Streptococcus agalactiae or group B streptococcus (GBS) is a significant bacterial pathogen of animals, largely known for causing mastitis in cows, but also commonly found in the human body (this is termed colonization). A type of gram-positive streptococcal bacteria, it is a frequent colonizer of the bowel, vagina, rectum, bladder or throat and has been found to be a major cause of neonatal infectious disease and an emerging cause of disease in non-pregnant adults. This type of bacteria (which shouldn’t be confused with group A streptococcus, which causes strep throat) doesn’t just impact humans – it also causes a variety of conditions in horses, camels, dogs, cats, monkeys, hamsters, mice, rabbits, lizards, and even fish which can possibly compromise food security and poses a zoonotic hazard. In fact, aquaculture losses from S. agalactiae have been estimated at US$10 million in the USA with US$100 million globally.
Though little is known about bacterial diseases of sharks, pathogenic bacterial infections are not uncommon. However, the number of shark streptococcosis cases are extremely limited. The first streptococcal infection in a cartilaginous fish was reported in 1992, where a long-term captive juvenile female nurse shark (Ginglymostoma cirratum) in the USA was diagnosed with Lancefield Group B Streptococcus septicemia. The mortalities caused by S. agalactiae involving five different species of wild and captive rays were also described in Australia (2009–2010).
Enter the sandbar shark (Carcharhinus plumbeus). A wide-ranging coastal species commonly found in tropical and temperate regions around the world, they are a robust grey-brown to bronzy shark with a very large first dorsal fin. Taken in commercial and artisanal fisheries throughout their range, sandbar sharks are heavily exploited for their fins in some regions. In July 2018, a female sandbar shark was observed moribund on the Netanya shoreline, dying a few hours later off the city in the Northern Central District of Israel. Despite being a common species, many mysteries still surround this species. This was the perfect time for scientists at the Morris Kahn Marine Research Station, of the University of Haifa, to learn more about the prevalence of diseases within the species.
The team took the carcass back to the laboratory, where they measured it and carried out a post-mortem examination. “As a part of our conservation efforts, we are trying to perform post-mortem examinations on every deceased shark that is stranded along the Israeli Mediterranean shore,” explained scientist Dr. Danny Morick of MKMRS (University of Haifa). “We had first assumed the shark was fished and unsuccessfully released back to the sea and died. But during the necropsy, no external signs of interaction with fishing nets or gear were observed.” For bacteriological examination, tissue samples of liver, spleen, kidney, uterine fluid, and brain were aseptically collected. “Surprisingly, in the initial stage of necropsy it was clear that something serious happened to this animal. The animal was skinny and gross pathological signs were observed both in the genital system and in the heart of the shark. We suspected it was a systemic bacterial infection and took many swabs from different organs. These samples were sent to the Kimron Veterinary Institute for further identification and characterization of the pathogen.”
The probable cause of death for this female sandbar shark? Severe meningoencephalitis. Meningoencephalitis in sharks isn’t new; meningoencephalitis caused by Carnobacterium maltaromaticum-like bacteria was reported in juvenile salmon sharks (Lamna ditropis) that stranded themselves along California’s shorelines between 2002 and 2007. While this seems drastically different from Streptococcus agalactiae, it really isn’t! C. maltaromaticum-like bacteria is related to the S. agalactiae that was detected in this new study because the Carnobacteriaceae family are in the same order of Streptococcaceae family (Lactobacillales). “Our investigation revealed a bacterial multi-systemic infection caused by S. agalactiae in a female sandbar shark,” said Morick. “Streptococcus agalactiae is well known as a primary pathogen of both homoeothermic and poikilothermic organisms, yet to the best of our knowledge, this is the first description of a fatal Streptococcosis in sandbar sharks. The source of the infection remains unclear. Possible routes of infection and transmission for the S. agalactiae infection in wild sharks may include horizontal infection through waterborne exposure or oral transmission through the ingestion of contaminated prey. Although the sandbar shark is one of the most common shark species in the Mediterranean Sea, the available knowledge regarding its health status and its susceptibility to pathogens are sparse and, therefore, this study broadens our knowledge regarding the health status of elasmobranches in this area. As S. agalactiae is a zoonotic agent, and this bacterium can be transmitted from fish to humans, fishermen and researchers working with sharks should be aware of the transmission potential of the disease.”
The ecological impacts of this disease on marine ecosystems are unknown. The researchers stress that more epidemiological studies are needed to provide insight into the likelihood of inter-species transmission of strains that are associated with fish, marine mammals and humans.