Walking On Volume 5, Issue 7, July 2018 | Page 6

For the Health of It Laboratory Diagnosis of Strangles Reprinted with permission from the July 2017 issue of Equine Disease Quarterly Highly contagious equine strangles is transmit­ted by inhalation or ingestion of Streptococcus equi originat- ing from discharges of the nose or abscess of an infected horse. Nasal shedding begins approximately 4-16 days afrer initial infection and continues for two to three weeks in most horses. However, survival of the organ- ism in pus located in the guttural pouch may continue for months or years and be associated with periodic escape of the organism through the nasal passages. Per- sistent carrier animals may therefore serve as long-term sources of infection for naive, susceptible horses with which they have contact. Some carrier horses may be recog­nized by an intermittent unilateral nasal discharge, cough, or have palpable swelling in the throatlatch area below the larynx. Numbers of viable S. equi in infect- ed guttural pouches become very few and detection of carrier horses generally requires direct endoscopic examination and sampling of the pouch. Bacteriologic Culture: Nasal swabs and nasopha- ryngeal 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 char- acteristic watery colonies are easy to recognize on appropriate selective 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 demonstra­tion 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 establish presence of the pathogen and 6 • Walking On mitigate effects of poor sample quality. In contrast to its value in acute phase diagnosis, culture has low sen- sitivity in detection of chronic carrier horses. This is explained by massive die-off of S. equi in pus-filled gut- tural pouches in combination with infrequent drainage into the nasopharynx. Polymerase Chain Reaction (PCR): A variety of for- mats and gene targets based on PCR have been shown to be at least three times more sensitive than culture in detection of S. equi in diagnostic samples from the nasopharynx 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 during collection and in the laboratory. Nevertheless, PCR is by far the most sensitive diagnos- tic aid in detection of pos­sible guttural pouch carrier horses. Detection of Serum Antibody: S. equi specific an- tibody 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 antibodies-decline to near baseline by 30 weeks. Ideally, antibody responses to two or three proteins of S. equi should be mea­sured in combination for greatest sen- sitivity and allow for differences in responses of indi- vidual horses. A positive level of antibody may indicate infection or vaccination within the previous six months or possibility of persistent guttural pouch carriage. Se- rology is especially helpful in diagnosis of occult (bas- tard) strangles abscesses and S. equi associated immune mediated vasculitis (purpura). Affected horses usually have very high antibody levels to S. equi proteins. Se- rology 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. Ti money, DSc, MVB, MS, PhD [email protected] | (859) 218-1106 Maxwell H. Gluck Equine Research Center University of Kent ucky, Lexington, KY