The bacterium that causes strangles in horses owes its long-term survival to one important factor, and researchers are keen to find out why.
Streptococcus equi subsp. equi (SEE) is a host-restricted bacterium that causes the common infectious upper respiratory disease known as strangles.
SEE does not live long in the environment, nor can it infect other host mammals or vectors, and infection results in short-lived immunity.
Its survival appears attributable to its ability to persist in symptom-free carrier horses — what the researchers call inapparent carriage.
Ellen Ruth Morris and her fellow researchers, writing in the journal PLOS ONE, said it remains poorly understood whether pathogen-related factors enable SEE to remain in horses without causing clinical signs.
The study team used next-generation molecular-based sequencing technologies to characterize the genome, methylome, and transcriptome of isolates of SEE from horses with acute clinical strangles and inapparent carrier horses — including isolates recovered from individual horses sampled repeatedly.
The researchers wanted to identify pathogen-associated changes that might reflect specific adaptions of SEE to the host that contribute to the bacterium being carried in the symptom-free horses.
The researchers used isolates from outbreaks in two different continents — American samples from an outbreak in Pennsylvania, and European samples from cases in Sweden among Icelandic horses.
The researchers found no significant or consistent differences between acute clinical and inapparent carrier isolates of SEE.
The findings, they said, indicate that adaptations of the SEE to the host are unlikely to explain the carrier state of the infection.
“Although genomic differences were observed between the two geographical regions, no changes in the genome, methylome, or transcriptome were identified that could be interpreted as reflecting a consistent mechanism of adaptation of SEE to the host resulting in inapparent carriage.
“These findings indicate that host-associated differences are a more likely explanation of the bacterium’s ability to persist in horses without resulting in either clinical signs or a robust immune response (i.e., the presentation of clinical disease).
“Efforts to understand the carrier state of SEE should instead focus on host factors.”
They said further evaluation of host immune responses to SEE is warranted to learn how to identify and eliminate chronic carriers in a bid to prevent the spread of the disease.
The study team comprised Morris, Ashley Boyle, Andrew Hillhouse, Ivan Ivanov, Angela Bordin and Noah Cohen, all with Texas A&M University; Miia Riihimäki, Anna Aspán and John Pringle, with the Swedish University of Agricultural Sciences; and Eman Anis, with the University of Pennsylvania.
Morris ERA, Boyle AG, Riihimäki M, Aspán A, Anis E, Hillhouse AE, et al. (2021) Differences in the genome, methylome, and transcriptome do not differentiate isolates of Streptococcus equi subsp. equi from horses with acute clinical signs from isolates of inapparent carriers. PLoS ONE 16(6): e0252804. https://doi.org/10.1371/journal.pone.0252804