EQUINE | Equine Disease Update
with which they have contact. Some carrier horses may
be recog¬nized by an intermittent unilateral nasal dis-
charge, cough, or have palpable swelling in the throat-
latch area below the larynx. Numbers of viable S. equi
in infected guttural pouches become very few and de-
tection of carrier horses generally requires direct endo-
scopic examination and sampling of the pouch.
Bacteriologic Culture: Nasal swabs and na-sopharyngeal
washes collected two to three days after onset of fever
in the acute phase of strangles and pus from abscesses
usually contain abundant S. equi. The characteristic wa-
tery colonies are easy to recognize on appropriate selec-
tive culture media within 18 hours of incubation. Sugar
fermentation assays can then be completed in three
hours to confirm identity. The wide availability, low cost,
and diagnostic certainty provided by demonstra¬tion
of the pathogen argue strongly for inclusion of culture
in strangles outbreak diagnosis. Ideally, three to five
horses in a nascent outbreak should be cultured to es-
tablish presence of the pathogen and mitigate effects
of poor sample quality. In contrast to its value in acute
phase diagnosis, culture has low sensitivity in detection
of chronic carrier horses. This is explained by massive
die-off of S. equi in pus-filled guttural pouches in combi-
nation with infrequent drainage into the nasopharynx.
Polymerase Chain Reaction (PCR): A variety of formats
and gene targets based on PCR have been shown to be
at least three times more sensitive than culture in detec-
tion of S. equi in diagnostic samples from the nasophar-
ynx and guttural pouch. PCR will detect DNA of S. equi
in numbers too few to be detectable by culture and is
effective in the presence of background contaminants.
However, in addition to cost and limited local availability,
a positive PCR reaction is not proof of presence of viable
S. equi and hence there is risk of false positive reactions.
Also, PCR is vulnerable to accidental contamination dur-
ing collection and in the laboratory. Nevertheless, PCR is
by far the most sensitive diagnostic aid in detection of
pos¬sible guttural pouch carrier horses.
Detection of Serum Antibody: S. equi specific antibody
responses are detectable in serum two to three weeks
following exposure, persist at high levels in most horses
for 10-12 weeks, and—with the exception of SeM anti-
bodies—decline to near baseline by 30 weeks. Ideally,
antibody responses to two or three proteins of S. equi
should be mea¬sured in combination for greatest sensi-
tivity and allow for differences in responses of individual
horses. A positive level of antibody may indicate infec-
tion or vaccination within the previous six months or
possibility of persistent guttural pouch carriage. Serol-
ogy is especially helpful in diagnosis of occult (bastard)
strangles abscesses and S. equi associated immune me-
diated vasculitis (purpura). Affected horses usually have
very high antibody levels to S. equi proteins. Serology
is also helpful in deciding whether to vaccinate. Horses
with a preexisting positive level of antibody are likely to
have protective immunity and a few of these will be at
risk of developing purpura if vaccinated.
Contact:
John F.Timoney, DSc, MVB, MS, PhD
[email protected]
(859)218-1106
Maxwell H. Gluck Equine Research Center University of
Kentucky Lexington, KY
Biosecurity at Equine Events
A disease-related “perfect storm” occurs when risk fac-
tors and a pathogen successfully in¬teract, resulting in
the introduction and spread of an infectious organism
among a susceptible population. In the world of equine
events, a perfect storm is plausible if:
1. Susceptible, stressed horses are expos ed to an in-
fectious disease agent.
2. The conditions and environment at the event sup-
port transmission and infection.
• Volume 19 no 3 • September 2017 •
21