Breakthroughs TO Curtail Whirling Disease In US Western Trout Streams

There are some big breakthroughs in the effort to curtail whirling disease (WD) in US western trout streams. WD, you’ll recall, is a parasitic infection that attacks the head and spinal cartilage of juvenile trout and salmon, deforming and eventually killing the subject. Gardner writes:

It was 14 years ago that I first blew the whistle in these pages about the spread of Whirling Disease around the Rocky Mountain West. The disease decimated wild rainbow populations and threatened native cutthroats too. I haven’t written about it in a long time because there was little to say, except despair that we could ever get rid of the WD parasite.

Now, I am delighted to report, a team of Colorado Division of Wildlife (CDOW) research scientists has achieved a breakthrough in developing strains of rainbow trout with a proven high degree of resistance to the WD organism in the wild. Within a few years, WD could cease to be lethal to naturally-reproducing wild rainbow populations in Colorado.

All this began with rainbow trout egg shipments from California and Colorado to Germany over a century ago. A small hatchery over there run by a family named Hofer nurtured these eggs in local water where WD is endemic. Over the course of 120 to 130 years, the Hofers selected and bred the best surviving specimens. Today, it is descendents of those hearty survivors that are being brought back home to western America with a high degree of resistance to WD.

A key player in this story is a scientist at the University of Munich named Mansour El-Matbouli, an expert on fish diseases who had studied the Hofer strain. With support from TU, he brought the news to a US fisheries conference in 2000, explaining that Hofer-strain rainbows did not become infected in waters where recently imported rainbow eggs became severely infected. After two more years of lab exposure, Drs. El-Matbouli and Ron Hedrick of UC/Davis proved that the Hofer lineage did indeed have a high level of resistance and might solve the WD problem in the US, at least for rainbows. The reception they got at a 2002 WD Symposium in Denver ranged from curiosity to skepticism to outright derision.

To its great credit, the Colorado Division of Wildlife took Dr. El-Matbouli seriously, acquired some Hofer eggs from quarantine at UC/Davis and grew a brood stock that is now the center of hope in the effort to defeat WD in the Rockies. In rich environments like hatcheries and ponds, Hofers were again proven dramatically more disease resistant. But after 120-plus generations of being coddled in hatcheries, there was concern that Hofers might have lost survivability in the rough natural environments of wild rivers. So, they were crossbred with successful (pre-WD) local wild-adapted strains such as the Gunnison and Colorado River lineages. First planted in 2004, these hybrids began reaching spawning age last year. Despite highly infectious levels of WD in streams where they were stocked, they not only survived but successfully reproduced. The hybrids inherited, and have now passed on, much of the Hofer strain’s genetic resistance to the disease.

Pure Hofers are now the norm in Colorado for put-and-take stocking, which should greatly reduce infectivity loads. They also grow bigger and faster, an advantage for hatchery production. They don’t live as long, but this doesn’t much matter for put-and-take fish. Hybrids with longer-lived Colorado River, Gunnison and other local strains will henceforth be the norm for rivers with potential for natural reproduction.

So, assuming all continues to go well, how long until WD can be defeated throughout the West? CDOW has cooperative agreements with Utah and California to supply eggs from their brood stock, and these two states are also on track to defeat WD soon. Other states are more wary, still skeptical of Hofers and/or highly protective of their ‘native’ rainbow strains. The research is still new, but again assuming things continue to prove out, key trout states like Montana should soon be convinced, though they will lag behind. Overall, the human-boosted evolutionary process in the West could be completed in five to 10 years.

What about native cutthroats? You can’t replace a greenback cutt with a Hofer rainbow. But there’s good news on this front too. Another line of CDOW research into the tubifex worm family, the intermediate host organism for WD, has demonstrated that some native subspecies of this worm are highly resistant to the WD parasite. When these subspecies are introduced into WD-infected waters, they appear to out-compete or replace infectable worms, essentially acting as living biofilters as they consume and deactivate the WD spores. Without a commodious worm belly to hatch and grow in, the aquatically-mobile TAM phase of the WD creature, which attaches to baby trout and eats their cartilage, cannot thrive and will eventually die off or so we can hope. Research thus far is still experimental, but looks very promising.