Human cystic echinococcosis, trichinellosis, and toxocariasis in Bulgaria: an update of data for 2015-2017

Study design

The study encompasses a period from January 2015 to December 2017 and was conducted at the Department of Parasitology and Tropical Medicine (DPTM) at the National Centre of Infectious and Parasitic Diseases (NCIPD), Sofia, Bulgaria. Aggregated data on all individuals in the country infected with Trichinella spp., Echinococcus granulosus, and Toxocara spp. were collected from the annual reports issued by the Regional Health Inspectorates (RHI) and from the own records of NCIPD on the performed through the period routine and confirmatory serological tests.

 

Patients

The study includes data of all confirmed cases of cystic echinococcosis and trichinellosis in Bulgaria, diseases that under national law are subject to mandatory notification. Also, we included DPTM data from the serologic tests for toxocariasis in subjects suspected of the visceral or ocular form of the disease.

 

Parasitological diagnosis

Diagnostic methods applied in cases of CE were fully consistent with the recommendations of the WHO [11]. Used were imagine techniques (ultrasound, computed axial tomography, magnetic resonance, radiography), immunological tests (commercial enzyme-linked immunosorbent assay (ELISA), and commercial Western blot, according to the manufacturer instructions), microscopic examination of hydatid fluid and methylene blue test and PCR methods for species identification of E. granulosus.

The parasitological diagnosis of trichinellosis and toxocariasis in humans is performed by immunological methods: commercial enzyme- linked immunosorbent assay, and commercial Western blot, according to the manufacturer instructions. Also, at NCIPD is carried out species identification of Trichinella larvae with biomolecular methods [12].

 

Data analysis

The data analysis focused on the incidence, age, and gender of patients. Mean, standard deviation, standard error, and confidence interval of mean (95%) were used. The statistically significant difference in age and gender distribution was analyzed by Mann Whitney test. Values of p<0.05 were considered statistically significant.

Cystic echinococcosis

For the studied by us period in Bulgaria were recorded 800 confirmed cases of CE, of which 715 (89.4%) primary and 85 (10.6%) cases of relapse. Table 1 presents the distribution of the cases by age and gender. Of all patients, 168 (21%) were children and adolescents. The most affected age groups were 25-29 years (N=67, 8.4%) and 30-34 years (N=74, 9.2%). By gender 51.5% of the patients were female and 48.5% male. Annual morbidity ranged from 4.35 in 2015 to 3.1 per 100,000 in 2017 (mean annual morbidity over the period, 3.56 per 100,000). During the period was recorded one case with a fatal outcome, a 34-year-old woman died from anaphylactic shock as a result of a spontaneously ruptured hydatid cyst in the liver. In terms of localization and number, a solitary cyst in the liver was found in 561 cases, solitary cyst in the lungs in 156 patients, in 13 the cyst was in the spleen, eight patients were with solitary cyst in the kidney, in two in the mesentery and in another 10 patients’ solitary cysts were found in the myocardium, CNS, mammary gland, subcutaneous tissue, and ovary. Ten patients were with cysts affecting the lung and the liver and 40 patients were with multiple cysts localized in different parts of the body.

 

Trichinellosis

Ten outbreaks of trichinellosis were recorded in 16 settlements of 10 regions of the country. Contaminated with Trichinella larvae meat was consumed by a total of 293 persons, of whom laboratory confirmed infection was found in 113 patients (38.6%), (Table 2). The age groups from 4 to 74 years were affected, but the largest share was of the groups 35-39 (15%) and 40-44 years (16.8%). The proportion of infected children and adolescents was 13.3%, and by gender distribution, the number of infected males was higher (67.3%). The registered annual incidence from trichinellosis was 0.31 per 100,000 (2015), 0.5 (2016), and 0.78 (2017). In five outbreaks source of infection was domestic pork, in two, meat from wild boar, and in three outbreaks the source remained unknown. In three trichinellosis outbreaks, the species of the causative agent was identified by biomolecular methods performed at the NRL of NCIPD. In one outbreak was isolated T. spiralis from the meat of domestic pig and in the other two outbreaks, T. britovi was isolated also from the meat of domestic pigs. In the other seven outbreaks, the Trichinella species was not established due to lack of material to be investigated. All infected persons with a laboratory-confirmed diagnosis were treated with albendazole in standard doses and regimens.

