EQUINE | URT endoscopy
The results of this study are, surprisingly, most
comparable to older racehorses within South Africa
(Saulez & Gummow 2009:431-435) with the only major
difference being the percentage of grade 2 horses
(14.49% compared to 1.8%) (Saulez & Gummow
2009:431-435). The reason for this may be that as horses
age, the wastage rate for horses graded 2 to 4 would
be high, with horses being retired or sent to stud. This
leads to the assumption that horses with higher grades,
still racing, are able to perform better irrespective of
their RLN grade. The current study is mostly similar
in findings to an Australian study of competing races
horses with the grade 2 RLN being the most dissimilar
category (14.49% grade 2 to 1.3% for combined grade 2
and 3 [Brown, et al. 2005:397-401]). The values for grade
4 for the New Zealand and North American studies of
yearlings and young horses are fairly comparable to
each other (0.71% vs 1%, 0% and 3% (Anderson, et al.
1997:188-192, Stick, et al. 2001:962–967, Garrett et al.
2010:669–673) however the other grades vary greatly.
The reasons for these discrepancies may be
multifactorial. Although the most similar studies to the
current yearling studies were in adult horses, this may
be due to a similar definition of RLN used by the studies.
Examination at a different life stage may play a part
since laryngeal hemiplegia is progressive (Embertson
2004:42-44). This may account for the larger grade 2
groups in other older populations studied. There is also
debate over the repeatability of resting endoscopy,
suggesting that there is moderate variation in repetitive
grading which may also influence the results of different
groups and that single day grading should be cautiously
interpreted (Perkins et al. 2009:342-346; Pollock et al.
2009:354-360).
In the South African context, there has been a large push
from both race horse veterinarians as well as stud farms
to perform endoscopic examinations (often multiple
per horse) before a yearling is put up for public auction,
particularly at the more prestigious sales. This is both an
effort to prevent horses with RLN deficits from entering
the auction to reduce the loss, and disappointment to
both the stud farm and the potential buyer on auction
day. The current study was performed at the two most
prestigious yearling sales in South Africa. So, while this
study compares well with others in older horses, this may
be because horses with RLN deficits have been removed
from the population group before examination at these
yearling sales. This would mean that the current study is
only relevant to the sales population and may not be the
same in the general racing population as horses in the
general racing population may have bypassed the sales
altogether or been obtained on other sales. The similar
prevalence to the Saulez & Gummow (2009:431-435)
study, however, suggests that it would hold true for the
older racing population in South Africa.
This study was limited due to the nature of the yearling
sale in that the horses presented for endoscopy may be
more valuable horses and horses not presented for postsale
endoscopy were considered ‘cheaper’ by the buyer
and not worth the examination even though these
were prestigious sales. Horses that failed endoscopic
examination before the sale would not present at the
sale. Therefore, this sample may not be a representative
sample of the Thoroughbred racehorse population in
South Africa. The large variation of grade 2 horses from
year to year may be due to the sample size for in each
year with 2013 having only 13.71 % of the horses offered
for purchase being examined versus 2019 where 26.85
% of the horses offered for purchase were examined.
The larger sample size in 2019 may have been a more
representative sample than 2013.
It is ultimately important for both buyers and sellers
of yearlings to be aware that a single endoscopic
examination may give a skewed impression of a
yearling’s RLN and serial examinations are more likely
28 • Equine Health Update •