(CNN) A multistate outbreak of campylobacter infections initially linked to contact with puppies sold through Petland, a national pet store chain, has spread to 55 people in 12 states and is resistant to common first-line antibiotics, the US Centers for Disease Control and Prevention announced Tuesday.
Campylobacteriosis is a common bacterial infection that can cause diarrhea, abdominal pain and fever. It's usually easily treated with common oral antibiotics, but the CDC said clinical samples taken from puppies and people sickened in the outbreak show signs of antibiotic resistance.
However, the CDC says that most people infected with campylobacter need only supportive care, such as fluids and electrolytes, to recover.
"Talk to your doctor if you think you have a Campylobacter infection and let them know if you recently purchased a puppy at a pet store, visited a pet store, or visited or live in a home with a puppy sold through a pet store before illness began," CDC public affairs specialist Brittany Behm wrote in an email. "If your doctor believes you need to be treated with antibiotics, they should do specialized testing to determine which antibiotic is the best choice for you."
As of Tuesday, 55 people have fallen ill in 12 states: Florida, Kansas, Maryland, Missouri, New Hampshire, New York, Ohio, Pennsylvania, Tennessee, Utah, Wisconsin and Wyoming. There have been 13 hospitalizations and no deaths reported. The first case within this outbreak occurred over a year ago, in September 2016.
Symptoms, which typically begin within two to five days of contact with the bacteria, last around a week, though some people don't experience any signs of illness.
While many cases go unreported, about 14 cases for every 100,000 people are diagnosed each year in the United States, according to the CDC. Overall, campylobacteriosis -- which occurs much more frequently in the summer months than in the winter -- is estimated to affect over 1.3 million persons every year.
Almost every patient recovers within five days without treatment, though drinking extra fluids is recommended. In rare cases, an infection can lead to complications, including paralysis and even death.
People with weakened immune systems, such as infants, the elderly and those with cancer or other severe illnesses, are most at risk of serious infection.
People sickened in the current outbreak range in age from younger than 1 year to 86 years old.
Most -- 35 of the 55 -- either recently purchased a puppy at Petland, visited a Petland, or visited or lives in a home with a puppy sold through Petland. Fourteen are Petland employees from five states.
One person obtained the infection after sexual contact with a person with a confirmed illness, four were exposed to puppies from various sources, and one person had unknown puppy exposure.
Whole genome sequencing of samples of the campylobacter infection from Petland puppies was closely related to campylobacter samples taken from people sickened during the outbreak, said the CDC.
However, the CDC noted that no matter where a puppy comes from, it may carry a campylobacter infection.
"While the CDC has openly stated that there is a risk of Campylobacter from any source, not just Petland or a pet store, the focus has been on Petland," the chain's director of public affairs, Elizabeth Kunzelman, wrote in an email.
"The results now highlight cases showing up in Utah, Wyoming, New Hampshire and Maryland, states where Petland does not have a presence," she stated. "The report also indicates that 5 cases had nothing to do with Petland. We certainly hope for continued expansion of testing."
Most people become infected with campylobacteriosis through eating raw or undercooked poultry. Most infections are singular and not part of an outbreak.
By contrast, outbreaks of campylobacteriosis are often linked to unpasteurized dairy products, contaminated water, poultry and produce.
Sometimes, though, people get sick after coming into contact with the stool of an ill dog or cat. The CDC said the infection usually does not spread from person to person, but certain activities such as having sex with an infected person or changing an infected person's diapers can increase the risk.
Although puppies and dogs with a campylobacter infection might have diarrhea, vomiting or a fever, just like humans, they sometimes show no signs of illness.
To avoid contamination from your pet, the CDC recommends that you wash your hands thoroughly after touching dogs, their poop or their food. Extra care is needed so that children playing with puppies also wash their hands carefully. Pick up and dispose dog poop carefully, especially in areas where children might play. Finally, contact your veterinarian if you see signs of illness in your puppy or dog.
When the outbreak was first reported on September 11, Petland said it has many sanitation stations in each store and has strict kennel sanitation procedures and protocols put in place by consulting veterinarians.
After the latest CDC report, Petland said it has "re-doubled its efforts in educating staff and customers about proper hand sanitization."
The investigation is ongoing, according to the CDC, which is working with the US Department of Agriculture's Animal and Plant Inspection Service and several health departments.