Trichinella spiralis prevalence among wildlife of a boreal region rapidly reduced in the absence of spillover from the domestic cycle.

Carnivore and omnivore animals are the main reservoir hosts of nematodes of the genus Trichinella. Out of the twelve taxa identified so far in the world, four, namely Trichinella spiralis, Trichinella nativa, Trichinella britovi and Trichinella pseudospiralis, are circulating in Europe. All these four species were detected in Finnish wild animals and T. spiralis also in domestic pigs and synanthropic rats of Finland up to 2004, when the last T. spiralis infection was documented in a domestic pig. In the periods 1993-1997 and 1999-2005, T. spiralis was detected in 13.6% (6/44) and in 14.5% (46/317) of infected wild animals, respectively. The aims of the present study were to investigate the prevalence of Trichinella spp., especially T. spiralis in wild carnivores of Finland, 7-9 years after the eradication of Trichinella infection from Finnish domestic pigs, and to evaluate the impact of the disappearance of domestic cycle of T. spiralis on the sylvatic Trichinella cycle(s). Trichinella spp. larvae were detected in 34.7% (1081/3112) wild animals and 835 (77.2%) Trichinella spp. isolates were identified at the species level. Trichinella spiralis was detected in 9 animals (1.0% of identified infections) as single, double or triple infections; whereas, T. spiralis had been detected in 13.6% and in 14.5% of infected wild animals in the course of the two previous investigations. The reduction of T. spiralis prevalence from 1993-1997 to 2011-2013 is statistically significant (p < 0.001) both per total tested animals and per host species. The results of this study support the hypothesis of spillover from domestic pigs as a source of T. spiralis in wildlife. The eradication of T. spiralis from the domestic cycle in Finland in 2004 has probably established a virtuous cycle reducing the prevalence of this pathogen among wild carnivores. We can expect that within some years and in absence of T. spiralis reintroduction in the domestic habitat, it might even disappear from the Finnish wildlife.

Synanthropic Trichinella infection in Finland.

The first three human trichinellosis cases in Finland were recorded around 1890, and altogether eight cases were registered until 2008. The first infected Finnish swine was found in 1954. From the early 1980s, an increasing trend in the number of infected swine was seen, with the highest number registered in 1996, after which a decrease has been observed. Infected pigs were found yearly until 2004. Since 1954, all slaughtered pigs have been tested for Trichinella, regardless of subsequent export or domestic consumption purpose. All Trichinella infections revealed in pigs are, since 1998, analysed for species by multiplex PCR. So far, all larvae from pig infections have been identified as Trichinella spiralis. During the recent decreasing trend in prevalence, the number of pig farms has also decreased, while the yearly number of slaughtered pigs has remained stable or even slightly increased. For many decades, the Trichinella prevalence in Finnish wildlife has remained high. Foxes, raccoon dogs, wolves, and lynx in the southern part of the country exhibit prevalence exceeding 50%. The most common species in wildlife is Trichinella nativa, a species with very low infectivity to swine, but also, T. spiralis, Trichinella britovi, and Trichinella pseudospiralis occur in wildlife.

The prevalence of Trichinella infection in domestic dogs in Finland.

In this study, we examined the prevalence of Trichinella infection in domestic dogs in Finland, which is known to have a high prevalence of Trichinella infection in sylvatic hosts. Altogether 727 dog serum samples were tested serologically by ELISA with ES-antigen in dilutions 1:50, 1:100, 1:200, and 1:400. Additionally, muscle samples from 102 dogs' front leg extensors were tested parasitologically by HCl-pepsin-digestion. The tested dogs originated from the University Veterinary Hospital (244 of the sera and all of the parasitologically tested muscle samples) and from a serum bank of the National Veterinary and Food Research Institute (465 serum samples), representing healthy dogs mostly from southern Finland. As negative controls, we tested sera from 18 experimental dogs known to be negative for Trichinella infection. Trichinella spp. was isolated from one dog (1%) with a low infection intensity. High OD% were found in both groups of serum samples. Of the hospitalized dogs, 4.9% were classified positive, and 8.6% of the healthy dogs were classified positive (7.3% in total). The sex or breed of the tested dogs did not correlate with the positive results, but the age correlated positively in the serum bank group. The serological results suggest that dogs may be exposed to Trichinella spp. in Finland; however, confirmation of low-level infections requires confirmation by direct testing methods.

Associations between Trichinella species and host species in Finland.

Examination of 627 wild animals--raccoon dogs (Nyctereutes procyonoides), red foxes (Vulpes vulpes), European lynxes (Lynx lynx), brown bears (Ursus arctos), wolves (Canis lupus), and badgers (Meles meles)--revealed Trichinella spp. The prevalence varied according to geographical region of Finland (north; southwest, SW; and southeast, SE) and was the highest among lynxes (70%, SW). The risk of trichinellosis was higher in the SE (odds ratio, OR, 19.4) and SW regions (OR 14.3), as compared with the northern region (OR 1), with no difference between the former 2 regions. Foxes (OR 2.1) and lynxes (OR 1.9) had a higher risk than raccoon dogs (OR 1) of being infected. The distribution of different Trichinella species was evaluated in 87 wild and domestic mammals by multiplex polymerase chain reaction. Trichinella spiralis was detected more often in domestic and synanthropic animals than in sylvatic hosts. Trichinella nativa was detected only in wildlife. Trichinella pseudospiralis was found both in sylvatic and synanthropic hosts. Trichinella britovi was detected only in mixed infections with other Trichinella species. The raccoon dog was the sole host for all 4 Trichinella species and also carried the most intense infections.