A survey of ecto and endoparasites of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) fingerlings in Midmar reservoir, Adwa, northern Ethiopia

ABSTRACT

Fishes are hosts to a range of taxonomically diverse parasites. This survey was intended to provide insight into the occurrence, distribution, prevalence, and intensity of parasites infesting the fingerlings of the tilapia, in Tigray. A total of 384 Oreochromis niloticus specimens were collected from Midmar consisting of 227 males and 157 females. External and internal organs were examined for parasite infection. Squash preparation of fish eyes was also examined to reveal the organisms through microscopic examination. The major groups of parasites were trematode, nematodes, protozoa and hirudinea. A total of 3762 genus Diplostomum were recorded affecting the health of 93% of fishes. Contracaecum species was the second most prevalent distributed majorly in the intestine (77.8%), pericardial cavity (19.1%), swim bladder (2.4%), gill (0.2%) and mouth (0.8%). The prevalence and mean intensity of Diplostomum spp., Contracaecum, Myxosporea, Clinostomum spp., Eustrongylides and Leeches was 93%(10.5), 62.2%(5.3), 39.3%(8.3), 2.1%(8.8),1.8%(5.7) and 1.6% (2.5), respectively. The mean temperature, dissolved oxygen, conductivity, and pH of the dam were 24.4°C, 7.55 mg/l, 849.5µs/cm, and 7.5, respectively. Therefore, responsible bodies in fisheries should perform a pre-stock assessment and provide fish farming extension services for optimum production and marketing of fish products with management and production enhancement methods.

KEYWORDS:

• Fingerlings
• intensity
• Midmar dam
• Oreochromis niloticus
• parasites
• prevalence

Abbreviations:

• DO, Dissolved oxygen
• K, Fulton’s condition factor
• L, Length
• O, Niloticus: Oreochromis niloticus
• PC, Pericardial cavity
• pH, Power of Hydrogen
• SB, Swim bladder
• W, Weight

1. Introduction

Fish infections with parasites cause production and economic losses through direct fish mortality; reduction in fish growth; replica and energy loss; the fish's increased susceptibility to disease and predators, as well as the expensive treatment (Cowx 1992; Shinn et al. 2015). Facts regarding the modes of transmission and potential intermediate hosts are usually crucial to selecting the foremost acceptable management action to reduce or eliminate the matter (Aken’ova 2000).

Ethiopia has massive water resources, with an approximated calculable surface area of 733k km2 of major lakes and reservoirs, 275 km² of undersized small water bodies, and 7285 km long rivers inside the country (FAO 2005; Janko 2014; Ayalew 2018). As a result of these ecological variations, the Federal Democratic Republic of Ethiopia has been the house of extremely wide-ranging flora and fauna. Over 200 species of fish are best known to occur in lakes, rivers, and reservoirs in the Federal Democratic Republic of Ethiopia (Jerbe 2007). The country depends on its interior water bodies for the fish offered to its population.

Mostly, Nile tilapia (Oreochromis niloticus) has been introduced throughout the country because of its adaptational capabilities and its quality to match Ethiopian consumers’ preferences. It contributed roughly 40.9% of the 13,253 tons of commercial fish taken in 2007/2008 as a result of its natural incidence and introduction into entirely different water bodies (MoARD 2008; FAO 2014).

One of the issues of fishery management within the wild populations is parasites and disease conditions of fish. Parasitic diseases cut back fish production by poignant the traditional physiology and if left uncontrolled, it may result in mass mortalities or some cases, will serve as a supply of infection for humans and alternative vertebrates that consume fish (Ayotunde et al. 2007). The presence of an enormous variety of parasites in every fish may represent a true threat to the fish population and needs immediate action (Komar and Wendover 2007; Areda et al. 2019).

