Seroprevalance of Trichinella spp. in wild boars (Sus scrofa) from Bihor county, western Romania

The wild boar (Sus scrofa) has one of the widest geographical distributions of all terrestrial wild mammals with increasing populations in all European countries (Massei & Genov, 2004). Moreover, they are considered to be an important source of Trichinella spp. infection in humans (Boadella et al., 2011). In Romania, trichinellosis, is considered “a never-ending story” because of the local traditions and culinary customs that favor the parasites transmission to humans. Especially the consumption of undercooked meat and meat products has led to various outbreaks over the years (Neghina, 2010).

The magnetic stirrer digestion is the gold standard method for the direct detection of Trichinella larvae in meat samples (Gamble at al., 2000). This method is mandatory for meat inspection according to EU regulation (EU 2015/1375). Enzyme linked immunosorbent assay (ELISA) using excretory-secretory (ES) antigen is a highly sensitive test for the detection of circulating antibodies against Trichinella spp. in blood samples from pigs and wild boars (Kapel & Gamble, 2000; Kapel, 2001). On the other hand, Western blot allows the identification of Trichinella infection by highlighting bands characteristic of the genus (Cuttell et al., 2014). However, this method is time consuming and is mostly used as a confirmatory assay. Successful Trichinella spp. diagnosis and controls are important for ensuring food safety and for preventing future out-breaks (Gajadhar et al., 2009).

Trichinelloscopy and artificial digestion were previously used for the identification of Trichinella spp. in wild animals in Romania (Blaga et al., 2009a; Blaga et al., 2009b; Ciobotă et al., 2015). Therefore, the objectives of this study was to assess the presence and prevalence of Trichinella spp. in wild boars from Bihor County, Romania, by using artificial digestion and serological methods.

All applicable national and institutional guidelines for the care and use of animals were followed.

Among the 84 animals included in the study, 45 (53.6 %) were females (38 adults, 7 juveniles) and 39 (46.4 %) were males (30 adults, 9 juveniles). All the samples were negative when artificial digestion was conducted. In ELISA 54 samples (64.2 %; 95 % CI: 53.7 – 74.2) were positive and 6 samples (7.1 %; 95 % CI: 1.4 – 12.5) were doubtful. Western blot was performed on 26 random samples, from which only 6 samples (23.0 %; 95 % CI: 5.6 – 40.3) gave a positive result. All the positive samples had three bands which were situated in the margin of 46 – 56.8.kD. Furthermore, ELISA and Western blot showed different results in the 26 samples tested. Thus, from the six samples that were positive in Western blot, only five were positive in ELISA. In addition, 11 (42.3 %; 95 % CI: 31.1 – 78.8) of the samples that were negative in Western blot, were positive in ELISA, as shown in Table 1. A Kappa value of 0.18 was found between ELISA and Western blot results, which demonstrated a slight agreement between the two methods used. Also, a Kappa value of 0.0 was observed in case of Western blot and artificial digestion, but also in case of ELISA and artificial digestion, respectively showing no inter-rated agreement between the methods chosen for diagnosis. Moreover, the seroprevalence of Trichinella spp. was statistically higher in females than males, when ELISA test was used, at a p value < 0.01.

In case of artificial digestion, the lack of other regional muscles samples that are recommended in EU regulation can be responsible for the negative results. Kapel (2001) suggested that diaphragm and tongue muscles were the predilection sites in wild boars, independent of Trichinella genotype and infection level. Blaga et al., (2009a) observed that in Romania from 1997 to 2004, the prevalence rate of Trichinella spp. infection in wild boars tested by trichinelloscopy was 8.6 %. Another study conducted from 2010 to 2014 in Hunedoara county, Romania, indicated a prevalence rate of 0.82 % for Trichinella spp. infection in wild boars after the examination of intercostal muscle samples by trichinelloscopy and artificial digestion (Ciobotă et al., 2015). Moreover, another study in which the preferential muscles samples were tested with trichinoscopy and artificial digestion, found the presence of Trichinella larvae in five wild boars. In three of the animals, T. britovi was identified, while the remaining two animals were diagnosed with T. spiralis infection (Blaga et al., 2009b). Some recently collected larvae in Romania, during the artificial digestion of diaphragm samples from pigs and wild boars evidenced the circulation of T. spiralis and T. britovi in these animal species (personal communication, unpublished data).

