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Spleen samples from 14 mink that were trapped in 4 counties of Nova Scotia were tested for the presence of the Aleutian mink disease virus (AMDV) by polymerase chain reaction. Viral DNA was not detected in samples from Kings County (n = 2), but was detected in all the mink sampled from Colchester (n = 2) and Halifax (n = 6) counties, and 3 of 4 mink from Yarmouth County. The high level of AMDV-infected mink in Colchester and Halifax counties may pose a serious threat to the captive mink and wild animal populations. Because treatment of infected free-ranging mink is not an option, AMDV control strategies for the captive mink should be primarily focused on bio-security to protect clean ranches.
Enquête sur l’infection virale de la maladie aléoutienne des visons parmi les visons d’Amérique féraux de la Nouvelle-Écosse. Des échantillons de la rate de 14 visons piégés dans 4 comtés de la Nouvelle-Écosse ont été testés pour déceler la présence du virus de la maladie aléoutienne des visons (VMAV) par réaction d’amplification en chaîne par la polymérase. L’ADN viral n’a pas été détecté dans des échantillons provenant du comté de Kings (n = 2), mais a été détecté dans tous les échantillons des visons provenant des comtés de Colchester (n = 2) et de Halifax (n = 6) et chez 3 des 4 visons provenant du comté de Yarmouth. Le taux élevé de visons infectés par le VMAV dans les comtés de Colchester et de Halifax présente une menace sérieuse pour les populations de visons en captivité et de visons sauvages. Parce que le traitement des visons en liberté infectés ne représente pas une option, les stratégies de contrôle du VMAV pour les visons en captivité devraient d’abord se concentrer sur la biosécurité afin de protéger les élevages exempts de maladie.
(Traduit par Isabelle Vallières)
Aleutian disease (AD) is the number one disease problem for the mink industry in Nova Scotia. Despite many years of testing and elimination of infected animals, and implementing disinfection and bio-security measures, the problem has persisted on several mink ranches in the province. Although poorly documented, one possible source of persistent infection or re-infection of clean ranches in Nova Scotia may be the wild animal species, particularly mink that carry the Aleutian mink disease virus (AMDV) without showing signs of the disease (1).
A limited number of studies showed that AMDV infection in free-ranging mink in North America and Europe is widespread. In a survey of feral mink in northern Ontario (Canada), 61.7% (74 of 120) were seropositive for AMDV (1). Most recently, 44.4% (8 of 18) of free-ranging mink in Newfoundland, an island which was thought to be AMDV free, were positive for the virus (2). Percentages of feral American mink that were seropositive for AMDV were 23% (17 of 75) in France (3) and 52% (14 of 27) in southern England (4). In Spain, none of 14 American mink tested were positive for AMDV-antibody, but viral DNA was detected by polymerase chain reaction (PCR) in 2 of 5 mink carcasses tested (5). The objective of this study was to survey the free-ranging mink population in Nova Scotia, the largest producer of mink pelts in Canada, for AMDV infection.
Spleen samples from 14 free-ranging American mink were collected in 4 counties of Nova Scotia (Kings, Colchester, Halifax, and Yarmouth) between October 2006 and February 2008 (Table 1 and Figure 1). The mink carcasses were obtained from licensed professional trappers at the time of the annual wild fur harvest. In Kings county, spleens were harvested following pelting by the trapper. In other counties, the carcasses were obtained frozen after pelting and the spleens were collected by a technical person. The sampling instructions and supplies (scalpel blades, disposable gloves) were provided to trappers to minimize the chance of contamination. The location of sampling and the gender of the mink were recorded by the trappers (Table 1) using a data collection form that was designed for this survey. Samples were put into plastic bags, identified, and kept frozen at −15°C. Frozen samples were transferred to cryovials for long-term storage at −80°C.
