1. Introduction
Disease outbreaks in fish are the most devastating challenge for aquaculture production. Many freshwater fish species are seriously afflicted with various parasites, which results in high fish mortality and reduced aquaculture productivity and has a negative influence on the economy [1]. Infections caused by numerous fish parasites can impede the development of a culture system. The significance of fish parasites is directly connected to the significance of fish health. The host’s tissues or digested gut contents are the primary sources of nutrition for fish parasites, and upon favorable conditions, the parasites reproduce rapidly [2].
The rate of parasitic infection in fish is high among vertebrates, which is related to the aquatic environment in terms of dispersion, life cycle, and reproduction [3]. Parasites are ubiquitous in Asian countries and thus reduce aquaculture productivity, which is an important source of employment in many countries [4]. Parasites enhance their transmission in fish by altering the host’s eating, mating, and social behavior and migration patterns [5]. They also influence the neurological system of their host, interfere with the secretary functions of the alimentary canal and metabolism, and damage fish skin and gills by causing sores, ulceration, and tissue deterioration [6]. Parasites can influence the structure of fish communities [7]. Adult parasites are more dangerous, depending on the parasite load and size of the host, as well as the form of attachment [8]. In certain circumstances, parasites do not kill fish, but they have a negative impact on the population and individual fish. Protozoans, trematodes, and monogeneans are the most common parasites of fish species [9].
Protozoan parasites are of great importance as they vary in size and shape. Protozoan parasites have diversified classification [10]. Among the protozoan parasites, Ichthyobodo necator (Henneguy, 1883), Ichthyophthirius multifiliis (Fouquet, 1876), Trichodina sp. (Ehrenberg 1831), and Trichophyra and Hexamita (J. R. Uzmann, J.W. Jesse 1963) are some of the most significant pathogens that cause diseases in aquaculture [11]. They raise farm inputs due to increased handling costs and yield insufficient growth rates due to disease outbreaks [12]. Protozoan parasites cause severe diseases in freshwater fishes all over the world including ichtyobodiasis, coccidiosis, ichtyopthiariasis, and trichodiniasis [13]. The mortality rate of infected fish can reach almost 100% [14]. Parasitic diseases have gained much attention in research as fish consumption has increased in the last decade.
Fishbone trematodes cause serious infections in humans if they are consumed improperly [15]. Fish-borne trematodes affect the health of more than 40 million people in the world [4]. In 2005, 56.2 million people were infected with foodborne trematodiasis, including 7158 deaths [16]. Many farmers experience economic losses due to trematode parasites [17]. Trematodes belong to the phylum Platyhelminthes. Adult trematodes are obligatory parasites of many vertebrates. Trematodes complete their life cycle in four hosts [18,19]. Trematodes of the family Heterophyidae are intestinal trematodes. Their final hosts are mammals and birds. There are 22 species in the Heterophyidae family that cause infection in humans worldwide. Some previous studies reported that Haplorchis pumilio and Centrocestus formosanus were zoonotic species found in Sutchi catfish, but some unidentified species were also found, suggesting that there is a need for further investigation [20]. There is a need to record the distribution of parasites in relation to host size, season, and farm management [21]. The effects of the trematode metacercariae on fish include delayed growth of young fish and decreased immunity of fish, due to which secondary infections may also occur. Some other diseases include black spot disease, malformations in fish, inflammation of the liver, necrotic tissue change, displacement of organs, functional morbidity, and severe gill damage [22].
Monogeneans are a typically diverse group of ectoparasites of freshwater and marine fishes [23]. Monogeneans worms and their communities in cultured fish affect ecosystem health [24]. Monogeneans cause an increase in fish mortality due to various infections such as respiratory problems, anemia, and osmoregulatory dysfunction, and they also cause secondary microbial infection. Mongenean infestations in fish aquaculture bring about large financial losses [25]. The monogenean life cycle was found to rapidly increase in an artificial environment, which caused injurious infection in their hosts [26]. Depending on the fish species, monogeneans attach to the gills, the surface of the skin, fins, and eyes and typically fed on the blood, mucus, and epidermal cells of their host. Small- to medium-sized monogenean parasites complete their life cycle in a single host [27]. Different species of monogenean have been involved in the death of wild and cultured fish such as tilapia [28]. Members of Dactylogyridea, Ancyrocephalidae, and Gyrodactylidae have been reported in cultured and wild fish. Transmission of these parasites mostly depends on host-to-host interactions, although parasites may also occupy a new host by drifting with water currents or depending on water quality, which directly affects their infection processes [29]. In polluted water, parasitic infections commonly increase, and they provide an indication of water quality [30].
Relatively little research has been conducted on freshwater fish parasites. The identification of parasites is important for determining the specific etiology of sicknesses. Once the diversity of fish parasites is established, identification of the disease-causing agent and their pathogenicity will be easily accessible. After the identification of parasites, the risk of infection can be determined using the prevalence of parasites in fish and different water bodies. The present study was therefore designed to identify parasites of freshwater fishes in River Ravi, Pakistan
