Fungus and water moulds – what are they?
Fungus is a common fish disease that can prove fatal if not treated
early. Most fungal infections involve water moulds from the class
Oomycetes (a group of fungi with similar characteristics). Although
there are several water moulds that can affect fish, the most common and
significant water mould is Saprolegnia, a filamentous fungi. In
common with all moulds, it feeds by secreting digestive enzymes onto the
surrounding area. These enzymes break down cells and tissues enabling
the fungi to absorb nutrients such as proteins and carbohydrates. This
isn’t a problem if it's a piece of fish food – but it’s not so
welcome when it is one of your fish that is being slowly digested!
Fish fungus - Saprolegnia
click on pictures to enlarge them
Saprolegnia infection on the body of a koi
Water moulds are saprotrophs – that is they normally feed on dead
organic matter such as fish wastes, uneaten food etc, however they are
also opportunistic parasites, able to take advantage of damaged or
stressed fish. Fungal infections are called mycoses.
Water moulds consist of a tangled mass of individual filaments called
hyphae. As the mass grows it forms
compact tufts that are collectively called a mycelium.
The mass of fungi filaments or mycelium can be seen clearly without
a microscope. Moulds reproduce by releasing thousands of spores into the
surrounding water. The fungal spores are highly resistant to drying and
chemical attack and thus Saprolegnia spores and molds are common
in all ponds and tanks.
It is believed that the fish mucus contains effective fungicides that
prevent fungal growths under normal circumstances. It is worth pointing
out that large amounts of decomposing organic matter within the system
will encourage fungal growth.
A dangerous secondary infection
Saprolegniasis is mainly a secondary infection seen after damage to
the fish integument (skin and gills) caused by parasites, viruses,
bacterial infections and other skin damage. Other predisposing factors
include water pollution and overcrowding. Less commonly, Saprolegnia
can act as a primary pathogen infecting fish that haven’t shown signs
of previous damage. It is believed that such attacks are
temperature-dependant, usually occurring at low temperatures, possibly
as a consequence of a reduced immune response. I have seen two separate
cases recently involving orfe that were severely infected without any
other signs of damage.
As well as being a threat to fish, fungus can also affect fish eggs.
It is usually the infertile eggs that are first infected, but if
untreated the fungus soon spreads to nearby, healthy eggs.
Fish fungus appears as grey/white patches on the skin or gills that
resemble tufts of cotton-wool. At a later stage they may become brown or
green as they trap sediment or algae. If the fish is removed from the
water, the fungus appears as a slimy matted mass. Saprolegnia normally
establishes as small, focal infections that then spread rapidly over the
body or gills. As it spreads, healthy tissue is destroyed. There is
often little inflammation unless there is an underlying bacterial
infection. Microscopic examination shows broad, non-septate (no dividing
cell walls) hyphae of varying diameters.
Don't underestimate the danger
Although most infections are fairly superficial, rarely penetrating
beyond the superficial muscles, the danger comes from the speed at which
Saprolegnia can spread and the amount of surface area that is
often damaged. This in turns leads to serum and protein loss. There is
sometimes a tendency to underestimate the potential danger of fungal
infections and it is important to realise that even superficial damage
of the skin or gills can prove fatal if not treated. The chances of
recovery from saprolegniasis are directly related to the area of
integument affected, which is why prompt diagnosis and treatment is
Fish fungal infections are difficult to treat and mould cannot ever
be eliminated from any fish keeping systems. Any treatment plan must
take account of any predisposing factors and these should be resolved /
or treated at the same time.
Long-term bath treatments: