Improved techniques for identifying and treating strangles carriers are described in a report by researchers based at the Animal Health Trust in Newmarket.
Strangles is caused by infection with Streptococcus equi. Typical signs are increased temperature, loss of appetite, soft cough, purulent nasal discharge and swollen lymph nodes of the face, which may often abscessate and burst.
It has long been suggested that strangles can appear without the introduction of obviously sick horses. The infection may persist in affected groups of horses either in carrier horses or in the environment. There is increasing evidence that asymptomatic carriage of Streptococcus equi following strangles plays an important role in the spread of infection to susceptible animals. A recent study1 found that 15 / 22 outbreaks (68%) produced at least one horse which was a S. equi carrier (ie from which S. equi could be isolated more than 4 weeks after the disappearance of clinical signs.) Carrier animals may remain a potential source of infection for a considerable time. An earlier study2 found that some horses continued to harbour S. equi for many months after signs had gone. One horse remained infected for over three years.
Richard Newton describes investigations, funded by the Home of Rest for Horses, which he carried out in collaboration with Dr Neil Chanter and his team in the bacteriology department of the Animal Health Trust. "The overall concept of the study was to investigate in detail prolonged strangles outbreaks (i.e. those that had been going on several months) in order to eradicate the infection from the premises so they could return to normal activity" he said. Three naturally occurring outbreaks of strangles were included in the study.
The first step was to introduce control measures based on strict segregation and good hygiene practices.
* All movement of horses on and off premises was stopped
* Recovered cases and animals that had been in contact had nasopharyngeal swabs taken on three occasions at weekly intervals. These samples were cultured for Streptococcus equi and tested by polymerase chain reaction (PCR) for evidence of S equi M-protein DNA.
* Horses were placed in 2 groups according to results.
1. non-infectious : S equi was not cultured and at least the last PCR test was negative.
2. potentially infectious: S equi was cultured or detected on PCR
* To prevent cross infection, potentially infected and non-infected animals were kept separate. Strict hygiene measures were employed: eg dedicated equipment for each group, disinfection for stable staff, thorough disinfection of stable and equipment.
* The infected group was monitored including flushing of guttural pouches and bacterial culture and PCR on flushed fluid. treatment of carriers
Detection of carrier animals.
"We sampled a total of 135 animals including the clinical cases and known contact animals. At the time of our detailed investigations, we found evidence of S. equi infection in 45 horses (33%). Of those, 14 (31%) remained carriers after clinical signs of infection had gone. We got involved in all of the outbreaks at a fairly late stage, and so some of the earlier cases were no longer infectious."
Polymerase chain reaction (PCR ) is a very sensitive technique. It can detect an individual molecule of target DNA, and so is able to identify a single organism. PCR in conjunction with culture is much more sensitive than culture of swabs alone. In this study PCR detected almost twice as many cases as did bacterial culture. However the technique also detects dead bacteria. It was found to be most useful as a screening test to indicate which horses should be subject to endoscopic flush of the guttural pouches to detect the carrier state.
"Using PCR alongside culture of nasopharyngeal swabs as a preliminary step to endoscopy, we were able to identify a large proportion of horses that continued to carry S. equi after clinical signs of strangles had disappeared. The results confirmed previous observations that three consecutive negative swabs (at weekly intervals) were not always sufficient to detect S. equi carriers" said Newton. "People can`t rely on the fact that horses just get over strangles and no longer pose an infectious threat after a particular period of time. This is a concept that veterinarians are only just coming to terms with, and so it may be quite new to most horse owners" he added.
The guttural pouches were confirmed as the main site of S. equi colonisation in the carrier animals. The best way to detect these carriers was by guttural pouch lavage. Most carrier animals showed signs of disease in the guttural pouches - either purulent discharge or "chondroids". ("Chondroids" are hard dried aggregates of purulent material.) The investigations demonstrated the considerable length of time that carrier horses remained infected . The longest periods a horse remained a carrier in each of the three outbreaks were 8, 7.5 and 5.5 months.
Fourteen Streptococcus equi carrier horses were treated. Thirteen had signs of guttural pouch disease. Particular attention was paid to the elimination of S. equi infection from the guttural pouches.
Endoscopy and treatment was performed after sedation with detomidine and butorphanol. An incremental approach to treatment was employed.: ie the simplest and most practical treatments were tried first. If they were not successful further techniques were used.
