann said none of his company’s traps have snagged worrisome Aedes mosquitoes this year.
“It’s just the same old, same old,” he said.
Eric Aakko of the Weld County Health Department said the county’s surveillance efforts have found higher-than-normal levels of West Nile-carrying mosquitoes but no Zika concerns.
That leaves only one other prominent way that the virus can spread within Colorado, and it’s one Foy knows a little too well. While conducting research in the African nation of Senegal, Foy contracted Zika in 2008. He then came home and accidentally gave it to his wife, which led to a research article hypothesizing — correctly, it turns out — that Zika can spread through sex.
The experience, though, didn’t change his mind about the dangers of Zika in Colorado.
“The risk of all of a sudden having a massive outbreak in Denver is, I would say, infinitesimally small,” he said. “From mosquitoes, I can’t see it happening.”
