Streptococcus suis infection: 12 of 18 drugs failed
Two finishing pigs on one commercial farm carried a streptococcus suis infection that resisted 12 of the 18 antimicrobials tested. Gentamicin was the only drug that stayed fully susceptible in both isolates. Tetracycline resistance was part of the same pattern, so the panel and not the farm’s history has to decide which drug is still usable.
Why a streptococcus suis infection that resists drugs matters
Streptococcus suis is a Gram-positive coccus, and 29 serotypes have been described. It sits in the tonsils and the upper respiratory tract of healthy pigs, so its presence alone tells a herd owner very little. Where it goes and what it brings along are the parts that matter.
The species is zoonotic. It crosses the blood-brain barrier and causes meningitis, and it can reach the bloodstream and produce sepsis, endocarditis and arthritis. On a finishing floor that is a reason to move fast rather than to watch a pig for a day.
Genes for resistance travel on mobile elements such as integrative and conjugative elements, and those elements allow horizontal gene transfer. That is why the profile of one isolate cannot be predicted from the profile of another. A panel that closes 12 of 18 options leaves very little room to guess.
The two pigs and what the necropsy found
Both pigs were 10-week-old terminal-cross finishers from a single commercial swine farm, and both came from a commercial terminal line built as a three-way terminal cross. Both were euthanized on welfare grounds and sent for diagnostic necropsy.
One pig was lethargic and down, with forelimb paraparesis. The other showed generalized weakness and poor muscle tone in the hindlimbs. Nothing in the history separated these signs from the other common reasons a finishing pig goes down.
The table below sets the two necropsies side by side.
| Finding | Pig 1 | Pig 2 |
|---|---|---|
| Age and genetic line | 10 weeks, three-way terminal cross | 10 weeks, three-way terminal cross |
| Clinical signs | Lethargy, recumbency, forelimb paraparesis | Generalized motor weakness, decreased hindlimb muscle tone |
| Brain, gross | Moderate diffuse cerebral hyperemia | Moderate diffuse cerebral hyperemia |
| Lung, gross | Pulmonary congestion and edema | Pulmonary congestion and edema |
| Kidney, gross | Renal medullary enlargement, bilateral | Renal medullary enlargement in the right kidney only, plus a 1.2 cm congenital cyst |
| Stomach and gut, gross | Focal glandular-stomach ulcer about 7 mm across | Duodenal hyperemia |
| Histology | Non-suppurative inflammation with perivascular mononuclear cuffs | Same pattern, extending into cerebellar perivascular spaces |
| Bacteriology | S. suis from brain and spinal cord | S. suis from brain, Pasteurella spp. from lung |
Histology found non-suppurative inflammation with perivascular mononuclear cuffs, which fits meningoencephalomyelitis and myelitis. Mild multifocal mononuclear leptomeningitis reached the cerebellar perivascular spaces. There was no significant neuronal necrosis and no suppurative inflammation, and that detail matters later, once the streptococcus suis infection is confirmed by culture.
Culture brought S. suis back from the brain and spinal cord of pig 1 and from the brain alone in pig 2. Pasteurella spp. also grew from the lung of pig 2.
How the samples were cultured and identified
Swabs went onto chocolate agar, blood agar and MacConkey agar, with 5 mL brain heart infusion broth for enrichment. Incubation was 37 degrees Celsius for 24 hours, aerobic with 5 percent CO2. The colonies were beta-hemolytic, and identification used Gram stain, a negative catalase reaction and API 20 Strep.
The extended scheme ran longer: 72 hours in liquid enrichment, then 72 hours on selective and differential solid media. The minimum inhibitory concentration run on the automated Sensititre system needed another 18 to 25 hours of incubation.
| Step | Condition or specification |
|---|---|
| Specimens | Brain, spinal cord and lung |
| Primary plating media | Chocolate agar, blood agar, MacConkey agar |
| Enrichment | 5 mL brain heart infusion broth |
| First incubation | 37 degrees Celsius for 24 hours, aerobic plus 5 percent CO2 |
| Colony appearance | beta-hemolytic colonies |
| Identification | Gram stain, negative catalase, API 20 Strep |
| Extended culture | 72 hours in liquid enrichment, then 72 hours on selective and differential solid media |
| MIC platform | Automated Sensititre system, 18 to 25 hours |
| Panel | SENSITITRE BOPO6F, 18 antimicrobials |
| Breakpoints | CLSI-VET01S 2024 and EUCAST clinical breakpoints 2024 version 14.0 |
| Supporting references | Epidemiological cutoff values and MIC50/MIC90 distributions |
| Quality control | ATCC reference strains |
What the 18-drug MIC panel showed
The minimum inhibitory concentration is given in micrograms per milliliter. The table below lists the value and the call for each pig, with R for resistant, IS for intermediate and S for susceptible.
