Some filarioids, such as Dirofilaria immitis, Dirofilaria repens, Acanthocheilonema reconditum, Oncocherca lupi, and Cercopithifilaria grassii, belonging to the family Onchocercidae (Spirurida), are transmitted by arthropod vectors and cause canine filariasis (1). Worldwide, more than seventy species of the Culicidae family participate in the transmission of D. immitis and the main mosquito-vectors are: Aedes, Ochlerotatus, Culex, Anopheles (2). Competent vectors ingest microfilariae (MF) when they take a blood meal. In about 10 to 14 days, depending on the environmental temperature, the larvae develop into third stage (L3) and migrate to the head of the mosquito. When the mosquito takes the next blood meal the L3 migrates to the new definitive host. Once the new host is infected the L3 molts to L4 and in approximately 120 days young adults can be found in the pulmonary arteries and right chambers of the heart. Adult males and females’ mate and produce MFs that can be found in the peripheral blood stream approximately in 7 to 9 months (3).

Canine clinical signs are multifactorial. Most dogs are asymptomatic and when they become sick, coughing, weight loss and exercise intolerance are frequent. Severe disease includes signs of congestive right heart failure (4). An update on the South American seroprevalence showed that no infected dog has been reported in Chile and that in the other countries were the infection has been detected, prevalence rates range from 14.41% in Argentina to 1.6% in Colombia, 8.9% in Mexico, 5.5% in Peru and 15.2% in Venezuela (5). The overall prevalence of canine D. immitis infection in Brazil was 13.03% (6). Dirofilaria immitis canine infection is common in the coastal regions of Brazil, with a high prevalence of 23.1% (7).

In the State of Rio de Janeiro, during the active search for cases of canine heartworm disease, seroprevalence was recorded in some locations in the metropolitan region where no survey had been carried out. In the west zone, a study showed that 21.6% of canines were infected (8); another research showed that laboratories that received samples from different neighborhoods in the city of Rio de Janeiro reported only 7% of nematode infections in dogs (9) and occurrences were reported during veterinary care on Ilha do Governador showing that 14.5% of dogs were infected by D. immitis (10).

The Baixada Fluminense region was considered to be indene, until 2004 when a record of a case with a frequency rate of 0.9% in the municipality of Nova Iguaçu (11). After 2017, D. immitis has been detected at a higher frequency with autochthonous cases reported in this area (12, 13). The Baixada Fluminense region has recently been recognized as a new focus area for onchocercid infections (13). Undoubtedly, global climate change and anthropogenic actions favor increased human, canine, and mosquito population densities and, thus, the spread of the infection (14).

Traveling with dogs is increasing owing to the easiness. Some families travel with multi-species pets. Although this practice may be incentivized, the associated health issues must not be ignored. One way to counteract these health issues is through good pet care, including preventive measures that undoubtedly impose chemoprophylaxis on D. immitis. Therefore, infections monitoring and spreading awareness, particularly in areas without parasite circulation, must be prioritized locally. This study aimed to analyze the epizootiological factors including the prevalence of infections of D. immitis in domestic dogs in Baixada Fluminense, Rio de Janeiro, Brazil.

The analysis included 16,314 test results, of which 3.4% were positive for D. immitis (Table 1). The highest overall prevalence was observed in Magé, where 8.5% (270/3,162) of the dogs tested positive, followed by Duque de Caxias, where 2.5% tested positive (241/9,788) (Table 1). The space–time distribution shown in Figure 1 indicates that these municipalities have remained the same over the years, with a greater number of cases than those in the others. According to antigen tests, 25.3% (62/245) of the dogs were positive for the D. immitis, 17.5% (157/897) were positive for the modified Knott’s test, and 2.2% (335/15,172) were positive for unexpected findings of microfilariae (Table 1; Figure 2).

The results indicated a significant association between infection and all canine characteristics (Tables 1, 2). Compared to dogs treated at veterinary clinics in Magé, those treated in Duque de Caxias, São João de Meriti, Nova Iguaçu, and Guapimirim were 73% (OR = 0.270; CI95% = 0.226–0.323), 94.5% (OR = 0.055; CI95% = 0.023–0.134), 86.2% (OR = 0.138; CI95% = 0.061–0.312), and 81.8% (OR = 0.182; CI95% = 0.125–0.263) less likely, respectively, to have positive results (Tables 1, 2). Furthermore, dogs treated in São João de Meriti were 79.6% less likely (OR = 0.204; CI95% = 0.084–0.496) to be infected than those treated in Duque de Caxias (Tables 1, 2). Age was also a significant factor, with dogs aged 8–14 years being 30.3% more likely (OR = 1.303; CI95% = 1.052–1.614) to be infected than those aged 1–7 years. Male dogs were 38.6% more likely (OR = 1.386; CI95% = 1.168–1.644) to be infected than female dogs (Tables 1, 2). Moreover, the D. immitis antigen test showed 59.6% (OR = 1.596; CI95% = 1.141–2.233) more positive results than the modified Knott’s test (Tables 1, 2).

