Care must be exercised over the use of antibiotics for treating strangles cases in horses, with veterinary opinion split on the issue, a panel of experts says.
Strangles is a horse disease caused by the bacterium Streptococcus equi. It was first reported by Jordanus Ruffus in 1251.
It is a costly, worldwide, highly infectious upper respiratory disease, with infected horses often developing nasty abscesses in the throat region and side of head, where lymph nodes are located.
A just-published consensus statement from the American College of Veterinary Internal Medicine says the appropriate treatment of horses with strangles usually depends on the stage and severity of the disease.
“The majority of strangles cases require no treatment other than proper rest and a dry, warm stall and provision of soft, moist, and palatable food of good quality while letting the disease run its course,” Ashley Boyle and her colleagues said in the Journal of Veterinary Internal Medicine.
Food and water should be easily accessible to the horse, and fans should be used in hot weather to ventilate stables for infected horses with an acute fever.
“Veterinary opinion as to whether or not to use antibiotic treatment remains markedly divided,” the panel said, noting that clinical and experimental evidence evaluating the effects of antibiotic use in strangles was limited.
There were, they noted, no reported studies comparing horses treated with antimicrobials to untreated horses.
“In many cases, antibiotics are unnecessary, and several potential concerns regarding their use have been put forward.” These include the potential to delay the maturation of abscesses or a recurrence of them when antibiotics are stopped.
There was no clear data regarding the role of antibiotics on increasing the risk of occurrence of abscesses away from the normal head region, a condition known as bastard strangles.
Antibiotics could potentially inhibit the synthesis of protective antigens and affect the bacterial cell wall, which in turn could diminish the development of immunity. A less than ideal immune response could leave horses susceptible to reinfection, they said.
If not treated with antibiotics, about 75% of infected horses develop long-term immunity, and natural exposure to disease over time contributes to reboosting and herd immunity.
The panel said antibiotics may be appropriate in some cases, including acutely infected animals with very high fever and malaise before abscess formation, horses with profound inflamed lymph nodes and breathing difficulties, horses with abscesses arising elsewhere, and instances where it is deemed necessary to locally treat guttural pouch infections to stop a horse carrying the disease.
Antibiotics should not be used as a preventative measure in animals that may have been exposed, they said. “Overuse of antibiotics promotes resistance, provides a false sense of security, and convalescent immune responses may not be induced.”
The panel said active and recovering strangles cases were an important and easily recognizable source of new infections for susceptible horses through their pus discharges from lymph nodes, nose, and eyes.
Transmission occurs when there is either direct or indirect transfer of pus between affected and susceptible horses. It can occur through normal horse social behavior, or through sharing contaminated housing, water sources, feed or feeding utensils, tack, and so on. The clothing and equipment of handlers and veterinarians can result in spread of the infection.
“It is now recognized that transmission originating from outwardly healthy animals may be of greater importance than that from purulent discharges from sick horses in initiating new outbreaks or recurrences in previously affected herds because the source of infection is not obvious,” they said.
Some horses incubating the disease are outwardly healthy and potentially infectious, and go on to develop signs of strangles. It is assumed that nasal secretions are the source of infection in these animals.
Also, outwardly healthy horses can continue to harbor the organism after full clinical recovery.
“It is therefore appropriate to consider that all recovered horses may be potentially infectious for at least six weeks after their purulent discharges have dried up.”
In some outwardly healthy horses, carriage and at least periodic shedding of the bacterium occurs for prolonged periods after apparent full and uncomplicated recovery. These horses are commonly called long-term, subclinical S. equi carriers.
“There is strong evidence that they can be a source of new or recurrent disease in well-managed groups of horses.”
Effective strangles control measures require detection, segregation, and treatment of carrier animals, which can be identified through testing.
For new horses, the authors recommended a blood test be used to identify recently exposed or persistently infected animals. A second blood sample taken two weeks later can identify horses that have seroconverted and may have been incubating the infection.
“If the second sample is also negative and the horse remains free from clinical disease, then it is considered safe to enter the herd.”
If positive, the guttural pouches should be examined by endoscopy to identify signs of persistent infection, which would then require treatment.
The panel said limiting exposure remained the best method to prevent strangles infections.
Biosecurity measures should include quarantine and screening of all new arrivals, appropriate disinfection and cleaning of potentially contagious equipment, and education of caretakers on proper hygiene.
New arrivals should be isolated for at least 3 weeks. Additional screening for subclinical carriers by guttural pouch endoscopy, culture, and PCR testing should be part of any screening program.
The panel said scientific knowledge of S. equi and the mechanisms it uses to cause disease is growing at an unprecedented rate.
“The importance of identifying and treating persistently infected horses is clear at both national and international levels. However, there remains a relatively low level of adoption of even basic biosecurity and testing measures to prevent incursions of strangles or other infectious diseases.
“The ability to differentiate strains of S. equi in source tracing provides a basis for possible litigation and will serve as an impetus to galvanize horse owners and veterinarians into taking pre-emptive preventative action.”
They said vaccines that generate sterile immunity and serum antibody responses distinguishable from those induced by infection were urgently needed.
“Unfortunately, little is known about the protective immunogens involved and how they must be presented in the horse to induce high level protective immunity. More research in this area is necessary in order to improve the effectiveness of emerging vaccines.
“Studies are also needed to explain shedding of virulent S. equi from persistently infected guttural pouches.”
They said proper biosecurity, new testing, vigilant detection of subclinically infected carrier animals, and improving vaccines meant that prevention or efficient elimination of strangles from a property is becoming ever more feasible.
Boyle, A.G., Timoney, J.F., Newton, J.R., Hines, M.T., Waller, A.S. and Buchanan, B.R. (2018), Streptococcus equi Infections in Horses: Guidelines for Treatment, Control, and Prevention of Strangles—Revised Consensus Statement. J Vet Intern Med. doi:10.1111/jvim.15043