A mystery dog illness that spread across the U.S. last year and can lead to serious or fatal respiratory problems is still under investigation, but cases are starting to slow, experts say. The illness, which started to spread more widely in the summer and fall of 2023, causes coughing, sneezing and fatigue, among other symptoms, and can progress quickly to pneumonia. The dogs suspected to have it test negative for all of the typical causes of respiratory symptoms and often don't respond to treatment. In a statement to TODAY.com, the American Veterinary Medical Association says that based on "conversations with various sources ... the number of cases are declining," adding that an "ebb and flow" of respiratory illness in dogs over the course of the year is common, similar to cold and flu season in humans. The statement also explains that the U.S. Department of Agriculture has so far not seen that the rise in respiratory illness in dogs is linked to a new virus or bacteria. A statement from the Animal and Plant Inspection Service, part of the USDA, confirms to TODAY.com that the state-led testing that the agency is helping coordinate "has not indicated the presence of a novel pathogen or single infectious cause among these cases." Numerous veterinary diagnostic labs across the country are investigating the outbreak, from Oregon to Kansas to New Hampshire. The APHIS spokesperson adds that the illness is not regulated by APHIS and therefore the branch doesn't have data on case numbers. The Oregon Veterinary Medical Association also shared in an Feb. 14, 2024, update that "cases seem to be waning, if not back to normal," citing veterinarian Dr. Scott Weese's Worms and Germs blog. At last count, at least 19 states had reported cases of the illness, according to various expert groups. The AVMA explained in an earlier statement that the mystery dog illness is difficult to track because there's no national surveillance system. In November 2023, veterinarians in critical care settings told TODAY.com that they'd seen dozens of cases since the fall. Late last year, multiple groups — including the Colorado and Oregon departments of agriculture, the AVMA, the Rhode Island Department of Environmental Management and the Los Angeles County Department of Public Health — issued warnings about the mystery illness in dogs. “We don’t know what is causing this situation, where dogs are presenting at veterinarians with what private veterinarians would call kennel cough,” Rhode Island's state veterinarian Dr. Scott Marshall told NBC affiliate WJAR. “What’s different about this situation is that dogs are presenting with little bit more severe signs, not responding to usual treatments, and unfortunately had a small number, still a number, of dogs that have succumbed to them.” “It seems to happen very, very quickly — to go from this cough that’s just won’t go away ... and then all of a sudden they develop this pneumonia,” Dr. Lindsey Ganzer, veterinarian and CEO at North Springs Veterinary Referral Center in Colorado Springs, Colorado, told TODAY.com. She said she treated over a dozen dogs with what she believes is the condition between October and November 2023. If your dog develops a cough, do not panic, the AVMA advises. But stay vigilant about its progression and your pet's overall health, and contact your vet right away if anything seems off. What is the mystery dog illness? Dogs with this mystery illness usually have coughing, sneezing, eye or nose discharge, are abnormally tired, and do not test positive for any common causes of canine respiratory illness, the Oregon Department of Agriculture noted in a Nov. 9 press release. Typically, dogs with respiratory illnesses have a cough for seven to 10 days, but some vets saw an uptick in dogs with coughs lasting weeks to months that don't respond to treatment, the Colorado Department of Agriculture said in a Nov. 22 statement. One of the distinguishing characteristics of this mystery illness outbreak w