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Watch your Strep: Keep your guard up for this evolving bacterium

Few things remain the same for long on a hog farm. By the very nature of today’s production system, pigs are continuously moving on or off a site. Seasons change as do health challenges, and any number of stressors can surface to further test the pig population.

One thing that remains constant is Streptococcus suis (Strep suis), a ubiquitous, Gram-positive bacterium that can infect pigs at birth.1 It’s important to note that Strep suis is always evolving,2 which not only challenges pig health but management and treatment strategies as well.

“We have to understand that Strep changes; a producer may have a protocol in place for a year, and all of a sudden it seems like it no longer works,” Ashley Johnson, DVM, told Pig Health Today. “We have to be diligent in identifying and isolating the serotype so that we can decide on the best approach for that farm.”

In fact, controlling one type of Strep may lead to emergence of another serotype,3 causing frustration for long-term control strategies, she adds.

As a technical services veterinarian for Zoetis, Johnson sees lots of different production systems. There’s one thing they have in common, however, and that’s what producers frequently call “Strep pigs,” as if they’re resigned to living with the disease and its economic impact. She insists it doesn’t have to be that way. “We have lots of tools that we can use to attack Strep so that it’s not the norm.”

In fact, Johnson sees Strep suis as low-hanging fruit in terms of what to address and make an impact on herd health. Even better, by addressing Strep suis, you also improve the chances of having a positive impact on other disease issues, such as porcine reproductive and respiratory syndrome (PRRS), influenza or pneumonias.4

What’s different today?

Strep suis was identified as a new pathogenic species in 1987. Today, there are 35 known subtypes, with 2, 3, 4 and 7 the most common in the Midwest,5 Johnson noted. In recent years, more challenges associated with Strep are being reported in the farrowing house.

To provide context, Johnson shared some of the general changes that swine veterinarians have reported from the field:

  • 10 years ago — Strep suis was seen primarily in the nursery stage, with a low percentage of animals affected.
  • 5 years ago — Started to see an increase in the percentage of animals affected, including further into the nursery period (i.e., 50-pound pigs). Strep suis cases also increased in the farrowing house, with young piglets being affected.
  • Today — An increase of Strep suis cases has been reported across the country. Many farms are struggling with control and treatment measures.

According to recent swine veterinarians’ observations and veterinary diagnostic laboratory (VDL) reports, some shifts have occurred. For one, Strep suis submissions and cases are on the rise. Johnson reviewed the Iowa State University’s VDL historical data for Strep suis cases, calculated the annual percentages and found that cases increased from 4.9% in 2009 to 10.8% as of late 2018.

Some of the possible reasons for the increase in cases include greater PRRS virus incidence, “as well as other pathogens that cause injury to respiratory epithelium,” Johnson said. “Co-infections are a big issue; they open the door for Strep.”

Management and environmental issues can ignite Strep suis breaks, as can stressors such as commingling pigs from different sources or adding new sources, especially in the nursery or wean-to-finish phases.6

Again, the ability for Strep suis to change influences the number of cases, and notably, just as a herd can encounter multiple PRRS viruses, it also can be exposed to multiple Streps.

Johnson does not believe that changes in on-farm antibiotic use has had as much impact on the number or type of Strep suis cases as PRRS or other herd-health challenges, which are often secondary infections to Mycoplasma hyopneumoniae and other bacterial pathogens.

Some things remain the same

Strep suis is an opportunistic pathogen — one that can ride on the coattails of co-infections, stressors or hiccups in management. “People have the misconception that just because a pig is ‘normal,’ that it doesn’t carry Strep,” Johnson pointed out. “But it’s just waiting for an opportunity.”

Transmission of Strep is both vertical, from the sow to piglets, and horizontal between pigs via nasal and oral secretions, skin, dust, feces, flies and rodents.7 Wounds, such as those during castration or tail docking, create an entry opportunity for the bacterium to invade.

“Early weaning (7 to 24 days) and medicated early weaning are ineffective in preventing transmission to at least some piglets, which may then serve as sources of infection to cohorts during lactation and nursery phases,” Johnson added.

The sow’s maternal antibodies do offer piglets a period of protection, which is why the effects of Strep suis become evident after  weaning. However, co-infections such as PRRS, which can challenge the sow’s immune system, and reductions of the quality or quantity of colostrum may reduce her ability to pass on adequate immunity to piglets.

“The sow isn’t impacted so much, but the piglets will be,” Johnson said. “It can do a lot of damage to the pig.”

