A deadly disease is on the rise in Australian cat populations. Known as feline panleukopenia, or "cat plague," the sickness hasn't been an issue for cats down under for 40 years thanks to a vaccine developed during the 1970s. But in the last couple of years, cat plague has reemerged. And as veterinarian​s Mark Westman and Richard Malik write for The Conversation, it has the potential to spread quickly if something is not done.

Last weekend, Victoria's Royal Society for the Prevention of Cruelty to Animals (RSPCA) sent out a community alert urging owners to vaccinate their cats after the vets found the disease in several stray kittens brought to shelters around Melbourne. “Vaccination provides high immunity, which is why these recent confirmed cases of Panleukopenia are cause for concern—and action,” Australian Veterinary Association President Paula Parker says in the release. “It typically takes two days for an infected cat or kitten to become symptomatic, so the risk of transmission is extremely high."

According to the American Veterinary Medical Association, feline panleukopenia (FP) is a highly contagious virus that attacks cells that rapidly divide like those found in bone marrow, intestines and developing unborn kittens. If the disease attacks and destroys bone marrow cells, the cats can no longer produce white blood cells, an important part of the immune system. Infected cats then often develop serious secondary infections.

The virus can be transmitted through urine, feces or even fleas from other cats. Kittens, sick cats and unvaccinated cats are most susceptible. Symptoms include diarrhea, lethargy, fever, vomiting and dehydration. And once a feline is infected, there's no medication that can kill the virus. The hope is to help keep the kitties healthy long enough so they can naturally fight it off. Such supportive care includes IV fluids, opioid meds for pain, nutrition supplements and blood transfusions. Without treatment, the AVMA reports that ​up to 90 percent of FP-infected cats may die.

So why has this cat scourge reared its head again after 40 years?

Westman and Malik write that it’s likely it never really went away. Australia has six times as many feral cats as they do pet cats, and the virus may also be able to infect dogs and foxes. “Perhaps with an increased effort to rehabilitate and re-home 'fringe-dwelling cats,' it was inevitable that the virus would spill back from these unvaccinated cats into the general pet cat population, given waning herd immunity,” they write. Once immunization rates drop below a certain level—in the case of cats, this is around 70 percent—they lose what is known as 'herd immunity' or 'community immunity,' which can potentially protect non-immunized animals from infection.

The first outbreak in pet cats occurred in Mildura. According to Westman and Malik, the region is rural with a fairly low average income for residents. "It is our suspicion that the cost of vaccinating the family cat (currently more than $200 for a kitten requiring a course of two to three vaccines) exceeds the budget for many pet owners," they write.

From there, in early 2017, the disease found its way to the Sydney metropolitan area, where more than 50 cats in shelters died. “The current outbreak seems to be caused by a lack of mass vaccination, especially in shelter-housed cats,” Professor Vanessa Barrs of the Univeristy of Sydney said at the time. “The disease had previously re-emerged in Melbourne cat shelters a few years ago but despite warnings, cats have not been vaccinated in many shelters because their risk of disease was perceived to be lower than in dogs, when in reality the risk to cats is high.”

The disease used to be once widespread, but according to the AVMA, is now considered "uncommon." Occasional bouts have appeared outside of Australia in recent decades. Last year, shelters in North Carolina saw an increase in the virus. And in 2014, the disease struck the island of Maui, the first time FP was found in the state of Hawaii.

The effects of the virus may also be worsened by a spreading anti-vaccination movement in the pet community. But as Gavin Haynes at The Guardian reports, there is no strong evidence that points to the fact that vaccines cause the range of claimed negative side effects or diseases.

Overall, the key to stopping FP's spread is vaccination. As Liz Walker, CEO of the Victoria RSPCA says, "the importance of keeping your pet’s vaccinations up to date cannot be overstated."