Davis, Calif. — A bacterium common among horses kills infected shelter dogs at an alarming rate, researchers say, prompting an alert to clinical practitioners who encounter densely housed animals.
The latest confirmed outbreak of Streptoccocus equi subsp. zooepidemicus, commonly called strep zoo, struck in February at an animal shelter near Milwaukee, where seven dogs died in 10 days of acute clinical respiratory disease. Earlier that month, a dozen dogs died in a Miami shelter where the bacterium was identified.
But those outbreaks pale in comparison to a 2007 occurrence, when an estimated 1,000-plus shelter dogs succumbed to severe hemorrhagic pneumonia in Las Vegas.
Strep zoo outbreaks are rare — just three have been documented in the United States — but the onset of infection is dramatic and fast, says Kate F. Hurley, DVM, MPVM, director of the University of California-Davis' (UC-Davis) Koret Shelter Medicine Program.
While incidence has been limited to dogs in intensive housing systems, there's no reason private practitioners might not see cases, she says.
"Those dogs in Las Vegas dropped dead in a pool of blood, sometimes within 24 hours of the first clinical signs," Hurley says. "In general, we're recognizing that canine respiratory disease complex is just that — it's complex, with newly emerging causative agents. We don't know why this couldn't be seen like canine influenza, with patients being adopted and taken to private veterinary practices."
Diagnosing strep zoo
Clinical signs of infection include a short period of a moist cough, fever, anorexia and depression. Terminal cases involve hemorrhaging of the lungs associated with acute respiratory disease. The onset is rapid; some dogs are found dead before any clinical signs appear.
Unless practitioners specifically test for strep zoo, a diagnosis likely will be missed, researchers say. Yet the bacterium is sensitive to a wide range of antibiotics, including penicillin.
"When you're talking about diagnosing 100 dogs, you want to go with something easy," Hurley says. "We advise practitioners to treat early, even prophylactically. Once these clinical signs show, it's too late for treatment to work in many cases."
Strep zoo transfers dog to dog; an animal can appear normal until something, possibly stress, triggers the bacterium to act as a primary pathogen, says Patricia Pesavento, DVM, PhD, assistant professor of Clinical Anatomic Pathology in UC-Davis' veterinary medical program. It's a big, complicated bacterium that occurs naturally in the environment and even is found on the oral swabs of cats. In shelters with outbreaks, cage bars, floors, drains and other surfaces often are contaminated.
"We think there must be something in the environment of intensive-housing systems that allows it to do more than normal," Pesavento says. "We've collected a number of strains, but we have more work to do. We don't know the incidence of disease after exposure or the recovery rates. Are there co-pathogens or underlying viruses? The confounding disease state is still unknown."
Beating the disease
Hurley and her colleagues' investigation of strep zoo is reported in the January issue of Veterinary Pathology. While the three-page paper cites Bordetella bronchiseptica as the most common trigger for canine infectious respiratory disease, other bacterial agents like strep zoo now are being implicated because of enhanced detection, vaccination and/or natural evolution, the study says.
Husbandry changes, such as dramatic depopulation, facility cleaning and staff education, work to slow or limit outbreaks.
"It's very common in a shelter that there's more than one thing going on with canine respiratory disease," Hurley explains. "Practitioners need to move beyond the assumption that kennel cough is Bordetella and think more three-dimensionally."