Could Scavenging Be a New Transmission Route for Avian Cholera?

Avian cholera is an infectious disease of birds caused by the bacterium Pasteurella multocida. The current understanding of avian cholera in the marine environment is unclear, with transmission pathways similar to human cholera, such as consumption of contaminated water or food, or inhalation of bacteria-laden aerosols. However unlike human cases scavenging of infected carcasses may also be another important yet understudied avenue of transmission.

Observations by workers on vessels and oil platforms reported seabird mortality to the Canadian Wildlife Health Cooperative (CWHC) who initiated the study. Bird Carcasses were opportunistically collected off Eastern Canada around 300–400 km off the coast of the island of Newfoundland. Tissue samples from all birds were taken for bacteriologic analysis. Bacteria were then identified based on selective growth, colony morphology, Gram stain, and biochemical characteristics. Further histological tests were carried out on carcasses submitted individually and well-preserved carcasses from multiple specimen. Species were diagnosed as either having confirmed or probable avian cholera infections based on their decomposition state.

Various carcasses were found to be infected including a range of gull species and one raven. The first infected gull carcass was collected early January 2007, with continued mortality occurring until the final confirmed cases in mid-march 2007. In total 136 carcasses over 10 species were analysed for avian cholera, with 103/136 being scavenging gulls. P. multocida was isolated from 77 carcasses, 38 of which underwent comprehensive necropsy with 25/38 confirmed to have avian cholera. 24 of these confirmed infections were species of gulls. Along with the confirmed cases of Avian Cholera there were 60 probable infections with gull species making up 53 of them.

The outbreak studied by Wille et al. is the first known outbreak of avian cholera in a pelagic environment. This occurrence of avian cholera also seems to have been contained within scavenging populations, possibly indicating that scavenging individuals were important in transmission, perpetuation, and spread of the outbreak.

Evidence of scavenging based transmission was found during initial necropsies. A large majority of the confirmed cases had evidence of feather shafts found in their ventricular (i.e., gizzard) lumen. Analysing the population dynamics of the infected area, it became apparent that there was a clear bias towards the mortality of large scavenging gulls as despite large populations of colonial or water fowl there were no individuals diagnosed with avian cholera.

Understanding avian cholera outbreak dynamics is essential to understand the mechanisms that drive the spread of the disease, allowing us to formulate successful strategies to manage future outbreaks. This is especially important due to the heightened public health and food security concern that are linked with contagious disease, as local arctic communities often rely on marine species for food.

The complexities associated with tracking an outbreak became apparent throughout the study, with possible underestimations of the true extent of the mortalities, due to the isolated nature of the outbreak. Along with an inherent bias due to observation errors, a characteristic trait of outbreak investigation. This bias is covered by McClintock et al. 2010. To amend this bias and assess avian cholera dynamics adequately a more targeted approach is needed in future studies.

Edit: To make the results a bit cleared I reworded them here, if anybody is confused by the in-Text Results.

"In total there were 136 carcasses submitted, with 77 coming back positive with P. multocida. However, there were 25 confirmed cases and 60 probable cases, equaling 85 cases. This 85 cases is higher than the No. carcasses
with P. multocida because histology was not carried out on specimens due to carcass decomposition."

References:

Wille, M., McBurney, S., Robertson, G., Wilhelm, S., Blehert, D., Soos, C., Dunphy, R. and Whitney, H. (2016). A PELAGIC OUTBREAK OF AVIAN CHOLERA IN NORTH AMERICAN GULLS: SCAVENGING AS A PRIMARY MECHANISM FOR TRANSMISSION?. Journal of Wildlife Diseases, 52(4), pp.793-802.