DISEASES & PARASITES OF AQUARIUM FISH
By Jim E. Quarles.
Anyone who has kept tropical fish or even pond goldfish have crossed
paths with bacteria or parasites that cause illness or death in our fish.
Let me make a flat out statement: All fish, freshwater or marine types,
are hosts to a wide range of parasites. Generally problems only occur when
other factors come into play. Such as substandard water conditions, over
crowding, or new arrivals that may bring parasites into your collection
The average hobbyist need not become a pathologist to keep bacteria and
parasites under control in the fish room. But a general knowledge or broad
over view can save you big bucks in lost fish or expensive treatments.
It's hard to know were to start. I guess internal flukes is as good a
place as any, since these are of major importance in captive fish we all
try to keep and spawn.
They occur most often in the intestinal tract, but have been found in
blood, lungs, liver, bladder, lymph nodes, spleen, muscle and most other
tissues. Sometimes they are found as fully developed adults other times as
wandering larvae. The adult worms are microscopic or can be a couple of
inches or more.
Under normal conditions in nature, the numbers of parasites in any one
host are few. But even when the numbers are large, they may be harmless to
the fish. However, for our consideration the problem manifests itself when
we confine the fish to a small volume of water with other fish or species
that can act as hosts for the parasites, there by increasing the parasites
for any given volume of water or the population of fish. When a parasite
lives on only one kind of host, it is said to be host-specific but most
parasites live on many types of hosts. Flukes are less host specific. A
general rule to remember is that the less a parasite is host specific the
more likely it is to cause problems in your aquariums.
Presently about 9000 fluke species have been described and named, but
this factor should not be considered very noteworthy, since there are so
few parasitologist in the world this number of species is extremely low.
Flukes vary considerably, but the basic body plan is a sucker in front,
one in the middle of the bottom, nothing at the rear end. They have no eye
spots since they are not exposed to light. Spines are uncommon, and hooks
in most species do not occur.
Reproduction is almost always hermaphroditic, internal flukes produce
vast numbers of eggs. The high number of eggs is related to the low
likelihood that any one of them will succeed to produce another generation
of adults. The eggs have to be successfully expelled ( via the intestinal
tract, sputum or urinary system. Many factors the come into play, first of
all, internal flukes are captives of their own evolution, and that demands
that the life cycle be divided between sexual reproduction in a vertebrate
host. ( mammal, bird, fish, herptile,) asexual reproduction in an
invertebrate ( almost always a snail).
After the egg gets outside the vertebrate host life gets very
complicated. The goal is to get the larval product of that egg into a
snail, and then into another vertebrate, this indeed is a long shot. But
the worm has some wonderous tricks to manage this. The first larval stage
can get into the snail one of two ways. Either the inactive egg is sniffed
out and eaten by a roving snail, or the egg hatches and the swimming
ciliated larva actively zeros in on the chemical scent of a snail.
Interesting so far? If so read on Mc Duff, it gets more complicated for
the fluke and the fish!
Have you ever had swimmer's itch after swimming in a lake? That is
caused by schistosomes that normally live in ducks penetrating your skin,
then dying and inducing a hypersensitive allergic reaction. Merely a
nuisance. But if you ever go collecting fish in the waters of South
America, Asia, or Africa, you should think carefully about the danger
before wading unprotected, because human schistisines ( at least three
species in man ) occur throughout most of the tropical world.
The question is are digenetic trematodes a problem? Usually natural
infestation rates are not very high, but there are exceptions, but even in
captivity, the odds of epidemics occurring are remote.
WILD CAUGHT FISH
Wild caught fish will have digenetic trematodes in it's gut or in other
organs or tissues. Are they harmful in most aquarium fish? No! Should they
be treated with medications? No! You can eliminate them with some
anthelmintics ( anti-worm drug, ) But in most cases you should not do so.
Just leave them alone, the adverse effects on the host fish to the
chemicals probably exceeds the benefits of eliminating them.
One other note. It you find some neat snails in nature that you'd like
for your aquarium, do not just put them in with your fishes. They might
release cercaria. Instead, put them in their own tank or container for
propagation. After they have produced baby snails get rid of the adult
ones and use these clean baby snails for your fish tank.
In the next issue of Tropical News, I will cover some basics of
medications for fish.