Aims: To review 28 cases of human dirofilariasis reported in the last 30 years in the Old World and attributed, by their respective authors, to Dirofilaria immitis or a species of Dirofilaria other than D. repens.
Methods and results: Each case was analysed by examining the published accounts or by discussions with the authors, who were interviewed whenever possible.
Conclusions: On the basis of these analyses we conclude that there is as yet no proof demonstrating with certainty that Old World D. immitis plays a pathogenic role in humans. It remains to be explained why D. immitis causes pulmonary infections in humans in the Americas while, in the Old World, this location appears, instead, to be always associated with D.repens, even though the former species is at times more frequent than the latter both in dogs and in the vectors. To explain this apparently different pathogenic power, two hypotheses are proposed: (i) there are perhaps twin populations with different genotypes on the two sides of the Atlantic, with different infective capacity for man and dog; (ii) the infective capacity to humans of the parasite could be modified, only in the Old World, by some unidentified factor, possibly inherent to the vector, that affects the complex mechanism of the vector-parasite relationship, affecting the survival of the larvae.