Which of the following is a primary transmission route for the emerging fish pathogen Streptococcus?

The primary transmission route for the emerging fish pathogen Streptococcus is through horizontal transmission, which occurs via ingestion and contact. This means that the bacteria can spread from one fish to another through contaminated water or by direct physical interactions among fish. This method of transmission is especially significant in aquaculture, where fish are often in close proximity to each other, facilitating the spread of the pathogen.

In contrast, vertical transmission from eggs typically involves pathogens being passed from parent to offspring, which is not the primary route for Streptococcus. Additionally, while waterborne transmission can occur in various pathogens, Streptococcus is primarily known for horizontal transmission rather than being limited to waterborne methods. Lastly, airborne transmission is not relevant to Streptococcus in aquatic environments, especially since most fish respiratory pathogens spread through water rather than air, making this option unlikely.