Strangles, caused by the bacterium Streptococcus equi subspecies equi, is one of the most frequently diagnosed infectious diseases in horses across the world (Mitchell et al., 2021). Only the geographically isolated equid population in Iceland remains free of S. equi infection by virtue of a ban on the import of horses, which has been in force for over 1,000 years (Bjornsdottir et al., 2017). Elsewhere, the disease is endemic and often regarded as an inevitable rite of passage for horses (Boyle et al., 2018). However, the risks of strangles can be minimised through the effective use of biosecurity measures and diagnostic testing (Waller, 2014). The application of vaccines against strangles can prepare the specific immune response of horses for exposure to S. equi, providing an opportunity to further reduce the impact of this disease.
Clinical disease
Strangles is characterised by pyrexia and the formation of abscesses within the submandibular and retropharyngeal lymph nodes.
Pyrexia (rectal temperature of 38.5°C or above) is usually the first clinical sign of S. equi infection and occurs between 1 and 18 days after exposure to the bacteria, depending on the infectious dose and immune status of the horse (Boyle et al., 2018). Abscesses form in the lymph nodes of the head and neck over a period of between 4 and 22 days. As the abscesses grow and mature they can restrict the airway, hence the name “strangles”. Abscesses then rupture, releasing a thick purulent discharge that drains through the skin (Figure 1) or into the guttural pouch and out of the nostrils via the nasopharynx. A soft moist cough may accompany the release of purulent material (Boyle et al., 2018).
The overall mortality rate due to S. equi infection is typically between 1 and 2 percent but can be higher, particularly in populations of younger horses
Some horses may develop severe forms of strangles. This includes the dissemination of S. equi to lymph nodes beyond the head and neck (“bastard strangles”) and the immune-mediated conditions of purpura haemorrhagica and myositis (Boyle et al., 2018). These severe complications can occur in as many as 20 percent of infected horses (Ford, 1980). The overall mortality rate due to S. equi infection is typically between 1 and 2 percent (Boyle et al., 2018) but can be higher, particularly in populations of younger horses that receive a high infectious dose.
Transmission
As carriers can shed S. equi for several years, they undoubtedly play an important role in the transmission of strangles to naïve populations
Shedding of S. equi typically begins one to four days following the onset of pyrexia as the abscesses form and drain (Figure 2). Therefore, isolating horses that develop a rectal temperature of 38.5°C or above can minimise the transmission of S. equi to in-contact animals, so one clinical case does not lead to an outbreak affecting the entire yard.
Incomplete drainage of abscess material from the guttural pouch or sinuses can lead to the development of persistent infections in horses following their recovery from acute clinical disease. Such “carrier” horses appear healthy but can intermittently shed S. equi from the guttural pouch into the environment, where it can trigger new cases of disease. Although the levels of S. equi entering the environment when shed from carriers may be much lower than are achieved from horses with acute strangles, exposure to as few as 1,000 bacteria is sufficient to establish infection in some animals. As carriers can shed S. equi for several years, they undoubtedly play an important role in the transmission of strangles to naïve populations of horses and are responsible for recurrent outbreaks in yards with endemic disease (Newton et al., 1997).
Bacterial survival in the environment
S. equi can survive for up to seven days on surfaces that are not exposed to direct sunlight and as long as 30 days in drinking water (Durham et al., 2018). S. equi is sensitive to detergent, and strict biosecurity and cleaning protocols reduce the risk of transmission to other horses. The limited ability of S. equi to survive in the environment highlights the importance of transmission from direct contact with acutely or persistently infected horses.
Diagnosis
Until recently, most cases of strangles were diagnosed following the culture of S. equi (Bannister et al., 1985; Jones, 1919). The sequencing of DNA from S. equi (Holden et al., 2009) facilitated the development of more rapid quantitative polymerase chain reaction (qPCR) assays, which benefit from enhanced levels of sensitivity (Baverud et al., 2007; Webb et al., 2013). However, sampling of the guttural pouches is required to maximise the sensitivity of the identification of persistently infected horses (Pringle et al., 2022).
Serology can be used to identify horses exposed to S. equi during a recent outbreak regardless of whether they developed clinical signs
A serological assay is available to measure the number of antibodies directed towards fragments of two S. equi cell surface proteins: SEQ2190 and SeM (Robinson et al., 2013). Serology can be used to identify horses exposed to S. equi during a recent outbreak regardless of whether they developed clinical signs. This information can be utilised to direct additional sampling and testing by qPCR to determine if they remain infected. The assay has reduced sensitivity for the detection of long-term carriers (over 10 months) (Pringle et al., 2020). Despite this caveat, serological testing of horses for exposure to strangles accounted for the identification of 8.5 percent of all qPCR- and/or culture-positive horses in the UK between 2015 and 2019 (McGlennon et al., 2021).
