This endemic disease affects swine operations around the globe. Scientists continue to search for effective solutions.
by Kate Ayers
Although African swine fever has dominated the spotlight for a year, this disease is not the only one about which the swine sector worries.
Streptococcus suis (S. suis) has a silent but steady presence in North American swine herds, costing the industry millions of dollars annually, says Dr. Matheus Costa, an adjunct professor with the University of Saskatchewan’s Western College of Veterinary Medicine.He is also an assistant professor with the University of Minnesota.
As a result, stakeholders are researching ways to reduce the effects of this bacterium on the swine sector.
In Canada, S. suis is a constant cause of swine mortality, he says.
And the challenges stretch across borders.
National Pork Board and the Pork Checkoff, Des Moines, Iowa photo
“In fact, S. suis was recently considered the number one bacterial agent that poses the most risk for the U.S. swine industry,” says Dr. Maria Clavijo, a research assistant professor at Iowa State University’s Veterinary Diagnostic Laboratory (ISU-VDL). She also works as a part-time health assurance veterinarian at the Pig Improvement Co.
The disease “contributes to significant economic losses mainly associated with mortality, decreased feed efficiency and increased treatment costs.”
Producers have limited options to prevent and mitigate S. suis in their herds.
Fortunately, researchers and veterinarians around the world are working to gain a better understanding of the bacterium and how it causes disease.
Better Pork speaks with veterinarians, a pathologist and other herd health specialists to discuss the bacterium’s characteristics, treatment options and ongoing research that they hope will help producers battle this bug.
What is S. suis?
Researchers identified S. suis as a pathogen in 1987, an Iowa State University article says. This bacterium is the predominant streptococcal infection of pigs. While it is most commonly seen in nursing or newly weaned piglets, S. suis can affect all age groups.
Researchers have identified over 35 serotypes of the disease, and these strains range in virulence. Serotypes 1 through 9, including serotype ½, represent over two-thirds of the S. suis isolates found mainly in diseased pigs in North America, a Merck Veterinary Manual article says.
However, “serotype 2 is probably the most commonly diagnosed” in Canada, says Dr. Egan Brockhoff, the veterinary counsellor for the Canadian Pork Council and a partner at Prairie Swine Health Services in Red Deer, Alta.
S. suis, a gram-positive bacterium, is quite resilient. It “can survive in the environment for up to two weeks in water at 4 C (39.2 F), up to 104 days in feces and up to 54 days in dust at 0 C (32 F),” says Clavijo.
S. suis is a normal inhabitant of the “upper respiratory tract (tonsils and nasal cavities), genital tracts and possibly the gut,” adds Costa.
“Virtually every pig has Streptococcus suis and thus virtually every herd may have S. suis-associated disease.”
The pathogen is most often transferred through direct contact with carrier or infected pigs, says Lisa Becton, the director of swine health information and research for the National Pork Board in Iowa.
Piglets can contract S. suis when passing through the sow’s birth canal or nursing.
National Pork Board and the Pork Checkoff, Des Moines, Iowa photo
“S. suis is often described as an ‘early colonizer,’ as piglets are infected from their dams shortly after birth with endemic serotypes of S. suis present in the herd,” says Dr. Bailey Arruda, a pathologist at ISU-VDL.
“Ongoing transmission occurs as pigs age or are commingled.”
Pigs can also become infected through the transfer of respiratory secretions in nose-to-nose touching while in nurseries.
As a result, “when mixing two different populations of pigs, it is important to keep in mind that they might not share the same strain and might not have immunity to the other strain,” says Clavijo.
This situation could lead to an outbreak.
S. suis “can also be directly transmitted by equipment, feeders, flooring and things that pigs come in contact with such as gates and objects that are not properly disinfected,” Becton adds.
Clinical signs of S. suis
While most herds have a carrier rate near 100 per cent, the incidence of disease is generally around 5 per cent, the Merck Veterinary Manual article says. But if left untreated, S. suis can cause up to 20 per cent mortality.
A troubling aspect of S. suis is that animal infections are unpredictable.
Some cases emerge in the presence of other diseases such as porcine reproductive and respiratory syndrome or influenza. These viruses disrupt a pig’s immune system and leave it more vulnerable to infection from S. suis. Transport, stressful events or less-than-ideal environmental conditions may also play a role in disease predisposition.
