Francisella orientalis DNA detected in feral tilapia populations in Hawai'i

Francisella orientalis is a bacterial pathogen that is responsible for substantial mortalities in warmwater fish, such as tilapia, and has negatively impacted the aquaculture industry globally. Starting in the mid-1990s, periodic F. orientalis outbreaks in Hawai'i led to severe mortalities in cultured and feral tilapia populations on Oahu. In an attempt to limit the outbreak's impact on tilapia aquaculture in Hawai'i, the Hawai'i Department of Agriculture Plant Quarantine Division issued a moratorium on the export of tilapia from Oahu to the other Hawaiian Islands. Despite reported high incidences of F. orientalis from cultured tilapia on Oahu and from around the world, the moratorium continues to remain in effect to this day.

To assess the prevalence of F. orientalis in feral tilapia populations across the Hawaiian Islands, tilapia were collected from streams on Kauai, Oahu, Maui, Moloka'i, and the Big Island (Hawai'i) and were screened using a combination of molecular, bacteriological, and histological techniques.

Although signs of infection (i.e., granulomas) were observed in fish on all five islands, molecular screening using quantitative polymerase chain reaction only detected the presence of F. orientalis on the islands of Oahu, Maui, and Kauai.

These findings suggest that F. orientalis is prevalent in feral tilapia populations across the Hawaiian Islands.