Parasites are the most common cause of infectious diseases in fish. Most of the time problems occur when infected fish are brought into the laboratory or an intensive culture like smaller dams and earthen ponds situations. Not only are the fish outstandingly stressed but they are conjointly sometimes crowded and therefore the reproducing parasites do not seem to be distributed as they are within the wild. This means, that the closer the proximity of fish to one another the greater the probability of infection and mortality. Moreover, parasitic diseases of fish are classified into protozoan, crustacean, and helminthic diseases. Generally, most crustaceans are external parasites causing severe diseases while protozoans cause either external or internal diseases according to their habitats. The majority of monogeneans and annelids are external parasitic diseases, while the majority of digeneans cause internal parasitic diseases. Nematode, spiny-headed worm (acanthocephalan) and cestode infestations are generally internal parasitic diseases. Nevertheless, many parasites with larval stages in freshwater fish have a piscivorous mammalian carnivore as their normal final host and can infect humans because of the low host specificity of the adult stage (Aly 2013).

Metacercariae of the digenean fluke Diplostomum are the common parasites of wild and cultivated fish in temperate and tropical climates (Chappell et al. 1994; Hoogendoorn 2020). Fish heavily infected with these digenetic trematodes might experience loss of vision, reduced growth, thinness (Niewiadomska 1996; Islam et al. 2023) or deformation of the spinal column(backbone), brain tumour, cellular mortification and death (Machado et al. 2005). It has been shown that these parasites can cut back fish crypsis and escape response and therefore increase fish vulnerability to bird predation (Seppälä et al. 2005, 2012).

Parasites are important components of host biology, population structure, and indeed ecosystem functioning. They can be found in any fish species and within any type of aquatic and culture system. They range from protozoans such as flagellates, ciliates, and apicomplexans to metazoans including myxozoans, trematodes, cestodes, acanthocephalans, nematodes, and crustaceans (Marcogliese 2004). In Ethiopia, different genera of fish parasites have been reported from different water bodies. Fish parasites including genera of Trematodes (Clinostomum spp., Diplostomum spp., Neascus, etc.); nematode (Contracaecum spp., Eustrongylides, Camallanus, etc.), cestodes (Ligula intestinalis, Proteocephalus), Monogenea (Cichlidogyrus spp.), Acanthocephala spp., Argulus spp. and Trichodina spp. were identified and reported in different studies (Florio et al. 2009; Gulelat et al. 2013; Amare et al. 2014; Jossy and Daniel, 2015; Mohammed et al., 2015).

The most vital requirement of fish production is the convenience of healthy fish fingerlings of tilapias. It is evident from the offered literature that parasitic diseases have caused important damage inside the nursery system of fishes mainly the poignant fry and fingerlings (Gopalakrishnan 1961; Kaminskas 2022). The parasitic community of fish shows sizable changes in the environmental conditions inside the fish's lives (Hossain et al. 2008). Fish harbour a diversity of parasites particularly, protozoa, cestodes, trematodes, and acanthocephalans (Ali 1990; Tessema 2020), and also the degree of damage by infection is influenced to an outsized extent by the type and number of parasites existing (Ambani 2014). Once the parasites exist in giant numbers, they can typically cause gross pathological changes and damage to the host (Heckman 1996; Hoffman 1998; Gunn and Pitt 2022). Several environmental conditions are a lot of contribute to illness among these water temperature is one of the important criteria associated with unwell natural events and ill health incidence. As per Mitra and Haldar's (2004) 1st record of Chilodonella hexasticha in a province of India, parasites occur mainly within the colder months and in summer no infection was recorded. In addition, biological factors of the host, as well as the water quality, are responsible for a fish ectoparasitic infestation. The parasite infects adult fishes. Since the fingerlings are delicate, they are easily vulnerable to infection. The water temperature, pH scale, and dissolved oxygen are three water parameters that are related to disease infestation as they fluctuate more quickly (Banerjee and Bandyopadhyay 2010). Fish fingerlings become more vulnerable to infection because of their immature immune system (Anderson 1974; Ahmad and Kaur 2018), and it has been previously reported that shallow ponds and stagnant water favour the multiplication of ciliate like Trichodina become more prone to infection. A high stocking density of fingerlings is one more reason for ectoparasitic sickness eruption (Hossain et al. 2008). High stocking density increases the prospect of epizoan transmission from fish to fish simply. The provision of hosts for ectoparasitic infection increases with the increasing stocking density. Therefore, it was concluded that water quality has a great impact on the abundance of pathogens and their ability to survive on the host (Banerjee and Bandyopadhyay 2010). Furthermore, with high fish stocking densities under commercial fish production, parasite outbreaks will certainly increase (Michel 1989; Meyer 1991; Bondad-Reantaso et al. 2005).

