The influence of parasite on its host

       Every parasite living in or a fish exerts some degree of harmful influence on its host. This influence may result in extensive change in individual organs or tissues, or it can take on the character of a general effect. In either case the parasites causing these changes is regarded as pathogenic. On the other hand, this influence can be so slight that it does not cause the appearance of any external sings. In such cases parasites are commonly considered as non-pathogenic. However, small the noxious effect, every parasite is harmful to its host. It must also be remembered that any deflection from the parasite of an unusual site within the host, or sudden increase in the numbers of the parasite or changes in the physiological condition of the host can cause supposedly non-pathogenic parasite to assume a strongly pathogenic character. For example, Dactylogyrus minutum, a small and allegedly harmless monogenean infesting the gills of the carp occurs only in small numbers in natural waters but is capable of prolific reproduction in aquaria, where it might cause the death of its host.


Parasite can influence the body of the fish in many different ways. Usually one type of activity is responsible for various pathological changes, indicating the extent to which the fish as a whole has been affected. For example, the mechanical obstruction caused by the cysts of Myxobolus cyprinid in the renal blood vessels of the carp results both in disturbances of the blood functions and of the water exchange processes. The ways on which parasites may affect the body of the fish can be grouped under several headings.

  1. MECHANICAL EFFECTS:  can be seen in the damage to the tissues, in rupturing of the protective layers, complete or partial atrophy , of the internal organs, obstruction of the alimentary canal or vascular system, etc.

Simple irritations of the tissues can have many different effects. For example, mass infestation of the skin with ectoparasitic protozoa (Costia, Chilodonella and Trichhodina) or Monogenoidea (Gyrodactylus) causes a copius effusion of mucus. This coating or mucus disturbs the respiratory function of the skin and ionic exchanges of the blood.

Tissue damage is encountered much more frequently. It can be caused by the representatives of many groups of parasites. The skin can be damaged by the ciliates of the genus Ichthyophthirius. The young stages of this parasite burrow into the skin of the fish penetrate the sub epithelial layer and there undergo reproduction. In mass infection of the skin with Ichthyophthirius the upper layers of the skin are sloughed off in large pieces.  A leech, piscicola geometra, can cause emaciation of the carp, when infesting it in great number during the winter.

   Parasites can cause serious mechanical injuries also to the gills and the walls of the branchial chamber. The latter often bears small wounds and ulcers caused by leeches (pisccola, Cystobranchus, etc.), as well as the argulid carp lice. Small swellings are formed around the points of attachment of parasitic lernaeopodids, like Salmincola nordmanni on Stenodus leucichthys nelma and Basanistes Woskoinikowi on the Baltic salmon.

    Mass infection of the gills with Ichthyophthirius brings about anaemic and necrotic conditions of extensive areas of the gill filaments. Injuries, epithelial hypertrophy and an abnormally high production of mucus occur at the point of attachment of many monogeneans, causing disturbances in the respiratory function of the gills. These parasites also frequently cause anaemic by feeding on the blood of their hosts. Prominent among them is Dactylogyrus vastator producing mass epizootics of young carp.

     A somewhat less serious part in diseases of fish is played by the parasites of the alimentary canal. They exert influences of two kinds: obstruction of the lumen of the gut and damage to its walls. Eubothrium crassum, the tapeworm infesting in great numbers the gut of salmon, is considered capable of causing at least partial obstruction. The mass infestation of trout with the acanthocephalid, Echinorhynchus truttae, this parasite virtually obstructed the gut of some fishes in the region of the pyloric caeca.

Conspicuous damage can be caused to the wall of the gut by the attachment organs of parasites belonging to many groups of helminths. Attaching itself to the inner wall of the stomach of pike (Esox lucius) and predatory salmonids, a large trematode Azygia causes ulceration easily seen by the unaided eye, with associated hyperaemia of the adjacent tissues.

