Northern Prairie Wildlife Research Center
The nematode Dioctophyma renale (Goeze, 1782) infects the kidneys of wild mink (Mustela vison) in many areas (Woodhead and McNeil, 1939; Sealander, 1943a; Hallberg, 1953; Schacher and Faust, 1956; Miller and Harkema, 1964; Crichton and Urban, 1970; Fyvie, 1971; Mace and Anderson, 1975; Jorde, 1980). However, the only record in Minnesota was a 6.9% prevalence in fur farm mink (Erickson, 1946). We have found no information about prevalence of D. renale in mink of different ages. We report the first prevalence of D. renale infection in wild Minnesota mink and the first analysis of the ages of infected mink. Because D. renale can be fatal (Graves, 1937; Meyer and Witter, 1950; Mace and Anderson, 1975), this information may allow a better understanding of mink population dynamics and ecology.
Skinned carcasses of 121 mink caught by fur trappers in northeastern Minnesota (Cook, Lake and St. Louis counties) and of 17 mink from east-central Minnesota (Pine and Kannabec counties) during fall 1998 were examined for kidney worms. Only the kidneys and abdominal cavity were examined. Male mink were aged by examination of bacula (Petrides, 1950; Greer, 1957), but the females were not aged. Worms from the northeastern Minnesota sample were counted and half of them were measured. The sexes of the worms were not determined in this study. Two specimens were confirmed by the University of Minnesota Veterinary Diagnostic Laboratory as Dioctophyma renale (J. E. Collins, pers. comm.).
The difference in prevalence of D. renale in male vs. female mink was not significant. The number of D. renale found in individual male mink ranged from 1 to 13 and in females 1 to 5 (Fig. 1). The mean numbers of worms in males (2.5 ± 0.5 SE) and females (1.8 ± 0.7 SE) were not significantly different. The average length of the worms was 24.6 cm. Two 50.8 cm long worms, the longest in the study, were recovered from two male mink from northeastern Minnesota. Most worms inhabited the right kidney, but in 14% of cases, were found in the abdominal cavity (Table 2).Thirteen D. renale were found in the right kidney of one male mink, representing the highest documented infection intensity of a single wild mink host.
|Table 1. Prevalence of Dioctophyma renale in Minnesota mink during 1998|
|Immature male||Adult male||Total males||Females||Total|
|n||Positive (%)||n||Positive (%)||n||Positive (%)||n||Positive (%)||n||Positive (%)|
|Northeast||46||14 (30)||36||11 (31)||82||25 (30)||39||6 (15)||121||31 (26)1|
|Eastcentral||7||3 (43)2||||||7||3 (43)||10||3 (30)||17||6 (35)1|
|Total||53||17 (32)||36||11 (31)||89||28 (32)3||49||9 (18)3||138||37 (27)|
not significant between NE and EC samples (χ²
= 0.71; df = 1; P = 0. 40)
2 May have included one adult male
3 χ² = 2.76; d.f.= 1; P = 0.10
|Fig. 1. Number of Dioctophyma renale in 121 mink from northeastern Minnesota in 1998.|
|TABLE 2. Location of Dioctophyma renale in Minnesota mink during 1998|
|Location||Male mink||Female mink|
Although we did not find a significant difference between prevalence of Dioctophyma renale in male and female mink, our ratio of infected male prevalence to infected female prevalence (1.8:1) was greater than in Mace and Anderson's (1975) study (1.5:1), which did find a significant difference. One hypothesis for this gender-based dissimilarity is that males may have larger territories than females (Eagle, 1989), which could mean more exposure to infected prey. Another possibility is that males may consume more intermediate and paratenic hosts. Sealander (1943b) reported, however, that the diet of male and female mink varied only in size of prey. Many of the prey taken by both sexes are paratenic hosts for D. renale.
Lumbriculus variegatus, an oligochaete, is believed to be the only intermediate host required by Dioctophyma renale before infecting mink (Karmanova 1959, 1960, 1962 vide Mace and Anderson, 1975). Infected oligochaetes can carry the infection to second and third paratenic hosts that mink feed on (Mace and Anderson, 1975; Measures and Anderson, 1985). Lumbriculus variegatus is pollution tolerant (Lafont, 1989; vide Lafont et al., 1996) and oligochaetes, in general, have been used as indicators of level of eutrophication of a water system (Howmiller and Scott, 1977; Milbrink, 1983). However, L. variegatus inhabits primarily mesotrophic environments (Särkkä, 1987).
The relatively high prevalence of Dioctophyma renale in Minnesota mink, the serious effect of the infection, and the lack of information about many aspects of the ecology of the relationship among mink, worm and hosts, emphasize the need for further research on this subject.