Determination of infective dose (50%) of Trueperella (Arcanobacterium) pyogenes in sheep

    Information about the infective dose of an infectious agent is required for experimental trials of vaccine developed against that agent. Despite the versatility of Trueperella pyogenes as an agent of disease in animals, little is known about its infective dose for domestic animals.  In order to determine the infective dose of T. pyogenes in sheep, this study was carried out. For this purpose four groups (A to D) of 2-3 months lambs (three lambs per group), were intravenously infected with 10⁷, 10⁸, 10⁹ and 10¹⁰ cfu of T. pyogenes, respectively. The fifth group of lambs (group E) was injected with sterile phosphate buffered saline (PBS) and served as control. Blood samples were taken daily from all animals for bacterial cultivation till 10 days post infection. Clinical signs were recorded and all animals were euthanized ten days post infection, autopsy were done and samples (from liver, spleen, mesenteric lymph nodes, heart and lung), were collected for bacteriological and histopathological examinations. The results showed that although septicemia is not a constant criterion of experimental intravenous T. pyogenes infections, but it can lead to a consistent pattern of liver infection, ten days after inoculation. Post-mortem examinations revealed that all animals of group D had developed typical liver infection. No liver infection was observed in other lambs. Fifty percent lamb infective dose of the bacterium was calculated to be 3.162 × 10⁹ cfu. This finding is the basis for experimental trials of vaccine developed against the bacterium.