Last week one of your patients had a high fever, over 104°F. Thankfully, you were called to the farm immediately and ensured he was isolated away from the others. Within 24 hours of spiking that fever, your patient developed a runny nose that progressed to a mucopurulent discharge. After you have diagnosed Streptococcus equi subsp. equi (S. equi) and reported the outbreak, your patient is now “EDCC famous” and your county is listed as one of the many that report the strangles outbreaks that occur every month throughout the United States.
Following the timeline of a typical case of S. equi, your patient became anorexic, dull and depressed and developed the tell-tale submandibular and retropharyngeal lymph node swelling. He stood with his head outstretched and appeared uncomfortable. Together, these clinical signs popularized the nickname “strangles.”
Luckily, you were wise to isolate the horse immediately after identifying the fever, and none of the other horses on the property developed similar clinical signs of this incredibly infectious bacterial disease.
Surreptitious Spreading of Strangles
As we can see from this hypothetical—yet realistic—scenario, diagnosing strangles is relatively straightforward in most cases. The high fever, depressed demeaner, swollen lymph nodes and nasal discharge are fairly telling.
But now your job becomes a little tougher: You should determine where he got this pathogen and how he needs to be managed so he doesn’t spread it to other horses once he has recovered.
It usually takes between 3 and 14 days after being exposed to the bacteria for clinical signs of disease to develop in a horse. So, what was your patient up to 3 to 14 days ago? If your client cannot recall the patient coming in contact with an obviously sick horse, you need to consider subclinical shedders.
“Subclinical shedders are horses that previously had strangles, recovered from infection, but still harbor S. equi in their guttural pouches,” said Ashley G. Boyle, DVM, DACVIM, an associate professor in the field service section at the University of Pennsylvania’s New Bolton Center. “The guttural pouches become infected with S. equi secondary to retropharyngeal lymph nodes draining directly into the pouches in clinical cases.
“Intermittent guttural pouch shedding from apparently healthy horses can persist for years after that horse has recovered from infection,” noted Boyle. “These horses serve as an important source of this highly contagious bacterial respiratory tract infection.”
The subclinical shedders can therefore serve as a reservoir of infection for any naïve horse.
Approximately 10-40% of horses become carriers following a strangles outbreak, which is a substantial number.
In a study by Delph et al. (2018), 9 of 28 horses (32%) involved in a strangles outbreak had persistent guttural pouch infections. Only three of those 12 had endoscopically visible chondroids. (Chondroids are stone-like structures that form from hardened pus within the guttural pouches.)
As we can see, identifying these inapparent carriers would help protect naïve horses that have not previously been exposed to S. equi.
Detecting Inapparent Carriers
“Detecting these inapparent horses, however, can be exceedingly tricky without appropriate diagnostics,” warned Katherine Delph, DVM, MS, DCVIM, a clinical assistant professor from Kansas State’s College of Veterinary Medicine.
There are several potential sources of infection on the farm that might explain why the hypothetical patient described above had strangles. For example, horses brought onto a property might initially appear healthy, then develop a fever and subsequent clinical signs of disease. In addition, apparently healthy horses brought onto a property might be shedding S. equi and never develop external signs of infection (i.e., be inapparent shedders).
“I think not testing horses from unknown backgrounds is a common source of introducing carriers into a population which can then infect the other horses,” said Boyle.
Alternatively, apparently healthy resident horses that are carriers can spread the bacteria to newly introduced horses if they are naïve to the disease.
In a competition setting where horses are mixing, naïve horses are at risk either from 1) horses that have S. equi but haven’t yet started showing clinical signs of infection, or 2) inapparent shedders.
Flushing out the Offenders
A number of tests are available that can help diagnose S. equi in apparently healthy horses.
Swabs and Lavage. “Pharyngeal swabs may identify S. equi in subclinical shedders. However, false negatives can occur if the horse is not actively shedding at the time the sample is taken,” Boyle warned.
Pharyngeal washes are better for diagnosis than swabs. However, even the washes can be inconsistent in picking up carriers because of intermittent shedding.
There is also a point-of-care PCR test that can be used in the clinic. According to the manufacturer (Fluxergy), nasopharyngeal samples can be tested in 50 minutes, with a sensitivity of 88.7% and a specificity of 100%.
“This point of care test was evaluated using nasal secretions from actively sick animals,” said Boyle. “Clinically ill horses carry a much higher number of bacteria than some of the inapparent carriers do. And while it is theoretically possible to test guttural pouch lavage samples using this point-of-care test, it has never specifically been tested on samples with low numbers of bacteria, so we do not know how well it will perform in that setting and could miss some positives.”
According to Boyle, endoscopically guided examination of the guttural pouch combined with nasopharyngeal lavage PCR is the best way to test.
“We found that testing the guttural pouch lavage fluid with PCR was 50 times more likely to find carriers in horses recovering from strangles,” said Boyle. “The horses used in that study were known to be recovering from strangles and were a few to many months out.”
She added, “This combined guttural pouch exam, flush and nasopharyngeal lavage is a test veterinarians can and should use if they are planning on testing the carrier status of new horses before introducing them to a herd.”