However, some cases occur due to a single S. suis bacterial infection.
“It’s a complex bacterium in terms of its ability to evade the immune system. Its natural presence in pigs doesn’t always stimulate strong immunity either,” Brockhoff says.
S. suis “can fly under the radar, not really triggering the animal’s immune system despite universal presence. And then some third factor or driver, like a viral disease or environmental stressor, may trigger a severe infection.”
External factors such as “ventilation issues, piglet chilling or weaning stress all seem to be capable of driving this bacterium to go clinical,” he says.
Pigs can become colonized with the bacterium but show no disease, she says.
In pigs that show clinical signs, the animals’ central nervous systems will often be affected.
Some signs may include septicemia (blood poisoning), acute meningitis, polyarthritis (arthritis affecting several joints) or bronchopneumonia, the Iowa State University article says.
Signs may differ between cases, depending on what parts of the body the bacterium infects and the serotype that invades the animal.
For example, if a virulent strain of S. suis infects the lungs, pigs may have bronchopneumonia. If S. suis infects the brain, pigs may exhibit neurological signs such as “tilting of the head and paddling,” says Dr. Alejandro Ramirez, an associate professor of veterinary diagnostic and production animal medicine at Iowa State University. Incoordination and inability to stand are other neurological signs.
“The other symptom we see is swollen joints. As S. suis gets into the bloodstream, the pathogen can travel and hide in the joints. This infection may then result in lameness,” he adds.
The range in clinical signs can make S. suis a challenging disease for veterinarians to definitively diagnose in pigs. To make matters worse, S. suis can present like other swine pathogens, including African swine fever, Brockhoff says.
“Sometimes you’ll just see purple ears or cold feet. But S. suis commonly causes meningitis in piglets and nursery pigs, and the bacterium is one of the classic causes of heart failure in pigs due to destruction of the valves.”
In severe cases, S. suis “causes sudden deaths because it attacks so quickly,” Brockhoff adds.
But scientists do not yet know what factor(s) influences the virulence of different strains.
“What we don’t fully understand is why certain strains of the bacteria are more pathogenic or more damaging to the pig and actually cause disease,” Becton says.
“The question is: when does S. suis go from being a normal skin bacterium to actually invading and creating problems?” he asks.
“We don’t know what triggers it.”
Controlling S. suis
Using antibiotics is still the most common and effective method of treating pigs that have clinical signs of S. suis.
Kelli Jo/iStock/Getty Images Plus photo
“It’s a disease you don’t eradicate from a farm. You just manage it with immune stability, managing the environment, minimizing stress, or in certain circumstances with some use of antibiotics,” says Brockhoff.
“One of the key things is finding the infection early on,” Ramirez says.
“If diagnosis takes too long and the disease establishes itself in the animal’s brain or in the joints, it’s hard to (a) get drugs into those organs and (b) effectively treat the disease. The damage can still be left behind.”
However, before farmers and their veterinarians administer medications, they should submit tissue samples to a diagnostic lab, Becton says.
Due to the vast range in serotypes, farmers must know what strain their animals carry to most effectively treat the disease.
The lab will create a plated culture and test its sensitivity to determine which antibiotics will work the best.
The situation “comes down to making sure you have the right diagnosis and supporting information to help guide treatment,” Becton adds.
Once lab technicians identify the serotype, farmers and their herd management staff can more effectively administer medications at the right time and dosage, improving animal prognosis and treatment.
Veterinarians and laboratory staff can also conduct post-mortem examinations to help with diagnosis.
“There may be multiple serotypes on the farm but only one or two are often the pathogenic and true cause of the clinical signs,” Brockhoff says.
“Because the bacterium is a normal pathogen in pigs, you really have to look for the bacteria in lesions. For example, you could find multiple serotypes in the lungs, but you’ll only find one serotype in the brain. Getting necropsies or post-mortem examinations done” on tissues related to the symptoms that the pigs present will help to determine the serotype that causes the disease.
Farmers may also administer anti-inflammatories to sick pigs to help reduce internal inflammation associated with the infection and ensure animal welfare, Costa says.
