The following article is
acknowledged with thanks from R.E. Carlson of
KoiVet.comSaprolegnia - Water Fungus
Like all fresh water environments, koi and goldfish ponds are living
ecosystems. They contain plants, algae, invertebrates, protozoa, bacteria and
molds or fungi. Many of the life forms in our ponds are dependent upon each
other. This is known as symbiosis (living together). An extreme form of
symbiosis is parasitism, where one life form lives at the expense of another.
One of the more common forms of parasitism in our ponds is saprolegnia or water
fungus. For the average ponder, great zoological detail about saprolegnia is
not required to understand what it is, what it does and how to treat the pond
and fish if it becomes a problem, therefore not much detail on the zoology of
saprolegnia is offered here: just good-to-know information on how to identify
and treat it as well as some tips to help prevent it from affecting your fish.
What It Is Saprolegnia (or sap) is a freshwater fungus
which simply means it lives in fresh water environments and needs water to grow
and reproduce. Sap can also be found in brackish water and moist soil. It is
often referred to as a cold water fungus as it flourishes in colder
water, but it lives well in a wide range of water temperatures extending from
37 deg F to 91 deg F (3 to 31 deg C). While it has been described also as a
mold, sap is a genus of fungus, with the difference being a
mold is a mass of fungi.
Under the
microscope, sap is composed of filaments that tend to have spherical ends. It
is these spherical ends that house the zoospores, or the seeds of
sap, that allow it to proliferate and spread. The filaments are called hyphae
and give sap its cotton-like appearance. It is the hyphae, or more exactly, the
hooked, foot-end of the hyphae, which penetrate the tissue of the fish as they
seek nutrients. In the lower-power micrograph below, you can see the hyphae
with the spherical ends. With a 400x microscope, the structure will look
basically the same.
In the water, sap looks like fluffy cotton;
however, out of the water it appears to be a matted mess of slime. Sap starts
out either white or grey in color. The grey appearance also may indicate the
presence of bacteria growing with saps structure. Over a short period of
time, sap may turn brown or green as organic particles in the water (such as
algae) adhere to the filaments. Note the picture below. This is a classic
presentation of sap that has invaded a bacterial infection of the dorsal fin
area of a koi. The green color is actually embedded algae particles within the
saps filaments.
By appearance, sap can be easily
confused with epistylus, a freshwater parasite that presents as a white cottony
substance growing on the skin of the fish. Epistylus does not gather organic
particles and will remain white. Confirmation of epistylus versus sap should be
done with a microscope. The following is a microscope shot of epistylus at
400x: The favorite food of sap is dead organic tissue. We can usually see
evidence of sap on dead and dying fish, live and dead fish eggs and even food
left in the water so long that it has begun to rot. Typically we see infertile
koi eggs being infected first with the fungus then it spreads to kill live,
fertile eggs. Infected eggs have a typical fluffy cotton wool-like covering.
Sap also likes to feast on exposed and decaying tissue caused by bacterial
infections, such as ulcers. It is most prevalent on the head and fins of the
fish as these are the areas that offer the least amount of natural resistance
provided by the mucous coat.
Primary and Secondary Invader
Typically we think of sap as a secondary invader. This
means that something else has violated the integrity of the fishs skin
allowing bacteria to enter and provide direct tissue access for the sap hyphae
to embed themselves. In treating sap, it is imperative to treat BOTH the sap
and the underlying primary cause. This will be discussed in more detail in the
treatment section below.
However, sap can also be a primary invader under the right
conditions and this is where it gets the reputation as a cold water
fungus. We all know that cold water conditions, including dramatic temperature
changes, cause great stress on the fish and suppress the fishs natural
immune system. As stated above, sap flourishes in colder water by producing and
releasing increased zoospore counts into the water. The combination of stress,
suppressed immune systems, and increased spore counts give sap the ability to
cause major problems in colder water. Sap infections are commonly the cause of
winter kill.
The following is a list of conditions that
support the spread of sap:
- Overcrowding stress and too many organics in too little
water
- Handling stress and removal of the mucous coat on the fish
- Epidermal integrity open wounds that provide direct access
to tissue
- Parasites and pathogens parasites cause wounds that allow
pathogens (like bacteria) to enter the tissue thus giving sap a change to take
hold and stress
- Pollution stress and reduced water quality
- Spawning stress and physical damage
- Water quality stress and reduced physiological conditions
- Water temperature changes stress.
Obviously the underlying theme in the above list is
stress and this is the one thing that we need to guard against
first and foremost to keep our fish healthy. In healthy conditions, our fish
have some natural protection against sap with the mucous layer being the most
effective first line of defense. The mucous layer provides the ability to
reject a sap attack by sloughing off a layer of mucous and sending the sap with
it. The mucous coat also provides a natural fungicide at the cell-level. So you
can see that improper handling or any activity that reduces the mucous coat
offers an increased opportunity for sap to take hold.
Death by
SAP An increased morbidity of fish caused by sap can be traced to
three major things:
- Low water temperatures
High concentrations of sap zoospores
Mechanical stress
We know that sap attacks fish weakened by any of the above and the
suppressed nature of the immune system under cold water or high stress
conditions leaves the fish mostly defenseless. However, what kills the fish in
a sap attack is a condition known as hemodilution. By definition, hemodilution
is a decreased concentration (as after hemorrhage) of cells and solids in
the blood resulting from gain of fluid from the tissues.
this
causes the blood to loose electrolytes (blood salts) and make it less than
life-supporting. Then as the sap hyphae penetrate the tissue layers of the
skin, water begins to enter the fish thereby further diluting the fishs
blood salts. This explains why fish grossly affected by sap appear lethargic
and often lose their equilibrium.
Once it has (literally!) taken root,
sap can spread rapidly over the surface tissue of the fish. While it is rare
that sap will penetrate deep into tissue layers, even superficial damage to the
fishs initial tissue layers (and particularly the gills) can be deadly.
Obviously, the more wide-spread the sap infection becomes, the higher the rate
of hemodilution and greater chance that the fish will not recover. Therefore,
managing the sap infection quickly becomes the key to saving the fish.