Whirling Disease"the AIDS of trout"
Whirling disease is a parasitic disease affecting salmonids. It is
caused by Myxobolus cerebralis, a myxozoan parasite that
penetrates the head and spinal cartilage of fingerling trout where it
rapidly multiplies, putting pressure on the organ of equilibrium.
This causes the fish to swim erratically (whirl),
and have difficulty feeding and avoiding predators. Other
characteristic signs of the disease include a blackened tail and
spinal and cranial deformities.
The renewed interest in whirling disease results from its association with catastrophic declines among wild rainbow trout populations in premier angling waters of the intermountain west of the United States. The potential threat to US salmonid aquaculture due to whirling disease was viewed so strongly that M.cerebralis was listed as one of only two pathogens in US federal legislature governing importation of salmonids in the 1950's. Whirling disease has gained a lot of attention and therefore monetary support for research which is why it can be considered as the AIDS of trout.
Myxobouls cerebralis was first reported among rainbow trout in Germany in 1903. The parasite was reported for the first time in North America in 1958 among hatchery-reared trout in the eastern USA, presumably arriving with frozen trout shipments from Europe. The disease has now been found in 22 states and is present in many salmonid rearing regions of Western and Eastern Europe, the former Soviet Union, Australia, New Zealand, and South Africa.
Whirling disease mainly affects rainbow trout, one of America's prime game fish, but also susceptible are the steelhead rainbows, cutthroat trout, chinook salmon, kokanee salmon and brook trout, all of which are salmonids native to North America. Whirling disease does not, however, affect all salmonid fishes: Coho salmon, lake trout, and brown trout so far have been resistant, but these resistant species can serve as carriers.
The primary host of M.cerebralis is an
inch-long aquatic worm, Tubifex
tubifex. As many as 10,000 tubifex worms can thrive in a square
yard of silty river bottom. M.cerebralis has a complex
two-host life cycle, which
begins
when waterborne triactinomyxon spores released from the infected
oligochaete Tubifex tubifex contact a susceptible trout. Upon contact
the triactinomyxon spores attach to the
fish and release their sporoplasm cells into the skin. These travel
along the nerves to the central nervous system and to cartilage found
in the spinal column and cranial regions. After a period of 20 days,
parasites begin to emerge from nerves to attack cartilage. Young fish
possess a skeleton with abundant cartilage and therefore suffer the
most devastating effects of whirling disease. As fish age, and the
skeleton turns from cartilage to bone, they become more resistant to
clinical signs of the disease. The final developmental stage or
myxosporean spore developing in the salmonid is a compact and sturdy
form prepared to withstand harsh environments once released from the
fish. The spores can be released from infected fish only after the
fish die or are eaten by predators. When the trout dies, the spores
which are heavier than water sink to the bottom where they can be
eaten by the tubifex worm to begin the life cycle anew. Even if the
spores do not immediately enter a tubifex worm, they can survive for
20 years in the mud. A wading angler who stirs up the bottom can send
hundreds of thousands of spores downstream to infect more fish.
At present, there are few options available to control whirling disease in wild trout populations. One approach is being worked on by a Montana fisheries biologist, Dick Vincent, who is working with trout to get them to spawn earlier in the spring so that they will hatch and begin to mature in water too cold for ideal whirling-disease spore dispersal. Temperature is a dominant factor controlling the development of the parasite in the worm and the fish. Water temperatures near 16 degrees C are probably optimal for development of the parasite in both hosts. By the time water temperatures rise and millions of infectious spores are present, the young fish will hopefully have matured enough to fend off a serious whirling-disease attack. Disinfection of water containing spores with ultraviolet light has been demonstrated as a potential method for treatments of hatchery water supplies. Active and prophylactic treatments with drugs and chemicals have also been attempted with whirling disease, with fumagillin shown as potentially the most effective. Feeding the drug to young trout prior to the onset of clinical signs in hatchery trials has been very effective.
By: Chelsea Byrd, for MB 492 class, Spring 2000