Anisakis simplex: histopathological changes in experimentally infected CBA/J mice

Third-stage juveniles (L3) of Anisakis simplex, surgically implanted into the abdominal cavity of CBA/J mice and necropsied at 7, 14, or 21 days postinfection (PI), embedded in the gut mesentery and only rarely invaded viscera. Histologically, intense aggregations of neutrophils adjacent to the parasites were noted at Day 7 PI. At Day 14 PI, mature granulomata consisting mostly of eosinophils and large numbers of fibroblasts and associated collagen were observed. Granulocytes and occasionally multinucleate giant cells occupied the host-parasite interface. At 21 day PI, lesions displayed the predominance of connective tissue. Multinucleate giant cells were found adjacent to the L3 with eosinophils adjacent to parasite remnants or scattered within the walls of the granulomata. Most L3 were viable at Days 7 and 14 PI; however, at Day 21 PI the L3 were dead and invaded by inflammatory cells. Hematological findings indicated that infected mice had a neutrophilia of varying magnitude regardless of the number of worms implanted. Eosinophil levels as a percentage of the total leukocyte pool in peripheral blood always remained at or below normal limits. On Days 7 and 14 PI, the peripheral blood showed an increase in neutrophils that began to return to normal values at 21 day PI. Conversely, peripheral blood eosinophils decreased on Days 7 and 14 PI and returned to normal values on Day 21 PI. Surgical implantation of A. simplex L3 into mice produced both a hematological and histological picture consistent with that seen in human anisakiasis.