Whirling disease

Signs of disease

Important: animals with disease may show one or more of the signs below, but disease may still be present in the absence of any signs.

Disease signs at the farm level

• mass mortalities in fry

Disease signs at the tank and pond level

• convulsive movements
• faster breathing
• jerking backwards movements

At 3–8 weeks postinfection:

• many fish swim with a whirling motion ('tail chasing')
• erratic, nervous darting movements until exhausted
• darkening of skin from the anus to the tail ('blacktail')

Clinical signs of disease in an infected animal

At 5–6 months postinfection:

• spinal curvature
• skull deformation
• shortened gill plates

Disease agent

Whirling disease is caused by Myxobolus cerebralis, a parasitic protozoan that affects salmonids.

Lifecycle

• Myxobolus cerebralis has a complex lifecycle involving two hosts: fish (trout or salmon species) and its definitive host, the tiny bottom-dwelling tubifex mud worm (Tubifex tubifex).
• Infected fish develop Myxobolus spores (myxospores) that are very persistent and can survive in moist environments for many years.
• Worms ingest the myxospores, which develop in the worm's intestine and multiply rapidly.
• The infected worms release a fragile waterborne spore stage of the parasite (triactinomyxon stage), which must infect a fish within a few days or perish.
• When released by the worm, the spore infects a susceptible fish by attaching to the fish's body. The parasite then migrates through the fish's skin to the central nervous system, and ultimately into the cartilage.
• When an infected fish dies and decomposes, spores are released into the water, beginning the cycle again.
• Spores can also survive passing through the digestive tract of predators and can be transferred from place to place on muddy boots or other equipment.

Host range

Fish known to be susceptible to whirling disease:

* naturally susceptible (other species have been shown to be experimentally susceptible)

The presence of clinical signs in all these species can depend on many factors, particularly the age of the fish when first infected. For example, rainbow trout if infected when greater than nine weeks old will generally show no clinical signs. Chinook salmon, on the other hand, show no clinical signs beyond the age of three weeks, showing the reduced susceptibility of this fish to infection.

Presence in Australia

EXOTIC—not present in Australia.

Epidemiology

• Whirling disease is chronic and mainly affects young fish.
• Water temperature, age and species of fish influence susceptibility.
• Young fish are at greatest risk because the parasite attacks their soft cartilage, causing nerve damage, skeletal deformities and sometimes death.
• Clinical signs of the disease are not readily seen until fish are about 6–7 cm long.
• When fish are 8–10 cm long, cartilage forms into bone and they are no longer susceptible to disease but remain carriers of the parasite.
• The definitive host, the worm Tubifex tubifex, is found in Australia.
• Because tubifex worms live in mud, the disease can be partly controlled in trout farms by growing young fish in concrete raceways.
• The parasite spreads mainly through the stocking of infected fish and also through the alimentary tracts of fish-eating migratory birds.
• The parasite Myxobolus cerebralis is highly resistant in the myxospore stage. The spore stage released from the worm is, however, quite susceptible to destruction—the spore released from the fish can withstand freezing and desiccation, and may survive in a stream for 20 to 30 years depending on the conditions.

Differential diagnosis

The differential diagnostic table and the list of similar diseases appearing at the bottom of each disease page refer only to the diseases covered by this field guide. Gross signs observed might well be representative of a wider range of diseases not included here. Therefore, these diagnostic aids should not be read as a guide to a definitive diagnosis, but rather as a tool to help identify the listed diseases that most closely account for the gross signs.

Similar diseases

Viral haemorrhagic septicaemia

Further images

Sample collection

Because of uncertainty in differentiating diseases using only gross signs, and because some aquatic animal disease agents might pose a risk to humans, you should not try to collect samples unless you have been trained. Instead, you should phone your state or territory hotline number and report your observations. If samples have to be collected, the agency taking the call will advise you on what you need to do. Local or district fisheries/veterinary authorities could advise you on sampling.

Emergency disease hotline

For your state or territory emergency disease hotline number, see Whom to contact if you suspect a disease.

Further reading

www.tu.org/site/c.kkLRJ7MSKtH/b.3596607/

wildlife.state.co.us/Fishing/Management/WhirlingDisease.htm