Pitfalls and difficulties in histological diagnosis of human dirofilariasis due to Dirofilaria (Nochtiella) repens.

The authors have conducted a histologic study of over 90 cases of Dirofilaria infection in man due to Dirofilaria repens, the most extensive study of its kind in the world. On the basis of their findings, they illustrate the diagnostic difficulties that may arise when regressive phenomena render the parasite's morphologic characteristics largely or completely unrecognizable. These phenomena are due to the death of the filaria inside the inflammatory nodule weeks or months prior to surgical removal, and to the consequent invasion of inflammatory cells. They start at the nematode's natural orifices (mouth, vulva, anus, cloaca) and gradually spread throughout its body. In cases of difficult diagnosis, therefore, it is recommended that the nodule be sectioned at different points comprising caudal, cephalic and middle body aspects, so as to reveal the few morphologic features that are still recognizable and which may afford a correct diagnosis. The authors present examples of histologic sections in color plates in which the nematode was in an excellent state of preservation and others illustrating the full range of regressive phenomena, from slight to total disintegration of the bodily structure.