LOWELL — Almost 20 dogs in this Merrimack Valley city have died of a highly contagious disease called parvovirus in the last three weeks, and more are expected to die before the outbreak is contained.
For unvaccinated dogs, the virus is almost always fatal.
Vaccination is key to survival, though vaccinated dogs can also contract the disease. Animal care providers urged caution.
“I’d avoid dog parks, and maybe even avoid any area where dogs congregate, even a day care,” said Dr. Nancy Kopec, a Wignall Animal Hospital veterinarian who practices in Lowell.
It is unclear why Lowell is experiencing the outbreak. Viruses can mutate over time, Kopec said.
“They’re sneaky,” she said. “For some reason, this seems to be kind of a hotbed for parvovirus.”
The virus might have come into the city through a visiting dog or a newly purchased dog, said Lowell Animal Control Compliance Officer Darleen F. Wood. It also can lurk for months on inanimate objects, such as a bowl at a dog park, she said.
“It could’ve been 100 different reasons,” she said.
Wood said outbreaks tend to happen in compact, urban areas with many stray dogs and people who, because of financial constraints, neglect to bring their pets to the veterinarian.
Spread when a dog comes in contact with an infected dog’s feces or vomit, parvovirus attacks the gastrointestinal system. Dogs experience severe vomiting, lethargy, loss of appetite, and bloody diarrhea with a particularly foul smell, according to the American Society for the Prevention of Cruelty to Animals.
Canine parvovirus has no known effect on humans. Dogs tend to show symptoms three to seven days after exposure to the virus, Kopec said.
“The dogs are very, very lethargic,” she said. “Some will come in, and they can’t even stand up.”
Even with treatment, which can take a week or more, some dogs don’t survive.
More than 50 percent of young dogs die. Older dogs, with a hardier immune system, have a better chance of battling the virus, Kopec said.
This outbreak has claimed a variety of dogs, from pit bulls to Jack Russell terriers, Wood said.
Treatment includes intravenous fluids and drugs to control vomiting. Home treatment is not recommended.
The virus can come as a surprise to some dog owners who don’t see the symptoms as abnormal.
“My dog will have diarrhea or throw up occasionally, and you think nothing of it until it continues for two, three, four days, and maybe you say, ‘OK, something’s wrong,’ ” said Paul Sweeney of Lowell, who has a shih tzu-American Eskimo mix named Sammi. “And they’re already at the point of terminal, you know.”
To contain the outbreak, local groups are providing vaccinations Saturday in Lowell. Healthy dogs can get vaccinated for $10. Sick dogs should immediately be taken to the vet.
The clinic, at Kathryn P. Stoklosa Middle School from 9 a.m. to 2 p.m., is open to anyone.
Vaccines take seven to 10 days to build in the immune system, Wood said.
“What we’re hoping is over the next few weeks it really gets contained, and then ends,” she said.
Crystal Arnott, shelter manager of the Lowell Humane Society, said she has seen several dogs suffering from parvovirus brought to the clinic to be euthanized.
Losing an animal to the disease is devastating, said Wood, the animal control officer.
“Animals are so much of people’s lives. You always want to protect your children and your animals,” she said. “When you feel that you can’t protect them, I imagine it would take some kind of toll.”
Sitting on a Shedd Park bench while her daughter’s Yorkshire terrier, Cheech, nosed around in the mulch, Ana Santos said she could not imagine losing her dog.
“It would be strange without him now,” said Santos, 45, of Lowell.
She hadn’t heard of the outbreak, but she wasn’t worried because she keeps her dog vaccinated. So does 42-year-old Eric Hand, whose pit bulls Dio and Cooper played with Hand’s 12-year-old son, Danny, in a parking lot near their Lowell home Friday.
“It’s kind of like paying insurance on anything else,” Hand said. Despite the precautions, he said, it can be hard to keep tabs on pets.
“Those dogs will eat anything, so that’s the problem,” he said. “You can’t talk to them, so you have to be on guard all the time.”
The outbreak has not worried him much. Still, he is trying to be careful.
“I’d be heartbroken,” he said. “It would be like losing a family member.”Globe correspondent Zachary T. Sampson contributed to this report. Claire McNeill can be reached at claire.mcneill @globe.com.