1. removal of inflammatory material + systemic potentiated sulphonamide
2. removal of more inflammatory material + topical + systemic potentiated sulphonamide
3. topical + systemic penicillin or ceftiofur
Removal of inflammatory material :
Treatment consisted of removing the inflammatory material from the guttural pouch - either by flushing purulent exudate or physically removing chondroids.
A. Irrigation and aspiration of the guttural pouches.
Two methods were used:
* large volume. An indwelling Foley catheter or rigid catheter was used to flush the guttural pouch with up to 3 litres of saline. This was not well tolerated in some animals.
* small volume. Between 50-200ml phosphate buffered saline was administered through a catheter in the biopsy channel of an endoscope and suctioned out again.
B. Endoscopic removal of chondroids.
Two methods were used:
* The chondroids were removed whole using an endoscope with forceps, a basket snare or a polyp retrieval basket. The memory - helical polyp retrieval basket was found to be particularly useful in removing large numbers of chondroids. In only one case was this approach not adequate. This was because of adhesions at the nasopharyngeal opening of the guttural pouch which prevented adequate drainage.
* The chondroids were macerated and flushed out.
This was used only after all inflammatory material had been removed and the guttural pouches appeared clean. Drugs used were:
* 1.3g trimethoprim + sulphadiazine (Duphatrim paste) in phosphate buffered saline
* 20ml 3% (w/v) gelatin in PBS +5mega units of penicillin
* Acetylcysteine (40ml of 20%w/v) in 2 horses with severe empyema and chondroids. Topical acetylcysteine was used for its
* mucolytic properties, but was not found to be very useful when chondroids were present. It also caused increased inflammation of the guttural pouch lining.
The horses` heads were kept elevated for 20 minutes after treatment.
A Nd-YAG laser was used to try to break down adhesions which obstructed the drainage in one case. This was unsuccessful and conventional surgery was needed to provide adequate drainage.
Work over many years in the Animal Health Trust laboratory has shown that resistance to antibiotics commonly used against S. equi is very rare. Culture and sensitivity testing was therefore not carried out and the choice of antibiotic was based on factors such as convenience and cost. For example:
* oral trimethoprim / sulphadiazine for 21 days (in some cases the course was repeated)
* intramuscular procaine penicillin for 7-10 days
* intramuscular ceftiofur for 7-10 days
Treatment was deemed to have been successful and the horses were considered free from infection if there was no endoscopic evidence of disease and S. equi was not detected by culture or PCR in at least 3 consecutive lavage samples.
The overall success of the control and treatment regime was 100% . (One horse was excluded from the figures as it was moved off the premises against strong advice. ) All outbreaks were rapidly brought under control, and once appropriate screening, segregation and hygiene measures were introduced all premises returned to normal activity. It was only necessary to resort to surgery in the exceptional circumstances of finding that the pharyngeal guttural pouch openings were occluded and could not be safely broken down by laser surgery. Otherwise all the treatment was conducted on the affected premises using sedation. " We feel this offered considerable benefits over the previously accepted norm that horses with severe guttural pouch disease required general anaesthetic and invasive surgery in the hospital environment, which posed a risk of infection to other, possibly, sick horses."
" Treatment of future cases would be most effectively and rapidly achieved by the removal of inflammatory pathology combined with systemic and topical use of bactericidal antimicrobial drugs, especially penicillin." advised Mr Newton.
1. J R Newton , J L N Wood , M N De Brauwere, N Chanter, K Verheyen, and J A Mumford. Detection and treatment of asymptomatic carriers of Streptococcus equi following strangles outbreaks in the UK In: Equine Infectious Diseases VIII: Proceedings of the 8th International Conference. R & W Publications Ltd, Newmarket. pp82 - 87
2. J R Newton ,J L N Wood , K A Dunn , M N DeBrauwere , N Chanter . Naturally occurring persistent and asymptomatic infection of the guttural pouches of horses with Streptococcus equi. Veterinary Record (1997)140, 84 - 90
for further details see:
Control of strangles outbreaks by isolation of guttural pouch carriers identified using PCR and culture of Streptococcus equi. J R Newton, K Verheyen, N C Talbot, J F Timoney, J L N Wood, K H Lakhani, N Chanter. Equine vet J. (2000)32 (6) 515 - 526
Elimination of guttural pouch infection and inflammation in asymptomatic carriers of Streptococcus equi.K Verheyen, J R Newton, N C Talbot, M N de Brauwere, N Chanter. Equine vet. J (2000) 32, (6) 527-732.