SACRAMENTO - Darlene LaDell stroked Sarah's strong neck and whispered soothing words in the horse's ear. Still, the mare pranced around the stall, wide-eyed and tense in anticipation of the needle's prick.
From LaDell's point of view, Sarah's nervousness was a small price to pay for protection against West Nile virus, a disease that in California kills nearly half of the horses it infects.
"I'd be sick if my horses got West Nile virus," said LaDell, of Granite Bay. She has had Sarah since the foal's birth 19 years ago, and the mare's offspring, Shulaba, for 13 years. "They're a part of the family."
Since introduction of the equine West Nile virus vaccine in 2001, hundreds of thousands of horses such as Sarah and Shulaba have been inoculated against the virus -- contributing to a dramatic drop in West Nile illness and death among horses.
Health officials can only hope that a vaccine for humans will one day do the same for those at highest risk for the most serious consequences of the disease.
Unfortunately, developing and testing a human vaccine is far more complicated, requiring years of clinical trials. Even if the Food and Drug Administration approves a vaccine, persuading people to get inoculated may be tricky; in many parts of the country, the incidence of West Nile virus -- and public concern over it -- has diminished significantly.
The federal government, however, has a huge investment in fighting the mosquito-borne disease, spending $43 million on human vaccine development this year alone.
"The virus is here to stay in the United States," said Patricia Repik, program officer for emerging viral diseases for the National Institute of Allergy and Infectious Diseases. "Even though there is not as much disease right now doesn't mean it's not going to increase at some point or in certain populations."
Along with dead birds, infected mosquito pools and sentinel chickens with antibodies to the virus, horse infections and deaths have long been signals to health officials that West Nile virus is active in a community. However, the apparent success of the equine vaccine has muddled the equation used to prompt mosquito-control measures such as insecticide spraying.
Although all other indicators are clearly up in California this year, horse infections are down dramatically.
By this time last year, 48 horses in California had tested positive for the virus and 24 had died. By season's end, the state reported 456 cases, with 200 deaths.
In contrast, just eight horses in California have tested positive for West Nile virus this year; four of those animals have died.
Bakersfield veterinarian Laura Blanton said horse owners who had initially resisted vaccinating are getting wise to the threat posed to their prized animals.
"One client resisted vaccination and had a horse come down with the disease and got a $1,200 vet bill," she said, contrasting that with the $30-per-dose cost of the vaccine. "The disease has been ugly enough that almost everyone knows somebody whose horse has gotten sick from West Nile."
Ben Sun, California's public health veterinarian, said an estimated 750,000 to 1 million horses in California have been vaccinated, though it is not known how many are getting annual booster shots, which are critical to maintaining immunity.
Early on, some horse owners worried that the vaccine was causing spontaneous abortions and birth defects, but studies have failed to find an association. All the same, vaccination of mares is not recommended during the first 60 days of pregnancy, said David Wilson, a UC Davis veterinarian who specializes in equine infectious diseases.
Among horses infected with West Nile, one-third to one-half get encephalitis, a brain infection that also afflicts a small fraction of humans who contract the disease.
"They get funny muscle twitches or odd, herky-jerky behavior," said veterinarian Greg Fellers, owner of Loomis Basin Equine Medical Center, whose practice has seen about 70 infected horses since 2004.
Some infected horses will seem depressed, become less aware of their surroundings and will not respond normally to their owners, while others become hyperactive and overly sensitive to noise or light, added Wilson.
"Once they're down and can't eat or get up, they have to be lifted up and put in a sling and fed intravenously or they are going to die," said Fellers.
From 25 percent to 45 percent of horses that do survive suffer long-term effects, including problems with coordination, muscle weakness and personality changes, Wilson said.
Some horses exposed to the virus through mosquito bites develop natural immunity -- the same as with humans -- a phenomenon that also contributes to a decrease in West Nile disease among horses.