The first ever case of zoonotic fish-associated S. dysgalactiae subspecies dysgalactiae (SDSD) infection was isolated in our lab in 2005. This was during a period when we were actively looking out for cases of Streptococcus iniae, another fish-associated zoonosis (see an earlier post).
We had isolated a Streptococcus from the blood culture of a breast cancer mastectomy patient with cellulitis ascending her upper limb to the chest following a puncture injury of her index finger on the same side. She had been cleaning fish (Red Tilapia) and shrimps at the time. This is a typical setting for S. iniae infection. However, the isolate was alpha-haemolytic, PYR negative, and Lancefield group C. So this was not S. iniae which is beta-haemolytic, PYR positive, and not groupable, The API 20 STREP had identified it as S. dysgalactiae subs. dysgalactiae/equisimilis which is entirely in keeping with a large colony Group C Streptococcus. So nothing special here-or perhaps…?
After getting over my initial disappointment, I read that Streptococcus dysgalactiae (SD) comprise 2 distinct subspecies. The more commonly encountered Streptococcus dysgalactiae subspecies equisimilis (SDSE) are large colony β-hemolytic Group C and Group G streptococci that cause pharyngitis, skin and soft tissue infections in humans, much like Group A Streptococcus pyogenes.
On the other hand, SDSD are classically described as α-hemolytic Group C Streptococci that cause disease in animals, especially cattle. And just recently, SDSD had also been found to cause disease in farmed fish…SDSD are not mentioned in textbooks of human clinical microbiology. So we were potentially sitting on the first case of zoonotic SDSD infection!
To cut a long story short we eventually were able to prove our isolate was indeed a fish associated SDSD by comparing pathogenicity gene sequences with those of fish isolates (Koh TH, 2009).
DB49998/05 the first case of zoonotic SDSD infection.
We have since picked up a few other SDSD isolates.
DB60705/15 was an alpha-haemolytic Group G Streptococcus isolated in 2015 from blood culture of a patient with right breast cancer and lymphedema of the arm on the same side. This patient was discharged before we could interview her but Providencia stuartii, a known fish pathogen, was isolated from the same blood culture. Interestingly, this isolate also had the same multi-locus sequence analysis (MLSA) and emm type profile as DB4998/05.
STREP 97/15 was isolated from a sample of fish meat (Red Tilapia) purchased from a supermarket. This was during a large outbreak of Group B Streptococcus (GBS) in non-pregnant adults in Singapore (see https://10minus6cosm.tumblr.com/post/132590510556/serious-group-b-streptococcal-infection-in-adults), and we were investigating fish meat as a source of GBS. I was intrigued to find a beta-haemolytic (usually a sign of pathogenicity) Group G Streptococcus in the fish meat, and even more surprised to find it was SDSD (remember they are supposed to be alpha-haemolytic). However I subsequently found an article that showed that other researchers were also finding strains of SDSD that were beta-haemolytic (Jensen, 2012). Of course finding SDSD in a sample of fish meat does not prove that it was originally in the fish as we cannot exclude contamination by human handing of this fish. However when we did the molecular analysis, the pathogenicity gene profile, and alignment of gene sequences did indeed cause DB49998/05, DB60705/15, and STREP 97/15 to cluster with fish rather than human isolates.
DB53393/17 was a beta-haemolytic SDSD (non-groupable because of auto-agglutination) isolated in 2017 from a patient who had a mastectomy for breast cancer and presented with upper limb oedema and cellulitis on the same side. This time I had a chance to speak to the patient but try as I might I could not elicit a history of fish contact from her. She was a food stall owner who only handled chicken meat. As far as I could tell from my review of the literature till then, SDSD did not infect chickens. However when we did the molecular analysis, not only did DB53393/17 not cluster with human isolates, it was also closely related to an isolate that had been isolated from a chicken! In the late 1980s-early 1990s, there had been an investigation of focal dermatitis and cellulitis in broiler chickens in Canada that yielded beta-haemolytic SD. This article was easy to miss because SD were not explicitly mentioned in the title.
The genomes of these zoonotic SDSD have all been sequenced and it is clear that they form a separate cluster similar to animal isolates and distinct from human isolates (Koh TH, 2019).
Clustering of human and animal isolates of SD based on tuf (elongation factor Tu) gene sequences. The length of the horizontal lines is in proportion to the differences between sequences (ie a longer horizontal line indicates sequences that are more dissimilar).
So summing up.
1. SDSD is a zoonotic pathogen.
2. While it is likely to be rare, it is possible that SDSD is under-recognised in clinical microbiology labs as it can appear beta-haemolytic like the human pathogen SDSE. Phenotypic methods including MALDI-TOF are not good at discriminating SDSD from SDSE.
3. Women who have mastectomy and lymph node clearance for breast cancer are particularly susceptible to ascending infection and sepsis, and need to take care to avoid penetrating injuries when handling meat or fish.
Koh TH, Sng LH, Yuen SM, Thomas CK, Tan PL, Tan SH, Wong NS. Streptococcal cellulitis following preparation of fresh raw seafood. Zoonoses Public Health. 2009 May;56(4):206-8. (no free access)
Jensen A, Kilian M. Delineation of Streptococcus dysgalactiae, its subspecies,
and its clinical and phylogenetic relationship to Streptococcus pyogenes. J Clin
Microbiol. 2012 Jan;50(1):113-26. (free access)
Messier S, Quessy S, Robinson Y, Devriese LA, Hommez J, Fairbrother JM. Focal dermatitis and cellulitis in broiler chickens: bacteriological and pathological findings. Avian Dis. 1993 Jul-Sep;37(3):839-44. (no free access)
Koh TH, Binte Abdul Rahman N, Sessions OM. Comparative genomic analysis of Streptococcus dysgalactiae subspecies dysgalactiae, an occasional cause of
zoonotic infection. Pathology. 2019 Dec 16. pii: S0031-3025(19)30432-5. doi:
10.1016/j.pathol.2019.09.016. [Epub ahead of print] PubMed PMID: 31859005. (no free access)