Ex., Dipartimento di Patologia Animale, Igiene e Sanità Pubblica Veterinaria, Sezione di Microbiologia e Immunologia, Università di Milano, Milano, Italy
The role of Dermanyssus gallinae (D.g.) as vector of various agents of infectious diseases has been emphasized by some authors, particularly in the past (1, 3, 4, 5, 6).
Aim of this paper is just to contribute to the study of the role of such parasite in the transfer and maintenance of Salmonella gallinarum (S.g.) in a large layer farm.
Materials and methods
The farm under control consisted of about 1 million red-hybrid layers distributed in 6 houses.
The mites were collected from four houses of the farm, where a variable infestion of D.g and a high mortality due to S.g. infection occurred, in spite of 2 or 3 vaccinations with non-pathogenic SG9R. A portion of parasites was suspended in saline; a portion was washed two times with saline, and then suspended in 4% formaldehyde solution for 7 minutes (7). After removal of disinfectant and two washings with saline, the mites were grinded in a sterile mortar and resuspended in saline. The washing fluids after treatment and the homogenate suspension of mite tissues were seeded in enrichment medium (selenite broth), then or directly in solid medium (Gassner).
The isolation of S.g. was performed biweekly in birds of 6 different houses, with or, apparently, without mites.
The identification of isolated microorganisms was done by biochemical and antigenic testing.
The percentage of specific mortality due to S.g. from 18 to 60 weeks of age was calculated in the different houses on the ground of lesions found at necropsy, often confirmed by bacteriological controls.
Results and discussion
The examined mites, obtained from 4 mites infested houses, resulted positive for S.g. The vaccination (also 3 times) demonstrated to be not sufficient to control the disease and prevent the transmission of S.g. in presence of mites in the houses. The contamination of outer and inner organs of the mites was demonstrated, so suggesting the localization of S.g. also inside the mite body. After treatment with formal-dehyde the washing-water resulted negative for S.g. The results of isolation of S.g. from parasites are reported in Table 1.
The transmission of Salmonella by arthropods, the mites mainly by bite, is a quite known phenomenon (2). It has been demonstrated that S.g. can survive in mites, even for 4 or over months after contact of them with the host (2). The spontaneous infection of mites' population with S.g. shows the importance of a correct and continuous disinfection and disinfestation of poultry houses and farms for the successful control of Salmonella infection, also in case of specific vaccination of the birds. In fact the vaccination proved to be rather effective, greatly reducing the incidence of the disease, as observed in the houses without or with rare D.g. infestion.
References
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