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The salmon louse Lepeophtheirus salmonis (Caligidae) as a vector of Aeromonas salmonicida Novak, Colin William


The sea louse, Lepeophtheirus salmonis, has been hypothesized to be a vector of fish pathogens and previous studies have isolated viral and bacterial pathogens from L. salmonis parasitizing farmed salmon. To examine the potential transmission of A. salmonicida by preadult and adult L. salmonis via parasitism of Atlantic salmon (Salmo salar), an in vivo bacteria-parasite challenge model was tested. Two pathogen challenge trials were performed, in which sea lice from donor (Aeromonas salmonicida-injected) fish were allocated among recipient fish for 14 days. Three hypotheses were examined: (i.) L. salmonis can acquire A. salmonicida from donor fish via parasitism; (ii.) A. salmonicida-exposed sea lice can transmit the pathogen to recipient Atlantic salmon via parasitism and (iii.) L. salmonis and A. salmonicida infections can cause synergistic effects on host fish. Sea lice acquired A. salmonicida externally (Trial 1 and 2, 100%) and internally (Trial 1, 100%) from parasitizing donor fish. Trial 1 (~44g fish) demonstrated a successful transfer of bacteria from lice to salmon (mucus, 100%; kidney, 77.3%), with a decrease (t = 5.29, df = 6, p = 0.00186) in mean fish condition factor and 59.1% cumulative fish mortality. Conversely, there was no evidence of bacteria transfer, no fish mortality and no decrease in mean fish condition factor in Trial 2 (~155g fish). In addition, histological examination revealed widespread inflammatory responses in small salmon (~46 g) infected with A. salmonicida and sea lice. Thus, preadult and adult L. salmonis can acquire and transmit A. salmonicida to recipient fish via parasitism under experimental conditions. However, the following conditions of pathogen, environment and host are required: (i.) a large inoculum of A. salmonicida (10⁶ - 10⁷ colony forming units (CFU) mL-¹), (ii.) internal acquisition of bacteria by sea lice and, (iii.) young Atlantic salmon post-smolts (~44g) as hosts.

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