Stray dogs of Sofia (Bulgaria) could be an important reservoir of heartworm (Dirofilaria immitis)

Dirofilaria immitis is a parasitic nematode that causes heartworm infection. It is a vector-borne disease transmitted by culicid mos- quitoes and that mainly affects dogs and cats. Furthermore, D. immitis is a zoonotic parasite that causes pulmonary dirofilariosis in infected humans (McCall et al., 2008; Simón et al., 2012). Heartworm is a worldwide distributed infection. Due to the humid- ity and temperature required by mosquito vectors, the disease is mainly present in tropical and subtropical areas. In Europe, D. im- mitis is endemic in southern countries (Genchi et al., 2009); howev- er, heartworm is considered an emerging disease and is spreading toward eastern, central and northern Europe, affecting countries or areas previously considered free of the parasite (Morchón et al., 2012). Several factors have been reported as possible causes of this spread, such as climate change, the development of human activity for agricultural or urban uses in new areas, the growing movement of microfilaremic dogs throughout Europe or the introduction of new species of mosquitoes able to act as vectors. Also, infected wild animals can be reservoirs of the disease (Morchón et al., 2012; Simón et al., 2012).

Bulgaria is located in southeastern Europe. The country has mainly humid continental and oceanic climates (Kottek et al., 2006; Penin, 2007). Sofia is the capital of the country and is located in western Bulgaria, in the Sofia Valley, surrounded by the Balkan Mountains to the north, and Lyulin, Vitosha and Lozenska mountains to the southwest. The average altitude of the valley varies from 500 to 2290 meters and, unlike most of the European capitals, Sofia has no large rivers crossing it, but several small ones. Sofia has a humid continental climate, with cold, snowy winters and warm, sunny summers. The average annual temperature within the city is 10.6°C (Kottek et al., 2006; Penin, 2007). There are 1.5 million people living in the city and 1.7 million people living in its metropolitan area (Eurostat 2018a; 2018b).

Previous publications have shown presence of heartworm in Bulgaria. The first cases in dogs were documented in 1997 – 1999 and, since then, the presence of D. immitis has been described and studied in some regions of the country (Pantchev et al., 2015). Georgieva et al. (2001) reported a canine prevalence of 7.4 % in the Stara Zagora region. Between 2001 and 2006, microfilaremia was detected in 8.62 % of dogs from different Bulgarian districts (Kirkova et al., 2007) and there were 87 registered cases of canine heartworm in Plovdiv and surrounding regions (Kostadinov, 2007). Between 2012 and 2014, 34.3 % of the samples sent to a parasitology laboratory tested positive for D. immitis (Iliev et al., 2017) and a study determined 15 % of prevalence in 33 dogs from a shelter in Sofia in 2013 – 2014 (Radev et al., 2016). In 2015, a canine prevalence of 16.2 % in the Stara Zagora region was described by Pantchev et al. (Pantchev et al., 2015). The disease has also been observed and studied in wild carnivores, being described high prevalences in foxes and jackals (Kirkova et al., 2007; Mirchev et al., 2013; Panayotova-Pencheva et al., 2016).

These studies show the presence of heartworm in Bulgaria. However, there is a lack of current epidemiological data of canine D. immitis in Sofia. Therefore, the aim of this study was to obtain the prevalence and distribution of Dirofilaria immitis in dogs in the capital of Bulgaria and its metropolitan area.

The present study included 80 stray dogs from different parts of the city of Sofia and its metropolitan area. The animals were captured between December 2017 and February 2018 for routine neutering campaigns that take place in Sofia. A complete record was kept for each animal, including identification, age, sex, breed and location. The inclusion criteria were being dogs over 7 months of age and had not been treated with macrocyclic lactones.

Of the included dogs, 45 % were female and 55 % were male. The age ranged from 1 to 19 years old (mean: 7.1 years). Animals were further divided into 3 groups of age, from 1 to 4 years (n=25), from 5 to 9 years (n=29), and from 10 to 19 years (n=26). There were 60 mongrel dogs and 20 pure-bred dogs. According to the distribution, 38.75 % dogs were living in the city of Sofia while 61.25 % dogs were living in the metropolitan area.

Blood samples were collected from the cephalic vein of every studied animal and all the dogs were tested for circulating D. immitis antigens using a commercial immunochromatographic test kit (Uranotest Dirofilaria®, Urano Vet SL, Barcelona, Spain). The tests were performed according to the manufacturer’s instructions. The data were analyzed using the SPSS Base 25.0 software for Windows. The descriptive analysis of the variables considered was carried out studying the proportions in the qualitative variables. The chi-square test was performed to compare proportions. In all the cases, the significance level was established at p<0.05.

The design of the study was approved by the ethical committee of Veterinary Medicine Service of Las Palmas de Gran Canaria University and was carried out in accordance with the current European legislation on animal protection.

