Rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) represent two salmonid genera separated for 15–20 million years. cDNA sequences were determined for the classical MHC class I heavy chain gene UBA and the MHC class II β-chain gene DAB from 15 rainbow and 10 brown trout. Both genes are highly polymorphic in both species and diploid in expression. The MHC class I alleles comprise several highly divergent lineages that are represented in both species and predate genera separation. The class II alleles are less divergent, highly species specific, and probably arose after genera separation. The striking difference in salmonid MHC class I and class II evolution contrasts with the situation in primates, where lineages of class II alleles have been sustained over longer periods of time relative to class I lineages. The difference may arise because salmonid MHC class I and II genes are not linked, whereas in mammals they are closely linked. A prevalent mechanism for evolving new MHC class I alleles in salmonids is recombination in intron II that shuffles α1 and α2 domains into different combinations.

The prevalence of infection and susceptibility of the aquatic oligochaete Tubifex tubifex to Myxobolus cerebralis, was examined in 2 studies on the upper Colorado River, Colorado, USA, where whirling disease occurs in wild trout populations. In the first study, the prevalence of infection ranged from 0.4 to 1.5%, as determined by counting the number of T. tubifex releasing triactinomyxons of M. cerebralis directly following their collection from the field. The susceptibility of those T. tubifex not releasing triactinomyxons was assessed by the number of these oligochaetes releasing triactinomyxons 3 mo following experimental exposures to spores of M. cerebralis. The prevalence of infection following experimental exposures of these T. tubifex ranged from 4.2 to 14.1%. In a second study, all T. tubifex collected at 2 different times directly from the 2 field sites in Colorado were exposed to spores of M. cerebralis. Individual oligochaetes representing those groups of T. tubifex releasing and those groups not releasing triactinomyxons at 3 mo were screened with molecular genetic markers. T. tubifex populations found at the 2 study sites consisted of 4 genetically distinct lineages that varied with respect to their susceptibility to experimental exposure to M. cerebralis. Lineages I and III contained the most oligochaetes susceptible to M. cerebralis and were the most prominent lineages at Windy Gap Reservoir, a site of high infectivity for wild rainbow trout on the upper Colorado River. In contrast, at the Breeze Bridge site which is below Windy Gap Reservoir and where M. cerebralis infections are less severe in wild trout, oligochaetes in lineages V and VI that are resistant to M. cerebralis were more prominent. These results suggest that certain habitats, such as Windy Gap Reservoir, are conducive to large and more homogenous populations of susceptible T. tubifex lineages that may serve as point sources of infection for M. cerebralis. Although not a direct objective of this study, there was no evidence of M. cerebralis infections among any oligochaetes other than those that would be classified as T. tubifex by standard morphological characteristics. KEY WORDS: Whirling disease · Myxobolus cerebralis · Tubifex tubifex Resale or republication not permitted without written consent of the publisherDis Aquat Org 51: [113][114][115][116][117][118][119][120][121] 2002 clines among wild rainbow trout populations in the states of Idaho, Montana, Colorado and Utah, USA (Nehring & Walker 1996, Vincent 1996, Hedrick et al. 1998. M. cerebralis is the best known of 1300 parasites grouped in the phylum Myxozoa and the first shown to possess a 2 host life cycle including salmonid fish and an aquatic oligochaete, Tubifex tubifex (Markiw & Wolf 1983, Wolf & Markiw 1984, Wolf et al. 1986). The parasite has been observed among wild fish in 23 states in the USA, but without the same disastrous effects observed in the intermountain west (Nickum 1999). The differences in the impacts on wild fish between geographic region...