The pathological influence of parasites can be observed also in other internal organs. It is particularly strong in the case of the larval stages of various trematodes, cestodes and nematodes. The liver, gonads and kidneys are among the most frequently involved organs. Mass infestations of the liver with the larvae of some nematodes, mainly of the genera Contracaecum, Porrocaecum and Anisakis occur not infrequently in marine fishes. The larvae of Contracaecum aduncum infesting the liver of cod cause serious loss of weight both of the organs infested and of the fish.

Mechanical damage to the reproductive glands of fish might cause their partial or complete castration. It was found that Ligula, parasitic in the visceral cavity of various cyprinids, suppresses the development of the ovaries of the host at the maturity stage two. Similar castration is caused by Amphilina foliacea, infesting the body cavity of acipenserids.

 Obstruction of the blood vessels and of renal tubules in the kidneys of fishes may be caused by parasites. The eggs of the trematode, Sanguinicola, circulating in the blood stream, are trapped in the capillaries of the kidneys, causing hypertrophy of the connective tissue around them. The resulting disturbance of function brings about abdominal dropsy, exophthalmus and bristling of scales. Infection of the renal tissues with some myxosporidians, for example Myxobolus cyprini and Hoferellus cyprinid in the carp, is responsible for similar signs.

The destruction of the eye lens by the larvae of Diplostomum spathaceum (the so-called parasitic cataract) must also be considered as mechanical damage. Sight can be impaired, too, as a result of infection of the eye with some myxosporidians (e.g. Myxobolus magnus, Thelohanellus oculi, Henneguya schyzura, etc.)

2. CONSUMPTION OF THE HOST’S FOOD. This occurs to a significant extent only when the parasites are comparatively large or occurs within the host in great numbers. For example, the weight of the ligulids or of Schistocephalus in one host individual might at times reach 1/4, 1/3, or even 1/2 of the total weight of the host. Studying ligulosis of Carassius carassius, it was observed by scientists that the worms reached as much as 76% the weight of the fish, being on the average about 36% of the weight. When occurring in great numbers, intestinal parasites undoubtedly convert to their use a considerable proportion of the host’s food. Among such parasites are the tapeworms Eubothrium crassum, E. rugosum, Thaenophorus and many others.

3. TOXIC EFFECTS. Very little is known as yet about the toxic effects of parasites on their fish hosts. They can be divided into two groups: those due to the secretions of special poison glands and those due to the metabolic activity of the parasite. The first group is exemplified by the activity of the carp louse, Argulus. The mouth armament of this parasite includes a poison gland, the secretion of which, in cases of mass infestation, may kill small fish. Some authors believe in the toxic influence of Nitzschia on the gills of acipenserids. Such influence could explain the inflammation of area of the gills not directly occupied by the worm. Considerable swelling of the gill filaments in areas free of the parasite was also observed by several authors.

In recent years several authors have studied the influence of parasites on the blood of the host. It has been found that the parasites, whatever their habitat, have considerable influence on the blood of fish. In all the investigated cases the parasites caused decrease in the haemoglobin content and erythrocyte count.

Parasitic toxins might influence the endocrinology of the fish. It is found that ligulosis results in a considerable lowering in the production of gonadotropic hormones by the pituitary gland of the fish, a well as causing pathological changes in the basophilic cells of the anterior part of that gland. The underdevelopment of thegonads in igulosis is, therefore, due as much to the mechanical suppression by the parasite as to the toxic effects on the glands of internal secretion.

4. PARASITES AS VECTOR OF OTHER PARASITES. A parasite is sometimes harmful not only due to its own activity but also because it disseminates agents of some infectious and parasitic diseases. Among the best known of these parasites are leeches which infect fish with various trypanosomes and haemogregarines. For example, the common leech, Piscicola geornetra, is a vector of Trypanoplasma cyprini. It has been shown in recent years that fish ice (Argulus), leeches and other blood-sucking parasites might act as vectors of the red disease of carp.

