Equine Health Update EHU Vol 19 Issue 3 - Page 21

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 jtimoney@uky.edu (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