Alternatively, three negative nasopharyngeal lavage PCRs can be used to catch 90% of the intermittent shedders if scoping is not available. False negatives can still occur because of intermittent shedding from the guttural pouch.
Serology. It is very important to appreciate that measuring antibody titers for the surface protein SeM will not help diagnose inapparent carriers.
In a 2018 study by Delph, et al., all 12 persistently infected cases of S. equi had SeM antibody titers ³6,400, and eight of the 12 persistently infected horses had SeM antibody titers ³12,800 eight weeks after the outbreak. The sensitivity and specificity (number of false negative and false positive tests) were very low (67% and 42%, respectively) when titers were ³12,800. Even when the cut-off was decreased to a titer of ³6,400, the specificity was unacceptable (26%).
“The elevation in SeM in horses with persistent guttural pouch infections following a strangles outbreak could be due to continued exposure to S. equi eliciting an immune response,” suggested Delph.
Instead of being used to “screen” for subclinical shedders, the SeM test is more typically used to detect evidence of recent infections by at least a four-fold increase in paired titers taken 10 days apart. The SeM test, however, cannot differentiate between horses that have been vaccinated and those that are naturally infected.
An indirect enzyme-linked immunosorbent assay (iELISA) measures both Antigen A and Antigen C of the S. equi bacterium and could also be useful for diagnosing S. equi in some situations, but it is not currently available in the United States.
“In countries where the test is available, iELISA can identify animals without clinical signs of S.equi that may be carrying the bacteria. If positive, it could be used pinpoint horses that should undergo guttural pouch examination,” said Boyle.
Two factors must be considered when performing the iELISA:
One of the problems with this test is that most vaccines on the market cross-react with it, said Boyle. “We published a study in Equine Veterinary Journal in 2021 that showed that even horses that were not vaccinated for strangles had some cross-reactivity to this test,” she said.
A second study found that the combined Antigen A and Antigen C iELISA did not identify subclinical S. equi carriers, and therefore concluded that this test could not be used to confirm a carrier status. Instead, it only identifies potential horses that should undergo guttural pouch examination (Durham et al. 2021).
Sussing out New Additions
If you’re dealing with a farm setting, a new addition should be isolated for about three weeks before allowing the horse to join the herd. This falls within the 14-day incubation period. If a horse has S. equi, it should develop clinical signs of infection within that three-week isolation. If the horse does develop signs of strangles, it should be isolated for at least another three weeks following cessation of clinical signs of disease (i.e., nasal discharge). Also, an endoscopic examination of the guttural pouches and PCR of guttural pouch lavage fluid should be negative at that time.
Even if a newly introduced horse does not develop overt strangles, it still could be an inapparent shedder. That would put naïve horses on the farm at risk of infection.
“Some farms, especially those that have a history of dealing with strangles, elect to simply perform endoscopic examination and guttural pouch lavage on all new horses arriving on a premises,” said Boyle. “This testing involves qPCR testing.”
If positive, the horses can be treated appropriately based on the endoscopic findings. If no underlying pathology exists, then they can receive the recommended treatment of lavage and local application of an antibiotic (penicillin).
If underlying pathology exists, then additional treatment is needed. This could involve manually removing chondroids or empyema via endoscopic guidance. The guttural pouches must be copiously lavaged with a 20% solution of acetylcysteine in buffered saline. Manual removal of chondroids or empyema might be required endoscopically or even surgically in some cases. Horses will require local antibiotics (penicillin) and systemic antibiotics (procaine penicillin or potassium penicillin) for 7 to 10 days.
“In these horses, repeat the guttural pouch lavage and qPCR testing three weeks after cessation of treatment,” advised Boyle. “If using the guttural pouch lavage with endoscopic examination, then visually clear guttural pouches and a negative PCR at this recheck examination is considered sufficient.”
If, however, the horse is being tested with nasopharyngeal samples, then three negative test results are recommended before deeming a horse “clean.”
Screening for Shedders After a Strangles Outbreak
Immediately isolating horses with fevers and using appropriate biosecurity principles will minimize strangles outbreaks. After an outbreak, however, the high number of horses likely carrying persistent guttural pouch infections is concerning.
In the hypothetical horse described at the start of this article, should that horse be tested for a persistent infection afterward even if he appears healthy?
Boyle said yes.
“If the owners can do it, I recommend scoping all horses recovering from strangles,” she advised
“Effective strangles control measures require detection, segregation and treatment of carrier animals,” summarized Boyle. “And the best test is guttural pouch evaluation via endoscopy and guttural pouch sampling/testing.
“Some barns that have had problems with strangles require testing (scoping) prior to entering their barns,” she noted.
Another good time to perform endoscopy of guttural pouches is after a quarantine of horses that come from unknown backgrounds, such as auctions or rescues.
Clearly, it would be ideal to have a non-invasive test capable of identifying subclinical carriers (i.e., a test less invasive and cumbersome than endoscopic evaluation of guttural pouches). In countries where it is available, the iELISA is as close as we currently have, and still it is only used to identify horses that were exposed to S. equi.
“There is clearly a pressing need to develop an easily-performed, non-invasive screening test capable of identifying carriers and/or a 100% effective vaccine against S. equi to prevent continual spread of this highly contagious organism,” concluded Delph.