Evidence of vector borne transmission of Salmonella enterica subsp. enterica ser. Gallinarum and fowl typhoid mediated by the poultry red mite, Dermanyssus gallinae (De Geer, 1778)

Background: The poultry red mite Dermanyssus gallinae (De Geer 1778) is a major ectoparasite of poultry. Infestations are found in most laying hen farms in Europe, and breeder flocks have also been reported to be affected. Mite infestation has detrimental effects on animal welfare, are a cause of significant economic losses, and D. gallinae is often considered as a pathogen vector. Despite suspicion of a close relationship between the poultry red mite and Salmonella enterica subsp. enterica ser. Gallinarum biovar Gallinarum ( S. Gallinarum), the causative agent of fowl typhoid (FT), there has been no definitive proof of mite-mediated transmission. Therefore, an investigation was conducted to determine if D. gallinae -mediated transmission of S. Gallinarum could be demonstrated among four different hen groups.Methods: Two groups of 8 hens (A and B) were experimentally infected with S. Gallinarum in two isolators. After seven days, when birds showed signs of FT, about 25,000 mites were introduced. After 3 days, mites were harvested and used to infest other two hen groups of 8 (C and D), in two separate isolators. Health status of hens was constantly monitored; detection and quantification of S. Gallinarum were performed by PCR and qPCR from mites and organs of dead hens. The maximum likelihood estimation of the infection rate and mite vectorial capacity were calculated.Results: Clinical disease was observed in groups infected with S. Gallinarum (A and B) and in hens of groups C and D infested with mites harvested from the isolators containing groups A and B. In all 4 groups, S. Gallinarum was detected from liver, spleen, ovary and cecum of hens, thus confirming the diagnosis of FT. Mite analysis demonstrated the presence of the pathogen, with an estimated infection rate ranging between 13.72 and 55.21 infected per thousand mites. Vectorial capacity was estimated to be 73.79.Conclusion: Mites harvested from birds infected with S. Gallinarum were shown to carry the mite, and then to transfer S . Gallinarum to isolated groups of pathogen-free birds that subsequently showed signs of FT. Mite vectorial capacity was high, demonstrating that D. gallinae should be considered an effective vector of FT.