DISEASES & PARASITES OF AQUARIUM FISH
Part 1
21-04-99
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 with them.

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.

INTERNAL FLUKES

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.

EPIDEMICS

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.