Although artificial digestion was negative, ELISA and Western blot showed high rates of positivity. Indirect ELISA is commonly used for the surveillance of Trichinella spp. infection (Elbers et al., 2000; Gamble et al., 2004; Kim et al., 2015) and it can detect infection levels as low as 1 larvae/ 100g of tissue (Gamble et al., 1983). Møller et al. (2005), by using indirect ELISA, detected Trichinella specific antibodies in sera and muscle fluid samples from wild boars. Moreover, ELISA based on carbohydrate tyvelose or excretory/secretory antigens, have been considered the most suitable antigens available (Forbes et al., 2004). However, the assay yields false positive results due to cross-reactivity of the secondary antibodies (Nöckler et al., 2004; Davidson et al., 2009; Széll et al., 2012). Therefore, ELISA positive results are recommended to be confirmed by Western blot due to its specificity of 100 % and the capacity to eliminate ELISA false-positive sera results (Gómez-Morales et al., 2012; Cuttell et al., 2014).

Taken together, these results indicate the circulation of Trichinella larvae at a low level in wild boar population of Bihor County. The larvae burden might be too low to be detected by direct artificial digestion whereas serological approaches allow to detect circulating antibodies or the presence of Trichinella spp. specific antigens. However, other studies observed a higher seroprevalence percentages, than the positivity rates observed in Western Blot or muscle digestion (Davidson et al., 2009).

In Romania, the human population can acquire trichinellosis mainly by consuming raw pork and wild boar meat. The meat from these animals are usually prepared accordingly to local habits and traditions. Commonly, the wild boar meat products (dried raw salami, raw dried sausage, smoked pastrami e.g.) are not subjected to thermal processing, exposing the population to parasitic infestations. In 1993, controlled pig meat was mixed with infested wild boar meat, causing an extensive outbreak in Romania (Cironeanu & Ispas, 2002). Moreover, in January 2007, in the city of Timisoara, 5 humans acquired trichinellosis after consuming sausages with infected wild boar meat mixed with pork (Neghina, unpublished data).

Epidemiological actions that limit the spread of the infestation to humans and animals should be widely available and correctly applied by pig breeders, hunters, and consumers to prevent future outbreaks. These measures should include hygiene conditions in backyard pig breeding and raising, testing of wild boar meat from hunters and the consumption of only controlled wild boar meat by the local population (Neghina, 2010). However, in countries with high percentages of Trichinella spp. infection, in both animals and humans, an effective diagnosis method is essential for providing food safety and preventing the onset of human outbreaks. Even if trichinelloscopy is a widely used method for Trichinella spp. detection, its sensitivity is lower than that of artificial digestion (Li et al., 2010). The digestion assay is considered to be the most reliable procedure and it can be widely applied in practice (Gamble et al., 2000).

The EU Regulation 2015/1375 highlights the importance, in case of wild boar meat, of using a sufficient amount of samples from the predilection sites, meaning foreleg, tongue or diaphragm muscles. In this regard and giving the recommendation of using at least 10 g of muscle tissue from each animal, is essential to ensure a correct training for the personnel that is responsible for the sample collection and inspection.

Serological evidences confirmed the exposure of wild boars from Bihor County, to Trichinella spp. Therefore, the consumers might be at risk to ingest low larvae burden as animals are misdiagnosed after the parasitological diagnosis by artificial digestion. To prevent future misdiagnosis by using artificial digestion, only trained personal should perform the sampling and the digestion assay protocol. Human trichinellosis is continuously reported in Romania, consequently, risk factor assessment and surveillance should be improved in wild boars and other wild species to monitor the parasite presence and prevalence.