Generally, fish is vital to the human population in trade and economy; it is required within the diet as a basis of protein for diverse countries, especially within the tropics and subtropics where undernourishment is a major drawback (Alune and Andrew 1996; Davies et al. 2006). Tilapia is becoming the foremost cultured fish native to Africa (Agbeko et al. 2014) and among various factors that affect the production of Cichlids notably Nile tilapia, one vital issue that is commonly unnoticed is parasitic infections and diseases. Parasitic infection and diseases are some of the factors obstructive high productivity in fish farming (Kayis et al. 2009). Diseases caused by parasites are widespread and cause loss of fish in the intensively stocked ponds and aquariums (Tokşen 2006; Koyuncu and Toksen 2010). Moreover, parasite-affected fish decrease food intake and lose weight. Growth is stunted and damage or injuries to fish organs reduce their productivity. These disease problems are more prevalent in fish production with high biomass over limited space (Okaeme et al. 1986; Mitiku et al. 2018).

In Ethiopia fish stocking and aquaculture practices are increasing from time to time as demand for fish protein is increasing, however, there is no preliminary study on the fingerlings and their original habitats whether they are free of ecto and endoparasites or not. In Tigray, the most suitable water body for seining fingerlings to stock in reservoirs and ponds is Midmar Reservoir. This is shallow and open for domestic use and human activities around its source and the eastern part of the reservoir. Almost entire stocking activities practiced by different organizations in Tigray were done from this reservoir. Tigray agricultural research centre, Tigray agriculture offices, and our university particularly our team. Up to now the results from earthen pond aquaculture were not satisfactory. The weight and length relationship and increment were small or retarded growth was observed. This might be due to health problems (the effects of ecto and endoparasites). In our visit to the dam at different years, we saw some parasites, particularly Ascaris affecting the physiological activities of fingerlings. As parasites can affect the growth and reproduction of fish it is advisable to take a preliminary study before stocking. Moreover, the stocking density and water quality should be maintained properly to avoid a parasitic infestation in the ponds and reservoirs we planned to stock.

So far, very few diseases have been described in fish in the Ethiopian water bodies. Moreover, there is no report on the disease and prevalence of parasitic infection in fish in Tigray, particularly in the Midmar reservoir. Therefore to fill this gap a preliminary study on the prevalence of parasites on fingerlings before stocking is advisable. Because stocking infected fingerlings leads to economic losses and allows other healthy species to be contaminated. Hence, this survey aimed to determine the prevalence, diversity, and infection intensity of ecto and endoparasites in Oreochromis niloticus fingerlings in Midmar reservoir.

2. Materials and method

2.1. Study area description

The study was conducted at Midmar reservoir (14^o 12’23.30’’N latitude and 38^o54’52.48’’E longitude) which is located at an altitude of 1986 metres above sea level. Midmar Reservoir is about 7 km far from the centre of Adwa town. It was constructed in 1997 by the Tigray Regional Government as a source of water supply for Adwa town (Eskendir 2015). The surface area of the reservoir is about 6km² (NFLARRC 2002; Tigabu 2010) and it was primarily constructed for the Adwa town water supply but currently, it also serves as the water supply for Axum town (Figure 1).

Figure 1. Google map of the study area (the light blue filled isosceles triangles are the sampling points).

