Here is another equine update from guest blogger, Dr. John Prescott of the University of Guelph.
Research presented at the Ninth International Equine Infectious Disease conference last week in Lexington, Kentucky, highlighted the dramatic impact that the latest inexpensive genome sequencing techniques are having on understanding microbial disease.
This is well illustrated by an epidemic of S. zooepidemicus upper respiratory tract infection in horses that occurred in Iceland in 2010, described by lead author Sigrídur Björnsdóttir of the Icelandic Food and Veterinary Authority. The infection itself was relatively mild but lasted about 4 weeks, and over the course of the summer affected a large proportion of Iceland’s horses. It caused a headline-stopping movement and export of horses. The disease presented as a laryngitis and persistent dry cough, with a serous [watery] nasal discharge. The infection started at a facility with a water treadmill, and was rapidly disseminated by horses moving from there to 18 other centres across the country, with a high transmission rate to horses within these stables. Since S. zooepidemicus was isolated from the nasal exudate only as the disease progressed, veterinarians investigating the outbreak thought at first that these isolates were opportunistic or secondary invaders, layered on top of an unknown virus infection.
This is where bacteriologists Andrew Waller and Carl Robinson from the United Kingdom’s Animal Health Trust in Newmarket and Matthew Holden from the Sanger Centre in Cambridge became involved. Incredibly, these researchers sequenced the genomes of 290 isolates from the outbreak as well as from an earlier national collection. They used this information to find, to their astonishment, that S. zooepidemicus Sequence Type 209 was responsible for the outbreak. This strain was isolated from the affected farms all over the country, as well as from a case of miscarriage in a person. It could clearly be linked epidemiologically to the outbreak; the date that the infection started was pinpointed to within 5 days. The epidemic occurred so quickly that the genomes of this strain showed almost trivial variation compared to S. zooepidemicus strains that were more established in Icelandic horses.
This is the best description ever of the impact of introduction of a more virulent S. zooepidemicus into a naïve horse population, and will change forever the way that equine veterinarians will think about this underrated pathogen. It illustrates the enormous power of genome sequencing in bar-coding the bad guy.
Andrew Waller also gave a really interesting talk about the diversity of S. zooepidemicus. Sequencing and “sequence typing”, based on multilocus sequence typing (MLST), has identified over 300 sequence types of this species. What is emerging from this is the recognition that difference types cluster with different diseases. For example, strains (sequence types) of S. zooepidemicus that cause abscessation of pharyngeal lymph nodes (“mild strangles”) belong to sequence types that have acquired a bacterial virus carrying a superantigen gene. These are, of course, outclassed by the true strangles organism, S. zooepidemicus subspecies equi, which possesses four bacteriophages with these virulence genes, as well as other nasty characteristics.
Andrew Waller told me that it cost him about $35 to sequence an entire streptococcal genome, which for Canadian veterinarians is now about the cost [Weese comment: or much less than the cost] of sending a swab to a diagnostic lab for culture and sensitivity testing. However, don’t start ringing up the lab to ask for a genome sequence just yet, since the real cost is for the analysis, which is still labour intensive. However, it points the way to the future, which has clearly now arrived, and is changing the way we think about S. zooepidemicus and infection in the horse.