Outbreak of Pasteurella multocida septicaemia in neonatal lambs

sistent with a reaction to chronic pituitary-dependent hyperadrenocorticism. Histological screening of samples of the kidney and myocardium was undertaken and revealed slight chronic interstitial nephritis. The most significant changes were present in the liver, which was enlarged, with rounded edges and a pale, mottled exterior. Histological examination of representative sections revealed micronodular hyperplasia, interspersed with areas of parenchymal collapse and parasitic granulomas or'nests' These lesions measured up to 1-5 mm across and contained eggs and possible remnants of worms, which were embedded in necrotic material and surrounded by a fibroblast capsule (Fig 1). The eggs (Fig 2) were barrelshaped, with a slightly greater circumference on one wall and bipolar plugs, although the plugs themselves were no longer visible and the egg contents were degenerate or absent. The egg shell was two-layered; the inner layer was dense and homogeneous, and the outer layer was striated in cross-section and reticulate on the surface. Eleven eggs that had both plug regions visible measured an average (se) of 56-8 (0-4) Mtm x 31 (0-6) Mim, which was consistent with a diagnosis of C hepatica; the egg size was similar to the original description by Hall (1916) of eggs from rats of 55 x 30 to 35 Htm, and that of Wright (1930) from a dog, but was larger than eggs from some other infections described in dogs (Stokes 1973). The normal hosts of C hepatica are rodents, with the most common being rats. The prevalence of infection ranges from 0-2 to 88 per cent, and is usually higher in Rattus norvegicus (>55 per cent in many surveys) than in Rattus rattus. The prevalence tends to increase with the density of rodents and is highest in urban areas (Spratt and Singleton 2001). The life cycle of the parasite is unusual; larvae in ingested eggs hatch and migrate to the liver, where adult worms live for a few months, producing eggs in the liver parenchyma. Development of the eggs is possible only on their release from the liver and exposure to oxygen. Starvation and cannibalism in rodent nests are at high levels in late winter, and may explain why the highest incidence of infection is seen in the spring. The unembryonated eggs pass intact through the cannibal and, when passed in faeces, can embryonate in the warmth of the nest. Decomposition of carcases also releases eggs. Egg release through predation by domestic carnivores was considered to be of secondary importance (Farhang-Azad 1977). Examinations of rodent populations in Britain, some including rats, have either not examined the liver or have noted the presence of only Taenia taeniaeformis metacestodes. Egg nests, if present, might have been visible as white spots but may not have been further investigated. Fewer than 20 cases of C hepatica in dogs have been recorded worldwide and so infection is uncommon even where prevalence in rats is common (Stokes 1973). The presence of this infection in a dog suggests that examination of rodent populations for C hepatica would be timely. In 1992/93, 4-8 per cent of properties in the UK were infested with rats (Meyer and others 1995). Dogs hunting rodents could contaminate the soil in gardens, while there are anecdotal reports of rat numbers increasing. Human beings can become infected and in some cases this can develop into a serious disease.