5. PARASITES AS INDIRECT CAUSES OF DISEASES. By damaging the surface of the body and internal organs of fishes and producing various wounds and ulcerations, parasites favour the penetration of other pathogenic organisms, mainly fungi and bacteria, less often Protozoa. Wounds caused by leeches and copepods can frequently be seen to be infested with saprolegniaceous fungi. The agent of the red pest of carp penetrates the blood of that fish through similar wounds. It has been observed that red pest occurs particularly frequently in traumatised fish. The damage caused to the intestinal mucosa by the acanthocephalids and coccidians has been thought to favour the infection. Damage of the skin also allows the entry of the putrefying bacteria, as well as encouraging the settling at the injured site of some unicellular parasites. Great concentrations of the flagellate Costia necatrix and of Saprolegnia have been observed around wounds caused by the copepod, Lernaea cyprinacea.

6. THE INFLUENCE OF INFESTATION OF NON-SPECIFIC SITES. It is well known that parasites are characterized by adaptation of definite habitats within their hosts. There are several examples of fish parasites infesting the gut without causing any observable changes in it were found in the kidneys of Rutilus rutilus in the Sea of Azov. The parenchyma of the kidneys was seriously damaged and as a result the water balance of the fish was impaired and severe abdominal dropsy developed.

7. THE INFLUENCE OF PARASITES ON THE GROWTH RATE AND CONDITION OF FISHES. The pathogenic activity of the parasite necessarily affects the growth rate and the condition of the host. It was found that mass infestation of coregonids with Ergasilus sieboldi. During the five years of investigation, this infestation resulted in a marked lowering of the growth rate of the fish. Lernaeocera branchialis, parasitic in the branchial chamber of the gadoid fishes, influences the whiting, Gadus merlangus as the head of the infested fish becomes relatively larger and the body shallower than in the uninfested fishes.

8. INFLUENCE OF PARASITES ON THE SIZE OF FISH POPULATIONS. The density of fish populations might be considerably affected by parasites, particularly in the event of mass mortality. It has frequently been reported that the copepod, Ergasilus siebold4 causes almost 100% mortality of Tinca tinca in the infested ponds. A mass epizootic of triaenophorosis in the lake Kandry-Kul of the Bashkir Republic, USSR was reported.

Reaction of the host fish

Cell and Tissue Reactions

The body of the fish can react to the parasite by hyperptrophy of individualtissues, by metaplasia, by inflammatory processes and, finally, by the development of immunity. Though in relation to fishes, these phenomena have been studied insufficiently. They can be divided into cell and tissue reactions on the one hand and humoral reactions on the other.

1. HYPERTROPHY. This response very frequently evoked by the penetration of the parasites, particularly small one like Protozoa, is the hypertrophy of one or several adjacent cells of the host. The causes of parasitic hyperptrophy are to be sought, probably in the mechanical and chemical influences of the intracellular parasite, which bring about functional changes in the invaded cells. Abnormal stimulation of the cellular mechanism leads to gigantism of the infected cell. Poorly specialized tissues, such as epithelium and connective tissue, are best suited to this type of reaction. The best example of epithelial hypertrophy is provided by the reaction of the gill epithelium to infestation with various species of Dactylogyrus. D. vastator which infests the tips of the gill filaments of carp, causes hypertrophy of the tips, with the formation of very long and thin epithelial processes. Hypertrophy can be regarded as the specific reaction of the gills to this kind of parasite.

2. INFLAMMATION. The reaction to the presence of parasite is often expressed by the process of inflammation, developing at the point of penetration or at the sites of permanent localization of the parasite. Very often a connective tissue capsule is formed around the parasite, more or less isolating it from the surrounding tissue. Such a capsule is formed round most of the quiescent stages of parasites in their intermediate host. The body of the fish responds to all types of injuries by a more or less definite process of inflammation. As in other vertebrates, this process consists of an increased blood supply to the damaged part (hyperaemia), penetration of white blood corpuscles from the vessels directly into the tissues and a flow of plasma through the vessel walls (transudation). For example, general inflammation of the alimentary canal is generally observed in cyprinids infested with Pomporhynchus laevis. The same effects are caused by mass infestation of trout with trematode Crepdodostomum farionis.