The prevalence of D. immitis in the canine population in the studied area was 31.25 %. By sex, male dogs showed a higher incidence (36.4 %) compared to females (25 %). By breed, the most affected dogs were mixed-breed dogs (33.3 %) against purebred dogs (25 %). There were no statistically significant differences by sex or breed.

Positive dogs were found from 3 to 19 years, with an average age of 8.7 years. When age ranges were considered, the highest seroprevalences were found in the oldest part of the population, with 42.3 % of positives in dogs from 10 to 19 years old, followed by 37.9 % in dogs between 5 and 9 years. The lowest number of positive cases (12 %) was represented by dogs from 1 to 4 years (p<0.05).

The prevalence was higher in the metropolitan area of Sofia (34.7 %) than in the city of Sofia (25.8 %), although the differences were not statistically significant. The distribution of the positive cases of canine heartworm in the studied area can be seen in Figure 1.

Fig. 1

The present study reports the presence of heartworm in the stray dogs of Sofia and the metropolitan area. According to previous research, which reported prevalences from 7.4 % to 9.2 % in client-owned dogs in different regions of Bulgaria (Georgieva et al., 2001; Kirkova et al., 2007) and from 10 % to 15 % in stray dogs (Georgieva et al., 1992; 2001; Radev et al., 2016), the current results reported an increase in the prevalence of heartworm infections in the country. These results are consistent with studies that reported an increasing prevalence of D. immitis in Europe (Genchi et al., 2009; Morchón et al., 2012).

The institutions of Sofia are focused on decreasing the population of stray dogs, and there are no established control and prophylactic measures aiming to prevent the presence of different diseases, including heartworm. That is why the high prevalence reported in this study is probably influenced by the fact that all dogs were constantly exposed to mosquitoes and not receiving any prophylactic treatment. Probably, for the same reason heartworm infection was higher in the group of older animals. In 2015, it was estimated that 3844 (±10 %) stray dogs lived in the city, being one of the greatest problems of the capital (Ivanova & Gechev, 2015). Although the presence of microfilariae was not evaluated in the studied dogs, it is estimated that between 75 – 95 % of the infected dogs that do not receive chemoprophylaxis have microfilaremia (Stogdale, 1984), so these stray dogs may act as an important reservoir for heartworm, increasing the risk of infection of client-owned animals and humans living in the city.

The abundant vegetation and water reservoirs (green parks, artificial lakes and water ponds) present in the city of Sofia and in the metropolitan area may offer a perfect environment for reproduction and proliferation of the mosquito vectors which could favor the spread of the infection; furthermore, in the city of Sofia high levels of urbanization can cause the phenomenon called Urban Heat Islands (Gago et al., 2013; Yang et al., 2016) which retains heat and increase the temperature inside the city. This indirectly influences the development of D. immitis larvae in mosquito vectors during the colder months and thus enlarges the transmission season (Arnfield, 2003). In Sofia, the average annual temperature of the urban area is 0.5°C higher than that of the peripheral area; moreover, in winter the temperature in the center of the capital is 1° – 1.5° higher than that of the rural regions (Kovachev, 2005).

Although the results of this work cannot be determined as definitive, given the small size of the sample studied, they are undoubtedly indicative of a widespread presence of the parasite in the city and indicative of the need for a broader study. In this regard, there is a lack of studies on heartworm in client-owned pets in Sofia and, according to the results obtained in this study, a high prevalence of D. immitis infection in these animals should be expected. This probably is due to the lack of knowledge of the general population about the disease, updated data on its epidemiology and prophylactic measures and campaigns aimed to avoid infection. Considering the growing trend of heartworm in different regions of the country, demonstrated by this and other recent studies (Pantchev et al., 2015; Panayotova-Pencheva et al., 2016; Iliev et al., 2017), a nationwide study to learn the current distribution of heartworm disease in Bulgaria in pets should be done.

There is a high risk of human dirofilariosis in areas of high canine prevalences (Simón et al., 2012; Cabrera et al., 2018). This can be observed in the increasing publication of cases of human infections by D. repens in Bulgaria (Harizanov et al., 2014; Velev et al., 2019). This is a zoonotic parasite that is increasing in Europe and the most frequent localizations are the eye region, subconjunctival or subcutaneous tissues in other body regions, forming nodules (Genchi and Kramer, 2017). Cases of pulmonary dirofilariosis by D. immitis have not yet been reported in the country; however, the apparent increase in the canine prevalence may increase the risk of infections in humans. Therefore, awareness of the disease should be promoted through the implementation of educational and prophylactic campaigns among veterinarians and owners. Also, competent authorities should be aware and alerted about the epidemiological situation in Sofia. Furthermore, being a zoonotic disease, health agencies and institutions of the country, as well as physicians, should consider the repercussions of this infection and include human dirofilariasis in the differential diagnosis of pulmonary nodules.