On Thursday at dusk, about 100 people lined the Rillito bike path beneath the Campbell Avenue bridge as thousands of bats dropped and swirled just a few feet overhead.
In the dry riverbed, children ran and screamed as the nightly exodus from the bats’ seasonal nursery roost began.
Bats streamed westward to the farm fields along the Santa Cruz River to feast on moths and any other insects within reach.
A pregnant or lactating bat will eat her weight in insects each night to produce a rich milk that is 20 percent fat, said bat researcher Eran Levin. Baby bats grow to the size of their mothers within three weeks, Levin said.
That’s why these Mexican freetailed bats are here in the humid days of summer. They locate their nurseries where insects abound.
The 28 species of bat that call Arizona home are protected by state law. They are critical to agriculture and to desert ecology.
Two nectar-eating species pollinate our signature desert plants.
The 26 insect-eating species hold down the agricultural pest population and hold mosquito populations to a level more acceptable to human desert inhabitants.
Bats are useful, fascinating creatures.
We should admire them, naturalists say, not fear them. Discard the myths. Bats are not blind. They don’t get tangled in your hair or suck your blood.
We should also respect them and never handle them.
They, like just about any mammal, can carry rabies, and they are the animal most likely to spread rabies to humans.
Most bats are not rabid. Some are, and that’s enough for health officials who say you should be vaccinated for rabies if you have contact with a bat.
Rabies tests, usually conducted on bats that have come in contact with humans, show a 3-percent positive result.
In the wild, less than 0.5 percent test positive.
The odds are good, but health officials note that rabies is a fatal disease. There is no treatment for it once symptoms appear. If you get it, you die.
The disease is rare in humans. The last rabies death reported in Arizona was in 1981, according to the Arizona Department of Health Services.
The U.S. Centers for Disease Control analyzed 19 rabies deaths recorded in the United States in a 10-year period ending in 2006.
Bat encounters accounted for 17 of those deaths, and the majority occurred when bats got into people’s homes and people removed them without taking proper precautions.
In Arizona, health officials record about 70 positive rabies tests in animals each year, with most of those in bats.
If you encounter a bat, leave it alone. If it is in your home, remove it carefully or call a professional.
Bats in the wild are very good at avoiding you. In addition to keen eyesight, they navigate the dark with echolocation, that famous bat sonar that allows them to find food, and “see” down to the thickness of a hair.
The Arizona-Sonora Desert Museum, which staffs two local bridges with bat experts during summer Thursdays, has never had a report of a bat encounter with a human, said Rosemary Prawdzik. “They know we’re there, and they don’t fly into us,” she said.
Wildlife biologist Janet Tyburek said she lived in Austin, where the Congress Street Bridge hosts the largest urban bat colony in North America each summer.
“For years, we tried to get the county parks department to put up signs in English and Spanish telling people not to handle downed bats. Then the inevitable happened.”
A group of kids picked up a bat and played with it. It tested positive for rabies.
They all had to be given post-exposure rabies shots, and a cry went out to seal off the bridge.
Fortunately, said Tyburec, the naturalists’ argument won out, and the city simply put up the signs.
Health officials urge post-exposure shots for any encounter with a rabid bat or with a bat that is not captured for testing. If a bat is found in a room where an infant sleeps, the child should get the shots, said Jose Sanchez, enforcement operations manager for the Pima Animal Care Center.
Pets that come in contact with a rabid bat must be quarantined or euthanized, Sanchez said. If the pet has been vaccinated, the quarantine can be done at home. If not, the animal must be housed at the animal-care center or a vet clinic at the owner’s expense.
The rules are tough for a reason, he said. “Rabies is fatal.”
Naturalists like Tyburec and Levin say they understand the need for caution, but don’t like seeing bats singled out for rabies danger.
Bats account for more than half the positive rabies tests in most years, but they are the mammals most likely to be encountered in urban settings, said Tyburec.
When rabies cases spike, as they did in Southern Arizona in 2010, it’s usually because of other animals, she said.
In 2010, skunks accounted for 66 of the 114 positive cases, according to state health department statistics.