Parasitic zoonoses are diseases posing a major challenge to public health systems in endemic countries. For Bulgaria, of great medical and social significance are the cystic echinococcosis and trichinellosis. For the studied by us three-year-period the average incidence was 3.56 per 100,000. Comparing the data from our previous study covering the period 1996-2013 with an established mean incidence of 6.7 per 100 000 [13], it can be ascertained that there is a nearly twofold reduction in human morbidity. This is also confirmed by the European Union Synthesis Report on Trends and Sources of Zoonoses, Zoonotic Agents and Food-Related Epidemic Outbreaks for 2016, according to which cases from Bulgaria decreased by 30.3% from 2008 to 2016 [6]. We consider that the decreasing annual incidence of CE in the country is a result of the recently implemented National Program for Control of Cystic Echinococcosis in Humans and Animals (2004– 2008). Nevertheless, the incidence of CE in Bulgaria is much higher than in other EU member states. Whereas in 2015 the total annual incidence in the EU Member States is 0.2 per 100,000, the incidence in Bulgaria is 4.35 per 100,000 [14]. Similar is the situation in 2016 with a recorded incidence in the EU of 0.19 per 100,000 and 3.23 per 100,000 for Bulgaria [6].

 

Of great concern is the high relative share of infected children and adolescents: 21% for the period 2015-2017, and 21.4% for 1996-2013, because this is an indicator of high disease transmission rates [13]. On one side, the possible reasons are the insufficient hygienic habits of the children and their frequent contact with dogs for play. On the other, the high incidence of CE in the country is due to a large number of dogs (wandering, yard, sheep) that are not regularly dewormed, particularly in areas with developed livestock breeding and especially sheep breeding [15]. The highest average incidence rate of CE was recorded in three regions: Sliven (14.11 per 100,000), Kardzhali (13.4 per 100,000), and Dobrich (9.24 per 100,000). In all three areas, sheep production is well developed. The territory of Sliven and its region falls within South-East Bulgaria, where 21.5% of sheep are grown in the country, Kardzhali is within the South-Central region, where 23.2% of the sheep are grown, and Dobrich is located within the North-Eastern region of the country, where 14.9% of the sheep are bred [16]. It is clear that the areas with the highest incidence for our survey period are located in territories where about 60% of sheep in the country are grown. It is estimated that in 2016, 4.36% of the sheep in Bulgaria are infected with E. granulosus [6].

 

Another interesting fact is that these areas are also characterized by ethnic heterogeneity of the population in them. In all of them, live compact masses of Roma origin. In our view, the cultural and social peculiarities of these groups have a significant impact on the prevalence of the disease among people. This view is supported by the fact that in the Northwest and Southwest regions of the country where the relative share of sheep breeding is over 30%, the incidence of CE is significantly lower - 2.57 per 100,000 and 3.02 per 100,000 [16]. The prevalence and the intensity of the parasitic infections are important indicators for the level of the epizootic process. In countries where these indicators are low (Germany, Sweden, Norway), echinococcosis among farm animals and humans is rare. The epizootiological and epidemiological significance of the dogs as a source of echinococcosis is determined by their purpose. The shepherd dogs are the most important as final hosts, because of the relatively high frequency of infection and their constant and close contact with the sheep [15]. There is no single register for the number of dogs in Bulgaria. Data from annual dog enumeration display a decrease in the number of wandering dogs in the country from 31,435 in 2011 to 26,890 in 2017 [17].

 

Regarding the clinical manifestation and organ localization of the hydatid cysts, our study has the following data: 93.75% (N=750) of patients had a single cystic lesion located in a single organ. The liver was affected in 70.13% of cases. The second common place for cyst localization was the lung, 19.5%, followed by the spleen, 1.62%. With the involvement of both, lung, and liver were 1.25% of the patients, and 5% (N=40) were with multiple cysts localized in different parts of the body. In this respect, our data are similar to those of other authors [18-20]. The relative proportion of recrudescence was 10.62% (N=85), and our findings are similar to those of Velasco- Tirado et al., according to which the relative share of relapses after invasive treatment procedures is 11% in Spain [21], and Prousalidis, et al. who observed relapses in 8.7% of the patients with CE in Greece [22]. In Bulgaria, all persons treated with surgery or PAIR technique (Puncture, Aspiration, Injection, and Re-aspiration) are assigned with relapse preventing prophylaxis with albendazole, the duration is determined individually in each patient, depending on the clinical manifestation of the disease. Nevertheless, the number of relapses is relatively high. Regrettably, in the three-year period of our study, there was a case with a fatal outcome by anaphylactic shock, a consequence of a spontaneously ruptured hydatid cyst in the liver. Echinococcosis is a serious social problem with many dimensions: human, medical, veterinary, economic, and environmental. We believe that from the side of human medicine, there are no particular difficulties with the diagnosis, treatment, and surveillance of the disease. Measures to reduce the incidence include enforcement of the preventive actions by the public health authorities with media collaboration and involvement.

 

Trichinellosis is the second most important parasitic zoonosis in Bulgaria. Outbreaks of human trichinellosis are recorded annually in the country [23]. Through the period of our study were recorded ten outbreaks with 113 infected persons, of which 86 had clinical symptoms and 27 were with an asymptomatic form of the disease. The food-specific attack rate we established for each year is 9.64% (2015), 36.92% (2016), and 37.24% (2017). When determining this indicator, we included only cases with clinically manifested illness. Nearly all age groups were affected (Table 2), and the proportion of infected adults was significantly higher (86.73%) than that of infected children and adolescents (13.27%). The age of infected patients varied from 4 to 74 years, with the largest number infected in the 30-44 age group (N=48, 42.48%). The gender distribution displays that majority of the patients were males (N=76, 67.26%) than females (N=37, 32.74%) but without statistically significant difference between them (Mann Whitney test: p=0.9356). The annual incidence displayed a trend to rise for our study period: from 0.31 per 100,000 (2015) to 0.79 per 100,000 (2017). By this indicator, Bulgaria also holds a leading position among the EU member states, where the average annual incidence in 2015 was 0.03 per 100,000 and 0.05 in 2016 [6,14].

 

Meat from domestic pigs was a source for infection in five of the outbreaks, wild boar meat was a source in two outbreaks and in tree source was not identified. In our opinion based on our previous studies [23] and the present one, a prerequisite for occurrence of outbreaks and cases of trichinellosis among the population in Bulgaria is the widespread in rural areas practice to breed domestic animals at home and to slaughter them in the back yards without provision of veterinary sanitary control over the harvested meat. Тhe lack of stringent sanctions against violators of the meat production and poachers of wild animals is also important. Despite the systematically conducted educational work by human and veterinary doctors on trichinellosis among the population, the knowledge about the disease and its prevention is still far from a satisfactory level.

 

Toxocariasis is a parasitic zoonotic disease with various clinical manifestations, protracted course, and common recurrences. In Bulgaria the exact incidence of human toxocariasis still not established, but significant information has been gathered over the last 10 years [24]. For 2013-2014, by ELISA method for anti-Toxocara antibodies were tested 466 persons. Most of them were with clinical symptoms. A positive result was found in 90 of them (19.3%) [25]. Data obtained by the present study are similar. The proportion of positive patients is 19.1%, and most of them are males (59%). The relatively high number of seropositive individuals could be explained by the fact that subjects of the study were not randomly selected, but were patients with different clinical symptoms that could be associated with pathology caused by Toxocara larvae. Despite numerous literature data that younger people are a more vulnerable group in terms of toxocariasis [26], in our study the relative share of seropositive adults was almost equal to this of seropositive children and adolescents. Out of 78 children and adolescents, seropositive was 17.95%, whereas out of the 371 adults, 17.4% were with the presence of anti-Toxocara antibodies, and there is no statistically significant difference between the two groups (Mann Whitney test: p-value=0.1000). In contrast to age distribution, our data is similar to the literature regarding seroprevalence between urban and rural residents. Of the 363 surveyed urban residents presence of antibodies was found in 17.35% (N=63), while among 45 residents of rural areas the established seroprevalence was 33.33% (N=15). In Bulgaria, however, this division is quite conditional, because many urban residents have close links with rural areas (possessions and relatives) and frequently visit them on weekends and vacations [27].

 

The parasitological diagnosis of human toxocariasis is hampered by the fact that the parasite does not reach sexual maturity in the accidental human hosts, their eggs are not laid in the feces and microscopic methods cannot be used. For this reason, immunological methods (along with clinical evaluation) are leading in the diagnosis of the disease, mainly using ELISA to prove specific IgG antibodies and Western blot [9]. When interpreting the results of serological tests, it should be borne in mind that the establishment of a positive result in a serological survey has limited diagnostic significance. Only patients with clinical signs compatible with toxocariasis are appropriate for treatment due to the fact that with the available immunodiagnostic tests it is not possible to distinguish between recent and latent infection. Therefore, the serological test should be accompanied by the assessment of the level of eosinophilia and, if possible, by determining the total serum IgE. All patients with clinical symptoms of toxocariasis and positive results in ELISA and Western blot have been treated with albendazole in classic doses and regimens in combination with antihistamines and/or corticosteroids.