In summary, the choice of clinical sample and test depends on the clinical scenario:
- If there are signs of pyrexia, nasal discharge or cough, then a nasopharyngeal swab or wash for qPCR is optimal, accepting that S. equi shedding is intermittent during the early phases of disease
- If there is an external abscess or profuse mucopurulent nasal discharge, then a needle aspirate/swab of pus or nasopharyngeal swab/wash for qPCR are ideal
- Serology can be used to screen and identify horses exposed to S. equi following an outbreak or prior to transport in the absence of overt clinical signs
- A guttural pouch lavage sample tested by qPCR can be used to identify persistently infected carriers and to confirm infection-free status post-treatment
Control
Reducing the exposure of resident or in-contact horses to S. equi is an effective method of controlling strangles. Therefore, quarantining new animals for three weeks and screening for the disease prior to or following arrival is an essential tool. Monitoring the body temperature of horses while they are in quarantine on arrival, or following their return from an equestrian event, provides an early indication of rising temperatures which precede other clinical signs. Note that such quarantine measures are also effective at reducing the risk of incursions of equine influenza virus and equine herpes virus 1.
Reducing the exposure of resident or in-contact horses to S. equi is an effective method of controlling strangles
If an outbreak of strangles occurs, it is vital to set in place a biosecurity control programme which includes:
- Isolation of horses with temperatures of 38.5°C or above from others without clinical signs. This can be as little as an area of a paddock separated by a double layer of electric fence to eliminate nose-to-nose contact and the sharing of drinking water
- Cessation of movement on or off the yard. This should continue until all cases, and other horses that were exposed to S. equi, are declared free of infection
- Implementation of suitable biosecurity, including cleaning of equipment between horses
- Implementation of a traffic light system for horses on the yard (Figure 3). The temperatures of horses in the amber and green groups should be taken twice daily, and any animals that become pyretic (temperature of 38.5°C or above) should be moved to the red group immediately
Identifying persistent infection
Approximately 10 percent of horses affected in a strangles outbreak have a failure of guttural pouch drainage and develop a persistent infection that can lead to recurrent cases of disease (Newton et al., 1997).
From three weeks after an outbreak, the following measures can be used to identify and resolve persistent infection of the guttural pouch. Serology testing of horses in the amber and green groups (Figure 3) can identify those that were exposed to S. equi during the outbreak. qPCR testing of guttural pouch lavage samples from clinically affected or seropositive horses will identify persistently infected horses. It will also confirm infection-free status following the resolution of clinical signs or the treatment of infected guttural pouches. Pus and chondroids that are identified in the guttural pouch by endoscopy can be flushed by instilling saline or be physically removed using a basket snare placed in the guttural pouch via the biopsy channel of the endoscope. The instillation of penicillin into the guttural pouch can be used to resolve any residual infection (Newton et al., 1999; Verheyen et al., 2000).
Treatment
The use of antibiotics for the treatment of acute cases of strangles remains controversial. The vast majority of cases do not require antibiotics and resolve their infection with appropriate nursing care. Approximately 7 percent of S. equi isolates have acquired mutations within the pbp2X gene that encodes a penicillin-binding protein (Morris et al., 2021), and evidence of a reduction in the susceptibility of S. equi to penicillin has begun to emerge (Fonseca et al., 2020).
Cases where antibiotics could be considered include:
- Acutely affected animals with profound lymphadenopathy and respiratory distress that requires immediate intervention
- Cases of disseminated infection (“bastard strangles”)
- Cases of purpura haemorrhagica
The use of NSAIDs may help to counteract the inflammatory processes induced by S. equi and provide relief from some of the clinical signs of disease.
Immunity to strangles
Following the deliberate exposure of two groups of foals to S. equi, there was a 74 percent reduction in the number of foals that developed strangles in a group that had previously recovered from strangles six months earlier when compared to a similar group of naïve foals (Hamlen et al., 1994).
Following the deliberate exposure of two groups of foals to S. equi, there was a 74 percent reduction in the number of foals that developed strangles
A live attenuated vaccine, which is administered by submucosal injection into the upper lip, and a multicomponent fusion protein vaccine, which is administered via intramuscular injection, are available in Europe. Both strangles vaccines are indicated to reduce the severity of clinical signs of strangles and the formation of abscesses in the lymph nodes of horses following exposure to S. equi. The latter vaccine does not contain S. equi cells, and its administration does not trigger positive culture, qPCR or serology results.
Conclusions
S. equi infection is endemic in populations of horses throughout the world. While the majority of horses recover from strangles, complications can arise, with the disease killing between 1 and 2 percent of affected animals. The routine monitoring of body temperatures in resident herds can assist yards in identifying potential cases of disease, facilitating their isolation to minimise both the size and severity of an outbreak. Quarantining newly arrived horses coupled with screening by serology and qPCR can greatly reduce the likelihood of incursions of S. equi. Vaccination is a further tool to reduce the severity of disease and the number of lymph node abscesses, providing further opportunities to get to grips with strangles.