Unfortunately, the efficacy of available treatment options varies.
“Some herds may use autogenous vaccines, which are often produced using one specific serotype. Serotypes can affect the efficiency of (such) vaccines, while the presence of resistance genes affects the efficiency of antibiotics. Thus, if there’s more than one serotype circulating, the vaccine won’t protect against it,” says Costa.
“Antibiotics are usually delivered to many animals simultaneously, to help prevent and treat disease.
“The choice of which antibiotic to use is made by the herd veterinarian and takes into account multiple factors including the susceptibility of the specific strain circulating in each herd,” Costa adds.
“Research is underway to develop new preventative and treatment methods.”
While S. suis has challenged swine herds for over three decades, the bacterium’s prevalence in the swine industry has risen recently.
From 2016 to 2018, “S. suis diagnoses either due to pneumonia or systemic disease are considered primary or secondary participants in roughly 10 to 12 per cent of all porcine pathology cases submitted to the ISU-VDL that have histologic evaluation,” says Arruda.
National Pork Board and the Pork Checkoff, Des Moines, Iowa photo
“Over the past five years, it appears, based on data from the ISU-VDL, that the frequency and percentage of cases with a S. suis diagnosis have increased,” she explains.
Indeed, the epidemiology and ecology of the bacterium and how it causes disease are complex.
“We need improved tools, techniques, protocols, models and vaccines, all of which first rely on a concerted effort in the identification of pathogenic strains,” Arruda adds.
So far, no vaccine is commercially available for producers to prevent S. suis in pigs.
“There is interest globally for a vaccine to be produced,” says Brockhoff. “So, a lot of research is focused on knowledge blocks that are needed to produce an effective vaccine.”
The movement toward reduced antibiotic use in Canadian and American swine herds has put more market pressure on producers to go antibiotic-free, Brockhoff adds.
And “this disease often shows up when you take antimicrobials out of a farm as soon as there is a big stress event.”
“The recent changes in the Veterinary Feed Directive in the United States and overall societal pressures from the public and government have led to a downward trend in anti-microbial use in North America,” she says.
“We could speculate that this situation has potentially caused an increase of cases which, in other times, would have been under control. Therefore, the pork industry will have to develop novel ways to minimize the impact of S. suis.”
The industry needs more research to better understand the genetic variations of the bacterium, and develop effective preventative and treatment options.
“As the industry evolves and antibiotics are scrutinized more and used less, we are going to need the foundational knowledge we currently lack to prevent and mitigate disease through a better understanding of innate and acquired immune or resistance mechanisms,” Arruda says.
Researchers are also examining the zoonotic potential of S. suis.
“Although unheard of in Canada, this disease has the capacity to affect humans,” says Brockhoff.
Indeed, S. suis “is the leading cause of human bacterial meningitis in southeast Asia, where several outbreaks have been reported,” Costa says.
A person “can be infected if ingesting contaminated pork products such as food containing raw pork or if manipulating contaminated biological material with open skin lesions.”
However, such infections are rare in the western world.
“People in North America who work in pig barns their whole lives are typically not impacted by this pathogen.
“But farmers need to know that it can cause disease in humans and take appropriate precautions, such as wearing gloves when handling sick animals and remembering to wash their hands with soap and warm water,” Brockhoff adds.
Advances in overall herd health management have also recently brought more attention to S. suis.
Over the past few years, “the industry evolved, which led to better management practices, diagnostics and vaccines. These advances resulted in better control over other diseases,” Costa says.
“As we moved forward with healthier herds, we began to fine-tune things, such as S. suis-associated disease. It has always been out there, but we may have had other diseases eclipsing it.
“Unfortunately, we didn’t spend enough time and resources to tackle it and are now trying to catch up.”
So, scientists are searching for answers and trying to develop effective tools for swine industry stakeholders to enhance control of this pathogen.
Costa looks forward to the discovery of new treatment options.
“Many approaches are being investigated: microbiota manipulation, alternative vaccine strategies and non-antibiotic treatments,” he says.
“We are trying to gain a better understanding of the disease’s mechanisms: how does S. suis cause meningitis, for example. We hope that this new knowledge will soon lead to field tools.” BP