New insight in the epidemiology of avian botulism outbreaks: necrophagous flies as vectors of Clostridium botulinum type C/D.

Avian botulism outbreaks spread through the bird carcass-maggot cycle, in which Clostridium botulinum and blowflies interact to ensure their reproduction in a mutualistic relationship where neurotoxin/spore-bearing maggot is one of the keystones. Here we investigated the hypothesis that adult blowflies may also play a significant role in botulism outbreaks by carrying C. botulinum cells between carcasses. We carried out a field experiment placing bird carcasses free of C. botulinum type C/D in containers only accessible to necrophagous flying insects in wetlands where avian botulism outbreaks were occurring and in control sites. Additionally, we performed laboratory trials to evaluate if blowflies may carry C. botulinum type C/D and for how long. Maggots bearing C. botulinum type C/D developed in 27.5% of carcasses placed in wetlands during botulism outbreaks. Calliphoridae flies in laboratory trials were able to transfer C. botulinum between two points and excreted it in their spots for up to 24 h after an infective feeding. Our results confirm that adult necrophagous flies play a role in the spreading of botulism outbreaks, which have implications in the epidemiology of this disease.