Cover Story | Infectious Diseases

Obtaining a Good Sample Essential for Diagnosing Strangles

By Marie Rosenthal MS

The diagnosis of strangles is done primarily by looking for the causative organism, Streptococcus equi subspecies equi, so obtaining a good sample at the right time is essential for increasing the sensitivity of the tests, Camilo J. Morales, DVM, MS, DACVIM, said at the AAEP’s 70th Annual Convention, held in Orlando, Fla.

The bacteria sit in the nasal cavity for 3 to 5 days, when it might be possible to obtain a good nasal swab. They travel through the nasal pharynx and into the guttural pouches, where S. equi will stay there for long periods, so that is often the best place for a sample.

“This is important to know because that is going to help us determine where we are going to get the sample from,” said Dr. Morales, who is a staff veterinarian at UC Davis School of Veterinary Medicine, in California.

In a study done at UC Davis among 65 patients hospitalized with strangles, the most common clinical signs were nasal discharge, lymphadenopathy, fever, external abscesses and cough, according to an abstract presented at the ACVIM Forum 2024, in Minnesota (abstract E05). “We were able to culture the bacteria in 45 of them. The most common place for culturing was the guttural pouches, the submandibular abscess; for bastard strangles, the neck and links were the most common sites,” Dr. Morales explained. (Bastard strangles is a complication that can occur if S. equi spreads from the lymph nodes to other parts of the body if the abscesses in the nodes rupture.)

Strangles is endemic in the United States, and it’s prevalence is increasing, albeit slowly, according to biosurveillance (Vet Sci 2023;10[2]:78). In 11 years, cases rose from 6.4% to 7.6%. The disease is often self-limiting, but horses can have serious complications and disseminated disease. In addition, a silent carrier state can also occur after infection.

When deciding how to proceed, Dr. Morales suggested answering these questions:
• How sick is the horse?
• How long has the horse had clinical signs?
• Is there nasal discharge?

A complete blood count and chemistry, as well as serum amyloid A (SAA) levels can be useful, but the definitive diagnosis is made with quantitative polymerase chain reaction (qPCR), culture or a combination of both.

Depending on the timeline, samples can be taken with a nasal swab, nasopharyngeal lavage, guttural pouch swab, or a guttural pouch or upper respiratory tract lavage, according to Dr. Morales.

Flushing the guttural pouches to force the fluid into and out of the nose can be useful. “I think that’s the one that provides you more contact surface and maybe increases the sensitivity of the test,” he said. Remember to wear personal protective equipment and practice good biosecurity to avoid contaminating other horses.

If measuring titers, there are several things to consider. Between 2 or 3 weeks after infection, titers can be measured by an enzyme-linked immunosorbent assay (ELISA). However, those titers can be influenced by vaccination, which will need to be considered. The live-modified vaccine will increase titers for up to 3 weeks, whereas the intramuscular vaccine can increase titers for up to 12 weeks. So, it’s very hard to use ELISA titers to diagnose a horse with strangles.

There are researchers working on an ELISA that can distinguish between titers from natural infection and those from vaccination, so that is something to look forward to. These researchers are looking for antigen A, which is unique to infection (Aust Vet J 2020 April 23).

He suggested a second ELISA be done 4 to 5 weeks after infection (about 2 weeks after the first ELISA) to see if they increase, which would signal the horse has strangles. An immunocompetent horse should be on the road to the recovery by that time. However, horses with strangles can test positive on ELISA, even after 6 months.

“So, having a horse with positive titers doesn’t mean that the horse is sick. It could have become a silent carrier, which is truly a problem for us to treat,” he said.

He finds ELISA useful in detecting bastard strangles.

The consensus statement by the American College of Veterinary Internal Medicine provides the pros and the cons of each sampling type. For instance, aspiration of a lymph node is the easiest way to take a sample, and it has a high yield, but the horse must have an abscess to aspirate.

“The one that is the best to detect silent carriers would be the guttural pouch lavage. However, it needs equipment, training and you need to be able to get into the pouch and get the sample from there,” Dr. Morales said. If possible send the sample for culture and qPCR, but if only 1 test can be done, he suggested qPCR. “It has a higher sensitivity,” he explained.

