Pulmonary Lesions in Fetuses Exposed in Utero to Porcine Reproductive and Respiratory Syndrome Virus

The salient features of a porcine reproductive and respiratory syndrome (PRRS) epizootic include respiratory disease followed by reproductive failure. The respiratory component is usually recognized first in finishing pigs or in the breeding herd as a mild flu-like disease with a high morbidity for all ages and high mortality in younger pigs. 5 In breeding herds, acute maternal reproductive failure often follows the onset of respiratory disease. PRRS is characterized by a sudden increase in early farrowings, late-term abortions, stillborn and mummified fetuses, weak neonates with high mortality, late returns to estrus, and repeat breeders. 5 No consistent histopathology has been obseived in fetal tissues from epizootics or following experimental infections of susceptible dams. In this report, we describe fetal histopathology associated with an experimental PRRS virus (PRRSV) infection. Eight pregnant gilts underwent surgery at different stages of gestation: 2 at 34 days, 1 at 45 days, 3 at 49 days, 1 at 65 days, and 1 at 85 days gestation (Table 1) (Lager KM, Mengeling WL, unpublished). All fetuses in 1 uterine horn (identified as principal fetuses) were exposed to a well-characterized PRRSV isolate 1 by transuterine injection of virus into their amniotic fluids. All fetuses in the contralateral uterine horn (identified as control fetuses) were exposed to a sham inoculum in a similar fashion. The gilts were necropsied 1731 days postsurgery, and their fetuses were recovered for study. Virus isolation was attempted on sera from live fetuses, thoracic fluids from dead fetuses, and tissue composites from each fetus (Lager KM, Mengeling WL, unpublished). Tissue samples for histopathologic examination were collected from lung, kidney, liver, and spleen and placed in neutral buffered 10% formalin. Tissues were processed routinely, cut into 8-µm sections, and stained with hematoxylin and eosin (HE). Sections of lung, liver, and kidney were mounted in embedding medium, a frozen, and sectioned on