Many parasites from the phylum Nematoda are responsible for respiratory, enteric, cutaneous, and circulatory infections in animals and some cases, humans. Species of nematodes from the genera Dirofilaria and Achanthocheilonema are worldwide recognized and reported as common parasites of dogs, and potentially humans, responsible for circulatory/respiratory and cutaneous disease, respectively (Gang and Hallem, 2016; Ramos et al., 2016). The hosts harbor the adult parasites in the circulatory system and cutaneous and intramuscular tissue, where they reproduce (mostly by parthenogenesis) and release the early stages (microfilariae) into the bloodstream (Simón et al., 2017).
The main blood filarial disease of dogs is dirofilariasis, also known as heartworm disease or cardiopulmonary dirofilariasis, caused by Dirofilaria immitis. Even though many animal species can be infected (e.g. foxes, wolves, coyotes, cats and wild felids), this parasite is known to cause extremely pathogenic infections in dogs, with life-threatening disease (Farkas et al., 2020). The course of the infection depends on the duration of the infection, parasitic load and the immunocompetence of the host, but is characterized as a multisystemic disease that affects the heart, lungs, kidneys and liver and possibly causes the animal’s death (Adebayo et al., 2020).
Mosquitoes from many genera (e.g., Aedes, Anopheles, and Culex) are the main vectors in the cycle of transmission of dirofilariasis (Bendas et al., 2019; Santos et al., 2021). Even though the prevalence of this disease is higher in tropical and subtropical areas (with environmental conditions favorable to the vector development and survival) in the last twenty years the reports of D. immitis infections in dogs have increased even in non-endemic areas (Panarese et al., 2020; Simón et al., 2012). Increases in world temperature and climate change are important factors that favor the development of mosquitoes and consequently the spread of infections (Cazaux et al., 2019; Genchi and Kramer, 2020). Suitable temperatures are also a key factor for the development of the microfilariae infectious stages (L3) in the mosquito’s proboscis (Silaghi et al., 2017). Furthermore, the presence of wild reservoirs and stray dogs in cities and urban environments increases the contamination of other animals and humans (Kravchenko et al., 2016; Potkonjak et al., 2020).
Acantocheilonema reconditum is a low-pathogenic nematode, responsible for causing dermatological disease in dogs. Those parasites have a global distribution, and even though infections rarely are life-threatening, this filarial worm species are the most widely spread (Dumitrache et al., 2021). The transmission of this parasite depends on may flea and lice species, who work as vector and intermediate hosts, and opposite to the severe disease caused by D. immitis, most dogs infected are asymptomatic or in rarely heavy infestations can have pruritis, alopecia, and dermal abrasions (Hoseini et al., 2020; Otranto et al., 2013; Tahir et al., 2019).
Even though adult stages of D. immitis and A. reconditum occupy different niches in the definitive hosts, both parasites microfilariae (which are very similar) can be identified in the bloodstream (Engelmann et al., 2019). Besides the need of practice and attention while distinguishing the parasitic larval forms, this demonstrates the importance of using different techniques to obtain a more precise diagnostic (Argôlo et al., 2018). The use of different diagnostic tools is important, especially in cases of ‘’occult heartworm’’ where dogs present amicrofilaremia. Other techniques besides the direct identification of circulating microfilariae through Knott’s technique are serological assays as enzyme-linked immunosorbent assay (ELISA) and immunochromatographic tests (ICT), molecular analyses, and complementary tests such as radiography and ultrasound (Bamorovat et al., 2017; Panarese et al., 2020).
It is important to emphasize the zoonotic potential of D. immitis. Zoonotic infections by A. reconditum are uncommon (only one case reported in a human eye), but human dirofilariasis caused by D. immitis is an emerging zoonotic disease, especially in areas with climatic conditions favorable to mosquito development and the presence of canine reservoirs (Fontes-Sousa et al., 2019; Huynh, Thean & Maini, 2001). As humans are not suitable hosts for Dirofilaria species, the parasites cannot fully develop and are often found in nodules in the pulmonary and subcutaneous tissue, joints, and synovial fluid, and ocular region (Falidas et al., 2016; Gebauer et al., 2021).
In Brazil, the prevalence of D. immitis infections ranges from 1 to 45% in over 12 of the total 26 states (Bendas et al., 2017). The city of Aracaju, capital of the state of Sergipe, northeastern region of Brazil, is a coastal city with a characteristic tropical climate (high temperatures and humidity) and the presence of many natural areas (mangroves, forests, and beaches) in close contact with the urban environment (Anjos et al., 2017; Santana et al., 2020). The city has about 672 thousand inhabitants and an estimated canine population of 45 thousand animals, of which approximately 10% are stray (IBGE, 2020). As seen, these environmental, climatic and demographic characteristics are factors directly related to the development of vector mosquitoes and the spread of filarial diseases to animals and humans.
Given the wide distribution of filarial diseases worldwide, along with all the harm caused to animal and human health, the present study objectivated to report the occurrence of the main species of circulating microfilariae in blood samples of dogs from the city of Aracaju, associating the prevalence with age and sex of the analyzed animals.