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Friday 20 October 2017

Guarding against the scourge of strangles

Highly contagious, painful and potentially fatal, beware of this condition that can strike any horse, donkey or pony and stock of all ages

A typical case of strangles showing swollen or enlarged lymph nodes in the neck or throat area. All pictures courtesy of UCD equine veterinary team and Professor Derek Knottenbelt at the University of Liverpool
A typical case of strangles showing swollen or enlarged lymph nodes in the neck or throat area. All pictures courtesy of UCD equine veterinary team and Professor Derek Knottenbelt at the University of Liverpool
Strangles symptoms include a discharge from the nostrils and abcess formation
Strangles causes swelling of the throat region and the first sign is often a spike in the horse's temperature

Dr Joe Collins

Strangles is so called because the sensation of having a bad dose must feel like being strangled -- or so the old story goes.

It's certainly true to say that it's a very unpleasant condition and that some horses affected with the condition struggle really badly to breathe, before some die.

It's a highly contagious, serious, but rarely fatal infectious disease that affects all, but usually young, equids. It is a disease that is endemic in Ireland, and is sometimes best left untreated. It can be prevented; and rarely recurs but carries a significant risk of setting up a 'carrier state'. By this I mean:

  • That it is caused by an infectious agent (a bacterium called Strep equi)
  • That it can readily pass between horses, ponies and donkeys
  • That it is more common (as a clinical disease) in weanlings to three-year-olds
  • That it causes significant illness, inconvenience and loss of performance but rarely death
  • That it is considered a 'normal' or common disease in this country
  • That it is well treated (with antibiotics) only in some animals
  • That it can be managed by a combination of detection, biosecurity and vaccination
  • That it can exist in a dormant form whereby a horse 'carries' the infection, shows no clinical signs but acts as a reservoir of infection spreading the disease to susceptible equids.

I'm often asked how common strangles is and that's a difficult one to answer because it all depends on what group of horses you are talking about and whether you include subclinical (carrier) cases as well as clinical disease cases.

Outbreaks occur on a sporadic but regular basis on stud farms, in livery yards, on training establishments and where just one horse is kept on private premises. Some would have it that the rate of infection has been on the increase recently; this may be so, but we just don't currently have the data to know.

In truth, there are likely to be peaks and troughs of infection, and the rate of disease seen will vary with the production, sales, movement, management and general health of our population of horses.

Strangles can vary between groups of horses, ponies or donkeys. Young horses from weaning up to four years or so of age seem to be more susceptible to clinical disease. Horses in poor health or bodily condition are more likely to get the disease and also to show it in a more severe form. And premises where there is frequent movement of horses (of varying age, type and origins) on/off or around the farm are distinctly more at risk of an outbreak.

Strangles carriers that show no clinical signs are more generally older horses. It is estimated that up to 10pc of horses that recover from a bout of the disease go on to become a carrier.

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How serious is strangles?

The seriousness of the condition varies greatly. There can be a total absence of signs in the carrier state, while in contrast the infection can spread through the body in cases of so-called 'Bastard strangles' and result in death.

Most typical, however, is where infection is confined to the head and upper neck region and recovery occurs after an unpleasant bout of illness. The throat is the first point of entry and the first sign is usually a spike in the horse's temperature.

The bacteria spread from here to the lymph nodes (glands) in the region, the body's defence mechanisms go into overdrive and you get all the classic signs of inflammation -- heat, pain, swelling and redness (calor, dolor, tumor et rubor, for the latin scholar).

A feature of strangles is abscess formation -- a combination of bacteria and the body's response forming a liquid mass of defence cells walled-off by a capsule. Over time these usually rupture, either into the nasal cavities, leading to a purulent white/yellow discharge from the nostrils, or direct to the outside under the lower jaw or below the ears. Spreading down the neck, possibly into the chest or abdomen is a very serious complication.

If it's endemic does it matter if we export or import it?

Wisely, we have a particular focus on not allowing in exotic disease (diseases we don't currently have such as rabies or African Horse Sickness) or new strains of conditions we do have. That shouldn't make us complacent about the serious conditions we currently have and the real impact these have on the health and welfare of our horses.

Neither should we under-estimate the harm done to our reputation as the breeders and exporters of quality horses when the introduction of an Irish horse is proven to be the source of a strangles outbreak.

Why only treat some affected horses with antibiotics?

Even though Strep equi strains are still highly sensitive to treatment with basic penicillin antibiotics, conventional wisdom suggests that once abscess formation has commenced we should let nature take its course.

Penetration of penicillin into pus-filled, walled-off abscesses is poor. Indeed, it might be best to encourage the abscess to form, rupture and then heal.

Where antibiotics seem to have a better role is in the treatment of in-contact animals showing no clinical signs yet, in the hope of preventing these from developing abscesses (usually by giving penicillin to them by intramuscular injection), or in the treatment of carrier cases (usually by the injection of penicillin through tubing up the nostrils).How can I detect if my horse has it?

By getting your vet to test for it. The classic test is to take culture swabs of pus from a discharging abscess, from a suspicious nasal discharge or from the back of the throat, and see if Strep equi grows.

A positive result will only be confirmed if there are enough live bacteria on the swab to grow into colonies in the lab.

Tests using newer technologies look for fragments of bacteria (alive or dead).

These tests can be readily performed on swabs, as above, but also on blood samples where our great hope is that it will be effective at identifying carrier animals.

These silent carriers are the scourge of managing strangles outbreaks on a premises.

Vaccination?

There are two types of vaccination generally for any infectious disease -- the immunity that can occur following natural infection, and the acquired immunity that can be conferred by vaccination.

Strep equi robustly stimulates the immune system and very rarely does a horse get a second bout of clinical strangles. Worldwide, strangles vaccines have been developed that can be administered by intramuscular injection, by squirting up the nose or into the mouth and by injection into the skin inside the lip.

None are as effective as naturally occurring infection, only the lip-injection form is available in Ireland, all have to be repeated at least annually.

The carrier state

An estimated 10pc of horses that recover from a bout of clinical disease retain live bacteria typically in the guttural pouches at the back of the throat. Every time the horse bends to eat/drink and swallows, the flap guarding the guttural pouch opens releasing some bacteria, ready to be breathed in by an unsuspecting, immunologically naïve horse, and the cycle of infection starts over again.

The carrier never shows any clinical signs. The classic test was to swab the throat area with special swabs on long handles, or sample the guttural pouch using an endoscope and tubing and then test in a laboratory.

Three tests taken at weekly intervals was deemed sufficient to declare the horse 'safe'. But, of course, no such testing is 100pc accurate and the prospect of a more reliable single blood test is to be welcomed.

Dr Vivienne Duggan, chair of Veterinary Ireland's Equine Group and a lecturer in equine veterinary medicine at UCD, has been involved in trial work looking at the prevalence of the carrier state around Ireland, by blood sampling horses at equestrian events. We await the results with interest.

Further information?

The Irish Thoroughbred Breeders' Association (ITBA) produces a code of practice annually with advice on the management of infectious diseases in horses.

It contains specific guidelines on strangles that are applicable across the equine industries. Infectious disease cannot be tackled without some joined-up thinking and a co-operative approach.

Even so, individual horse owners should be careful about what they bring home from the fair to their healthy horses.

Dr Joe Collins MRCVS Veterinary Ireland president 2010

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