2.2. Study methodology

2.2.1. Sample collection

Fish samples (fingerlings) were collected using a seine net from the shallowest area from four representative sites. Sampling was done during January and April 2016. Collected fishes were transported in the icebox to Mekelle University Fishery and Aquatic Ecology Laboratory, for analysis. In the laboratory, total length (cm) and weight (g) were measured; the sex of the fish was determined by internal examination of testes and ovaries (Smyly 1957; Mohammed et al. 2015). The length (L) of the fish was taken from the tip of the snout to the posterior tip of the caudal fin and was measured to the nearest ± 0.1 cm. The weight of the fish was measured to the nearest gram using an electric (sensitive) balance (Mohammed et al. 2015).

The desired sample size was calculated using the formula given by Thrusfield (2005). By considering a 95% confidence interval, 5% desired absolute precision, and 50% expected prevalence. Since there was no previous research done in this area, 50% expected prevalence, 95% confidence interval, and 5% precision were used to estimate the sample size. Hence, a minimum of 384 fish was considered in this study.

2.2.2. Laboratory examinations and identification of parasites

A squash preparation of fish eyes was examined to reveal the organisms through microscopic examination. Using a hand lens skin smear of the fish was examined for parasites. The smear was made by the scrapping of the skin and observed under the microscope at ×40 and ×1000 magnification (Paperna 1980, 1996; Khalil 1971; Bichi and Ibrahim 2009) for identification. A fish parasitology guide was used to aid the identification of each specimen examined and compared to the plates as described by Barker and Cone (2000). The gills were dissected, removed, and examined under the microscope. Moreover, according to the methods described by Noga (2010), Amare et al. (2014) and Mitiku et al. (2018) each fish was opened dorso-ventrally and its internal organs were fully examined for the presence of internal parasites. The entire digestive system was removed and placed in a Petri dish with physiological saline (0.9% NaCl solution), for parasite recovery and the gut was divided into sections. The muscles, gonads, liver, and heart were examined with the aid of a dissection microscope and a light microscope at 10 and 40 magnifications. Parasites were counted, their location recorded, and preserved in 70% ethanol. Identification of most parasites was made immediately following standard keys in the literature (Klinger and Francis Floyol 2002; Pouder et al. 2005; Roberts 2012). Taxonomic identifications were mostly limited to genus level as the fish harbours mostly larval stages of many nematodes and trematode parasites and could not be distinguished to species level morphologically (Mitiku et al. 2018; Hoffman 2019). The helminthic parasites were identified morphologically and parasitologically using standard identification keys and pictorial guides (Kabata 1985; Yamaguti 1985; Yanong 2002; Chandra 2004; Pouder et al. 2005). In addition, an identification key modified from Paperna (1996) was used for the identification of the major taxa of adult and larval parasites of fish.

2.2.3. Measurement of physicochemical parameters

Organisms known as parasites exist at the expense of their hosts, but not to the point where they can kill them. Fisheries, the kind of habitat, fish biology, and climatic circumstances are all factors that affect the severity of parasite assaults on fish (Kennedy 1976). Parasitic infection is influenced by environmental conditions such as salinity, temperature, pH, and other water quality elements (Philips 1996). The majority of fish health issues are caused by environmental factors, such as poor water quality, overcrowding, nutritional inadequacies, or ‘stress’. Preventive care is the best treatment for any fish health issue.

The physicochemical parameters of the reservoir were measured with common multimetres available in our laboratory. Dissolved oxygen (DO), pH, conductivity, and turbidity were measured in situ from four regions where the seining was performed. Dissolved oxygen was measured using an oxygen metre (HQ 40d multimeter). pH (model No: pH-013), conductivity (model No: SX713), and turbidity (model No:8000-001) were measured with their corresponding electronic metres (Marcogliese 2002).

