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Nodular Gill Disease (NGD) in highly intensive farming systems plays a major role in production losses and its outbreaks can result in cumulative mortality exceeding 60%, especially during the winter months and early spring. NGD was described in rainbow trout farms of North America and Europe and appears to be caused by different species of amoebae, for example both testate (Roghostoma minus) and naked amoebae belonging to five genera. In the last few years, NGD is widespread in rainbow trout farms of northern and central Italy. In March 2017, during the epidemiological investigations into NGD in salmonid farms of northern Italy, the first cases of branchitis referable to amoebic infection in juvenile brown trout (Salmo trutta L.) were observed. Twenty‐five live and moribund brown trout from each tank were collected at monthly intervals from March to May 2017 for a total of 150 fish. The samples were subjected to necropsy, parasitological analysis, histology and bacteriological and virological examinations for diagnostic purposes. Histological examination of gills from diseased and moribund brown trout showed characteristic pathological signs of NGD infection. The most significant change was the development of heavy gill lesions with variable degrees of lamellar fusion, especially in the distal part of filaments (clubbing), with obliteration of interlamellar spaces. A limited number of amoebae were detected along the surface of the gills. The lesions observed in fresh mounts and in histological sections obtained from the gills of the infected brown trout confirmed to be less serious than those detected in affected rainbow trout of the same farm. Therefore, this investigation, which reports for the first time NGD in brown trout, has shown that S. trutta would be less susceptible to this pathology than rainbow trout.

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.