Researchers ‘look beyond the normal’ to identify emerging disease

The first case of Streptococcus equi subsp. zooepidemicus (Strep. zoo) was identified in the US pig population last fall, and it’s not an organism to be taken lightly. At that time, the clinical signs observed in pigs included “lethargy, weakness, high fever, swift spread among pigs from highly varied sources within the affected premises and rapidly escalating mortality levels approaching 30% to 50%,” reported the US Department of Agriculture. “Although rare, it can cause severe illness in humans exposed to infected horses or other infected species. This includes exposure through consumption of associated unpasteurized milk products.”

The highly virulent strain showed up in the US for the first time in a buying station in Ohio last September, and then at a slaughter plant in Tennessee the next month as part of a follow-up.

About 40% of the sows had died within a 2-day period, Eric Burrough, DVM, PhD, associate professor at Iowa State University (ISU), told Pig Health Today. Tissue samples were sent to the Iowa State University Veterinary Diagnostic Laboratory in Ames, Iowa.

“The first thing that was unusual for us was the level of mortality — that got our attention,” Burrough said.

The ISU lab worked with USDA to evaluate the samples and the history of the case, which indicated the pigs had large spleens.

“In today’s world with African swine fever as a concern, [large spleens were] on our differential list as well as other causes of septicemia,” Burrough said. “We didn’t identify Strep. zoo right off the bat because it’s a beta-hemolytic strep, and the pathogenic streps in pigs are usually alpha-hemolytic.

“On the first pass-through, our bacteriologists assumed it was a commensal or background [organism] and didn’t pursue it,” he continued.

It wasn’t until a second case came a few days later that the diagnosticians identified it as Strep. zoo. They took the isolate and along with researchers at the USDA put it back into both adult pigs and growing pigs.

“We saw high mortality within 48 hours,” Burrough said. “It’s a very unique, highly pathogenic organism.”

When researchers did genetic analyses of the organism, they found it was highly related (99%) to a strain identified in China that was associated with the death of 300,000 pigs in the Sichuan province in 1975, Burrough said.

Where has Strep. zoo been for 50 years?

As researchers dug in deeper, they discovered there had been an outbreak in Canada in June or July 2019. Those herds also saw high mortality events.

“As we thought about the epidemiologic investigation, animals from that province in Canada would likely have made their way to Ohio, and then from the Ohio buying station to the slaughter plant in Tennessee,” Burrough said. “As far as how it got into Canada, that’s under investigation right now.”

Once the diagnosticians and laboratory technicians at ISU ruled out other pathogens and discovered Strep. zoo, they were concerned about how widespread the organism had become.

“Is it something that we’ve perhaps overlooked or haven’t been investigating?” Burrough questioned. “Since then, we’ve started to screen for it proactively and look for it in all appropriate tissues.

“Interestingly, and fortunately for the industry, we haven’t found it in a single diagnostic submission sent to our laboratory since the initial cluster of cases,” he added. “It tends to be very epidemic when it happens…but thankfully it’s not very widespread.”

Underlying manifestation

The Strep. zoo organism was found in animals that were not receiving antibiotics or had not received antibiotics. In other words, the pigs were at a buying station and then they were at a slaughter plant, so they would not have recently had antibiotics in their system, Burrough said.

“We did check the antimicrobial susceptibility of the organism and found it’s highly susceptible to beta-lactam antibiotics, so the common antibiotics that people would use are effective against it,” he said. “The good news is, if it happens and is caught early, it can be treated.”

Vigilance is important

Researchers were able to identify the Strep. zoo organism within 3 days, which helped keep it from spreading.

That’s an important lesson for the industry.

Being vigilant and looking for things beyond the normal are critical, Burrough said. In a case like this it would be easy for the pigs to also be positive for porcine reproductive and respiratory syndrome or some other co-infection, he pointed out.

“We needed to look at the nuances and really understand what was different in this case,” Burrough said. “You shouldn’t have that many animals be sick and die within that time frame.”

He stressed the importance of paying attention to details and communicating those details with the diagnostic lab, so diagnosticians know exactly what is happening at the farm, buying station or packing plant level.