| Antimicrobial | MIC pig 1 | MIC pig 2 | Call pig 1 | Call pig 2 |
|---|---|---|---|---|
| Spectinomycin | 32 | 32 | R | R |
| Gentamicin | less than or equal to 1 | less than or equal to 1 | S | S |
| Neomycin | greater than 32 | 16 | R | IS |
| Ampicillin-amoxicillin | greater than 16 | 8 | R | R |
| Ceftiofur | 8 | 8 | R | R |
| Penicillin | 8 | greater than 8 | R | R |
| Florfenicol | greater than 8 | greater than 8 | R | R |
| Danofloxacin | 0.25 | 0.5 | S | IS |
| Clindamycin | greater than 16 | greater than 16 | R | R |
| Tilmicosin | 32 | greater than 64 | R | R |
| Tulathromycin | 32 | 16 | R | IS |
| Tylosin tartrate | 32 | 32 | R | R |
| Tiamulin | greater than 32 | 4 | R | IS |
| Sulfadimethoxine | greater than 256 | greater than 256 | R | R |
| Trimethoprim-sulfamethoxazole | greater than 2/38 | less than or equal to 2/38 | R | S |
| Oxytetracycline | greater than 8 | greater than 8 | R | R |
| Chlortetracycline | greater than 8 | 8 | R | R |
| Enrofloxacin | reported resistant | reported resistant | R | R |
Both isolates qualify as multidrug-resistant under the definition used here, which counts resistance to at least three different classes of antimicrobial agents.
Reading the panel: 12 drugs failed against both isolates
Twelve antimicrobials came back resistant in both isolates, and they belong to unrelated classes: the beta-lactams ampicillin-amoxicillin, ceftiofur and penicillin, the macrolides tilmicosin and tylosin tartrate, the tetracyclines oxytetracycline and chlortetracycline, along with florfenicol, clindamycin, sulfadimethoxine, enrofloxacin and spectinomycin.
Gentamicin was the only drug that held full susceptibility in both pigs. Pig 2 was also susceptible to trimethoprim-sulfamethoxazole, and four drugs sat at intermediate in that pig: neomycin, danofloxacin, tulathromycin and tiamulin.
The two results are not the same result. The streptococcus suis infection in pig 2 stayed in the brain, while pig 1 also gave the coccus from the spinal cord, and every one of the five drugs that separated the pigs leaned toward less resistance in pig 2. Intermediate is not a comfortable place to sit, since it lies between the two breakpoints.
The counterintuitive result: tetracycline resistance is now ordinary
Neither tetracycline worked. Oxytetracycline read greater than 8 micrograms per milliliter in both isolates, and chlortetracycline read greater than 8 in pig 1 and 8 in pig 2. Resistance to this class no longer singles out a strange isolate, because it reads exactly like the beta-lactams and the macrolides next to it on the same panel.
The drug that survived was gentamicin, an old aminoglycoside, at less than or equal to 1 microgram per milliliter in both pigs. Ceftiofur and florfenicol, both newer and both more expensive, were resistant in the same two isolates.
Resistance was also uneven inside one farm on one day. Neomycin, danofloxacin, tulathromycin, tiamulin and trimethoprim-sulfamethoxazole all gave different calls in the two pigs. Tiamulin fell from greater than 32 to 4 micrograms per milliliter, and trimethoprim-sulfamethoxazole went from greater than 2/38 to less than or equal to 2/38.
Saying that tetracyclines do not work on this farm can be broadly true and still leave a pig treated for the wrong reason. Gentamicin’s clean result, meanwhile, rests on two animals rather than on a farm policy.
What a farm should do with a result like this
- Send brain and spinal cord, not just lung, when finishing pigs show nervous signs. Only pig 1 gave an isolate from the spinal cord here, and a lung-only submission would have missed the picture in the central nervous system.
- Ask for a full MIC panel instead of a single disk diffusion result. The information that decided this case was spread over 18 drugs and eight chemical classes.
- Read the result pig by pig. These two animals differed on five drugs, so one panel cannot stand in for the group.
- Keep gentamicin on the shortlist only where the panel calls it susceptible, and take that call from the farm’s own isolate rather than from another site’s report.
- Drop tetracyclines as a default when both tetracyclines on the panel come back resistant.
- File the panel with the farm name and the sampling date, and repeat it whenever the treatment program changes.
Limits of this diagnostic report
Two animals show what a resistance profile can look like. They cannot show how often it occurs, so nothing here is prevalence data for the farm or for the region around it.
The material was archived diagnostic tissue. There was no control group and no experimental design, so nothing can be compared against pigs that were managed differently.
Neither serotyping nor sequence typing nor whole-genome work was done on the two isolates. The resistance determinants are unknown, and so is the relationship between the two isolates.
