Prevalence of HPS was recorded by surveying 55 broiler farms in and around Lahore between September 1990 to December 1990. Out of these 55 broiler farms 12 farms were affected with HPS. From 12 affected farms 15225 birds died due to HPS, so the average mortality recorded was 16.36 percent. Indicence of HPS recorded was maximum between 3rd to 5th to week of age, however birds of younger & older age groups were also affected with HPS. Different breeds & feeds have no affect on prevalence of HPS. HPS inoculum prepared from morbid livers, when inoculated into 10 days old, twenty embryonating eggs with 0.2 ml through chorioallantoic membrane route, caused 100% mortality within 3-6 days post inoculation.

Day old, 120 broiler chicks were reared and divided into four groups A, B, C, D each comparising of 30 chicks. At the age of 21 days, broiler chicks of A, B and C groups were inoculated against HPS through various routes i.e. oral, subcutaneously and Intramuscularly, respectively, while D group remained control. Mortality was recorded for the experimental period of 12 days. It was noted that the broiler chicks in group A, 3 birds died having 10% mortality, in group B, 17 birds died having 56.66% mortality while in group C, 14 birds died having 46.66% mortality.

Mortality was maximum within first 5 days, whereas mortality become minimum in the last 4 days of the experimental period. Common clinical signs observed were sudden increase in mortality, depression, yellowish diarrhoea, huddling of the birds.

The histopathological changes in affected broiler chicks included ballooning of the pericardial sac with blood stained (amber) fluid. The pericardial fluid varied from 0.5 to 2.5 ml. The heart become flabby, rnisshaped and enlarged. Microscopically, cardiac muscles showed degeneration, mild congestion, leukocytic infiltration. Liver appeared swollen, enlarged, friable, dark brown in colour and show mottling. Pin point subscapular haemorrhagic lesions observed in markedly fatty livers. Microscopically, liver showed fatty changes, massive necrosis, dilatation of sinoids, nuclei become larger than normal. Light basophilic intranuclear inclusion bodies of different sizes observed in liver parenchymoal cells. The kidneys become swollen, enlarged, friable and dark brown to pale yellow in colour. Microscopically, kidneys showed tubular dilatation, degeneration and necrosis. The glomeruli were misshaped and showed glomerunephritis. The thymus did not exhibit any gross or histopathological changes.