2.3. Data analysis

The data obtained from the laboratory findings were summarized and then analyzed using SPSS version 16 analyzing software. The difference in parasite intensity between sexes was tested using the t-test. The effect of the parasites on the health of their host was determined by calculating Fulton’s condition factor (K), a measure of an individual fish’s health that uses standard weight. Proposed by Fulton (1904), it assumes that the standard weight of a fish is proportional to the cube of its length. K = 100(W/L³), Where W is the whole body wet weight in grams and L is the length in centimetres; factor 100 is used to bring K close to a value of one. Pearson correlation was used to find the correlation between the body conditions of fish with the number of parasites (Mohammed et al. 2015).

In addition, prevalence, mean intensity, and abundance concepts as suggested by Margolis et al. (1982) and Mohammed et al. (2015) are used in the present study and the formulas are given below: $\mathrm{Mean}\mathrm{intensity}={\displaystyle \frac{\mathrm{Total}\mathrm{number}\mathrm{of}\mathrm{parasites}\mathrm{in}\mathrm{a}\mathrm{given}\mathrm{host}}{\mathrm{Total}\mathrm{number}\mathrm{of}\mathrm{hosts}\mathrm{infected}}}$Mean intensity is the arithmetic mean of the number of individuals of a particular parasite species per infected host in a sample. $\mathrm{Prevalence}\left(\text{\%}\right)={\displaystyle \frac{\mathrm{No}.\mathrm{of}\mathrm{Infected}\mathrm{fish}\left(\mathrm{s}\right)\mathrm{x}100}{\mathrm{Total}\mathrm{number}\mathrm{of}\mathrm{fish}\mathrm{examined}}}$

3. Results

Out of 384 specimens examined, 364(94.8%) were found to be infected. The taxonomic group of the parasites was trematodes, nematodes, protozoa and hurnidea. The number (prevalence) of fishes infected by these taxonomic groups were 365(95.1%), 246(64%), 151(39.3%) and 6(1.6%), respectively. The eyes of the fishes were observed to be more infected by Diplostomum spp. parasites; the common trematodes in fish's eyes commonly known as eye fluke (Tables 1–3; Figure 2). Contracaecum species were the second most infective parasites (Figure 2) and Myxosporea (Myxobolus cysts) (Figure 2), Clinostomum spp., Eustrongylides, and leech were the third, fourth, and fifth, respectively.

Figure 2. Diplostomum spp. metacercariae in the vitreous humour of eyes, Contracaecum spp. in the intestine, and Myxosporea (Myxobolus cysts) beneath the scales or on the surface of the skin from fingerlings of O. niloticus.

Table 1. The prevalence and mean intensity of Diplostomum spp., Contracaecum spp., Myxosporea, Clinostomum spp., Eustrongylides and Leech.

Parasites Taxonomy	No. of infected fishes	Prevalence (%)	Total No. of parasites recovered	Mean intensity
Diplostomum spp.	357	93.0	3762	10.5
Contracaecum spp.	239	62.2	1254	5.3
Myxobolus cysts	151	39.3	1056	7.0
Clinostomum spp.	8	2.1	70	8.8
Eustrongylides	7	1.8	40	5.7
Leech	6	1.6	15	2.5
Total	768	–	6197	8.3

Table 2. Distribution, locations and number of ecto and intestinal parasites O. niloticus fingerlings recovered in Midmar reservoir.

Parasite	Taxonomic group	Location	No (%) fish infected	No of Parasites	Range
Diplostomum spp.	Trematode	Eyes	357(93.0)	3762	0–59
Clinostomum spp.	Trematode	Gill, Brachial cavity	8 (2.1)	70	0–6
Contracaecum spp.	Nematode	Int., PC, SB, Mo, Gi	239(62.2)	1254	0–22
Eustrongylides	Nematode	Muscle/peritoneum	7(1.8)	40	0–3
Myxobolus cysts	Protozoa	Scale	151(39.3)	1056	0–256
Leech	Hurindea	Gills	6(1.6)	15	0–4

PC = Peri cardial cavity, SB = Swim bladder.

