There is a complex relationship between host and parasites, so this relationship depends upon various factors which are important for such interaction. Initially, the parasites try to develop within the host, later they resist the infection through defense mechanisms.
There are two important factors, first is host susceptibility and other is resistance so both of them determine that the infection is developed or not. Here we are talking about the relationship of fish and parasites. There are other factors which are also related to host are age, physiological, immunological, feeding habits and location of water body.
So, here we have some parasite factors which can affect the relationship of fish and parasite are parasite’s mode of entry, ability to evade the hosts defense, the nutritional requirements of parasite and response of parasite to host immune system.
There are some environmental factors which are also important such as temperature of water, crowding and habitat changes that could also influence the relationship. Here we will discuss some parasitic diseases of fish. The protozoans are the major group who cause infections in fish species.
These parasites are present in immense number, either on surface of fish, within the gills or both. If they are present in gills they cause respiratory issues which some other stress inducing environmental factors are present.
Hexamita sp.:
These parasites are pyriform to oval with tapering toward the posterior end. sometimes, rounded individuals can be identified. They have three pairs of anterior flagella which are approximately one and one-half times the length of the body. The flagella originate from the blepharoplast at the anterior end of the axostyles.
These organisms can reproduce by longitudinal binary fission as well as undergo schizogony within the epithelial cells of the ceca. This parasite causes disease within the gastrointestinal tract of fish and affected fish will have clinical signs related to malnutrition and emaciation.
A poorly understood parasite thought to be Hexamita-like is thought to be responsible for Freshwater Hole-in-the-Head and Lateral Line Erosion.
Trichodinia sp.:
There are three genera which form the Trichodina complex: Trichodina, Trichodonella and Tripartiella. Commonly known as Trichodina. These parasites are equipped with cilia all around and they are “frisbee” shaped.
They comprise a denticular ring in body, which give characteristic shape. In this case the fish suffers from severe respiratory problems and osmoregulatory problems. Fin erosions are observed in chronic cutaneous infections.
Ichthyophthirius multifilis:
This parasite is very common in almost all species fish. This parasite has direct life cycle. The disease this parasite cause is “white spot”. There are lesions in the gills and these are due to epithelial hyperplasia. And cutaneous lesions are also seen in this disease.
Ambiphyra sp.:
These are ciliated protozoal organisms which are thought to be free-living but, have been known to cause infection in fish. They are sessile organisms with a cylindrical to conical body with oral cilia and a permanent unmovable equatorial ciliary fringe. They range in size from approximately 60-100 mm and attach to the epithelium of the skin.
Disease and death of fish have been associated with chronic infections of the gills due to mechanical blockage of respiratory epithelium. Diagnosis of this parasite is dependent upon identification of this organism within the skin or gill scrapings or histopathology.
Myxozoa Parasites:
This is a very immense group of parasites which can cause disease in a wide variety of aquatic organisms. They are obligate parasites of tissue (histozoic forms that reside in intercellular spaces or blood vessels that reside intracellularly) and organ cavities.
The Myxozoa include development of a multicellular spore, presence of polar capsules in their spores and endogenous cell cleavage in both the trophozoite and sporogony stages. The method of transmission of myxozoas is unknown, but evidence suggests that at least some pathogenic myxozoa’s have an indirect life cycle.
This life cycle may require the completion of two different life cycles involving a vertebrate (fish) and an invertebrate (annelid) host with each life cycle having its own sexual and asexual stages. Severe infestations by these parasites can result in disease or death of the host fish. Each parasite is somewhat species specific as well as organ specific.
Some other Parasites
- Trematodes
This group of parasites has a complex life cycle with several successive larval generations, alternating sexual and asexual generations and changes of hosts to develop into the adult in its primary host. The life cycles of trematodes involving fishes may either use fishes as the primary hosts or as intermediate hosts. Adult trematodes may infest the intestine or gall bladder of fishes.
- Diplostomum spathaceum:
The life cycle of this parasite begins as an adult trematode in the intestine of gulls or birds which like fish eating. The body of the adult is 0.3-0.5 cm in length and distinctly divided into a flattened anterior forebody and a cylindrical and narrower hind-body. Eggs are shed and passed in the feces of the bird to the water.
The eggs hatch in approximately 21 days into free-swimming ciliated miracidia. The miracidia infest aquatic snails as the first intermediate host by penetration of the snail’s hepatopancreas. The miracidia then become a mother sporocyst, followed by one or more daughter sporocysts.
Each daughter sporocyst produces many cercariae which are released into the water. These cercariae seek a second intermediate host by penetrating the fins, skin, gills or cornea of small fishes.
Primary host fish which ingest the initially infected fish (second intermediate host) become infected and the life-cycle is completed when the host fish are ingested by fish-eating birds.
- Cestodes
Cestodes are a taxonomic class of organisms in which the adult stage usually lives in the intestinal tract of vertebrates. Intermediate stages live in a wide variety of body locations in both vertebrate and invertebrate hosts.
The bodies of most cestodes are ribbon-shaped and divided into short segments called proglottids, hence the name “tapeworm”. Diagnosis of cestodiasis is dependent upon demonstration of the parasite within the intestinal tract of the fish.
Clinical signs of cestodiasis include emaciation, anemia, discoloration of the skin, and susceptibility to secondary infections. Low numbers of pleurocercoids may be located in vital organs such as the brain, heart, spleen, kidney, or gonad and have a devastating effect on the fish (photo demonstrates a larval cestode within the liver of a fish).
Authors: Hammad Ur Rehman Bajwa1,*, Muhammad Kasib Khan1, , Zaheer Abbas1, Nauman Iftikhar1, Shehryar Shahid2 , Gulfam Younas2
1. Department of Parasitology
2. Department of Livestock Management
University of Agriculture, Faisalabad-38040 Pakistan