Comparative pathology of pulmonary hydatid cysts in macropods and sheep

The development and appearance of hydatid cysts of Echinococcus granulosus in experimentally infected tammar wallabies (Macropus eugenii) and sheep during the period 9-17 months post-infection (mpi) were studied. Cysts of unknown age were also examined from mature, naturally infected sheep. The cysts grew more rapidly and became fertile within a shorter period in wallabies compared with sheep. Cysts from the wallabies were larger in absolute size and were larger relative to the size of the lungs. Microscopical examination revealed that wallaby hydatid cysts developed in small bronchioles. Hydatid cysts in the wallabies had a thicker germinal membrane, with more nuclei and a thicker laminated layer (LL), than hydatid cysts of similar age found in sheep. In contrast, the adventitial layer was thicker in the ovine cysts, comprising a hyalinized layer of degenerate collagen and necrotic cellular debris surrounded by a layer of granulation tissue that was largely absent from lesions in the wallabies. Multilocular cysts were present in sheep, but not in wallabies. The greater thickness of the germinal membrane in wallaby cysts suggests greater parasite activity, which may explain the more rapid growth rate in this host, whereas the thicker adventitial layer in sheep cysts may be restrictive to growth while simultaneously protecting the hydatid from the host immune response. These differences in the parasite-host relationship between macropods and sheep may reflect the relatively recent introduction of the parasite into Australia.