Prevalence and seasonal variations of canine dirofilariosis in Portugal

Dirofilariosis is a severe vector-borne emergent disease that is spreading worldwide and becoming a serious threat to human and veterinary public health. Portugal, a Mediterranean country, has favorable climate conditions for mosquito development and survival. At present, accurate data on the prevalence and epidemiological pattern of dirofilariosis in Portugal is scarce and outdated. To study these trends, a project was developed to assess the current prevalence of Dirofilaria immitis infection in shelter dogs as well as the prevalence of Dirofilaria species present in three coastal regions in central Portugal: Coimbra, Santarém, and Setúbal. Blood samples were collected from 696 shelter dogs during three consecutive years: 2011, 2012, and 2013. A rapid immunomigration technique was performed to detect female D. immitis antigens. Concurrently, to detect and identify circulating microfilariae, a modified Knott's technique and acid phosphatase histochemical staining were also performed. Of the 696 dogs sampled, 105 were positive for D. immitis, with an overall prevalence of 15.1%. Forty of the 105 dogs were antigen negative but were positive for D. immitis microfilariae. Three animals were co-infected with D. immitis and Acanthocheilonema dracunculoides, and there was also one dog infected only with A. dracunculoides, all confirmed by polymerase chain reaction. The highest prevalence of canine heartworm during the 3-y period was in Setúbal (24.8%), followed by Coimbra (13.8%), and Santarém (13.2%), with significant inter-district differences. Our results demonstrate a higher prevalence of dirofilariosis compared with findings of previous studies and show an increasing rate of infection in the southern areas of Portugal attributed, at least in part, to bioclimatic and ecological factors. The present study updates the epidemiological situation and correlates the risk of dirofilariosis transmission within each region. These findings are highly relevant to both human and veterinary public health, contributing to the general awareness of pet owners and veterinarian practitioners and reinforcing the need for effective control measures against vectors and preventive therapy in companion animals.