According to a previous report using multiplex PCR, at least 93.5% of dogs in the study area were infected with D. immitis (13). Hence, in this study MF detected by conventional tests displaying the morphology of the anterior and of the posterior ends in agreement with D. immitis description (17, 18) were assumed to be D. immitis by the laboratory. This incomplete identification of the larvae morphology can be considered a limitation in this study.

Male dogs were infected more frequently than female dogs. This has been previously reported (19) however, no hypothesis has been proposed to explain this difference (19, 20). Empirical observations have shown that spaying and neutering dogs in the study area are rare, suggesting that females need to be better cared for to avoid unwanted litter and that male dogs are mainly restricted to backyards or sometimes allowed to roam free, as observed elsewhere (19, 20). This human behavioral manner, along with the predisposition of male dogs, suggests that the difference may be attributed to the exposure to infected mosquitoes instead of the sex.

The frequency of infections among older dogs (8–14 years) may have been higher than that among younger dogs by chance. When moderately challenged as observed (frequency of 3.4%), the longer the exposure to the vectors, the higher will be the risk. This contrasts with the results of a previous study conducted in long-known focus areas for D. immitis high-challenge transmission (frequency > 20%). In those areas, the length of time the dogs lived in the focus did not increase the infection frequency, perhaps because the focus was established, and transmission was quick (7). Therefore, it may be inferred that Baixada Fluminense is an area where D. immitis transmission is a recent event as a possible result of global environmental changes that demand extended periods of transmission (14).

In addition to the human population density, the presence of microfilaremic dogs (21) conditions the establishment of an enzootic cycle and the emergence of cases of human pulmonary dirofilariasis in areas of socio-environmental vulnerability, making it a worrying factor according to the One Health concept (22, 23). Therefore, implementing public policies for the management of environmental sanitation, control of vector mosquitoes by the endemic sector, and educational planning for health professionals by the local authorities is of paramount importance (24, 25). The study area once considered free of heartworm transmission, currently presents data suggesting the existence of this parasite (13). Therefore, once transmission in the area has been established, veterinarians must be prepared to guide pet owners to adhere to prevention and treatment measures.

With the occurrence of infected dogs in Baixada Fluminense documented herein, factors related to anthropogenic and climate, in addition to the presence of infected dogs (26, 27), may facilitate the establishment of competent mosquito populations and enhance the transmission of D. immitis in the region. Considering that Baixada Fluminense is a section of the state’s lowlands tangential to the oceanic coast and is a permanent conservation area, wild animals may also be affected (28). Most of the D. immitis infections documented in Brazil are in coastal areas (7, 29, 30), although infections are not restricted to these environments (31). The geographical dispersion of the parasite D. immitis in Brazilian previously indene regions are scarce (12, 13, 32, 33), however in Europe this spreading receives attention and is seen as a possible consequence of global climate changes (14, 34).

Thus, a broad epidemiological investigation must be conducted to monitor the prevalence of D. immitis in local dog populations by performing specific routine laboratory tests for detection and this filarioid identification. The rapid tests for antigen research are readily available and have greater specificity and sensitivity for the detection of D. immitis and to be recommended for clinical and epidemiological research (7, 30, 31).

The general recommendation is to request the modified Knott test (15) associated with antigen test (3) to detect and confirm parasitism in case of “occult infection” once 30% of the canine population will never be microfilaremic and because the predictive value of the antigen tests may provide false-positive results in a low-frequency area (30, 35).

The recently detected D. immitis infection in dogs in the lowland Baixada Fluminense region makes the area a candidate for canine heartworm transmission. This reinforces the need for an integrative approach among health professionals with a broad one-health perspective to implement public policies that promote health.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal studies were approved by Animal Use Ethics Committee of the Oswaldo Cruz Institute/ Oswaldo Cruz Foundation. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent was obtained from the owners for the participation of their animals in this study.

VA: Conceptualization, Data curation, Project administration, Investigation, Methodology, Writing – original draft. PS: Writing – original draft, Methodology. ÉP: Formal analysis, Methodology, Writing – original draft. PA: Methodology, Writing – original draft. NL: Methodology, Formal analysis, Writing – review & editing. GG: Methodology, Writing – review & editing, Conceptualization, Project administration, Supervision. AM: Conceptualization, Data curation, Project administration, Supervision, Writing – review & editing.