The prevalence of Trichinella spiralis in farmed minks (Neovison vison) associated with exposure to wild rats (Rattus norvegicus) in Shandong province, China

Background Both of American mink (Neovison vison) and wild rat (Rattus norvegicus) is considered reservoir hosts carrying many endoparasites. Trichinella is a successful parasitic nematode including nine species and three genotypes with a worldwide distribution. However, little is known about the prevalence of Trichinella infection in mink (Neovison vison) and rat (Rattus norvegicus) in China. Methods In total, 289 muscle samples of minks and 102 rat carcasses were collected between April 2017 and December 2019 in Weihai city of Shandong province, China. The food of minks including chicken skeleton and mashed marine products was also collected at 3 batches. All the samples were used to examine for the appearance of Trichinella by the pooled articial HCl-pepsin digestion method. The isolates from minks and rats were identied as T. spiralis by multiplex PCR. Then, the phylogenetic tree was constructed based on the sequences of 5S rDNA inter-gene spacer regions. Results Muscle larvae were detected in 20 out of 289 minks (6.92%) and in 2 of 102 wild rats (1.96%), respectively. The intensity of Trichinella in mink samples was ranged from 0.025 to 0.815 lpg, while the larval burden in rats was 0.17 lpg. The isolates from minks and rats were identied as T. spiralis by multiplex PCR. Sequence analysis revealed a 100% identical alignment of the 5S rDNA inter-gene spacer regions from the two isolates. The phylogenetic tree conrmed the two isolates from minks and rats belonging to T. spiralis based on analysis of the 5S rDNA inter-gene spacer sequence. Conclusions The present study represents the rst report of T. spiralis infection in American mink (Neovison vison) and wild rat (Rattus norvegicus) from Shandong province, China. The farmed minks would be vulnerable to Trichinella infection through exposure to the wild rats. The prevalence of T. spiralis in wild rats may raise a public health concern for the potential zoonotic risk for the domestic animals.


Background
The zoonotic trichinellosis is a parasitic disease of public health signi cance, caused by infection with larvae of the genus Trichinella [1]. The parasite can infect more than 150 animal species including birds, mammals and reptiles [2,3]. To data, there are nine species and three genotypes consisting of Trichinella genus [4]. In China, trichinellosis is still recognized as a re-emerging disease [5], and more than 600 outbreaks were reported during 1964-2013, affecting nearly 40 thousand people [6]. The high prevalent setting of human trichinellosis in China is mainly related to the consumption of raw or under-cooked pork, and sporadically to vagarious ingestion of meats from dogs and wild animals as delicacies [7].
American mink (Neovison vison) is one of important economic animals for providing better marten, which is very popular in international fur market. Breed of minks is more increasing in China due to the abundant economic bene t [8]. Owning the largest breeding industry of minks in China, more than 70 percent of marten are yielded in Shandong province, especially around the coastal regions including Yantai, Weihai and Qingdao cities. Meanwhile, mink is also considered a crucial reservoir host for endoparasites in view of its wide food niche breadth ( sh, birds, amphibians, small mammals and invertebrates) [9,10]. Several investigations have showed Trichinella infection in the farmed minks in other countries, but data are limited on the prevalence of Trichinella in farmed minks (Neovison vison) in China.
In China, free-ranging pigs are at high risk for Trichinella spp. infections in humans in the place without any mandatory test [11,12]. Meanwhile, the domestic animals are assailable to Trichinella infection through exposure to wild reservoir hosts. Wild rats (Rattus norvegicus) can act as the primary reservoir host of Trichinella that trigger the onset of the transmission of zoonotic bacteria, viruses or parasites to the domestic cycle derived from the sylvatic environment when the carcasses are scavenged [13][14][15]. Rats commonly abundant lives wherever humans live, especially in primitive conditions less than desirable hygiene. However, little report could be available about Trichinella prevalence in wild rats (Rattus norvegicus) in China.
In this survey, we aimed to examine the prevalence of Trichinella infection in farmed American mink (Neovison vison) populations from Weihai city of Shandong province, China, and determined species of the isolates by multiplex PCR and phylogenetic analysis. In order to further investigate the source of infection, we examined Trichinella by arti cial digestion of the diets of minks including mixed mash and chicken skeleton, and the rats living closely with minks.

Sample collection
The investigation of Trichinella infection from 289 minks (Neovison vison) was conducted between April 2017 and December 2019 in Weihai city (36°41' ~ 37°35' N, 121°11' ~ 122°42' E) of Shandong province ( Fig. 1). Each mink was raised in a cage, farming at a semi-closed house. Samples used for testing mink (Neovison vison) were included tongue, diaphragm, forearm and hind limbs, and muscle tissues of 5 ~ 50 g were taken from one animal [16]. A batch of muscle tissues was stored at 4 °C, keeping no more than 15 days. Due to the minks were fed on chicken skeleton and mixed mash (mainly including little seash, shrimp et al.), we collected 600 g of mixed mash and 800 g of chicken skeleton by 3 batches.
Rats (Rattus norvegicus) (n = 102) around/inside mink farm were trapped between January and December 2019 using mousetraps that placed surrounding the rat holes. The captured rats were kept frozen at -20 °C until examination. The carcass of rats was obtained before laboratory analysis.