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Recent failures in recruitment of rainbow trout Oncorhynchus mykiss in the Colorado River in Middle Park, Colorado, USA, led to studies of the effect of the myxosporean parasite Myxobolus cerebralis, the causative agent of whirling disease, on the wild trout fishery in the river. conducted field exposures of sentinel fish to examine the vulnerability of seven species or subspecies of salmonids exposed to the parasite in the Colorado River. During 1995-1996, brook trout Salvelinus fontinalis and Colorado River cutthroat trout O. clarki pleuriticus experienced 85% or higher mortality within 4 months of exposure. Groups of fish introduced at greater mean weight or later in the summer tended to survive better than others, but mortality often was severe among these groups during the second summer. By April 1996, dead fish or sacrificed fish representing seven groups were shown by histology to contain mature M. cerebralis spores. During 1996-1997, three groups of cutthroat trout O. clarki spp. experienced poorer survival than brown trout Salmo trutta and rainbow trout introduced at similar weights. Snake River cutthroat trout O. c. bouvieri introduced in July survived well and developed fewer M. cerebralis spores (prevalence and mean concentration) than any group except brown trout. Rainbow trout parented by fish that recruited after M. cerebralis was established in the drainage developed significantly fewer myxospores than those parented by fish that recruited before establishment of M. cerebralis (P Յ 0.0001), but survival was not different between these groups (87% versus 89%, respectively). The poor performance of parasitized brook trout and cutthroat trout suggests that establishment of M. cerebralis in high-country streams may have serious consequences for these species.

We analyzed the geographic distribution of Tubifex tubifex from various river drainages in central Colorado by genetic screening with specific mitochondrial 16S ribosomal DNA (mt 16S rDNA) markers. Four distinct mt 16S rDNA lineages are evident. The sites varied with respect to land- and water-use practices. All sites represented habitats presumed capable of supporting oligochaetes. At the locations where whirling disease has had the greatest impact on resident rainbow trout, T. tubifex, representing lineages I and III (genotypes known to be susceptible to Mxyobolus cerebralis), were most commonly found. In contrast, at sites less affected by whirling disease, T. tubifex of lineages V and VI that are more resistant to M. cerebralis infections were more abundant. The predominance of resistant T. tubifex worms (lineages V and VI) at low-impact sites supports the conclusion that when these genotypes are in greater abundance, the potential for more severe effects of whirling disease on wild rainbow trout populations may be diminished.

A total of 686 Tiris half-duplex passive injectable transponders (PIT) of two sizes (23 and 32 mm) were randomly injected s.c. in three positions, armpit, ear scutulum, and upper lip, in 343 fattening calves (1 to 3 mo old). Injections were performed by two trained and two untrained operators. Losses and breakages on the farm were recorded at wk 1, 3, 7, 11, and 15 in restrained animals using two types of hand-held transceivers with a stick antenna. Dynamic reading efficiency (DRE) in animals running through a raceway was also evaluated at wk 1 and 3 and monthly until slaughter, using a stationary transceiver working at 137 dB x microV x m(-1) at 3 m. The total number of PIT that fell or broke in the slaughtering line, the location method, and the recovery time were also recorded. Results on the farm showed low breakages on average (0.4%) and differences (P < 0.05) in losses according to position (armpit, 1.7%; ear, 5.2%; and lip, 14.0%). An interaction (P < 0.05) between position x size was observed, and losses were greatest using a 32-mm PIT in the lip. The DRE was affected (P < 0.05) by PIT position and size, and values were greater for the 32-mm PIT in all positions (armpit: 99.9 +/- 0.1 vs 95.8 +/- 4.9%; ear: 93.8 +/- 2.2 vs 81.9 +/- 4.6%; lip: 66.8 +/- 4.9 vs 53.4 +/- 4.7%, respectively, for 32 vs 23 mm). Recovery of PIT in the abattoir was on average 96.7, 96.7, and 99.2% for armpit, ear, and lip, respectively (P > 0.05). Most of the PIT injected in the armpit were recovered by sight or palpation, but 31.9% were recovered after cutting the muscles around the area and 10.7% were recovered on the internal side of the hide, which jeopardized carcass identification. Recovery of PIT injected in the ear was 23.4% in the hide and 76.6% in the auricular muscles of the head. The easiest recovery was in the lip, 8.9% of PIT were located in the hide and 91.1% in the head. Recovery time was affected (P < 0.05) by position: the quickest was lip (27 +/- 2 s), followed by ear (52 +/- 5 s) and armpit (78 +/- 7 s). In conclusion, taking into account retention and reading performances, injection of a 32-mm PIT into the armpit showed the best results on the farm, but a careful and longer recovery was needed in the abattoir. Improvement of recovery methodology and time would be necessary in order to recommend injection of PIT in the armpit instead of in the ear for cattle tracking or monitoring.

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