One morning last July, Brookline resident Neal Heffron found Dani, his 18-month-old Australian Shepherd with a sleek coat and unbridled energy, collapsed in her bed. She had been vomiting, suffered a bout of diarrhea, and overnight had become too weak to lift herself out of her crate.

After several visits to the vet, a test came back positive for anaplasmosis, a tick-borne illness that can lead to fever, intense lethargy, and joint pain in humans as well as dogs. Antibiotics cleared the symptoms, but a little over a month later, Dani was back at MSPCA-Angell Animal Medical Center in Boston, virtually paralyzed and unable to eat or drink. This time, she tested positive for Lyme disease.

Within a few hours after the first dose of a 30-day course of doxycycline, which can also be used to treat Lyme in humans, Dani was walking again.

“It was very expensive and also very traumatic for us, for my daughter to see her dog just lying there hooked up to an IV,” recalled Heffron, vice president of a Cambridge-based computer sales company.

That dogs are enormously susceptible to the ravages of ticks is no surprise, but there is uncertainty over just how many dogs that are bitten by ticks end up as sick as Dani, and why. Canine veterinarians and researchers who study Lyme disease in animals often cite the statistic that 95 percent of dogs exposed to Lyme never show symptoms, even though a screening test may turn up positive.

“There’s a difference between infection and clinical signs,” said Michael S. Stone, an internal medicine veterinarian who teaches at Tufts University’s Cummings School of Veterinary Medicine. “When we talk about a dog being ‘infected’ with Lyme, it still doesn’t mean he needs to be treated or has an illness from it.”

Still, for cautious dog owners living in one of the many tick-infested New England communities, a case like Dani’s can be frightening, as well as confusing. As tick season approaches and new species carrying new diseases spread into all corners of Massachusetts, many state residents are becoming increasingly skittish about the growing threat of Lyme and other tick-borne illnesses to their pets.

We've got a terrible problem, and it's going to get worse and worse,” said Sam Telford, a professor at Cummings who studies tick-borne diseases. “Dogs are extremely, highly, super-exposed to ticks, and repeatedly so.”

In humans, Lyme is tricky to diagnose, even with symptoms present. A controversial idea, that the pathogen can evade antibiotics and keep people sick for years, persists, leaving people confused about how and when to seek treatment.

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Unlike humans, dogs benefit from a Lyme vaccine. While research hasn't confirmed the vaccine to be 100 percent effective, for dogs it is still a relatively cheap fortifying measure. A similar vaccine is unavailable to humans after being pulled off the market in 2002, despite overwhelming support for its use from doctors and scientists.

In Norfolk County, where Heffron resides, at least 12,900 dogs have been identified as Lyme-infected since 2007, according to IDEXX. The Grafton company maintains a national database, by county, of all dogs that have screened positive for Lyme with its popular test. Experts agree that number is still underreported and has certainly increased over the years.

At Angell, Lyme is part of a dog’s annual heartworm screening. The jury is still out on whether or not to treat all dogs that test positive for Lyme regardless of their symptoms. Doxycycline, the primary antibiotic used to combat Lyme, can be prohibitively expensive for some owners.

For Heffron, Dani’s hospital visits and treatments totaled $5,500.

“Here she was, a one-and-a-half-year-old who runs five miles a day with me, suddenly unable to move,” he said. “If left untreated, she might've died.”

Shan Wang is a reporter at the New England Center for Investigative Reporting, an independent, nonprofit news center based at Boston University and WGBH News.