Isolates from minks ( Neovison vison ), mixed mash, chicken skeleton and rats ( Rattus norvegicus )
After arti cial digestion, Trichinella were obtained from minks and rats, but were not detected in mixed marine mash and chicken skeleton. The overall rates of Trichinella infection in minks (Neovison vison) and rats (Rattus norvegicus) were 6.92% (20/289) and 1.96% (2/102), respectively. Thereinto, about 150 larvae were isolated from a blending muscle tissue of minks, and then were reserved in Kunming mice by oral. The lpg of infected mink samples was ranged from 0.025 to 0.815. The larval burden in collected rats was 0.17 lpg.

Identi cation of Trichinella species by multiplex PCR
The obtained larvae were identi ed species by multiplex PCR. As shown in Fig. 2, the size of PCR production of larvae from mink and rat samples was identical to that of T. spiralis Henan isolate with approximate 200 bp in length, which was coincided with the expected size (173 bp) for T. spiralis as reported. The fragments from T. pseudospiralis Russin and T. pseudospiralis ISS141 were longer than 300 bp, respectively. The result indicated that the larvae isolated from American minks (Neovison vison) and wild rats (Rattus norvegicus) should be identi ed as T. spiralis.
Sequencing and phylogenetic analysis of 5S rDNA inter-gene spacer region The sequencing result of 5S rDNA inter-gene spacer region from minks was showed 100% identical with that from rats. The length of ampli ed 5S rDNA inter-gene spacer region from minks (accession no. in GenBank: MT734668) and rats (accession no. in GenBank: MT734669) was 568 bp after fragment assembly. The 5S rDNA inter-gene spacer region from minks revealed 1 transition (522 T to C) in alignment with the corresponding sequence from Henan isolate.
Phylogenetic analysis using MP based on the 5S rDNA inter-gene spacer region showed that Trichinella isolates from minks and rats were all grouped with T. spiralis strains, which further indicated they were belonged to T. spiralis (Fig. 3).

Discussion
The prevalence of Trichinella in minks has been reported in many countries [21][22][23]. In the present study, we reported the rst data on Trichinella infection in farmed American minks (Neovison vison) in China.
The overall Trichinella prevalence in minks here was 6.92%, which was lower than that in minks from Estonia (23%) [9] and Canada (8.3%) [24], but higher than that in minks from Poland (3.3%) [10] and Belarus (4%) [25]. The difference in Trichinella prevalence in minks among studies would be resulted from the simple size, geographic region, living environment, climatic conditions, as well as animal husbandry practices, socioeconomic and ecological conditions.
The species of Trichinella isolates from minks and rats were all identi ed as T. spiralis in the present study, which was different from another previous investigation, isolated T. britovi and T. pseudospiralis two more strains in minks [10]. T. spiralis is considered a preponderant species in China, and as many as 13 isolates are identi ed as T. spiralis obtained from mainland China until now [7]. After isolated T. spiralis from caged minks, we raise a question as to how the minks acquired Trichinella, the unique parasite that transmit to other animals solely by ingestion of muscles infected with encysted larvae [15]. The food of minks was then conducted to examine the presence of Trichinella, but no worms were detected.
Each investigated mink raises in one cage. The cages are setting under a semi-closed house, which one side of the house orientate to the open, facilitating the keepers add mink diets to the food box. In the mink farm, there are many rat holes around the feeding house. According to the keepers, rats often grab the diets of minks from the food box. The fact that the stealing rats could be suddenly preyed by minks sometimes provides the other possible route of minks acquired Trichinella. After arti cial digestion, T. spiralis was obtained from rat carcass, and was identi ed 100% identity of the partial 5S rDNA inter-gene spacer region to the mink isolate. We thus deduced that, for minks, T. spiralis could be transmitted from wild rats, although it should not be the only route.
Minks have been showed to carry Hepatitis E virus [26], SARS-CoV-2 [27], Cryptosporidium spp. [28] and several other zoonotic pathogens [29][30][31][32]. In the present study, we identi ed the presence of foodborne Trichinella in farmed caged minks. Even though the zoonotic T. spiralis infection does not in uence the furry quality, the carcasses of minks would increase the biomass of T. spiralis in the surrounding environment, if they are not adequately destroyed.
The overall prevalence of Trichinella infection in rats from Weihai city of Shandong province was 1.96%, which is lower than that in rats from Northwest Vietnam (2.8%) [33] and Poland (23.33%) [34]. T. spiralis infection in wild rats is also well investigated in Europe [35,36] and America [37]. However, the prevalence of Trichinella species circulating is usually overlooked in rats in China. The rats could extensively live from the sylvatic to the domestic environment, so as to play as a vector of Trichinella transferred the parasite between domestic animals and wildlife [15,35]. Although the low larval intensity is detected in the investigated rats (0.17 lpg), presence of T. spiralis in rats indicated that attention needs to raise for the pig and chicken farms in view of the public health concerns.

Conclusions
The present study rstly demonstrated T. spiralis prevalence in minks and rats from Weihai city of Shandong province, China. The wild rats would be responsible for the caged minks acquired T. spiralis in the ecological and genetic viewpoint. Supervisions and inspections of destroying mink carcasses should be urgent for veterinarians, farmers and governmental authorities, and also the prevalence of T. spiralis in rats should be further assessed its risk for the domestic animals. Figure 1 Map of China with the studied locality. The red area represents the location of the investigated location, Weihai city of Shandong province. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.