Parasitic eye worms are common among dogs, cats, pigs, sheep, goats, cattle and wild carnivores like foxes and wolves. The larvae are transmitted by female "face flies" that feed on the animal's eye secretions. "Tears are full of proteins of various kinds, so the flies get a lot of nourishment from those tears," explained Dr. William Schaffner, a professor of medicine at Vanderbilt University's Division of Infectious Diseases. "For a scientist, it's a fascinating ecological niche."
The worm larvae grow into adulthood and reproduce between the eye and the eyelid. Their offspring leave the host's body via more secretions from the inflamed eye, which the flies ingest, completing the life cycle.
"The early-stage larvae need to go through the fly's digestive system to be able to develop to a more advanced stage to infect another host," Bradbury explained. "It's a complicated life cycle."
Veterinarians treat the infection in pets and livestock with the anti-parasitic drug ivermectin. But in untreated animals, Bradbury says, the worms can live and reproduce up to 30 months, leading to vision loss or even blindness.
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