“At Davis, we test for both, and we are seeing a lot of mixed infection in these horses that have empyema in the pouches,” he said, adding that qPCR enables one to monitor the progress of the disease because it provides information about the bacterial load.

Another diagnostic technique is endoscopy, which requires training and special equipment. But one can see the lesions, as well as how bad the empyema is. Sometimes, the pus is so bad, the endoscope cannot get into the guttural pouch, he said.

In addition, there are emerging strategies that can help the veterinarian in the field. The loop-mediated isothermal nucleic acid amplification device was developed in United States, but the Australians have enhanced this technique and can provide a diagnosis of strangles in 7 minutes in the field (J Vet Intern Med 2021;35(3):1597-1603 https://pubmed.ncbi.nlm.nih.gov/33728675/).

“You just need to use this isothermal device with your smartphone and you will have a diagnosis. It has a percentage of detection of 91%, which I think is good enough for one to have it on the truck [when it becomes available],” Dr. Morales said, adding he has experience with the Fluxergy Assay.

“I think it is going to help us in the field to diagnose strangles [when a laboratory is not available],” he said. “We are using it, and we are finding very good results that are comparable with the actual laboratory.”

The test does not yet have U.S. Department of Agriculture approval, he reminded, but it is something to anticipate.

Any positive result means the barn must take bio-security precautions to avoid the spread of this disease, according to Dr. Morales. MeV

GUTTURAL POUCH MANAGEMENT
To remove the purulent material from the guttural pouch, Dr. Camilo J. Morales, a staff veterinarian at UC Davis, likes to flush the pouch through the nose and collect the material in a bucket. It is important to keep the horse’s head down with adequate sedation to avoid aspiration pneumonia, he said. He prefers a manual pressure pump rather than an athroscopy pump because he thinks it causes less irritation.

“I like the manual pump because I feel it does not irritate the guttural pouch as much as I’ve seen using the arthroscopy pump machines,” he explained. “They achieve higher pressure, but the pouch gets very irritated after that.”

If the material is very compacted, manual pumps may not be a good option. Usually when it is that compacted, however, the horse must be sent for surgery, he said.

Because the condition is self-limiting, typical treatment is supportive, which often includes nonsteroidal anti-inflammatories. Antimicrobials are not normally required. However, patients with bastard strangles require antimicrobials.

He bases his decision on the patient, he said. Does it have a fever? Is it eating? How high is the serum amyloid A?

If he does use an antimicrobial, he prefers a local penicillin gel that he can leave in the pouch. He thinks it might help prevent the carrier state. However, Dr. Morales admits there are no studies proving this idea. After he administers the gel, they hold the horse’s head up for 5 minutes hoping the gel will stay there, although frequently as soon as the horse lowers its head, the penicillin gel comes out the nose.

He has read about a reverse thermodynamic gel with benzylpenicillin that when it encounters the warm guttural pouch, the liquid solidifies and might stay in a little longer. However, there are no clinical trials about whether it helps.

The choice of systemic antibiotics needs to be considered carefully, because not all have good penetration in the pouch or are effective against Streptococcus equi. Trimethoprim-sulfamethoxazole, doxycycline, minocycline, penicillin and rifampin have high concentrations and will kill the bacteria.

Enrofloxacin is not a good choice “even if you have sensitivity tests saying that strangles or S. equi is susceptible to enrofloxacin; it does not work in vivo. So, we do not recommend the use of enrofloxacin for this disease,” he said. Gentamycin and amikacin are also not good choices.

He did not discuss surgical approaches. However, he said transpharyngeal fenestration may be necessary to remove chondroids or purulent material that is packed into the pouch. In addition, laser salpingopharyngostomy is a new technique technique that is producing good results.

For more information:
Sweeney CR, Timoney JF, Newton JR, et al. Streptococcus equi infections in horses: guidelines for treatment, control, and prevention of strangles. J Vet intern Med. 2005;19:123-234.
https://www.wormsandgermsblog.com/files/2006/11/J-Vet-Intern-Med-2005-Sweeney.pdf