3. METAPLASIA. Very frequently the tissues of the host respond to the presence of the parasite by metaplasia, i.e. a change in their structure and function. A very good example of it is shown by; the mucoid transformation of epithelial tissue associated with a great number of parasitic diseases. Epithelial tissues of fishes are known to contain single cells or groups of cells, secreting mucus. Harmful influences on the epithelial tissue result in the appearance of increased numbers of such cells, transformed from ordinary epithelial cells. In effect, a sharp increase takes place in the secretion of tissue. A copius secretion of mucus by the skin of fishes is observed in mass infection with Chiodonella and Trichod/na and infestations with Gyrodactylus and other ectoparasites.


This term normally means the ability of the host to resist infection by the pathogen of a definite disease. As it stands at present, this definition is not fully satisfactory, especially in relation to immunity in parasitic diseases. The definition covers also the problem of specificity, the inability to resist the parasite due to certain anatomical or physiological features of the fish, etc.

The same parasite infesting different fishes is able to reach a large size and high reproductive capacity in some of them (main hosts), while in others (subsidiary hosts) his size remains small and his fertility restricted. For example, Azygia lucii reaches a large size and is very fertile in the gut of Esox lucius, in which it is also usually found in great numbers, while in Perca, Acerina and Lucioperca it is much smaller in size and reproduces far less vigorously.

Many Soviet parasitologists established that changes take place in the parasite fauna with the age of the host, which led to the consideration of age immunity. Some parasites have been observed to occur more frequently on the young, others on the old fish. It is not always possible to explain this phenomenon by the acquisition of immunity. Very often it is attributable to other causes. Changes in the parasite fauna with the age of the host are sometimes connected with changes in its size, at other times with the ecology of the young fish. For example, the young of Coregonus lavaretus ludoga are very extensively infested with the tapeworm Proteocephalus exiquus, the intermediate hosts being small copepods which provide an important part of the diet of the young fish.

Acquired immunity depends on the development, stimulated by the presence of the parasite of resistance to re-infestation with this parasite (super-invasion immunity). Less often it is retained after the parasite has left the body of the host (post-infestation immunity). Immunity acquired in parasitic diseases is, as a rule, only partial, not totally preventing but diminishing the chances of recurrence. It can also effect retardation of development and of fertility of the parasite. It is quite certain that this type of immunity occurs in parasitic diseases relatively frequently and that it constitutes one of the factors limiting the numbers of parasites.

Scheuring (1930) recorded the development of super-infestation immunity in fish caused by the adults of T. nodulosis in the gut of Esox lucius. Discussing the pattern of the infestation E. lucius with this parasite is noted that infestation occurs only during the summer (May-June), although plerocercoids can be found in the fish throughout the year. He further considered this to be due to the formation in the intestine of the fish of antibodies preventing re-infestation.

The development of super-invasion immunity has been amply demonstrated in several marine fishes, especially those belonging to the families Serranidae and Lutianidae. The best known is the case of infestation of the eyes with a monogenean, Benedenia (Epibdella) melleni, first pointed out by Jahn and Kuhn.

These authors observed that marine fishes, especially of the two above-mentioned families, are especially strongly infested when in aquaria. The parasite attaches itself to the epidermis and conjunctiva of the eyes; in cases of severe infestation it is found also on the gills and in the nasal cavity. Up to 2000 worms were found on one fish. The results were often fatal for the fish. In the surviving fish of some species the infestation drops after some time spent in the aquarium. It is believed it to be due to the acquisition of immunity. The study of the resistance offered by fish hosts to various parasites proved particularly important during the work of introducing fish into new waters. Such examples as those of Nitzschia or Ichthyophthirius, which in recent years caused outbreaks of epizootic in river fishes, when introduced into the rivers with newly acclimatized fishes, show clearly that the study of immunity in parasitic diseases deserves a great deal of attention.



  1. Hello,I like to read more on this field. Thank you for publishing RELATIONSHIP BETWEEN HOST FISHES AND THEIR PARASITES | Zoology World.

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