New Vaccines for Aquaculturists
Like people, fish have their share of diseases and need vaccines
to keep them healthy. Agricultural Research Service scientists at Auburn,
Alabama, have developed several of these vaccines and are now closing
in on one that protects fish from a Streptococcus bacterium.
agency's Aquatic Animal Health Research Laboratory is developing a
new vaccine against Streptococcus iniae, says Phillip H. Klesius, who
the Auburn unit. "S. iniae is an emerging bacterial pathogen in cultivated
tilapia, hybrid striped bass, rainbow trout, yellowtail, eel, and turbot.
Worldwide, streptococcal infections are reported in 22 fish species,
both cultured and wild," he says.
S. iniae is recognized as one of the most problematic bacterial pathogens
in intensively cultured tilapia and hybrid striped bass in the United
of good health management practices and a vaccine to control it is a superior
approach to using antibiotics or chemicals, Klesius says.
Klesius, ARS molecular biologist Craig A. Shoemaker, and ARS aquatic pathologist
Joyce J. Evans are co-developing the vaccine. This team combines unique expertise
in fish disease prevention. Members conduct basic research to understand immunity,
transmission, and infection—particularly in hybrid striped bass and tilapia.
"We found the S. iniae bacterium possibly enters the nostrils of the fish from
the water," says Evans, who is with the Auburn unit but based at Washington College
in Chestertown, Maryland. She is researching fish health problems associated
with fish kills and aquatic pathogens. "Finding out how bacteria enter and travel
through the fish may aid in development of an effective vaccine," she says.
The higher the density of cultured fish, the more easily S. iniae is transmitted
and the higher the mortality, says Shoemaker. "Signs of the disease in fish are
abnormal behavior such as erratic swimming, whirling motion at the surface of
the water, darkening of the skin, blindness, popeyes, and small lesions on the
body, fins, and anus."
Antibiotics are currently used to control the streptococcal disease in fish.
Surprisingly, the team's research indicates there are certain negative effects
on fish health and immunity after antibiotic treatment for S. iniae. This indicates
that antibiotic treatment suppresses streptococcal disease signs but doesn't
completely eliminate the bacterium from treated fish.
Vaccine to the Rescue
new findings are important determinants for developing a successful vaccine
to fight S. iniae, which causes $150 million a year in losses. The ARS scientists
are designing it to provide lifelong protection. In laboratory studies, it
reduced mortality in tilapia and hybrid striped bass by more than 80 percent.
Popular in Asian countries, tilapia is showing up on more U.S. menus. Since
1997, U.S. fresh and frozen tilapia imports have increased 28 percent and U.S.
cultivation is expanding steadily. Hybrid striped bass consumption and production
through cultivation are also increasing rapidly because of rising consumer
demand for this excellent-tasting fish.
"We are currently developing plans to test the vaccine on a larger scale throughout
the United States," Klesius says. "We are testing effectiveness of both injection
and the bath immersion immunization that gives fish farmers more flexibility.
This vaccine could potentially save producers money worldwide."
The ARS team has filed for a patent on the new vaccine for use in both small
and larger sized fish.
More Catfish Disease Protection
Klesius and Shoemaker recently developed the first approved modified live-bacterium
fish vaccine, one that protects young channel catfish against enteric septicemia
(ESC). A major catfish disease caused by the bacterium Edwardsiella ictaluri,
ESC costs catfish farmers as much as $60 million a year in losses. This new
of a live E. ictaluri organism rendered unable to cause disease—prevents infection.
ARS has filed for a patent on it.
Also called "hole in the head," enteric septicemia is characterized by lesions
and holes in the fish's cranium, as well as by a bright-red color at the base
of its gills and belly. It accounts for 70 percent of disease losses in catfish
but has never been associated with human infection.
"In field studies," Shoemaker says, "the ESC vaccine reduced catfish mortality
by 80 percent."