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FORT COLLINS — The Centers for Disease Control and Prevention laboratory has confirmed that a Los Angeles County woman contracted West Nile virus. 

The Rocky Mountain News reported in Wednesday’s editions that researchers confirmed the case by testing a blood sample. 

The case is believed to be the first appearance of West Nile virus west of the Continental Divide. It’s a baffling finding because West Nile has not been seen in Utah, Arizona, Nevada or anywhere else in California. 

This week officials said a Houston-area man visiting relatives in Los Angeles fell ill in a probable case of West Nile virus. However, they believe the man was likely bitten by an infected mosquito in Texas. 

“How she got it, your guess is as good as mine,” lab director Duane Gubler said Tuesday. “This virus continues to surprise us.” 

Migrating birds could have carried the germ north from Central America or Mexico, Gubler said. However, under that scenario, Gubler said the virus would probably have been detected in sentinel chickens or horses. 

“California has one of the best, if not the best, surveillance system in the country,” Gubler said. 

The California woman lived near Los Angeles International Airport and worked for an air-courier company, Gubler said. 

“It’s possible, since it was in the area of the L.A. airport, that the virus came in a mosquito that hitched a ride on a plane.” he said. “It’s pure speculation.” 

No human West Nile cases have been reported in Colorado. 

Nineteen more Colorado horses tested positive for West Nile on Tuesday, bringing the statewide equine total to 117. At least 31 of those horses have died, according to the state Agriculture Department. 

Nationwide, 45 human West Nile deaths and 1,086 human cases have been confirmed this year by the CDC.