The "San Juan Worm"

        The often-maligned "San Juan Worm"actually represents an aquatic annelid closely related to common earthworms named Tubifex tubifex. These thread-like worms that live in the mud on stream bottoms are a critical host in the life cycle of WD.   In the absence of Tubifex worms, the life cycle is broken and the disease cannot prosper.  The severity of infection within a river's trout population could well depend on the abundance and distribution of Tubifex worms.   Daniel Gustafson, an aquatic entomologist at Montana State University, is studying how Tubifex tubifex intereacts as a host in the transmition of whirling disease.  He observes that in Europe this is a perfect parasite -- one that does not kill its host, the brown trout.  The two organisms have evolved amicably there; but not in North America, where cerebralis has suddenly become the scourge of Western coldwater fisheries, killing young-of-the-year fish from Colorado to Montana and on into the West Slope watersheds.   Here are some of Gustafson's preliminary observations:

• Tubifex worms are apparently not common in the West, only where man has modified the natural habitat.
• The worms favor aquatic habitat that remains uniformly cold throughout the year.
• In aquatic habitats that warm significantly during the summer the worms do not survive well.
• The worms flourish in cold-water habitats that are relatively impoverished, i.e. low in densities of other aquatic life forms.
• The European experience shows that when tubifex worm habitat is cleaned of human influence, worm densities drop. This may suggest that worm habitat modification can be a management tool available to fisheries managers.

Low Risk Areas

• Wild and unimpacted Rocky Mountain streams and rivers -- Streams and rivers that have their normal, highly diverse insect community intact make very poor Tubifex tubifex habitat. This includes many headwater streams, but fewer rivers. Note that an upstream tributary that has Tubifex and whirling disease, may cause problems in a stream that otherwise looks safe from whirling disease.
• Warm trout waters -- Tubifex appears to drop out of warm water before trout do, at least during the critical summer period when the young-of-the-year fish are most vulnerable to the parasite. In Montana these will generally be rivers rather than streams. For the entomologist, this transition occurs about when Claassenia outnumbers Hesperoperla. These waters will be fairly comfortable swimming nearly all day, during the summer. Ilyodrilus templetoni is often very abundant in these places. Again, an upstream tributary could be a TAM source and thus threaten these waters.
• Lake outflows- Tubifex is conspicuously missing from the outflows of large, natural lakes. Even when the inflow to the lake is loaded with Tubifex, the outflow has Tubifex greatly reduced. Many of these streams have poor communities and it seems probable that temperature is important. Rhyacodrilus coccineus is often very abundant in these places. Lakes also seem to block the downstream movement of the free parasite.

High Risk Areas

• Spring Creeks -- Tubifex tubifex is normally present in almost all Montana spring creeks where trout live. It usually occurs in great abundance along with handful of other species that are also adapted to the unusual biological conditions found in springs. Persons making a "quick visit before dark" to a spring creek after "a day on the river" should be especially careful not introduce viable biological material of any kind.

• Tailwater streams and rivers -- No other single human-caused disturbance so quickly and clearly reduces the high biodiversity of Rocky Mountain streams and encourages Tubifex tubifex. The tailwaters which are not already known to be infected with the disease should be protected to the extend possible. In particular, the Big Horn River, the Madison River between Hebgen Dam and Quake Lake, and the West Fork of the Bitterroot River all deserve special care.
• Impacted Streams and Rivers, and all canals -- Almost any stream or river that is significantly impacted by urban, agricultural or foresty practices is likely to support Tubifex tubifex. Grazing with its sediment and manure loading, is the most frequent cause of Tubifex in Montana streams.

Lakes