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Iridovirus
in Gouramis
By:
RuthEllen Klinger, Ruth Francis-Floyd, John Slaughter and Craig Watson
What Are Iridoviruses?
Iridoviruses are a family of viruses (130--300 nanometers in size) that
contain DNA as their genetic material and have an icosahedral (20-sided)
capsid. Iridoviruses have been found in a wide variety of fish, including
both freshwater and saltwater species. Some iridoviruses have been
associated with serious diseases (e.g., viral erythrocytic necrosis of
salmonids) while others have only been found in apparently healthy animals
(e.g., goldfish iridovirus). One iridovirus causes a disease called
lymphocystis which causes unsightly skin lesions on infected fish, but
otherwise is of little consequence.
Iridovirus in Gouramis
An iridovirus was found in spleen and intestinal tissue of gouramis from
the genus Trichogaster that were dying with signs of systemic
disease. Mortality rates of affected fish have varied from low (0.5--10%)
to moderate (50%) with death usually occurring 24--48 hours after the
onset of signs. Clinical signs associated with the presence of the
iridovirus have included darkening of body coloration and lethargy. Sick
gouramis often stop eating and the abdomen may be distended. Internally,
an enlarged spleen has been the most notable abnormality. The intestine
may be reddened, and a clear amber fluid may be present in the body
cavity. Laboratory examination for bacterial, fungal, or parasitic agents
has frequently been negative. Through electron microscopy (EM), abundant
iridoviral particles have been found in the spleens and intestines of
dying fish.
An iridovirus has been isolated in cell culture and cytopathic effect
(death of infected cells) has been observed. Although the iridovirus has
been implicated as a possible cause of disease in gouramis, efforts to
reproduce the disease under laboratory conditions have not yet been
successful.
Implications of Possible Iridovirus Infection in
Gouramis
It is not known whether or not the iridovirus observed with EM in tissues
of sick gouramis is actually causing a disease, but it is suspect. It is
also unknown whether the gourami iridovirus is capable of causing disease
in other species. Iridoviruses have been associated with systemic disease
and mortality rates of up to 80% in Ramirez dwarf cichlids ( Apistogramma
ramirezi ) and angelfish ( Pterophyllum scalare ). Until the
disease can be induced in the laboratory, it will be impossible to know
whether the gourami iridovirus is species-specific, or if the iridoviruses
reported in several species of freshwater tropical fish are all the same
agent.
The mode of transmission of the iridovirus infection is unknown. Other
iridovirus infections are believed to be transmitted by direct contact
(i.e., exposure to viable virus particles in the water or fomites such as
nets or siphon hoses), or ingestion of infected tissue (i.e., cannibalism
of dead fish). Also, it is not known whether fish that survive the
infection could carry the virus and serve as a potential source of
infection to other fish.
Management
Because the iridovirus found in gouramis has not been demonstrated to
cause disease, it is premature to make sweeping recommendations for
management. However, common sense would suggest that extra attention be
paid to gouramis which may be susceptible, particularly blue, gold and
platinum varieties. If any of these fish become sick, extra effort should
be made to determine the cause of mortality. Water quality, nutritional,
parasitic and bacterial problems should be identified and corrected.
Effort should be made to provide fish a high quality diet, maintain them
in clean facilities, and to keep sick or potentially infected stock
separate from other animals. Equipment, boots, and hands should be washed
with a disinfectant after handling. New fish should be isolated from
established stocks for at least 3--4 weeks in an effort to avoid
introducing infectious agents to established facilities.
Manipulation of environmental temperature has proven effective in
controlling a number of infectious diseases of fish, and may be a
potential tool that can be used to control the gourami iridovirus.
Anecdotal evidence suggests that the disease may be more active in warmer
weather (i.e., summer), however, this is speculative at the present time.
Until this gourami disease is reproduced under laboratory conditions, it
will be impossible to answer questions concerning the range of susceptible
host species, whether or not a carrier-state exists, and which management
strategies are most effective in controlling outbreaks of this disease.
Summary
Iridoviruses are a family of DNA viruses that have been found in a variety
of fish. Some iridoviruses have been associated with serious disease
outbreaks, while others have not. Iridoviruses associated with disease and
mortality of tropical fish have been reported in Ramirez dwarf cichlids,
angelfish, and, most recently, gouramis from the genus Trichogaster .
It is unknown whether the virus particles observed in tissues of sick fish
are actually responsible for disease. Also, we do not know whether each
iridovirus reported in freshwater tropical fish represents a different
virus or are all caused by the same one. When disease occurs in blue, gold
or platinum gouramis, efforts should be made to promptly submit a sample
of sick fish to a diagnostic facility. There are no medications which can
be used to cure viral infections. Early detection and treatment of other
problems, such as parasitism or bacterial disease, will improve the chance
of recovery of affected fish.
RuthEllen Klinger, Biological Scientist,
Fisheries and Aquatic Sciences; Ruth Francis-Floyd,
Associate Professor, Fisheries and Aquatic Sciences; John
Slaughter, Veterinarian, Hillsborough County Extension Service; Craig
Watson, County Extension Agent, Hillsborough County Extension
Service; Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville, 32611
Copyright:
This document is copyrighted by the University of Florida, Institute
of Food and Agricultural Sciences (UF/IFAS). date first printed 1996. |
