SEATTLE — Veterinarians thought it odd when they saw eight cases of the sometimes deadly canine parvovirus in November at a Seattle-area animal-emergency clinic.

That one-month figure was half what doctors at Animal Critical Care & Emergency Services, or ACCES, generally see over an entire year. They’ve treated 28 cases so far in 2012 — almost twice as many cases as they usually treat in a year.

Because most of the pets ACCES sees are referred to them by other vets who have informed owners that parvo treatment can cost thousands of dollars, ACCES medical director Dr. Beth Davidow started worrying that there could be a much larger outbreak.

"It’s extremely contagious, which is why it’s so important to diagnose it as early as possible," Davidow said of the virus. "Most adult dogs don’t get very sick from it, but it kills puppies."

For the first time in 17 years of practice, Davidow said, she saw a vaccinated 15-month-old dog come down with parvo.

So ACCES put out an alert to other vet clinics last week to push for better vaccination rates and to warn puppy owners to be careful about where they take dogs that aren’t fully vaccinated.

"We’re not sure if this is a particularly virulent strain, but there is a recent across-the-board increase at several (animal) hospitals," said Lisa McCollough-Dutt, a manager at the South Seattle Veterinary Hospital.

McCollough-Dutt said she thinks a major contributing factor is people’s budgets tightening up.

"We get a lot of low-income owners in here who tend not to vaccinate when they should," she said.

She also said the informal selling and giving of dogs through Craigslist has "definitely" increased the rate of unvaccinated puppies. If there isn’t paper documentation that a puppy has been vaccinated, new owners should assume the dog needs to be inoculated.

"Even if someone told you they vaccinated the dog already, it doesn’t hurt to do it again," she said.

Though vaccinated adult dogs are likely not at risk for contracting the virus — which causes bloody vomiting, diarrhea and dehydration — puppies are. Even puppies younger than 6 months that have had their first or second parvo vaccine can be susceptible, according to Davidow.

"Most of the cases we’ve seen have been puppies," she said. "Your puppy is not covered until they have a series of three parvo vaccines."

She said most were infected while at a shelter, a pet store or simply somewhere on a walk. Because it takes more than one inoculation for a puppy to be fully immune, the virus can sometimes spread at a shelter or store even if the dogs have been properly vaccinated.

Parvo has a symptomless incubation period that can last two weeks, so dogs carrying parvo can’t always be identified before they pass it on to others.

Parvo spreads easily to puppies or dogs with suppressed immune systems and can live for as long as six months in dirt, feeding dishes, collars, leashes or anything else onto which dogs can shed the virus. It is extremely resistant to heat, cold and just about every disinfectant, with the exception of bleach. One close whiff or lick of an infected area is all it takes for some dogs to come down with the virus, Davidow said.

Even after an infected dog has gone through the most severe phase of infection — usually a week or less — the dog can shed the live virus through feces for up to six months.

Socializing puppies is important, but owners should make sure their dogs’ playmates are fully vaccinated, Davidow said. It is possible for puppies to survive the disease, but many don’t without intensive treatment.

At ACCES, which has offices in Seattle and Renton, the cost of treating a dog with parvo ranges from $1,500 for two days to $7,000 for eight days. The survival rate is 90 percent with treatment that includes intravenous fluids, anti-nausea medication, antibiotics and plasma in severe cases.

Owners can nurse a sick dog at home with round-the-clock attention, but many people who can’t invest the time or money to keep a puppy alive wind up having their dogs euthanized, Davidow said.

A single parvo combination vaccine shot usually costs about $25, according to Terri Inglis, executive director for Homeward Pet Adoptions Center in Woodinville, Wash.

Because no public agency tracks the number of parvo cases, Davidow said, she believes it’s the responsibility of a large clinic like hers to keep track of how prevalent the virus is.

Davidow said there’s also a chance the spike in parvo cases is purely coincidental. While some smaller clinics she’s called haven’t seen any cases, some, such as one in Shelton, Wash., recently dealt with a spate of 10 cases linked to one pet store.