In late July 1980, a five-month-old Doberman pinscher puppy in Washington, D.C., started throwing up blood. It died the next day at an animal hospital, one of many pets that suffered that year from a new illness, parvovirus. “This is the worst disease I’ve ever seen in dogs,” a local veterinarian told The Washington Post, in an article describing the regional outbreak. It killed so fast that it left pet owners in disbelief, he said.

The world was in the middle of a canine pandemic. The parvovirus, which was first recognized in 1978, can live for months outside the body, spreading not just from animal to animal but through feces, sneaking into the yards of dog owners via a bit of excrement stuck to the bottom of a person’s shoe. It quickly traveled across countries and continents, infecting thousands and possibly millions of dogs in the late ’70s and early ’80s. Essentially every dog alive at the time caught it, Colin Parrish, a virology professor at Cornell University’s College of Veterinary Medicine, told me. And untold numbers  died: A single Associated Press report from August 1980 mentions the city of Chicago losing 300 dogs by July of that year, and South Carolina losing more than 700 in just two months.

A vaccine was quickly developed, but with doses in short supply, the outbreaks dragged on for years. Today, puppies are routinely vaccinated for parvovirus, and the 1978 canine pandemic has faded from public consciousness. Since then, no outbreak has unfolded on that scale, even as dogs have become more integrated into American households. Few people stay up at night worrying about what might happen if a new and devastating disease did appear. Yet, for a moment at the end of last year, it seemed like one might have.

In late 2023, veterinarians started noticing something odd. They’d seen an uptick in cases of dogs sick with respiratory symptoms responding poorly to antibiotics. Some would develop severe pneumonia quickly and die. Soon, cases of this suspected illness started popping up in states across the country. Around Thanksgiving, media reports began warning dog owners about a “mystery dog illness” spreading nationwide.

Many experts now suggest that there probably was no “mystery dog illness.” More likely, some mix of previously known illnesses were surging around the same time. Still, the case is not entirely closed, and the prospect of a deadly new disease has left dog owners fearful and jumpy: How much should they worry? Could that seemingly normal cough in the family pet actually be something much more dangerous?

And if a new disease had started a modern dog pandemic, the world’s first in almost 50 years, what would have happened next is not entirely clear. Unlike humans and livestock, companion animals do not have sophisticated, coordinated infrastructure dedicated to monitoring and managing their diseases. The technology and science might exist to fight a dog pandemic, but any response would depend on what kind of illness we found ourselves dealing with—and whether it could infect humans as well.

Because dogs don’t interact with one another as much as humans do, dog transmission networks are different from ours. They see one another on walks, in day cares, or in dog parks. Some might travel between states or even between countries, but many just stay in their backyard. Their cloistered networks make it hard for some viruses to move among them. In 2015 and 2016, outbreaks of a nasty canine flu called H3N2, which was traced to a single introduction in the United States from South Korea, never reached full pandemic status. “I just remember seeing so many of these pretty sick dogs, like every day,” Steve Valeika, a veterinarian and infectious-disease specialist in North Carolina, told me. “And then it just stopped.” Most of his cases were from one boarding facility.

A disease such as parvo, which can spread without direct contact, has a better chance of circulating widely. But even then, authorities could respond quickly, maybe even quicker than in 1978. The same mRNA tools that led to the speedy development of a COVID vaccine for humans could be used in a dog pandemic; the ability to test for dog diseases has improved since parvovirus. Information travels that much faster over the internet.

Still, as companion animals, dogs and cats fall into an awkward space between systems. “There is no CDC for dogs,” Valeika said. “It’s all very patchwork.” Typically, animal disease is managed by agricultural agencies—in this country, the USDA. But these groups are more focused on outbreaks in livestock, such as swine flu, which threaten the food supply, the economy, or human safety. If an outbreak were to emerge in companion animals, veterinary associations, local health departments, and other dog-health groups may all pitch in to help manage it.

The dairy and pig industries, for example, are far more coordinated. “If they said, ‘We need to get all the players together to talk about a new emerging disease issue on pigs,’ that’d be easy. They’d know who to call, and they’d be on the phone that afternoon,” Scott Weese, professor in veterinary infectious diseases at the University of Guelph, in Canada, explains. Organizing a conference call like that on the topic of a dog disease would be trickier, especially in a big country like the United States. And the USDA isn’t designed around pets, although “it’s not that they don’t care or don’t try,” he said. (The USDA did not respond to a request for comment.) No one is formally surveilling for dog disease in the way government agencies and other groups monitor for human outbreaks. At base, monitoring requires testing, which is expensive and might not change a vet’s treatment plan. “How many people want to spend $250 to get their swab tested?” Parrish asked.

Dogs aren’t human. But they are close to humans, and it is easy to imagine that, in a dog pandemic, owners would go to great lengths to keep their pets safe. Their closeness to us, in this way, could help protect them. It also poses its own risk: If a quickly spreading dog disease jumped to humans, a different machinery would grind into gear.

If humans could be vulnerable and certainly if they were getting sick, then the CDC would get involved. “Public health usually takes the lead on anything where we’ve got that human and animal side,” Weese told me. These groups are better funded, are better staffed, and have more expertise—but their priority is us, not our pets. The uncomfortable truth about zoonotic disease is that culling, or killing, animals helps limit spread. In 2014, after a health-care worker in Spain contracted Ebola, authorities killed her dog Excalibur as a precaution, despite a petition and protests. When the woman recovered, she was devastated. (“I’ve forgotten about everything except the death of Excalibur,” she later told CNN.) Countries routinely cull thousands of livestock animals when dealing with the spread of deadly diseases. If a new dog-borne pathogen threatened the lives of people, the U.S. would be faced with the choice of killing infected animals or dedicating resources to quarantining them.

A scenario in which pet owners stand by while their dogs are killed en masse is hard to imagine. People love their pets fiercely, and consider them family; many would push to save their dogs. But even in a scenario where humans were safe, the systems we’ve set up might not be able to keep pets from dying on a disturbing scale. Already, there’s a nationwide shortage of vets; in a dog-health emergency, people would want access to emergency care, and equipment such as ventilators. “I am concerned that we don’t have enough of that to deal with a big pandemic as it relates to pets,” Jane Sykes, a medicine and epidemiology professor at the UC Davis School of Veterinary Medicine and the founder of the International Society for Companion Animal Infectious Diseases, told me.

Congress has mandated that the CDC, USDA, and Department of the Interior, which oversees wildlife, work on strengthening “federal coordination and collaboration on threats related to diseases that can spread between animals and people,” Colin Basler, the deputy director of CDC’s One Health Office, wrote in an email statement. A new, deadly canine disease would almost certainly leave experts scrambling to respond, in some way. And in that scramble, pet owners could be left in a temporary information vacuum, worrying about the health of their little cold-nosed, four-legged creatures. The specifics of any pandemic story depend on the disease—how fast it moves, how it sickens and kills, and how quickly—but in almost any scenario it’s easy to imagine the moment when someone fears for their pet and doesn’t know what help will come, and how soon.