Chinese mink population has become popular in the world at present due to the introduction and propagation of mink in the late 1950s for the fur industry in China. So far, AMD has severely affected farmed mink in China as well as worldwide. Currently, there is no available vaccine, and the knowledge about AMDV diversity and molecular epidemiology will help to prevent AMD from spreading.
Previous studies reported that a large number of mink farms were affected by AMDV in China, based on VP2 or NS1 gene from samples of mink tissues, such as spleen, and plasma, especially during the pelting season [9, 10]. However, this epidemiological survey of AMDV is commonly limited by sampling time and quantity. Fecal swab sampling is easy to perform in a large population and causes small or no damage to animals. In this study, the fecal swabs samples were used to investigate the prevalence of AMDV in the major mink farming areas in northeast China, and the high prevalence (54.0%) of AMDV was found.
Nucleotide sequence comparisons of acquired fragments of the VP2 gene revealed that AMDV isolates from different farmed minks in northeast China were closely related to each other, but different from the nonpathogenic AMDV-G strain. There are 28 amino acid differences in amino acid alignment, which were distributed in all segments of the detected genes. Interestingly, of these amino acids, 9 amino acids were centrally present in the hypervariable region. Morever, 23 Chinese samples had similar changes in some fixed positions, especially in the hypervariable regions. These results indicated that Chinese AMDV strains had a high degree of similarity among themselves, while they showed the difference from ADMV strains in other countries, which is consistent with the previous studies [9, 15].
The neighbor-joining method was used to construct a phylogenetic tree. However, due to the low confidence of some branches (possibly due to the short sequences used), maximum likelihood and maximum parsimony methods were also performed to verify the reliability of the neighbor-joining phylogenetic tree. The trees constructed by maximum likelihood and maximum parsimony methods were the same or quite similar as showed in neighbor-joining phylogeny, and the bootstrap trees were relatively consistent, which enhance the reliability of neighbor-joining analysis .
All the Dalian samples were collected from one farm of Liaoning province. The phylogenetic analysis found that AMDV-Dalian isolates appeared in all three groups, which showed that different AMDV genotypes could coexist in the same farm. Besides, AMDV-Jilin isolates clustered in clades A and B, with the same as many of the AMDV-Dalian isolates (except for Dalian 4 isolate), revealing similar AMDV genotypic distribution in different farms from Jilin and Liaoning provinces, which further indicated that the distribution of AMDV strains was not related to the geographical origin. The AMDV distribution in northeast China probably results from the frequent mink trade among different farms and the introduction of both domestic and foreign minks, which contributes to improving local mink availability, but simultaneously leads to the enhancive virus spreading.
The phylogenetic analysis also revealed a regional-specific clade B, which only contained the isolates from China, and the clusters of the Chinese AMDV strains may reflect a long history in the breeding of domestic minks. Furthermore, the Chinese AMDV isolates were distributed in all three groups, indicating that the newly isolated Chinese AMDV strains consisted of both prevalent indigenous strains and isolates imported from abroad, which further implied that both local and imported AMDV species were prevalent in the farmed Chinese minks. Similar findings were found by previous reports [6, 10].