Equine strangles: how is it diagnosed?
Helen Whitelegg, Senior Campaigns Officer at Redwings, explains the diagnostic options for equine strangles
As with any horse health issue, knowing what we are dealing with is key to providing targeted treatment and patient care. In the case of infectious disease, it also enables us to reduce the spread of infection.
Research indicates that many horse owners are aware of the classic signs of equine strangles such as nasal discharge and lymph node abscesses. But, there is now greater awareness of the number of horses who only show mild signs of disease, while still being able to infect other equines. This means strangles should be added to the diagnostic differential in more cases of unexplained respiratory issues or fever. Nor should we discount strangles because it isn’t already circulating in the local area; our highly mobile horse population means strangles is always a risk.
Testing, testing, testing…
Tests help to answer three key strangles questions:
Has a horse become infected?
Is a recently infected horse now disease-free?
Is a healthy-looking horse an equine strangles carrier?
Vets will take a different diagnostic approach in each scenario. In addition to a physical examination and assessment of the horse’s history, there are currently three testing options:
Guttural pouch endoscopy
These can be used to collect samples from the nasal passage, the nasopharyngeal region at the entrance to the guttural pouch, or from a draining abscess. Swabs are most useful when a horse has developed obvious signs of disease such as nasal discharge or an abscess and, are the primary means of confirming a new infection. However, false negatives are possible so, strangles should not be ruled out on the basis of one swab result. In a healthy-looking horse who may be recovering from the disease or, that could be a strangles carrier, swabs are less reliable; three swabs taken at weekly intervals are recommended to reduce the risk of false negative results. However, multiple swabs will still be far less accurate than testing a single sample taken through guttural pouch endoscopy.
Guttural pouch endoscopy
This allows access to two air pockets situated at the top of the nasal passages. The pouches are a primary location for strangles bacteria and, a visual check can reveal pathology such as inflammation, developing or draining abscesses, or chondroids. During scoping, the pouch is also flushed with a sterile solution to provide a lavage sample that can be sent for PCR and/or culture testing. Guttural pouch endoscopy is the fastest and most accurate test for horses in the later stages of infection. It is not suitable for early diagnosis, as bacteria will not yet have reached the guttural pouches. Endoscopy is also unlikely to detect infection outside the respiratory system – so called ‘bastard strangles’.
Endoscopy is also the most reliable way to confirm that an apparently healthy horse is actually an established strangles carrier, though blood samples may be used to help identify raised antibody levels to target scoping for horses most at risk. A quarantine and testing protocol is recommended to screen new horses arriving at a to reduce the risk of strangles finding its way onto the premises. A demonstration of guttural pouch endoscopy is free to view at: https://www.youtube.com/watch?v=7E3o3JK-Ts4&t=3s
Blood tests do not detect strangles itself but, measure antibodies produced by the horse’s immune system in response to Streptococcus equi bacteria. It takes up to two weeks for significant quantities of antibodies to appear in the blood stream, meaning the test cannot detect early infection. However, a blood test at this stage can provide a useful yardstick to measure against subsequent tests. Discuss with your vet whether to run the first test straightaway or, to store and process when paired with a second sample.
Interpreting blood results is not always easy as immune systems are complex and sensitive to a range of influences. Comparing two blood samples taken around two weeks apart, is helpful as they will suggest whether antibody levels are falling, rising or, consistent. Contextual information such as health history, recent horse movements and, yard biosecurity protocols will also help the vet interpret blood results. Antibody levels are likely to remain elevated for several months after clinical strangles signs have cleared, meaning guttural pouch endoscopy is the best option to test horses recovering from clinical signs of disease.
Around 75% of horses retain a degree of immunity to strangles after having the disease, though this will wane over time. Once primed by infection, the immune system of these horses is able to reproduce antibodies quickly if they come into contact with strangles bacteria in the future, helping them avoid re-infection, or experience milder signs of disease. It is important to remember that a blood test showing increased levels of strangles antibodies is not a diagnosis. Further investigation, possibly using guttural pouch endoscopy, is needed to determine whether infection is present.
Quarantine is vital
When Streptococcus equi bacteria first enter a horse’s respiratory system, they quickly disappear into the nearest lymph nodes, where tests cannot find them. When signs of disease such as nasal discharge or abscesses develop, swabs can be used to take samples for testing, but infection may already have spread. Therefore, it is important if a horse that may have been exposed to the disease, has an unexplained fever or signs that could indicate strangles, quarantine measures are used as a precaution before disease is diagnosed.
Managing an outbreak
In an outbreak, in addition to swabbing horses with active signs of strangles, blood tests can be used to identify horses that may only have mild signs of disease but, are still able to pass the infection on.
If the source of the outbreak is unknown, blood tests may also help to identify any unidentified strangles carriers who require treatment to clear them of infection. It is imperative that all infected horses are checked once their disease symptoms have resolved to determine who can be safely released from quarantine, those who are still potentially infectious and identify horses who are at risk of becoming long-term strangles carriers. Only when all equines involved in an outbreak have been ‘diagnosed’ as negative for Streptococcus equi can the yard be confidently declared to be strangles-free.