How is it spread?

 

The bacteria is airborne; it will not be carried far by the wind. This means that it is spread through the air in nasal droplets or saliva from a horse shedding bacteria.  It then has to be ingested or inhaled by another horse either directly or from a contaminated source.  The most common sources are water troughs/buckets or people. Any shared implement is a risk though, including feed buckets, tack, mucking out kit, etc.  Basically, anything that moves about the yard; this includes dogs, cats and wild animals.  The incubation period is more commonly between 3-10 days but can be up to 14 days.

 

Will all horses exposed to the bacteria become ill?

 

No, not all. Some older horses and horses that have been ‘out and about’ a lot in their lives may have a stronger immune system and could fight off the infection without showing any signs of being unwell.

 

If a horse has had strangles before, will it be protected against re-infection?

 

Not necessarily; if it was a different strain before or if it was a long time before, it probably won’t retain enough of the correct antibodies to fight off re-infection.

 

When does an infected horse shed the bacteria?

 

A horse that has been infected can shed bacteria before it starts showing signs of ill health.  76% or infected horses will stop shedding 1 month after signs stop.  However, some can keep shedding 3 months after infection or even up to 10 months!

The scary situation is when a horse becomes a carrier and these can keep shedding intermittently for years.  

 

How far can it travel in a sneeze/cough?

This is debatable - some people believe that it won’t travel very far, however, in a laboratory experiment, in ideal conditions for the bacteria, it travelled as far as 30 yards.

 

How long can it last in the environment?

 

Again this is debatable.  It is designed to live in the horse and not in the environment so usually does not last long in the environment.  It is inhibited by organisms that live in the soil and so is less likely to live for a long time in pasture.   Some people recommend that a pasture be rested for a month after an infected horse has grazed in it.  However in ideal conditions for the bacteria when the weather is colder and when they are protected in moist discharge, they can last longer than that.

 

The bacteria are degraded in direct sunlight and so will survive for longer indoors.  One study showed the bacteria to survive outdoors for 1 to 3 days and indoors for up to 7 days. Wood cannot be easily disinfected and it was found to survive on wood at 20 degrees C for 48 days and at 2 degrees C for 63 days.  These were however obtained from sterile studies done in a laboratory so there were no other bacteria present that would normally help to inhibit their growth.  In reality, in a normal yard situation, the bacteria are not going to live as long.

 

It can survive in water buckets and troughs for up to 30 days, although more commonly up to 7 days.

 

What are the clinical signs of the infection?

 

They can vary widely and will depend on the age of the horse, the strength of its immune system, the strain of the bacteria involved and the way the animal is kept.

 

Classic signs are:

 

· COUGH

· NASAL DISCHARGE

· HIGH TEMPERATURE

· ENLARGED LYMPH NODES UNDER THE MANDIBLE, WHICH MAY

 ABSCESSATE  7 TO 10 DAYS LATER

· PROBLEMS EATING AND BREATHING

· INAPPETANCE

· LETHARGY/DEPRESSION

 

However, horses may only show some or even one of these signs and perhaps only for a short period of time. Signs usually last for up to 23 days, but could last for up to 8 weeks.

 

How is the infection detected?

 

With caution, on seeing the signs described and taking a history, some further tests should be performed.  There is a blood test available that will detect antibody levels in an individual.  These are parts of cells used to fight off the infection.  However, they can only really prove that the horse has had previous exposure to the bacteria.  A horse can retain high antibody levels 6 months after exposure and it may not have even been infected.  A second sample needs to be taken 10 days later and if the levels of the antibody are much higher, then it is likely that they have come into recent contact with a significant level of strangles bacteria.  It still doesn’t mean that they are infected hence shedding bacteria.  The next test to prove that the horse has strangles, is to try to grow active bacteria from them.  This can be done  by sampling the back of the throat (naso-pharyngeal swabs) 3 times one week apart.

 

This will detect bacteria in over 90% of infected cases.  It has to be done this many times because of the fact that they may only intermittently shed them from the guttural pouch into the back of the throat.  Another test that can be done to detect bacteria, is by sampling the guttural pouch (guttural pouch wash).  This is performed by passing a scope up the nose, into the pouch and then washing it with some sterile fluid that the bacteria will survive in.  This fluid is then processed in a lab to see if the bacteria grow.  However, this is not useful in the early stages of infection, because it takes time for the bacteria to migrate to this structure.

 

How can you detect a carrier?

 

Bloods are helpful for ruling out carriers.  If a horse showing no clinical signs has negative blood samples taken 10 days apart, it’s unlikely to be a carrier.  If a horse showing no clinical signs has a raised antibody level on its bloods, it has either been exposed to strangles in the last 6 months or it is a carrier.  The bacteria need to be found in the individual to confirm.  Either 3 naso-pharyngeal swabs should be taken one week apart or a guttural pouch wash needs to be performed.

 

How should a horse with strangles be treated?

 

The horse should be isolated.  In most situations, no treatment is needed other than tlc.  If they are running a high temperature, some anti-inflammatory drugs like bute may be helpful.  If the animal is very ill, sometimes they need to be put onto antibiotics.  However, if the horse has a strangles abscess this will slow the progression and so it is better to hot pack them to encourage them to burst sooner.  The antibiotic treatment will also interfere with results from tests done to look for the bacteria.

 

In rare cases, horses can develop a problem called bastard strangles after an infection.  This is when they develop abscesses at other internal sites and can cause more serious problems.  They can present in different ways, depending on where the abscesses are but it should be considered if a horse exposed to strangles shows signs like colic, intermittent temperatures, weight loss, inappetance and depression.

 

What preventative measure can be taken?

 

There was a vaccination available and there will be again soon in this country.  However, they do not guarantee the prevention of strangles in a vaccinated horse.

 

The best way to prevent infection is to prevent exposure.  This means that if there is an infected horse on the yard, it should be isolated immediately as best is possible and if a new horse is coming onto a yard with an unknown history, it should be kept isolated for a 3 to 4 week period to make sure that it is not going to start showing any clinical signs of disease.  This is however not going to identify a carrier.  

 

THIS INFORMATION HAS BEEN COMPILED FROM MY OWN KNOWLEDGE AND EXPERIENCE, FROM PAPERS WRITTEN AND FROM ADVICE SOUGHT FROM OTHER MEDICAL SPECIALISTS.  HOWEVER ,THIS SUBJECT IS COMPLEX AND THERE ARE OTHER VALID OPINIONS THAT MAY CONFLICT MY ADVICE. IF YOU HAVE ANY CONCERNS PLEASE DO NOT HESITATE TO CONTACT THE PRACTICE.                                              

Vicky O’Gorman BVetMed MRCVS