When it comes to monitoring for West Nile virus activity, health officials look toward the animals and mosquitoes, as well as human cases, as measurements of how prominent the disease is in the state.
Already this year, Maricopa County has collected double the number of positive mosquito samples than last year and the state is leading the country in human West Nile virus activity, according to the Centers for Disease Control and Prevention. The first human case was confirmed in June, though mosquito-positive samples were found months before that.
Humans, horses and some bird types are most susceptible to illness when bit by an infected mosquito. Genetics make other animals - like cats and dogs - immune to the disease. While there have been no confirmed cases this year, the virus has also been found in squirrels in northern Arizona in the past. Llamas and alpacas may also get ill, health officials say.
In most cases, humans infected with the virus show no to few symptoms, but it can turn deadly. This year, the state has confirmed 23 human cases, 22 of them in Maricopa County and one in Pinal County. There have been two deaths - both elderly residents of the East Valley.
The state Department of Health Services uses six methods to track the virus, which is passed from infected bird to mosquito to human or horse, said Craig Levy, the state's Vector Control Program Manager for the health services department:
• Mosquito trapping and testing. Maricopa County has more than 500 traps set up from the East Valley to the West Valley. There have been 110 positive samples as of July 26.
• Human case follow-up and investigation.
• Tracking horse cases. No horses have tested positive so far this season.
• Dead bird testing. County departments can test birds brought in that may be suspected of having the illness. Common pigeons and doves do not get the disease, but finches and sparrows can.
• Other animal testing. Last year there were cases of squirrels dropping dead out of tress. They tested positive for West Nile virus.
• Sentinel chicken flocks. While the chickens are outside, they get bit by mosquitoes, but they do not get ill from the West Nile virus. Their blood creates antibodies, however. When that's found, that means the virus has arrived. There are more than a dozen sentinel chicken flocks in southern Arizona.
Once the virus has been identified, counties can get to work, Levy said, with mosquito fogging and education for the public about ways to eliminate mosquito breeding sites.
Dr. Philip Blair, state veterinarian with the Arizona Department of Agriculture, said most horse owners are informed about the disease and keep their animals vaccinated. Like West Nile virus in humans, it cannot be passed from one horse to another, but only transferred through a mosquito.
And also like human cases, some animals don't show any signs or symptoms of being infected, Blair said.
"Horses are very susceptible to West Nile," he said. "Most horses can be given a shot and a booster shot in the first year, then every six months. Even when we have infections around, horses are relatively safe. It's not so much for people."
There is no human vaccine for the disease.
When a horse does get ill with the virus, it can manifest itself as a fever, much like in human cases.
"They'll quit eating. They may be depressed and have nervous (disease) symptoms like blindness. Mostly, they look dejected. They don't feel good," he said. "If it's the right time of year and they're not vaccinated, that's usually the first thing we suspect."
Antibiotics are ineffective on the virus in humans and horses, though they may be given to prevent a secondary infection, Blair said.