Fatal Streptococcus iniae Infection in a Juvenile Free-Ranging Short-Beaked Common Dolphin (Delphinus delphis)

SIMPLE SUMMARY: Streptococcus iniae (S. iniae) is a significant aquatic bacterial pathogen that has caused devastating losses to wild and cultured fish populations worldwide and is an important zoonotic pathogen in humans. Previously reported in captive dolphins only, this case report describes S. iniae associated infection and mortality in a free-ranging short-beaked common dolphin (Delphinus delphis). Unreported previously in wild marine mammals, its detection highlights a need for further investigation into the epidemiology of S. iniae to better understand the risks for wild marine mammal populations and potential transmission cycles between marine mammals, humans and wild and intensive fish populations. ABSTRACT: Streptococcus iniae (S. iniae) is a significant aquatic pathogen of farmed fish species, important zoonotic pathogen, and reported cause of disease in captive Amazon River dolphins (Inia geoffrensis) and a bottlenose dolphin (Tursiops truncatus). Here we report S. iniae as the cause of subcutaneous abscesses, sepsis and mortality in a juvenile free-ranging short-beaked common dolphin (Delphinus delphis) found deceased on a metropolitan Australian beach. Body surfaces were covered by multifocal, depressed, deep, irregular cutaneous ulcerations, which microscopically were characterised by ruptured subcutaneous abscesses with intralesional cocci. Routine microbiological investigations revealed a heavy growth of beta-haemolytic Streptococcus sp. identified as Streptococcus iniae in skin lesions as well as from heart blood, the latter supportive of sepsis. Tissues were negative for cetacean morbillivirus and no other disease processes were identified. S. iniae has not been reported in free-ranging marine mammals, nor in Australian delphinids, previously. More notably a pathogen of captive animals, this case report identifies S. iniae as a pathogen of wild dolphins also. In addition to expanding the host reservoir of a significant zoonotic pathogen, determining the source of infection as well as possible consequences for other marine mammals and wild and intensive fish stocks warrants further investigations.