Bacterial, and Viral Diseases
bacteria, and viruses are responsible for various diseases of
aquarium fish. Collectively, this group of disease agents are
referred to as microbes. All are so small that it is not possible
to see the organism without the use of a microscope. Identification
of diseases caused by these organisms must initially rely on external
signs such as lesions or nodules.
Microbial disease outbreaks in aquariums are correlated
with a deterioration of water quality, parasites that damage the
fish's skin or gills, or poor nutrition. Any trauma that weakens
the fish's disease resistance barriers allows invasion by microbes.
Microbial diseases are readily transmitted from one fish to another.
Fungal Infections: Fungal infections are not uncommon
in aquarium fish. Infection often follows prior damage to a fish,
caused by handling with a net or by an existing infection by parasites.
Aquarium hobbyists familiar with fungal infections of freshwater
fishes will be familiar with the characteristic white, cottonlike
growth of fungus on the skin or fins of infected fish. Known as
Saprolegnia sp., this fungus readily invades stressed freshwater
fish, but is not known to affect decorative fishes. Signs of fungal
infection in fish are therefore less dramatic and often difficult
When present, the fungus appears on fishes as a fine film,
dark pigmented areas, or a coating covering areas of the body
or gills. This coating can easily be confused with similar signs
that are caused by the presence of various types of parasites.
Treatment: Treatment of fungal infections of the
body and gills involves the use of various fungicides available
commercially. Malachite green and methylene blue have been used
successfully to control fungal infections in decorative fish.
Various drugs are also useful for controlling both fungal and
bacterial infections. Since fungus is a secondary invader, treatment
methods must also address the initial cause of the disease, including
trauma induced by deteriorated water quality, poor nutrition,
or poor handling of the fish.
Bacterial infections: These are also commonly associated
with preexisting trauma or stress of fish, including poor water
quality, handling trauma, and poor nutrition. Pathogenic bacteria-those
that can cause disease-as well as the beneficial nitrifying bacteria
are always present in aquarium water. The pathogenic bacteria
will initiate disease only if the fish's immune system is impaired.
Providing optimal water quality, controlling the presence of parasites,
and practising good aquarium management will prevent the outbreak
of bacterial diseases. The exact identification of bacteria
requires the use of sophisticated diagnostic tests routinely used
by laboratory technicians. The procedures involve growing the
bacteria on specialised media, then conducting various chemical
Many bacterial infections can be identified by various
signs they produce when they infect fish. Bacterial infections
can be divided into those causing external infections and those
primarily causing internal lesions. It must be noted that external
infections can rapidly worsen, spreading to the internal organs
and bloodstream. Bacteria can enter fish either through the skin
or by oral ingestion.
External bacterial infections (fin-and-tail rot and ulcer
disease) can be caused by various species of bacteria. Bacteria
most commonly associated with these infections include pseudomonas,
myxobacteria, and vibrio. The latter group is the major cause
of bacterial infections, especially in newly imported fish that
are still under extreme stress from capture and transport.
Collectively, the signs of external infections caused by
these bacteria are often referred to as "fin-and-tail rot"
and "ulcer" disease. The names refer to the signs of
infections from bacterial disease and not to their cause from
Treatment: Bacterial infections must be treated
promptly because of the rapid progression of the disease from
minor to severe. Various antimicrobials are available for treating
bacterial diseases of fish. Those recommended for the treatment
of external infections of the body surface include nitrofurazone,
nifurpirinol, and various sulfonamides. Although various antimicrobials
are available commercially for the treatment of bacterial infections,
not all are suitable for use in freshwater aquariums. Those that
are virtually useless for treatment of infections in aquariums
include ampicillin, penicillin, tetracycline, and erythromycin.
The latter can kill the nitrifying bacteria in your aquarium,
causing an increase in toxic ammonia. In addition to treating
your aquarium, you should also investigate other possible causes
of the outbreak, including the addition of non-quarantined fish,
excessively high water temperatures, an overcrowded aquarium,
low-dissolved oxygen, or other parameters that could have initiated
The signs of the disease differ with the species of fish
as well as the extent of infection. Typically, fish can develop
a pronounced swelling of the body with a change in normal body
coloration. The appearance of signs associated with mycobacterio-sis
is largely dependent on the stage of the disease. Fish in advanced
stages become emaciated, have swollen or cloudy eyes, and can
develop ulcers on the body.
Viruses: Decorative fish are also susceptible to
infection by viruses, although few have been identified from aquarium
fish. Viruses are smaller than bacteria and require the use of
sophisticated electron microscopes for their identification.
The major virus common among freshwater fishes and relatively
easy to identify from the lesions it produces is lymphocystis.
The disease is chronic, meaning that in the majority of cases
it will not kill the infected fish. The major consequence of lymphocystis
disease is disfigurement of the fish.
The disease agent preferentially infects the cells of the
skin and fins, causing the appearance of lesions. Once the virus
infects a cell, it takes over the activities of the cell, forcing
it to manufacture more viruses.
Treatment: There is no known cure for lymphocystis.
The use of medications including antibiotics appears to be useless.
If the lesions are restricted to the distal area of a fin, it
is possible to carefully trim off the infected portion and treat
with an antibiotic to prevent a bacterial Infection. In many cases,
however, the virus will reappear in the same area.