Aidan Pankey died June 12, 2013, hours after he was rushed to the hospital with severe stomach pains.
Medical examiners say he died from a streptobacillus moniliformis infection, commonly known as rat-bite fever, after exposure to an infected rat.
The Pankey family's attorney John Gomez said they are devastated by Aidan's death.
The lawsuit says Petco should have known about the rodent's health and did not adequately test for the disease. It also says that negligence led to the boy's death, which has caused his parents, Andrew Pankey and Vanessa Sauer, emotional and economic hardship, and that the retailer did not post adequate warnings about the potential risks, especially for children.
Petco says it's in the process of investigating these claims and is waiting for more information before it can respond.
The company released a statement expressing its condolences.
"We are deeply saddened by the Pankey family's tragic loss," Petco said. "The health and safety of people and pets is always a top priority, and we take the family's concerns very seriously."
The boy's grandmother purchased the male rat, which Aidan named Alex, because her only grandson wanted a mate for his female rat, Gomez said. The boy took the rat home May 27, 2013, and woke up the night of June 11 in severe pain with a fever and stomach problems. He was pale, lethargic and could barely walk, according to the lawsuit. He died at 1:09 a.m. the next day.
The lawsuit was not filed until now because attorneys were awaiting the lab results from the federal Centers for Disease Control, which tested the rat to confirm it was infected, Gomez said. The agency could not immediately confirm the results Tuesday.
The CDC says people can contract rat-bite fever from bites or scratches from infected rodents like rats, mice and gerbils, or even just by handling an animal with the disease without a bite or scratch. It can also be contracted by consuming food or drink contaminated with the bacteria. It is not spread from person to person.
Antibiotics, such as penicillin, are highly effective at treating rat-bite fever, and it is rarely fatal, according to the CDC.
The Associated Press contributed to this report.