Due to management practices, herd-health status and differences in Streps, piglet morbidity and mortality rates vary widely. Johnson has heard producers report preweaning mortality rates of 3% to 10%. Looking into the grow/finish herd, she advised that if a producer’s mortality rates start creeping up by 1% or more, Strep suis is worth investigating.

Symptoms have essentially remained the same, with systemic signs characterized by septicemia, endocarditis and polyserositis. Localized symptoms, such as lateral recumbency and paddling in young pigs, reflect the bacterium’s impact on the central nervous system. Older pigs show signs of ataxia — or unstable walking — as well as tremors and convulsions. At any age, infected pigs may have swollen joints and lameness (polyarthritis) and exhibit respiratory signs, including pneumonia.

“I’ve seen a lot of lameness due to Strep, but that’s hard to identify until it’s very far along. It really damages the joint,” Johnson added. “Unless you have a good eye to measure toe touching or a slightly off gait, you’ll miss it.”

Not a seasonal problem

Due to its interaction with other pathogens and associated flareups, Strep suis is often viewed as a seasonal challenge, but it can surface at any time.

Herd history and identifying clinical signs are good places to begin a diagnosis. Investigate whether there have been changes in pig health, the animal’s environment or other management areas. According to Johnson, tonsils should always be part of the lab submission, and during post-mortem exams, look for lesions in the lungs, brain and joints. Culturing samples in the lab to identify the predominant capsular type is critical to determining any Strep changes occurring within the herd and to select the treatment. Because Strep suis may differ in consecutive outbreaks, antibiotic-sensitivity tests often need to be repeated.

The key to treatment success is to identify affected pigs early. Treatments can be applied to a group of pigs or to individuals, depending on the numbers impacted. If just a few pigs are affected, then individual injections of antibiotics are a reasonable option. For groups of pigs, water and feed medications may be needed. While injectable, water and feed antibiotics are all viable options, producers should consult with their veterinarian for the best treatment program for their herd.

“If you see a lot of poor-doing or lame animals, it may be wise to run medication through the water for a while,” Johnson said. “Using antibiotics, whether injectible, water or feed, at weaning or placement can be very effective. But it’s all about timing.”

There is no commercial vaccine for Strep suis, so the only vaccination option is to develop an autogenous vaccine for the farm. Reports from the field indicate mixed results, but success is dependent on identifying the capsular type on the farm and ensuring the vaccine matches. That’s why repeatedly and diligently tracking strains on the farm is a must, she added.

Here are some additional control strategies to apply:

  • Reduce stress for piglets both pre- and post-weaning. onitor ventilation, eliminate drafts and stagnate air, keep humidity in check (below 70%). Walk the pens, not just the aisles, to check the pigs and evaluate what’s happening at their level.
  • Prevent overcrowding when transporting pigs and once they’re in the facility. “It’s important to check the records and confirm how many pigs are actually placed in the barn,” Johnson said. “Sometimes there can be a couple hundred more than a producer thinks.”
  • Sanitizing facilities between pig groups is a must; however, too often overlooked is the commitment to thoroughly sanitize tools used during piglet processing. Make controlling other primary pathogens, especially PRRS virus and influenza-A virus of swine, a priority. It may not be possible, but all-in/all-out pig flow in the nursery or at least minimizing pig sources will help with control.


Because Strep suis is everywhere, elimination isn’t feasible. Instead, you need “a good surveillance program to track Strep and any changes within your herd,” Johnson concluded. “It’s not something you can wash your hands and think you have addressed it. But, with the knowledge and the tools that we have, producers can work with their veterinarians to establish an effective control program.”



1Iowa State University, College of Veterinary Medicine, Veterinary Diagnostic and Production Animal Medicine. Swine Index of Diseases: Streptococcal Infections. Available from https://vetmed.iastate.edu/vdpam/FSVD/swine/index-diseases/streptococcal-infection.
2 Shelyakin PV, Bochkareva OO, Karan AA, Gelfand MS. Micro-evolution of three Streptococcus species: selection, antigenic variation, and horizontal gene inflow. BMC Evol Biol. 2019;19:83 Available from https://bmcevolbiol.biomedcentral.com/articles/10.1186/s12862-019-1403-6
3 Ibid.
4Iowa State University. Swine Index of Diseases: Streptococcal Infections. Available from https://vetmed.iastate.edu/vdpam/FSVD/swine/index-diseases/streptococcal-infection.






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Strep suis is ever-present and ever-changing. How do you control this evolving bacterium?

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Posted on May 27, 2020

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It’s not unrealistic to say that if you checked the nasal cavities or tonsils of any group of pigs, you would find Strep suis. While the strain and impact can vary widely, this commensal bacterium is on virtually every hog farm.

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