FISH IMMUNOLOGY
FISH DISEASES AND DEFENSES
By Jim E. Quarles
14-01-2000

While it is to be hoped that you will have few diseases in your tanks to worry about it always pays to know just a little more about disease defenses.

All animals live in an environment that is full of disease-causing organisms and parasites. Of course parasites are organisms that derive their living from another organism. ( like brother-in-laws ) A true parasite provides no benefit to the host. They are a diverse group that have shown a wide range of complexity. They range from viruses to arthropods.

In the interaction between the host and parasite, the objective of a parasite is to invade the host, and develop and reproduce.( In other words carry on it's own life functions). And of course the objective of the host, is to prevent the parasite from invading and reproducing. Parasites have evolved various structures and activities ( known as virulence factors ) which allow them to invade and survive in the host.

In turn the hosts have developed various structures / activities (defense factors ) which act to prevent the invasion and development of the parasite.

The result of this entry action can reach several conclusions . Either the parasite invades and becomes very active, leading to disease and possibly death of the host, a very undesirable result to both host and parasite. However the parasite might invade the host and it's activity remain mild, and the host in turn reacts mildly and both the parasite and the host survive for a long time. This result is desirable for the parasite and acceptable to the host. On the other hand another possible outcome is that when the parasite invades, the host's defense reactions overwhelms the parasite and eliminates it. Of course this is the optimal result from the host's point of view.

Since we are interested in the disease fighting factors of fish it should be understood that fish possess a highly developed and very efficient system of external and internal defenses against disease, this allows them to survive and prosper in an environment that contains a very large number of disease-causing organisms.

With the proper understanding of these factors it is possible to manipulate these defenses to prevent infections. Prevention is much more effective and cost-efficient that treatment of diseases. The defenses factors of a fish can be divided into two systems, the innate and acquired. Innate factors are constant no matter what parasite the host encounters or how many times the fish has previously in countered a given parasite. The innate system is composed of physical barriers such as skin, scales, secretions such as mucus, phagocytic cell such as macrophages, and proteins in the blood.

The acquired factors are components of the immune system of the fish that have the ability to react specifically with parasites, develop and remember the interaction.

The reaction to a parasite increases after a initial encounter with a given parasite.

Before a parasite can develop and reproduce, it must enter the fish. The host's first defense is, therefore, an epithelial membrane, either the epithelium of the gills or the digestive tract , or the skin or scales. These are hard to penetrate, and are covered with a moving layer of mucus which the parasite must penetrate before it can attach to the membrane. By peristaltic actions move the mucus along. On the surface of the fish, mucus moves from the head to the tail, where it is shed into the water.

A good number of parasites are trapped in the moving mucus and are shed away into the water without penetrating the fishes defenses. Fish often increase their production of mucus in cases of parasite invasion, when this occurs often you can see strands of mucus stringing out from the fins and tail of the fish.

If a parasite manages to attach to a surface membrane and penetrates into the tissues of the fish, it encounters another component of the host's innate defenses, the phagocytic cells. These cells are found in the circulatory systems, wandering in the tissues, and lining the vessels in organs such as the spleen and liver. These cells are the cleanup crew, they engulf and break down damaged host cells.

There are far to many parasites that affect ornamental fish to list them in this article but a few well known. One should be high-lighted: Hexamita, an important cause of mortalities in the home aquaria. They are to be found in the intestines, frequently the liver of angelfish are infected, they are also found in discus and gouramis fish.

While tapeworms are common in wild caught fish, they are rarely found in ornamental fishes. When they are present they are found attached to the intestinal wall by means of a scolex or head like structure with four suckers. But with some tapeworms hooks maybe present on the scolex. Generally tapeworms need a secondary host in order to reproduce. Such as waterfowl or snails.

Intestinal Nematodes are far more common in ornamental fish than tapeworms. These are often called roundworms, tubifex worms act as a secondary host for the round worm along with Cyclops, and Daphnia. Within these hosts the parasite is in its larval stage and when these host's are eaten by a fish the larvae develop into the adult or final form and become a disease vector.

Maintaining a clean aquarium with fresh well filtered water, along with massive and frequent water changes will do more to prevent bacterial infections than almost any other procedure you can follow.

Avoid feeding live foods other than brine shrimp and you will have prevented most of the conditions required for external and internal parasites. The number one live food to avoid at all cost is tubifex worms, black worms or what is known as sewage worms. Earth worms are perfectly safe to use and encouraged as food.

The study of fish immunology will greatly improve your success with keeping your ornamental fish. There are a number of good books written in lay-man's language that will give you a good working knowledge of the subject.

I recommend every fish keeper should add one or more to their collection.

Contributors,
J.B. Gratzek, E.B. Shotts Jr. and D.L. Dawe.
From Infectious Diseases and parasites of freshwater ornamental fish.