Histopathology of an opecoelid trematode infection in mayfly naiads

Naiads of the mayfly, Litobrancha recurvata, were experimentally infected with an opecoelid trematode resembling Allopodocotyle lepomis. Cercariae entered the naiads in relatively large numbers through intersegmental articulations or distal gill filaments and encysted. The host response involved repair of entry wounds and hemocyte accumulation and encapsulation, occasionally with melanin deposition. However, parasite mortality was usually low, and many worms grew and developed. Damage to the insect was slight and usually limited to tissue displacement.