Mosquito larvae (Aedes aegypti), Francisella orientalis and tilapia (Oreochromis sp.): Relationship of host-pathogen-carrier in disease transmission

Abstract

Piscine francisellosis, caused by the bacterium Francisella orientalis, poses a significant threat to various warm water fish. The coexistence of mosquito larvae, bacteria, and fish in aquatic ecosystems suggests the potential for interconnected pathogen/disease transmission. While mosquito larvae have been recognized as carriers of Francisella spp. in humans diseases, their role in transmitting francisellosis to aquatic animals remained unexplored. This study investigates whether mosquito larvae (Aedes aegypti) can uptake F. orientalis and transmit the disease to hybrid red tilapia (Oreochromis sp.). To assess uptake, naive mosquito larvae were exposed to F. orientalis-contaminated water or to experimentally infected tilapia, and specific PCR and culture techniques were employed to monitor F. orientalis. Rearing conditions at 25 °C and 30 °C were also examined to explore temperature influences. Results confirmed the presence of F. orientalis in larvae from both contaminated water and infected fish experiments. To assess their potential to transmit the disease, F. orientalis-exposed mosquito larvae were fed to naive tilapia. The challenged tilapia developed typical granulomas in the kidney and spleen after ingesting the F. orientalis-harboring mosquito larvae. Histopathological changes were more pronounced in fish kept at 25 °C than those at 30 °C. Corresponding with histopathology, all fish (n = 10) cultured at 25 °C were positive for F. orientalis-specific PCR assays, compared to 7 out of 10 samples at 30 °C. This study reveals, for the first time, that mosquito larvae can uptake F. orientalis and act as mechanical carriers for disease transmission in tilapia. Preventive measures such as mosquito control and water treatment might aid in eliminating larvae and contaminated bacteria, and reducing infection risks of piscine francisellosis. Additionally, this suggests that mosquito larvae could serve as a model for further studying the transmission of other aquatic animal diseases.