New Parasite May Have Come From Imported Pets

Reviewed by Brunilda Nazario, MD on October 08, 2015

Oct. 8, 2015 -- It sounds like science fiction: Your eye hurts. You look in the mirror and it has turned an angry red. You try some eye drops, but the pain doesn’t go away, so you head to the eye doctor.

There, under a magnifying glass, is a tiny worm squirming under the light.

That’s what happened to a 57-year-old man from Eugene, OR. “We took him to surgery right that day,” says Annette Sims, MD, the ophthalmologist who treated him.

Sims said while red, painful eyes are common at her practice, she’d never before seen one caused by a worm. And it wasn’t so easy to remove.

“It had, I’m going to call them barbs or hooks or something -- it didn’t want to let go. It had something it was attaching to the actual iris,” says Sims, who eventually got the worm out and shipped it to a lab. There, it was partially identified as a relative of the same worm that causes river blindness in Africa.

But the man hadn’t been to Africa or any other part of the world where parasitic infections are common.

Instead, experts say, the worm likely came to him, brought to this country by one of thousands of dogs that are imported each year. As international pet adoptions become more common, so too are the exotic diseases they can bring with them.

This week, doctors attending IDWeek, the annual meeting of the Infectious Diseases Society of America, will hear about the first six human infections in the U.S. of a worm that goes by the scientific name Onchocerca lupi. All the patients live in the Southwest -- including three cases in Arizona, two in New Mexico, and one in Texas.

The Oregon man isn’t officially counted as one of the six cases, because researchers were only able to partially identify the worm that was removed from his eye. But the partial ID pegs it as the same family as the other cases.

And eyes aren’t the only places these worms can show up. Five of the 6 cases being presented this week were in people with innocent-looking lumps or bumps under their skin. When doctors cut into the bumps, they found worms inside.

Most of the new cases are children. They include a toddler and a 5-year-old who each went to the doctor with a strange bump on the back of the neck where a worm had burrowed. In those cases, the worm was pregnant, so doctors are giving those kids regular treatments with ivermectin, an antiparasitic drug that helped researchers in the U.S. and Japan win a Nobel Prize this week. The ivermectin is a precaution to make sure no new worms hatch and reinfect them.

Also, a 10-year-old had a bump on the scalp. Another 10-year-old had an eye infection. A 13-year-old had a mass on the back of the neck. The only adult, a 50-year-old, had a lump on the forearm. Those patients were treated with the antibiotic doxycycline, which also kills the parasite, says Paul Cantey, MD, a medical epidemiologist at the CDC.

Symptoms varied by the location of the lumps. Kids who had them on their neck or scalps said they had neck pain and headaches. People report eye pain when the worms burrow there.

More Questions Than Answers for Now

Onchocerca lupi is also known to infect the eyes of dogs and cats. Since 1991 in the U.S., there have been about 12 cases documented in pet dogs and 2 in cats.

Researchers aren’t sure how it gets to people yet. So far, none of the infected people caught it from a pet, Cantey says.

But tests of biting black flies in California have found that about 3% carry the youngest stage of the worm. It’s thought that the flies may bite infected dogs, then bite people, spreading the infection. That’s the same way people catch river blindness.

Tests of worms removed from dogs in the U.S. found they were genetically similar to the species that infects dogs in Europe, suggesting the infection was brought into this country -- though researchers can’t be sure how recently that happened.

And right now, researchers say they have far more questions about the infections than answers.

“We don’t know how common it is,” says Scott Weese, DVM, a veterinary infectious disease specialist at the University of Guelph in Canada. “Are these just oddball infections or just the tip of the iceberg?”

Weese studies infections, like these worms, that can be passed from animals to people.

Many international pet adoptions “are rescue dogs,” he says.

“There are rescue operations out of places like Greece, Azerbaijan, and the Caribbean. These are kind and well-meaning groups, but they’re ill-structured, and they’re bringing in animals that are sick. They’re bringing in diseases that we don’t see here. We’re seeing diseases that are foreign to North America and ones that are rare here because of vaccination.”

The only requirement for bringing a dog into the U.S. is proof of a rabies vaccination. Only the state of Hawaii and Guam, a U.S. territory, require a quarantine for arriving animals.

Weese says no one is sure how many pets are imported to the U.S. each year, because there’s no formal count. But some rough estimates from Canada suggest the number is in the thousands.

“Part of it is because people are thinking they’re doing a good job. Part of it, I think, is the cachet. ‘I’ve got a Peruvian rescue dog’ is a little different than the mutt from down the road,” Weese says.

'Nothing to Panic About'

Scientists also don’t know if the Onchocerca lupi worm is widespread or confined to a certain region of the country, or if it’s widespread in dogs and cats. In addition to domestic animals, Weese says doctors are trying to figure out if the worm is also in wild dogs like coyotes and wolves.

Aside from a decidedly high "ick" factor, the infection doesn’t seem to have any dire or long-lasting consequences -- unlike its cousin, river blindness, which causes intense itching and can cause permanent vision loss.

So far, all known human patients in the U.S. have recovered with short-term drug treatment.

Until more is known, “it’s nothing to panic about,” Weese says.

He says the most important thing people can do to help stem the infection here is to keep an eye on their pets. Dogs get lumps or nodules in their eyes, along with tearing and weeping. Symptoms like that should be checked by a vet, he says.

“It’s a reminder that we have emerging issues when it comes to diseases that can be transmitted from pets or companion animals to us,” he says.

Show Sources


Onchocerca lupi: An Emerging Parasitic Infection in the Southwestern United States” IDWEEK 2015.

Annette Sims, MD, ophthalmologist, Eugene, OR.

Scott Weese, DVM, Professor, veterinary infectious disease specialist, The University Guelph, Ontario,  Canada.

Paul Cantey, MD, medical epidemiologist, Parasitic Disease Branch, CDC, Atlanta.

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