non-steroidal anti-inflammatory drugs (NSAIDS) like flunixine meglumine at 1.0mg/kg
has been effective, especially in cases showing acute hind limb discomfort.
Response time to treatment is variable, from immediate up to approximately 30
minutes, but no case has failed to respond. Recovery is indicated when the “glassyeyed” look disappears and the horse starts to relax. Horses that collapse should,
where possible, be held down until sufficient parenteral fluids have been
administered and they are able to stand unassisted. Sedation by means of alpha 2
agonists (never use phenothiazine tranquilizers) has been utilised by various racing
jurisdictions around the world (personal communication), but the authors have no
experience with this and none of the cases in this study were sedated.
The common cause seems to be an elevation of the core temperature of the horse
leading to cerebral hyperthermia. This is usually as a result of extreme exertion in
high ambient temperatures as shown in Figure 2, where the incidence clearly follows
the seasons of the year. Horses do not possess a carotid rete (rete mirabile), so rely
on other thermoregulatory mechanisms to avoid cerebral hyperthermia (Hodgson et
al. 1993). These include sweating and upper respiratory tract cooling by increasing
the blood supply to the nasal cavities. The guttural pouches also play a role, as the
carotid artery travels through them (McCutcheon et al. 2004). Horses showing
respiratory stridor will therefore be compromised by the reduced amount of airflow
through the nasal passages, thereby reducing the heat exchange by convection.
It was interesting to note that the prevalence differed at the different racecourses.
There is no obvious explanation for this, but pollution may play a role at the Vaal
racecourse, which has the highest incidence and is situated in an industrial area with
lots of pollution. Interestingly, we also appear to report a lot more abnormal
respiratory noises at the Vaal and this may play a role. There does not appear to be
any difference in prevalence when comparing horses racing on sand and turf at the
Vaal racecourse. The same period of racing at the Vaal was compared for both
tracks. This is different to the previous paper where there was a greater risk of PRDS
when racing on the sand. The larger case study may account for this as well as
comparing racing on the turf over the same period. It is interesting to note that the
prevalence of PRDS in Kimberley where there is only a sand track and the average
ambient temperature is higher than at the Vaal racecourse, is 0.17% which is lower
than the average of 0.23%. It would appear that the horses stabled in Kimberley
have adapted well to the higher temperatures experienced there and the extreme
heat is a “dry” heat with very little humidity. It has been reported that horses take 714 days to acclimatise to higher ambient temperatures (McCutcheon et al. 2004)
The longer the distance of the race, the higher the risk of PRDS. This was the same
finding as reported in the previous paper. This stands to reason, as the core
temperature will increase as long as the muscles are working. The increased
prevalence in males than females needs further evaluation. The role of testosterone
and the resultant increased muscle mass could be considered. The increased risk as
horses get older also requires further evaluation. It may be linked to the fact that
younger horses are not raced over the longer distances in the early part of their
careers.
The increased prevalence of PRDS the softer the tracks get when compared to firm
going can be attributed to the increase effort required as the going gets softer. This
can be compared to trying to run on the soft sand of the beach or running where the
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
29