DISTRIBUTION OF TRICHINELLA INFECTIONS IN PIGS AND TRICHINELLOSIS IN HUMANS IN SERBIA FROM 1994 TO 2018

Introduction . Trichinellosis is a disease in humans caused by parasites of the genus Trichinella , and these roundworms can occur in a variety of animals (over one hundred mammal species). Members of the genus Trichinella epizootiological regions, and only 12.69% were diagnosed in the non-epizootiological regions in Serbia. During the period 1994-2018 in Serbia, a total of 6,850 people were treated for Trichinella infection. Out of this number, 4,153 (60.63%) people were from the five epizootiological regions. The trend-line describing the presence of Trichinella in pigs was defined by a fourth degree polynomial function. Meanwhile, the trend-line describing the presence of trichinellosis in humans was defined by a sixth degree polynomial function. Trichinellosis in Serbia is most common during the winter season, from December to March.


INTRODUCTION
Trichinellosis is a disease in people caused by roundworms from the genus Trichinella. In Serbia, Trichinella has been present in the domestic pig population for a long time.
Over an observed period of 25 years, it was constantly present along the major river catchment areas in the country (Sava, Danube, Velika Morava, and Drina) (Sofranić et al., 1997). In the epizootiological Srem and Podunavsko-Braničevska regions, Trichinella is sporadic, especially along the left riverbank of the river Sava, and the right riverbank of the river Danube. In the last decade of the 20 th century due to the proximity of the war zone, the economic crisis and political instability, there was an expansion of trichinellosis in humans and Trichinella in pigs (Teodorović et al., 1999). The closure of large pig farms and poor veterinary and sanitation measures resulted in the free spread of the parasite. Other factors such as inadequate pig feeding protocols, extensive pig breeding and the lack of financial competition in the pig industry resulted in large numbers of infected pigs. This expansion of Trichinella infections reached alarming proportions.
The aims of this study were to determine the distribution of Trichinella infections in pigs and trichinellosis in humans in five epizootiological regions in Serbia over a 25-year period and to establish trends for Trichinella infections in pigs and humans. For this research we utilized the local administrative setup, and the organization of municipalities into epizootiological regions in which all veterinary health measures are carried out by veterinary clinicians and/or specialist institutes. Hence, according to the veterinary inspectorate and the veterinary scientific and specialist institute databases, five epizootiological regions are defined (Mirilović et al., 2005). In these areas, Trichinella infections have long been present in pigs and humans. The five mentioned epizootiological regions are West Bačka, South Bačka, Srem, Mačva-Kolubara and Podunavsko-Braničevska.

MATERIALS AND METHODS
Trichinella surveillance data  covered Trichinella cases registered in pigs and trichinellosis in humans. Data were obtained from the Veterinary Directorate (Ministry of Agriculture, Forestry and Water Management) on the number and location of infected pigs. Data on human trichinellosis cases were obtained from the Institute for Public Health of the Republic of Serbia (Health Statistical Yearbook of Republic of Serbia 1995Serbia -2018.
Analysis of the obtained data was conducted using relative numbers for structure and dynamics, indices and descriptive statistical indicators. Trends in the changes in the number of infected pigs and human cases in the examined period are given as linear or polynomial equations. The most appropriate trend line was chosen according to the value of the correlation coefficient. Statistical analysis was performed with the support of the statistical package IBM SPSS 18. The overall trend of Trichinella in pigs in Serbia from 1994 to 2018 is shown in Figure  1, and trend analysis reveals the occurrence of Trichinella in pigs in Serbia is falling.

Numbers of registered
The average decrease in the number of infected pigs for the whole study period  was 47.65 pigs per year. During the study period, the disease in pigs had several cycles, but for the entire study period, the best-fit line was a fourth-degree polynomial, with a correlation coefficient of 0.91.
Partial trends of Trichinella occurrence in pigs in Serbia from 1994 to 2018 are shown in Figure 2. Between 1994 and 2018, the occurrence of Trichinella in pigs had several cycles ( Figure 2). During the first five-year period (1994)(1995)(1996)(1997)(1998), Trichinella occurrences in pigs increased, on average, by 373.80 infected pigs annually. In the next five-year period (1999)(2000)(2001)(2002)(2003), an opposite trend was found ( Figure 2). With the exception of 1999, when there was an increase in the number of infected pigs, numbers of infected pigs declined throughout 1999-2003 by, on average, 17.70 infected pigs annually.
Between 2009 and 2013, the average annual reduction in the number of infected pigs was 28.70 animals. The last period from 2014 to 2018 saw Trichinella-positive pig numbers increase again (the average annual increase was 2.20 pigs).
Among 6,850 reported human trichinellosis cases from 1994 to 2018, 56% (3,865) were in male and 44% (2,985) were in female patients. In the first half of the study period (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005), a significantly higher number of patients (5,581) was reported than in the second half of the study period (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018), when 1,269 people became  ill. The distribution according to sex of human patients with trichinellosis in Serbia from 1994 to 2018 is presented in Table 2. The annual prevalences of human trichinellosis cases in Serbia from 1994 to 2018 are presented in Figure 3. Trend analysis showed the prevalence of trichinellosis in humans in the studied period declined by, on average, 27.84 cases annually. The best-fit line for the entire study period is a sixth degree polynomial with a correction coefficient of 0.85. The highest number of diseased patients was reported in 1997 (766 patients), while the lowest number was reported in 2018 when only 11 people became ill.
The age structure of patients with trichinellosis in Serbia from 1994 to 2018 is shown in Figure 4. The highest number of patients were between 20 and 39 years old (1,227, i.e. 36.00% of all cases). Among the age groups, people from 20 to 60 years of age most often suffered from trichinellosis ( Figure 4). During this life period, 4,742 people became ill, i.e. 69.3% of all patients.
The distribution according to reporting month of patients with trichinellosis in Serbia from 1994 to 2018 is shown in

