Pathogens Invasive Species

Viral Hemorrhagic Septicemia Virus (VHSV)

Viral hemorrhagic septicemia virus is a serious pathogen of fresh and saltwater fish. VHSV virus is a rhabdovirus (rod shaped virus) that affects fish of all size and age ranges. It does not pose any threat to human health. VHSV can cause hemorrhaging of fish tissue, including internal organs, and can cause the death of infected fish. Once a fish is infected with VHSV, there is no known cure. The clinical signs of VHSV may include tissue hemorrhaging (bleeding), unusual behavior, anemia, bulging eyes, bloated abdomens, and the rapid onset of death; however, these symptoms could apply to many different fish diseases. There is no clear visual diagnostic to confirm VHS. Not all infected fish show signs and may become carriers of the disease. The only way to confirm VHSV is to test the fish in a lab.

VHSV can be spread from one waterbody to the next through a variety of means, not all of which are known at this time. One known method of spreading VHSV is moving fish from one waterbody to another. This can be done by importation, stocking, or the use of bait fish. Other potential sources of VHSV spreading are natural fish movements, recreational boating/angling, bird assistance, ballast water discharge, and sampling activities.

Whirling Disease (Myxobolus cerebralis)

Whirling disease is caused by a microscopic parasite that was introduced to the United States from Europe in the 1950s and has spread to many streams across the United States. Whirling disease attacks juvenile trout and salmon, but does not infect warm water species. Rainbow trout and cutthroat trout appear to be more susceptible than other trout species. Brown trout become infected with the parasite, but they appear to have immunity to the infection and have not been as greatly impacted as rainbow trout. By damaging cartilage, whirling disease can kill young fish directly, or cause infected fish to swim in an uncontrolled whirling motion. This can make it impossible for them to escape predators or to effectively seek food.

The whirling disease parasite has been found in wild fish and fish hatcheries in 25 states. Once established in a stream, the parasite cannot be eradicated, nor can its worm host, without significantly damaging the ecosystem. Whirling disease has no known human health effects.