Cardiopulmonary dirofilariosis is a zoonotic and vector-borne disease. The occurrence of dirofilariosis in humans depends mainly on the presence of infected dogs and vectors for transmission within a given area. At the same time, other factors, such as increase in temperature and humidity owing to climate change; emergence of new disease-transmitting species; transportation of infected hosts; modification of the environment owing to human activities; agricultural practices and irrigation areas; deficiency and economic instability; and adverse meteorological events such as hurricanes or tropical rains in the area, affect the development of the disease. [1, 4, 25]. Further, it is important to note that although it is a vector-borne disease, not all L3 that come into contact with the host develop into adults, neither in dogs nor in wild hosts [9].
Although there are numerous studies that report the presence of D. immitis in dogs worldwide, most of the information regarding humans comes from clinical cases and retrospective reviews. In these cases, there are only data from the affected population that showed some type of clinical manifestation, excluding the infected population that does not have symptoms related to the disease or any clinical manifestation, making its study even more difficult. Seroepidemiological studies show the complementary part of the problem because they detect contact with the parasite by analyzing the anti-Dirofilaria and anti-Wolbachia immune response and are excellent tools to analyze the risk of infection among the human population residing in an endemic area (1,4,7,22,26,27,28].
In Colombia, dirofilariosis is a very poorly studied disease. Few studies have addressed this problem during the last 20 years [10, 13, 15]. Furthermore, prevalence values in dogs vary (0.91–16.12%) and have been obtained using different methodologies. In humans, there is only one clinical case of a patient from whom an adult worm identified as Dirofilaria sp. was extracted from the lung [14], and there are two studies that warn of the existence of human infections caused by D. immitis in communities from the Colombian Amazon where infected dogs have been found [15, 16].
In this study, we analyzed the presence of D. immitis in dogs and humans in the metropolitan area of Bucaramanga, Colombia by analyzing the presence of circulating antigens of D. immitis within the canine population, and the response of anti-D. immitis and anti-Wolbachia IgG antibodies in the human population as a study model for other places in Latin America.
The mean prevalence in dogs was 10.82%. The highest prevalence was found in the Floridablanca area (13.7%), followed by Girón (12.9%), Piedecuesta (12.5%), and Bucaramanga (7.6%). These areas are surrounded by vegetation and are characterized by an average annual temperature of 24 ºC, high humidity levels, several gullies in their proximities or even in their interior that accumulate water in the rainy season, and the presence of two rivers (Oro and Surata in the areas of Girón and Bucaramanga, respectively). These conditions could promote breeding of these mosquitoes and disease transmission in said areas. Furthermore, prevalence in microfilardemic dogs was 5.12%, which was heterogeneously distributed across both sexes, within the different age groups, in the four municipalities, regardless of the different socioeconomic status of their owners, and the dogs’ lifestyle; thus, none of the analyzed factors are primarily responsible for transmission in dogs. This situation is similar to that observed in other endemic areas where the disease has been reported (2,4,7,8,10,27,28,29].
Regarding human infections, mean seroprevalence was 6.71% and the highest prevalence was 11.7% in Piedecuesta, followed by 6.9% in Bucaramaga, where prevalence in dogs reached similar values (12.5% and 7.6%, respectively); moreover, we observed slightly lower seroprevalence values of 5.7% in Floridablanca and 3.1% in Girón. where the prevalence observed in dogs was 13.7% and 12.9%, respectively. These data are related to the geographical location of the samples, and humans with positive serology have been reported in the same location as infected dogs. In addition, there are spatial clusters in these areas with a relative risk of < 1 for humans, suggesting a positive association between the variables studied and a higher frequency of contact with the parasite. Thus, these data suggest a direct relationship between the presence of dogs and humans infected by D. immitis. This is similar to what occurs in other European areas such as Spain, Portugal, Romania, and Russia, where the risk of infection among humans has been studied [4, 7, 8, 27, 28, 29].
Regarding the variables evaluated, sex may be a risk factor for human exposure to D. immitis. In our study, we found that seropositive individuals were mostly women. There are several studies that suggest that serological screening in humans should be carefully interpreted [7, 8, 28, 29]. Further, we observed that age can be a risk factor. In our study, the population with the highest seropositivity was that from the age group of 16–34 years. However, other studies report that the risk of infection increases with age [4, 7, 8, 27, 28, 29]. Humans within this age group perform more activities outside their homes and in areas with vegetation. However, it is necessary to analyze this fact more closely, and more data and optimization of serological screening are required in future studies.
Furthermore, not only did this study allow us to address the problem from a biological point of view but also from a socioeconomic point of view in case of humans. The highest seroprevalence was observed in stratum 1, where sanitary hygiene conditions are not adequate (20%), followed by stratum 2 (8.3%), stratum 3 (5.7%), and stratum 4 (4.1%). Seropositive individuals were not detected in the last two strata where the sanitary hygiene level is optimal. Socioeconomic status has been associated with mortality and the use of health services, which indicates that a lower income reduces the application of prophylactic and preventive measures to vectors and canines that live with humans (González de Haro, 2006). Environmental sanitation elements, such as water; sewage, garbage, and waste disposal; sanitary landfills; and garbage treatment, influence the prevalence of parasitosis. These data allow us to associate the lack of sanitary hygiene with the development of dirofilariosis, which may become a socially determinant public health factor, as in the case of other vector-borne diseases in Colombia such as malaria, leishmaniosis, Chagas disease, and human ascariosis [9, 30, 31].