Evidence of vertical transmission and tissue tropism of Streptococcosis from naturally infected red tilapia (Oreochromis spp.)

• The first evidence demonstrating parents-to-offspring vertical transmission of streptococcosis in tilapia.
• Adult fish that are apparently healthy and even without showing any clinical symptoms of streptococcosis could be asymptotic carriers of the pathogen.
• Necessity to improvise management techniques and strategies for controlling streptococcosis in tilapia.

Streptococcosis is a highly problematic disease in the aquaculture of freshwater fishes, especially for tilapia. The possibility of vertical transmission of streptococcosis and the pattern of tissue tropism of this pathogen in various organs was examined in red tilapia (Oreochromis sp.). Healthy broodstock without any clinical signs of Streptococcus spp. were selected from a farm earlier reported to have the disease and a total of 10 pairs were forced spawned to provide samples of gametes and progeny for pathogen testing. A colorimetric LAMP assay was used to confirm whether the bacterial pathogens Streptococcus. agalactiae and Streptococcus. iniae was present in samples of milt, unfertilized eggs, fertilized eggs, and offspring at various stages of development, as well as internal organs of broodstock (reproductive organs, gill, liver, spleen, kidney and brain) as well as samples of water from culture systems. The majority of samples of milt (9/10) and unfertilized eggs (7/10) collected from the broodstock were infected with S. iniae at the time of spawning and was transmitted to all of their offspring. Nevertheless, when the same samples of gametes were analyzed for S. agalactiae, they were all found to be negative but the pathogen was found to be present in some 10-day-old larval offspring (4/10). However, when the pathogenic presence was analyzed from the reproductive organs of the parents, both S. agalactiae (11/20) and S. iniae (18/20) bacterium were common. Although, all broodstock were asymptomatic, almost all broodstock harboured the bacteria in many organs. Confirmation of vertical transmission of streptococcosis in tilapia means that intergenerational break cannot be used as a reliable and simple means of reducing or eliminating the prevalence of these difficult pathogens in aquaculture stock.