A 38-year study on Trichinella spp. in wild boar (Sus scrofa) of Latvia shows a stable incidence with an increased parasite biomass in the last decade

Abstract

BackgroundTrichinella spp. are zoonotic parasites transmitted to humans by the consumption of raw or insufficiently cooked meat of different animal species. The most common source of infection for humans is meat from pigs and wild boar (Sus scrofa). The aim of the present work was to evaluate the incidence of Trichinella spp. infections in wild boar hunted in Latvia over a 38 year interval (1976 to 2013).MethodsA total 120,609 wild boars were individually tested for Trichinella spp. by trichinoscopy and, in case of negativity, by artificial digestion of 25 g muscles, in the 1976–2005 period, and by artificial digestion of 25–50 g muscles in the 2006–2013 period. Trichinella spp. larvae were identified at the species level by multiplex PCR.ResultsIn the study period, the overall prevalence of infected wild boar was 2.5%. Trichinella britovi was the predominant (90%) species. The incidence of Trichinella spp. infection in wild boar exhibited two different trends. From 1976 to 1987, the incidence of infected/hunted wild boar increased from 0.23% to 2.56%, then it decreased to 0.19 in 1994. Thereafter, the incidence fluctuated between 0.05% and 0.37%. A statistically significant (P < 0.05) correlation (r = 0.54; p = 0.0199) was found between the trend of Trichinella spp. incidence in hunted wild boar and the number of snow cover days from 1976 to 1993. From 1997 to 2013, the estimated wild boar population of Latvia increased by 4.9 times and the hunting bag by 9.7 times, with a stable incidence of Trichinella spp. in the population. It follows that the biomass of Trichinella spp. larvae and of T. britovi, in particular, increased.ConclusionsThe incidence trends of Trichinella spp. in wild boar could be related to the role played by the snow in reducing the thermal shock and muscle putrefaction which increases the survival of the larvae in muscle tissues of carrion in the 1976–1993 period; and, in the 1997–2013 period, to the increased biomass of Trichinella spp. due to the increased carnivore populations, which are the main reservoirs of these parasites.

Trichinella spp. biomass has increased in raccoon dogs (Nyctereutes procyonoides) and red foxes (Vulpes vulpes) in Estonia

“…According to data covering these three months from six weather stations located in Harjumaa, Lääne-Virumaa, Pärnumaa, Saaremaa, Tartumaa and Võrumaa, the mean number of days with snow cover was 12.7% (from 4% in Pärnumaa to 26% in Saaremaa) higher in 2002–2011 than in 1992–2001 (data received on request from Estonian Environment Agency). The snow cover could reduce the destructive effect of freezing-thawing cycles on carcasses of infected animals and thus facilitate survival of Trichinella larvae [ 65 , 66 ].…”

Epizootic and epidemiological situation of Trichinella sp. infection in Poland in 2006–2015 in view of wild boar population dynamics

“…The over 4,500 times higher frequency of trichinellosis detection in wild boar meat samples than in domestic pig samples supports the thesis that wild boars still remain the natural reservoir of the parasite. They have been documented as representing an epidemiological risk factor in Poland and abroad ( 7 , 14 , 16 , 17 , 20 ). Although Trichinella is also identified in other wild animals, especially predators, considering the alimentary mode of infection, it does not constitute a direct threat to humans from these animals.…”

“…However, it is worth mentioning that an indirect threat exists because of Trichinella transmission from the domestic stock environment to natural habitats and vice versa . Notably, most frequently it is the result of inappropriate husbandry practices that do not comply with zoohygiene regulations ( 1 , 2 , 7 , 14 , 16 , 20 , 25 ). What should also be taken into account is that trichinosis can occur in other species of wild and domestic animals, in such herbivores as beavers whose meat is edible and thus are a potential source of parasite transmission ( 29 ).…”

Trichinella and trichinellosis in Europe

“…T. britovi larvae survive less than one year in swine muscles, whereas circulating IgG can be detected in swine sera for up to two years (Pozio E., unpublished data). The short survival time of T. britovi in swine explains why muscle larvae of this species are detected in a low percentage of wild boar, e.g., 0.02% in Italy and 2.5% in Latvia, even if the prevalence in wild carnivores can be up to 50% (Gómez-Morales et al, 2014;Kirjušina et al, 2015;Deksne et al, 2016). Out of 2,900 isolates of T. britovi from Europe identified at the ITRC, about 50% (1,456) originated from domestic pigs (279) or wild boars (1,177), and the other 50% originate from wild carnivores (Table 1) (Pozio et al, 2009).…”