Epidemiology and population structure of Campylobacter jejuni and related organisms in wild birds

Abstract:
Campylobacter jejuni is one of the most common causes to bacterial gastroenteritis in the industrialised world. Also other species of the Campylobacter genus give rise to human infections. C. jejuni occurs in many different animal hosts and can be isolated from water sources. This thesis deals with the occurrence and population structure of this bacterium and related organisms in wild birds. Using systematic sampling procedures I was able to investigate the occurrence of C. jejuni and other related bacteria in wild birds both at migrating and breeding times. I frequently obtained isolates of Campylobacter, but most isolates belonged to other species than C. jejuni. Carriage rates of Campylobacter spp. was highly variable among taxa and ecological guilds of birds, being more common in short-distance migrants that primarily forage on the ground. During the studies I also found several isolates of Helicobacter canadensis from Barnacle Geese Branta leucopsis. These isolates were the first obtained from a non-human host suggesting a zoonotic potential of this pathogen.



Using Multilocus Sequence Typing, I analysed the population structure of wild bird C. jejuni strains. I found strong genetic subdivision depending on host species for these strains among five major hosts (Mallard Anas platyrhynchos, Black-headed Gull Larus ridibundus, Dunlin Calidris alpina, Song Thrush Turdus philomelos and Blackbird Turdus merula). The structure was partly temporarily persistent between years and existed despite a rather low overall genetic variation. Initial diversification in a clonal complex was three times more likely to result from recombination than from mutation. Recombination occurred both between closely related genotypes and between more distant genotypes and the overall structure was concordant with an epidemic population structure.



Wild bird strains of C. jejuni were most often different (95% of isolates) to those known to be associated with disease in humans or characterised as carrier isolates in the poultry industry. There were, however, some shared genotypes between these sources, including sequence types of the clonal complexes most often associated with human gastroenteritis. An analysis of susceptibility of wild bird C. jejuni strains to ten antimicrobial compounds showed that only two of 137 isolates were resistant. These two, however, exhibited resistance to either ciprofloxacin or doxycyclin and likely originated from human-associated sources. My findings suggest that wild birds play a minor role in the dissemination of this zoonotic disease.