DNA was extracted from the spleen samples using the salt-extraction method (6) with some modifications and addition of an RNAse treatment step (2 μL of 10 μg/μL RNAse and incubation at 37°C for 30 min). Sequences of the conserved regions of the AMDV genome from strains that are known to be circulating in Nova Scotia (Farid, unpublished data) were used to design the following 2 pairs of primers for amplification of the viral genome: 40-F: 5′-TTT GCT GCT GGT AAC GGT, 40-R: GTC CCA TGT CTT TTA TAG TTG C and 60-F: 5′-GGG TGT ATG GAT GAG TCC TAA A, 60-R: 5′-CCC CAA GCA ACG TGT ACT. To increase the chances of viral detection, different amounts of DNA were used (1.7X, 1X, X/10, X/20, where X is 1.5 μL of the stock DNA in 15 μL final PCR reaction mixture) for each of the 2 pairs of primers, along with a positive and a negative control in each trial. In cases where no amplification was observed, the entire test was repeated (16 amplifications/animal). Polymerase chain reaction products were run on agarose gels stained with ethidium bromide and visualized under UV light.
Of the 14 mink tested, 11 (78.6%) were positive by PCR for AMDV. No virus was detected in the 2 mink from Canard River (Kings county), but the 2 mink sampled in Brookfield (Colchester county), 6 mink sampled in Middle Musquodoboit and Musquodoboit Harbor (Halifax county), and 3 of the 4 mink captured in Yarmouth county were positive for AMDV (Table 1).
Because many mink ranches in Nova Scotia have been infected with AMDV in the past, and high percentages of free-ranging mink were shown to be infected in both Canadian studies (1,2), the high prevalence of AMDV infection in mink in Nova Scotia was neither surprising nor unexpected. The percentage of AMDV-infected mink in this survey (78.6%) was higher than those in Ontario (1) and Newfoundland (2). The results may indicate a higher prevalence of infection in Nova Scotia compared with other provinces, or may be due to small sample size in this survey, or the method of detection.
The actual percentage of infected mink in the wild may be higher than the number reported here, because PCR may fail to detect very small amounts of virus in a sample. Although the spleen is one of the major sites of AMDV replication (7–8) and the virus has often been detected in this organ (7–11), replication of AMDV could have occurred in organs or tissues that were not tested in this study.
More than 80% of mink ranches in Nova Scotia are located in the 2 adjacent Yarmouth and Digby counties, and a considerable number of the ranches have been infected with AMDV in the last 10 y. The presence of a very high percentage of AMDV-positive mink in Yarmouth County was not surprising. In 2007, there were 2, 8, 12, and 11 mink ranches in Colchester, Halifax, Kings, and Yarmouth counties, respectively, and those in Colchester and Halifax counties were free of AD. Central Nova Scotia (Halifax and Colchester counties) is considered an AD-free region, and even allowing for the small sample size, the existence of such a high proportion of infected animals was unexpected.
It is not clear whether the virus has been native to North America’s wild mustelids, and was transmitted to the captive mink populations or if the wild mink were infected through accidental escape or deliberate release of infected captive animals. Regardless of the origin of the virus, the widespread AMDV-infection in free-ranging mink has important epidemiological ramifications. There is a possibility of AMDV transfer from free-ranging to captive mink, as was the case in 1996 in Iceland (12).
Because control of infected wild animal species is not an option, AMDV control strategies of the captive mink should be primarily focused on bio-security measures to protect clean ranches. Controlling AMDV in Nova Scotia is particularly challenging because vast areas of the province are inhabited by many wild, potential AMDV-carrier animals which roam areas surrounding mink ranches in search of food. These animals may come in direct contact with captive animals if they can pass through the perimeter fence. In addition, several biological vectors, such as insects (13) and birds may transfer the virus from carcasses or feces of infected free-ranging animals to the captive mink. CVJ
Financial support for this project was provided by the Canada Mink Breeders Association and the Nova Scotia Fur Institute.
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