Strangles

What it is

Strangles (equine distemper) is an infection of the upper airways and their lymph nodes caused by Streptococcus equi subsp. equi, a bacterium adapted to equids. It enters through the mouth or nose, attaches to the lymphoid tissue of the pharynx and within a few hours reaches the mandibular and retropharyngeal lymph nodes, where it forms abscesses. The mandibular nodes usually rupture to the outside under the jaw; the retropharyngeal nodes often drain into the guttural pouches, where pus can accumulate (empyema) and thicken into concretions (chondroids).

Around 10% of horses that recover remain carriers: they harbour the bacterium in the guttural pouches without signs for months or years and shed it intermittently. They are the main reason the disease persists and reaches clean premises.

Complications include bastard strangles (abscesses in other lymph nodes, such as the mesenteric or mediastinal nodes, or in organs), purpura haemorrhagica (an immune-complex vasculitis with oedema and haemorrhages), myopathies and airway obstruction by enlarged lymph nodes, which gives the disease its English name.

Who it affects

Horses, donkeys and mules of all ages, with more cases in young animals. Outbreaks usually start after the arrival of a new horse, often a healthy carrier. It spreads by direct contact and through water troughs, feeders, equipment, clothing and hands.

Anatomy involved

• Mandible: between its two rami, in the intermandibular space, lie the mandibular lymph nodes, which are palpably enlarged, hot and painful and where most abscesses form and open. Behind the rami, under the base of the skull, lie the retropharyngeal lymph nodes and the guttural pouches.
• Guttural pouches: diverticula of the auditory tube unique to equids, lying between the base of the skull, the pharynx and the atlas. The medial retropharyngeal lymph nodes are in contact with their floor, so their abscesses rupture into them. They are the site of persistence of the bacterium in carriers and where samples are taken to detect them.
• Trachea: retropharyngeal abscesses compress the pharynx and larynx and can obstruct airflow; in such cases an emergency tracheotomy is performed at the junction of the proximal and middle thirds of the neck.

Clinical signs

• High fever, depression and inappetence, often one or two days before the horse starts shedding the bacterium.
• Mucopurulent nasal discharge.
• Enlarged, painful mandibular lymph nodes that eventually abscess and drain thick pus.
• Dysphagia, extended head, respiratory noise and, if the retropharyngeal nodes are very enlarged, breathing difficulty.
• Mild forms in horses with partial immunity, with little fever and barely enlarged nodes.

Diagnosis

• Quantitative PCR and culture of pus from an abscess, a nasopharyngeal swab or a nasopharyngeal wash in acute cases.
• Carrier detection: guttural pouch endoscopy with lavage and PCR, which is much more sensitive than a nasal swab. PCR also detects dead bacteria, so a positive result after treatment is confirmed with culture or repeat sampling.
• Serology: ELISAs against antigens A and C are used to detect exposed horses, for example before they enter a premises; titres against the SeM protein help assess purpura haemorrhagica or metastatic abscesses.
• Ultrasonography of the lymph nodes and, in bastard strangles, abdominal or thoracic ultrasonography.

Treatment and prevention

• Uncomplicated cases: most resolve without antibiotics. Rest, soft moist feed, non-steroidal anti-inflammatory drugs, hot packing to bring abscesses to maturity, and drainage and flushing of mature abscesses.
• Antibiotics (mainly penicillin): in the febrile phase before abscesses form, in horses with breathing difficulty or severe dysphagia, and in complications. Giving them once abscesses have formed can delay maturation. Recommendations rest mostly on expert opinion.
• Carriers: guttural pouch lavage, endoscopic removal of chondroids and topical and systemic penicillin, until results are negative.
• Outbreak control: isolate sick horses, separate exposed from unexposed horses, take temperatures daily to detect new cases early, quarantine and test incoming horses, and keep restrictions in place until carriers have been ruled out.
• Vaccines: there are live attenuated vaccines and a recombinant subunit vaccine that allows vaccinated horses to be distinguished from infected ones (DIVA). They reduce the number and severity of cases but do not always prevent infection, and vaccinating horses with high SeM titres is not recommended because of the risk of purpura haemorrhagica.
• Drug use in horses is subject to each country's regulations and to withdrawal periods. In the EU the horse is a food-producing species, and its identification document (passport) states whether it is excluded from slaughter for human consumption, which determines which drugs may be given.

Key studies

• ACVIM consensus (Boyle et al., 2018): updates the 2005 guideline on signs, pathogenesis, epidemiology, treatment, complications and control. It highlights the role of carriers, redefines the reference standard for detecting infection with PCR and serology, stresses prudent antibiotic use and links vaccination to control programmes. It is expert opinion.
• Houben et al. (2023), systematic review and meta-analysis of 10 outbreaks (8 published). The estimated basic reproduction number was 2.2 (95% CI 1.9–2.5), and 2.7 (95% CI 2.1–3.3) when outbreaks in which every horse fell ill were included. It helps calculate the vaccine coverage needed. Most estimates rely only on final outbreak size and may underestimate contagiousness.
• Boyle et al. (2017), diagnostic study in 44 convalescent horses sampled at the same time in the nasopharynx and the guttural pouches. Isothermal DNA amplification (LAMP) was positive in 1 of 41 nasopharyngeal swabs, 6 of 38 nasopharyngeal washes and 24 of 44 guttural pouch lavages, where detection was 51 times more likely (OR 51.0). In the pouch lavages, real-time PCR was positive in 18 of 44 and culture in only 4 of 44. It is a single group of horses.
• Robinson et al. (2020), four experimental challenge studies in ponies with the recombinant vaccine Strangvac versus an adjuvant-only placebo. All 36 control ponies became ill; with the vaccine, 5 of 16 were protected 2 weeks after the second dose, 7 of 12 at 2 months, and 15 of 16 (94%) 2 weeks after a third dose. The response did not interfere with the exposure ELISA (DIVA). It is an experimental infection in ponies, and the study was funded by the manufacturer, with authors linked to it.
• Mitchell et al. (2021), genomic analysis of 670 S. equi isolates from 19 countries. It showed many transmissions between regions and countries linked to horse movements, driven by carriers that look healthy and are rarely detected at pre-purchase or transit examinations. It is a population genetics study, not a clinical one, and one of the authors works for a vaccine manufacturer.