This is one farm and one point in time. Breakpoints come from CLSI-VET01S 2024 and EUCAST clinical breakpoints 2024 version 14.0, with epidemiological cutoff values and MIC50/MIC90 distributions used only as supporting references. Another breakpoint table can flip a call that sits at the margin.
Bottom line
A streptococcus suis infection in two finishing pigs resisted 12 of the 18 antimicrobials on the panel, and tetracycline resistance was one row of that result instead of a special finding. Gentamicin alone stayed fully susceptible in both isolates, and two isolates do not make a policy. Test the farm’s own isolate if you want to know which drug still works, and note how far the two pigs drifted apart before extending one result to the whole barn.
FAQ
Is streptococcus suis infection zoonotic for people
Yes. Streptococcus suis is zoonotic, and human cases follow close contact with infected pigs or pig carcasses rather than the consumption of pork. It can cross the blood-brain barrier in people as it does in pigs and produce meningitis. Barn, transport and processing staff should treat gloves and wound care as routine practice around sick pigs.
What are the first signs of meningitis in a finishing pig
The earliest signs are often vague: lethargy, reluctance to get up and a weak or unsteady gait. In the two pigs described here the picture progressed to recumbency, forelimb paraparesis and decreased muscle tone in the hindlimbs. Any finishing pig that cannot stand should be checked for a central nervous system cause instead of being treated blindly.
How is Streptococcus suis confirmed from a dead pig
Culture from brain and spinal cord is the direct route, and here the organism was recovered from the brain of both pigs and from the spinal cord of one. It grows as beta-hemolytic colonies on blood and chocolate agar and is identified by Gram stain, a negative catalase reaction and API 20 Strep. Lung culture alone can miss the central nervous system infection and may grow a different bacterium.
Why does multidrug resistance change the treatment plan
Because a resistant call removes options before a drug is used rather than after it fails. In this panel 12 of 18 antimicrobials were resistant against both isolates, which crosses the three-class threshold used to define multidrug resistance. With that many classes closed off, the remaining choices are narrow enough that they have to be read from the panel instead of from habit.
Which antimicrobial still worked in this 18-drug panel
Gentamicin was the only antimicrobial that retained full susceptibility in both isolates, at less than or equal to 1 microgram per milliliter. Pig 2 also tested susceptible to trimethoprim-sulfamethoxazole, and four drugs returned an intermediate call in that pig. Two susceptible results from two animals are a starting point for a decision and not a farm-wide rule.
What does the MDR definition mean in this report
Multidrug resistance means resistance to at least three different classes of antimicrobial agents and not resistance to three individual drugs. Both isolates passed that threshold by a wide margin, because resistance spanned beta-lactams, macrolides, tetracyclines, a phenicol, a lincosamide, a sulfonamide and a fluoroquinolone. The label is a classification, and it says nothing about how the resistance was acquired.
Can tetracyclines still be used when the panel reports resistance
On this panel both tetracyclines failed: oxytetracycline was greater than 8 micrograms per milliliter in both isolates, and chlortetracycline was greater than 8 in pig 1 and 8 in pig 2. A resistant call against a validated breakpoint is a reason to take the drug out of the plan for that pig. Tetracycline-class resistance has become common enough in this species that it no longer separates one isolate from another.
How does susceptibility testing change drug choice on a farm
It replaces a barn-level habit with a result taken from the isolate that is actually causing disease. These two pigs differed on neomycin, danofloxacin, tulathromycin, tiamulin and trimethoprim-sulfamethoxazole, so the panel answered questions that a farm history could not. The test also shows when an old and inexpensive drug still works and when a newer one has already failed.
Why did the two pigs from one farm show different results
The two isolates returned different calls on five antimicrobials, including an eight-fold gap in tiamulin and a shift from resistant to susceptible for trimethoprim-sulfamethoxazole. They also differed in where they were found, with S. suis recovered from the spinal cord of pig 1 and from the brain only of pig 2. Two isolates from one site are enough to show variation and not enough to explain it.
How long does a culture and MIC panel take
In this report primary culture ran for 24 hours at 37 degrees Celsius, and the extended scheme covered 72 hours in liquid enrichment plus 72 hours on solid media. The minimum inhibitory concentration run on the automated Sensititre system then needed 18 to 25 hours of incubation. A farm should plan on several days between submission and a usable susceptibility result.
What are the limits of a two-pig diagnostic report
Two animals can illustrate a resistance profile and cannot measure how common it is, on that farm or anywhere else. The tissue was archived diagnostic material with no control group and no experimental design. No serotyping, sequence typing or whole-genome work was done, so the resistance determinants and the link between the two isolates remain unknown.
Should one isolate decide treatment for the whole barn
No. The two pigs in this report differed on five drugs, which is a direct warning against stretching one panel across a group. A barn-level plan needs several isolates from several pigs sampled at the same time, and the sampling should cover the pigs that are actually sick. One result is a data point for the pig it came from.
