The results of this study contribute to current understanding of the epidemiology of Dirofilaria spp. in an endemic area of north-eastern Italy. Based on the dog/human attractiveness, on the survival after feeding on dogs with high microfilariae concentrations and on the overall rate of positive mosquitoes, Cx pipiens was the most efficient natural vector of D. immitis in the studied area, where highly microfilaraemic dogs are expected [7, 23]. Although the small number of Oc. caspius collected does not allow any definitive conclusion to be drawn, the high mortality rate of this species following a blood meal on a highly microfilaraemic dog indicates that Oc. caspius might be an effective vector in non-endemic areas. Indeed, the survival of different vectors was linked to Dirofilaria microfilariae concentration in the mosquito species .
The results also indicate that in north-eastern Italy, where Cx pipiens is most prevalent , the risk of exposure to Dirofilaria spp. infected vectors can be very high for dogs and relevant for humans.
Indeed, based on our calculations, during the seasons characterized by higher biting pressure, the contact between an infected mosquito and a host may occur as often as every four nights for D. immitis in dogs, whereas the possibility for a human to come into contact with an infected mosquito occurs within two weeks of exposure. The higher risk of Dirofilaria spp. transmission was recorded in late July and August, in accordance with previous reports .
In contrast, in a study in which more than 40,000 culicids collected from May to October 2010 in the areas of site B and C were screened for D. immitis and D. repens, the rates of Cx pipiens infection did not vary significantly through the season, indicating that over spring a certain number of dogs may act as a source of infection to suitable vectors. This finding indicates that the mosquito abundance is one of the key factors in the epidemiology of dirofilariosis. Interestingly, the rate of Cx pipiens infection with D. immitis estimated in this study in 1999 (0.26%-0.38%, site C) was very similar to that calculated more than ten years later (0.21-0.33%) . This finding shows that, in spite of the availability of several chemoprophylactic treatments used for the prevention of canine dirofilarioses in endemic areas, prevalence of microfilaraemic dogs has not decreased significantly in rural areas.
However, since the most conservative approach was used to infer the risk of host exposure to D. immitis (i.e. assuming that mosquitoes harboring microfilariae in the abdomen would have suffered the same rate of mortality of Cx pipiens fed on high microfilaraemic dogs, 76.3%), is likely that in certain sites, the risk of exposure of dogs and humans is higher than indicated.
In addition, due to the introduction of Aedes albopictus [Stegomya albopicta] in Italy as well in other European countries  the risk of Dirofilaria transmission is higher throughout the day time, as a consequence of the fact that this species is diurnal and it acts as natural vector of both filariae in Italy [9, 27]. Consequently, categories of dogs and humans previously considered not at risk for dirofilariosis (e.g., animals kept indoors at night or children playing in private and public gardens), should be included in the population at risk of exposure.
Without any doubts, host attractiveness plays a key role in the determination of the population at risk of exposure. Indeed, mosquito host preferences have been well documented, both at species and at individual level . Accordingly, in the present investigation, Cx pipiens fed preferentially on dogs (70%) than on humans (26%), and was differently attracted by the three dogs used in the study, independently from their sizes. Besides the individual host attractiveness to mosquitoes, the microfilaraemic status of the dogs may enhance the host-preference of Cx pipiens. Accordingly, Cx pipiens displayed a higher feeding rate on a microfilaraemic dog, compared to a dog under preventative treatment (i.e., 47% vs 6.7-12.9%) . Similarly, the mosquito host attractiveness is also enhanced in human patients infected by Plasmodium spp. [29, 30] indicating that alterations of the physiological status (disease conditions) and consequently of physiological parameters, may ultimately change the animal cue attractants (fever, sweat and breathing rhythm and odour) to competent vectors. Although these factors might have an impact on the spread of the infection, they remain purely speculative for dirofilarioses.