Researchers Develop A Challenge Model For Strep Zoo, By Ed Metzger from South West Ontario Veterinary Services

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Streptococcus equi subspecies zooepidemicus (Strep zoo) is most commonly identified as a commensal bacterium of horses. Strep zoo was first reported as a severe clinical disease in swine in Canada in the spring of 2019.  This Manitoba case occurred in breeding herds and resulted in up to 40% sow and gilt mortality. At about the same time that the Manitoba case was unfolding, the United States Department of Agriculture (USDA) reported that a sow processing plant in Tennessee had experienced an outbreak of 40% sow mortality in lairage. Ultimately the initial sites in Canada and the US were linked through cull sow movements. Strep zoo has continued to spread to a number of states in the USA including Pennsylvania , Ohio and Indiana.  In December 2020, the first case of Strep. zoo in swine in Ontario was isolated at the Animal Health Laboratory, University of Guelph. The Ontario case was associated with septicemia and elevated mortality in sows. Strep zoo continues to be present in Ontario.

In the early stages of the emergence of Strep zoo in North America, the Strep zoo isolates were genetically similar to previously published S. zooepidemicus genomes and were 99.7% similar to strain S. zooepidemicus ATCC35246. This particular ST-194 strain was reported as being isolated from a pig during the 1976 outbreak in China. The outbreak in China was reported to have been associated with the loss of more than 300,000 pigs. This Strep zoo strain carries 11 different antimicrobial resistance genes as well as at least 5 known virulence factors. There have been other problematic strains of Strep zoo detected in the USA and these are not exactly the same as the historical Chinese and Indonesian strains.

These US researchers wanted to generate a disease model for Strep zoo infection that would facilitate the evaluation of  the susceptibility of healthy, conventionally raised pigs to various Strep zoo isolates. The researchers challenged sows and five-month-old pigs with an isolate from a 2019 Strep zoo mortality event. Pigs were challenged with a genetically similar guinea pig isolate or a genetically distinct horse isolate to evaluate comparative virulence. The researchers found the following

  • The 2019 swine isolate caused severe systemic disease in challenged pigs with 100 % mortality.
  • Disease manifestation in sows was similar to field reports of lethargy/depression, fever, reluctance to rise, and high mortality.
  • The guinea pig isolate also caused severe systemic disease but most of the five-month-old pigs infected with the guinea pig isolate recovered.
  • The horse Strep zoo isolate did not cause any disease and was readily cleared from the respiratory tract of the challenged pigs. This was not unexpected.

It is important to note that Strep. zoo does have zoonotic potential. The risk of zoonotic infection is quite low but it is not zero. We know that Strep zoo can be transferred from animals to humans via aerosol droplets, direct contact with infected animals or through the consumption of unpasteurized milk products. Humans may develop pharyngitis, kidney disease, skin and soft tissue infection, toxic shock syndrome,  infectious arthritis, and death. From 1982 to 1986 there were several cases of septicemia in Hong Kong that were linked to Strep zoo. Several of these cases appeared to be related to pork consumption and the Strep zoo isolates from human cases were identical to isolates taken from septicaemic pigs detected in abattoirs in the same district. It should be noted, however,  that it is not uncommon for Asians to snack on raw uncooked pork and this could explain this particular outbreak. It is unlikely that Strep zoo would survive in pork after proper cooking.

Take Home Messages

  • This research has resulted in the development of a challenge model that was able to replicate the clinical signs that were reported in the field.
  • The model was able to demonstrate the differences in virulence between various Strep zoo isolates. The Strep zoo isolate from the 2019 North American out break caused the most significant disease.
  • Strep zoo continues to be relatively rare but because of the severity of disease and the potential for transmission to humans this organism will continue to be watched very closely.

Ref:  Samantha J Hau , Kristina Lantz , Keira L Stuart , Panchan Sitthicharoenchai , Nubia Macedo , Rachel J Derscheid , Eric R Burrough , Suelee Robbe-Austerman , Susan L Brockmeier   Replication of Streptococcus equi subspecies zooepidemicus infection in swine Vet Microbiol . 2021 Nov 2;264:109271. doi: 10.1016/j.vetmic.2021.109271. Online ahead of print.