Defining the role of Caligus rogercresseyi in transmission and spreading of Piscirickettsia salmonis

The epidemiological role of the association of Piscirickettsia salmonis and Caligus rogercresseyi in infected salmonid fishes has not been clearly elucidated, which has led to speculation and insecurity regarding the design of efficient therapeutic strategies for dual infections. We therefore decided to assess if there is a functional association between transmission and/or spreading of the bacteria in dually infected fish under in vivo and in vitro conditions. This was done by kinetically analysing expression of the bacteria in the parasite from naturally as well as in induced infections. We observed that under any of the conditions assayed, biological expression of the bacteria could not be detected neither in any developmental stages of the parasite, nor in target organs of the dually infected fish. Thus, we suggest that Caligus rogercresseyi does not act as a biological vector for P. salmonis, neither as a mechanical vector, as in this study, P. salmonis was not able to remain on the lice for 1 h after becoming detached from the infected host. Notwithstanding, a transient association between the two agents was detected early in their encounters with the fish, a situation that might explain the ambiguities about their functional interactive roles in the literature.