Parasites pose a very real risk to to fish
All fish are potential hosts to many different species of parasites.
Small numbers of parasites are common and probably do little harm.
However, all parasites have tremendous reproductive potential and can,
under ideal conditions, quickly overwhelm fish in the confines of a tank
Types of parasites
We can broadly class parasites into two groups. Ectoparasites are found on the external
surfaces such as skin, fins and gills, while endoparasites
are found in the internal tissues and organs.
Endoparasites are uncommon in ornamental fish. However, there is a
wide diversity of blood parasites, worms that colonize the intestines
and other parasites that can invade various organs and tissues. Many
require an intermediate host, such as snails, birds or the introduction
of an infected fish, and so are rare in pond/ aquarium fish. There are a
variety of clinical signs which might indicate the presence of
endoparasites. Lethargy accompanied by emaciation is a common
sign, as are worms protruding from the anus. Identification of sporozoan
and protozoan endoparasites often requires microscopic examination of
Ectoparasites are a common problem
For hobbyists, endoparasites are the most common parasite problem.
With few exceptions, these parasites are not life-threatening in small
numbers, indeed small populations are fairly normal. The danger
from endoparasites comes from their tremendous reproductive potential.
In a natural environment this rarely leads to severe outbreaks, as
only small numbers of juveniles would survive and find a host
after hatching. Limited physical contact between fish would prevent
parasites being transferred to a new host, so limiting the spread of the
infestation although individual fish may be severely parasitised.
The confines of a pond or tank, with fish constantly coming into
contact with each other, provide ideal conditions for fish-to-fish
parasite transfer. The ready-availability of fish hosts results in high
survival rates of juveniles hatched from eggs and cysts deposited in the
pond. This can lead to serious parasite outbreaks affecting many fish.
Fish can control numbers
The mucus layer, or cuticle, provides fish with some protection
from parasites. In healthy fish it is continuously sloughed off,
making it difficult for parasites to get a good foothold. Additionally,
it contains various substances such as lysozyme, specific immunoglobins
and free fatty acids, which are believed to have anti-pathogen
properties. I stress that this assumes a healthy fish, living in a
The situation changes when the fish are stressed, or the cuticle is
being affected by adverse water quality such high ammonia/nitrite
levels, inappropriate pH or other toxins. Under such conditions the
consistency and protective properties of the cuticle can be severely
altered, allowing parasites to proliferate.
Cuticle doesn't protect against all parasites
Although the cuticle offers some protection against parasite
infestation, there are several parasites that are not hindered by its
presence. These are mainly the larger parasites such as Argulus
(fish louse) and Lernaea (anchor worms), but a very important
exception is Ichthyophthirius (white-spot).
In large numbers ectoparasites can cause severe damage to the
integument from their feeding activities and /or their constant movement
and attachment structures such as hooks and suckers. This in turn can
often lead to secondary bacterial infections, typically body ulcers and
bacterial gill disease.
The irritation caused by ectoparasites often leads to excess mucus
production, seen as a grey, slime film, and epithelial hyperplasia,
causing respiratory problems if the gills are affected.
Signs of parasite infestation
The larger parasites such as leeches, lice and anchor worms are
visible with the naked eye. Typical signs of parasite infestations
are rubbing, 'flashing' [there is a flash of silver from the underside
of the fish it turns its body to rub against something], focal redness
and inflammation on the body - often at the base of the dorsal fin,
flared operculum, respiratory difficulties, lethargy and bacterial
ulcers. However, it is important to realise that these 'symptoms' are
not exclusive to parasites and may be caused by poor water quality or
Identification, diagnosis and treatment
Most ectoparasites are too small to be seen with the naked eye. It
simply isn't possible to be certain that parasites are present without
taking a mucus and gill sample for microscopic examination.
This important procedure enables us to see which parasites are
present on the skin and gills and determine the severity of the
infestation. It is common to find more than one species present.
With this information we can implement an effective treatment plan
that takes account of the life cycle of the parasite. This is
particularly important with parasites such as white-spot, which have
complex life cycles, including a period off the host. Failure to
treat all stages will lead to re-infection.
Another consideration is that different parasites often need
different treatments. This can be important when more than one
species is involved. While general anti-parasite treatments are
effective with most parasites, they do not treat all parasites.
Sometimes, for various reasons such as under-dosing, water chemistry,
organic pollution or green water, treatments do not work. This is
why it is important to carry out a follow-up examination at the
conclusion of the treatment, particularly when one of the real 'nasties
such as white-spot, Chilodonella or Costia are involved.