DISCUSSION
The five observed epizootiological regions in Serbia spread over a total area of 20,777 km 2 , so cover 23.51% of Serbia's total area. Mačva-Kolubara is the largest epizootiological region (5,742 km 2 ), covering 27.63% of the five epizootiological regions, while the smallest epizootiological region is West Bačka, accounting for 11.65% of these examined regions. The total human population of the five observed epizootiological regions is 2,076,200, i.e. 27.69% of Serbia's total population.
The high percentage of infected pigs reported in the five epizootiological regions compared with Trichinella occurrences in pigs in the rest of Serbia is striking. Within these epizootiological regions, Trichinella occurred most frequently in the Srem and Podunavsko-Braničevska regions. Altogether, 65.45% of all Trichinella-positive pigs were diagnosed in these regions, but they account for only 9.72% of Serbia's territory. A similar situation was found for the occurrence of trichinellosis in humans. A total of 6,850 human cases were notified in Serbia during the study period. Of these, 4,153 human trichinellosis cases (60.63% of human cases in the country) were resident in the five epizootiological regions.
The overall decrease in the number of Trichinella-positive pigs is likely a consequence of intensive application of zootechnical measures in pork production and the continuous pest control measures implemented throughout most of Serbia. Nonetheless, there was a large increase in the number of infected pigs in the first five years of this study, likely largely a result of the civil war in the Balkans, which resulted in a large migration of wild animals from their natural habitats to areas where domestic pigs live. Additionally, during this period there was a decline in intensive pig production, pigs were fed inadequate feeds, and biosecurity measures in pig breeding were not adequately applied. Probably as a consequence of all these circumstances, there was a constant increase in the number of Trichinella-infected pigs.
In the next five-year period (1999)(2000)(2001)(2002)(2003), numbers of Trichinella-positive pigs generally declined, although with the exception of 1999, when the number of infected pigs increased. During this five-year period, pig production stabilized, a change from smallholder pig production systems to the modern indoor farming systems occurred, better rodent control and biosecurity measures were implemented and the animals were fed good quality diets.
After the turbulent first ten years of the examined period, there was a lull in the occurrence of Trichinella in pigs and a significantly smaller number of infected pigs were diagnosed in the next 15 years. From 2014 to 2018, Trichinella-positive pig numbers have increased again, but it is interesting to note that Trichinella in pigs now occurs in some new, non-epizootiological regions, primarily in the south and southeast parts of Serbia.
Declining numbers of human trichinellosis cases in Serbia could be due to raised awareness of the harmfulness and consequences of consuming insufficiently thermally processed meat and meat products. It seems likely that preventive measures, education programs, implementation of good production practices and regular inspection of slaughtered pig carcasses for Trichinella have been fruitful. The large polynomial of the best-fitting trend line for the human case data indicates the large, almost annual, variation in the incidence of trichinellosis in humans.
The age-dependent occurrence of trichinellosis in humans, whereby the majority of cases were between 20-60 years of age, was expected; people of this age are most often in contact with meat harbouring infectious Trichinella.
Trichinellosis in humans occurs seasonally in Serbia. Specifically, this disease occurs most frequently in the period from December to March. This was also expected because this is the month when meat and meat products are traditionally prepared for the coming year. People are probably most often infected by consuming undercooked cooked meat (tasting sausages, under-fried and roasted meat at the time of pork processing) or consuming cured products (sausages, ham, roast meat etc.) containing infectious larvae. In the other months (April to November), the infection rate is much lower since the intensity of pig slaughter and meat processing for household needs is also lower.

CONCLUSION
Altogether, 14,837 Trichinella-positive pigs were diagnosed in Serbia from 1994 to 2018. Of these infected pigs, 87.31% were diagnosed in five observed epizootiological regions. The number of Trichinella-positive pigs for the whole study period  decreased, even though the number of Trichinella-positive pigs increased during the first five years of the examined period.
A total of 6,850 people developed trichinellosis in Serbia from 1994 to 2018. Of these, 4,153 cases (60.63%) were resident in the five epizootiological regions. However, over the entire study period, numbers of reported trichinellosis human cases and Trichinella in pigs both decreased. This situation is likely the result of the implementation of continuous control and prevention measures. Trichinellosis mostly affected people in their most productive age range, from 20 to 60 years of age. Also most commonly, trichinellosis occurred in January, since in our study, 2,371 human cases were notified in this month, constituting roughly one-third of all patients in the examined 25-year period.