In the present aerosol experiment, assessment of the respiratory tract of 1-day-old birds as a natural route of infection for induction of Enterococcus faecalis bacteraemia and arthritis was performed. Second, the severity and type of arthritis produced through intramuscular infection in two different inoculation sites (musculus pectoralis versus musculus gastrocnemius) was studied. Third, the resulting bacteraemia was assessed qualitatively and quantitatively in relation to the occurrence of arthritis. Exposure of 1-day-old brown layer pullets to aerosolized E. faecalis with an estimated uptake of 104 to 105 colony forming units per chick resulted in bacteraemia; however, joint lesions were not induced. In contrast, 3/10 birds inoculated intratracheally with 108 colony forming units developed both bacteraemia and arthritis. This suggests the occurrence of a dose effect and a role for the respiratory tract as a natural infection route in young chickens. In both intramuscularly inoculated groups the incidence of arthritis was 10/10 birds and 9/10 birds, respectively. Birds inoculated in the m. pectoralis developed symmetric polyarthritis, which harmonizes with haematogenous colonization of joints. In contrast, m. gastrocnemius-inoculated chicks mostly had asymmetric (poly)arthritis of the injected leg and varus deformation of the contralateral leg, suggesting predominantly local spread. The qualitative and quantitative results of bacteriology of blood samples show that arthritis develops in those groups with the highest number of bacteraemic birds and the highest median bacterial colony forming units per millilitre of blood during the first 24 to 36 h after treatment.
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