The role of pulmonary intravascular macrophages in the pathogenesis of African horse sickness

African horse sickness (AHS) is a disease of equids, characterized by sever e pulmonary oedema and caused by an orbivirus. To determine the role of pul monary intravascular macrophages (PIMs) in the development of pulmonary mic rovascular changes in this disease, five horses were given an intravenous i noculation of 10(6) TCID50 of serotype 4 of AHS virus. Viral replication wa s detected in endothelial cells, PIMs, interstitial macrophages and fibrobl asts. Alveolar and interstitial oedema, and changes in pulmonary microvascu lature, consisting mainly of the sequestration of neutrophils and the forma tion of platelet aggregates and fibrinous microthrombi, were related to end othelial changes and to a high degree of PIM activation. This suggested tha t the PIMs, once activated, contributed to these vascular changes by releas ing chemical inflammatory mediators. (C) 1999 W.B. Saunders Company Limited .