Perkinsus spp. is one of the most important protozoan pathogens that can infect oyster and clam species, causing large scale mortalities worldwide, with a significant implication in both fisheries and aquaculture sectors, and resulting in severe economic losses. Histology is still considered the most important classic diagnostic method to detect Perkinsus spp. although PCR, ISH and TEM are recommended to provide specie specific diagnosis.
Perkinsosis is a disease of marine bivalve molluscs caused by protozoan parasites belonging to Perkinsus genus, phylum Perkinsozoa. In the present study, the infection caused by Perkinsus spp. was investigated in wild Grooved carpet shell Ruditapes decussatus collected from three different brackish environments of Sardinia - Italy (Calich, Porto Pozzo and Santa Gilla lagoons ) devoted to extensive aquaculture.
Tissue sections of each clam (n =300) were histologically analysed to determine the presence of Perkinsus spp., its prevalence and the immune response of infection in digestive gland, gills and kidney.
The i nfection by Perkinsus spp . was observed in all site and organs considered with different percentages of infection between sites. The highest prevalence was found in the clams from Calich lagoon in organs as the digestive gland (37%) and gills (68%).
Porto Pozzo and Santa Gilla lagoons showed a prevalence of 23% in gills . Overall, prevalence in the kidney was less frequent for all sampling sites and the presence of parasites ranged from 7% (Porto Pozzo lagoon) to 22% (Calich lagoon).
The presence of haemocytes infiltrate, in all sites and organs, was commonly correlated to the protozoan Perkinsus spp. ( p < 0.001).
Generally, bivalve molluscs with parasitized organs revealed an important host immune response: in our study, histological analysis revealed the presence of Perkinsus spp . which affected the tissues, typically characterized by encapsulation through severe haemocytic infiltration, especially of granulocytes.
Histopathology permitted to differentiate the different sampling lagoons for the prevalence of Perkinsus spp . Clams from different Sardinian coastal environments have constantly reported infection in all the considered organs with different degrees of distribution due to the presence o f Perkinsus spp. Moreover, our findings revealed that clams collected from Calich lagoon reported the highest prevalence of haemocytes infiltration linked to the presence of parasite.
The clams, R. decussatus , were confirmed to be a suitable biological indicator sensitive to environmental stressors tool, such as Perkinsosis. However, further studies are necessary to monitor the presence of parasite in Sardinian lagoons, in order to establish the species involved too.