Table 3. Parasite infestation in O. niloticus fingerlings and their numerical distribution in body parts.

Parasite	Eye	Int	PC	Br	SB	Mus	Sk	Gill	Mou	Total
Diplostomum spp.	3762	0	0	0	0	0	0	0	0	3762
Clinostomum spp.	0	0	0	26	0	0	0	44	0	70
Contracaecum spp.	0	975	239	0	30	0	0	2	8	1254
Myxobolus cysts	0	0	0	0	0	0	1056	0	0	1056
Eustrongylides	0	0	0	0	0	40	0	0	0	40
Leech	0	0	0	0	0	0	0	15	0	15
Total	3762	975	239	26	30	40	1056	61	8	6197

Key: Int = Intestine, PC = Peri cardial cavity, Br = Branchial Cavity, SB = Swim bladder, Mus = Muscle, Sk = Skin, Mou = Mouth.

3.1. Prevalence of ecto and endoparasites in Oreochromis niloticus fingerlings in Midmar reservoir

Throughout the study, we found a total of 151 fishes infected with Myxosporea and 6 with leeches. These are ectoparasites with a prevalence (mean intensity) of 39.3% (6.9) and 1.6% (2.5), respectively (Table 1). Diplostomum spp. and Clinostomum spp. were among the endo parasite trematodes retained in the study fingerlings. Moreover, endoparasite nematodes such as Eustrongylides, and Contracaecum spp. were recorded. Among these, Diplostomum spp., and Contracaecum spp. were the most prevalent and familiar parasites found infesting most of the Oreochromis niloticus fingerlings in Midmar reservoir. Diplostomum spp. showed the highest prevalence (93%) and mean intensity (10.5). Contracaecum spp. was also found distributed in different locations of the host body mainly inhabiting intestines (Tables 1 and 2).

3.2. Parasites’ diversity, distribution, infection intensity and prevalences

A total of 6197 parasites with Shannon diversity index H’ =  1.167 and species richness S = 6 were isolated from the 364 specimens of O. niloticus thus a mean of 17 parasites per fish. Of these 3853 with a mean of 16.97 were isolated from males and 2344 with a mean of 14.92 from females. The species of parasites isolated from O. niloticus and their distribution in body organs are shown in Table 2. Diplpstomum spp. and Contracaecum spp. were the most prevalent parasites. The infection intensity of the digenetic flukes Diplostomum spp. and clinostomum spp. were higher than the other parasites (Tables 1 and 2; Figure 3).

Figure 3. Prevalence and mean intensity of infection by parasite species.

3.2.1. The prevalence and intensity change with fish size or age group

In this study majority of the fish were between 10.5 and 14 cm and 96.1% of them were infected with parasites (Table 5). The body weights of the fishes ranged from 4 to 83 g and the highest number of parasites recovered in fishes having sizes of 24–33 g (Table 6).

3.3. Determine and relate the most parasitized organ to fish health

Based on our observation and survey in the field and laboratory examinations, the most parasitized organ of the fingerlings were their eyes with a prevalence of 91.6% and mean intensity of 10.5. The parasites responsible for this infection are eye fluke; Diplostomum spp. which are under the taxonomic group of trematodes. Metacercariae of Diplostomum spps were found actively moving in the vitreous humour and lens of the sampled fish. This movement was also observed during microscopic examinations. Following this group, Contracaecum spp. of nematodes with a prevalence of 62.2% and mean intensity of 5.3 was found distributed among the organs intestine, pericardium, swim bladder, mouth, and gills (Tables 1–3). The numerical distribution of this parasite in each body part is indicated in Table 3. The third most infective parasite is Myxosporea; a protozoan group that causes fish skin ulcers. It is covered with a white or yellowish-white cyst known as Myxobolus cyst beneath the scales of Oreochromis niloticus.

The six parasites were distributed among the fish in the reservoir. The total and multiple infections were calculated, hence, out of the 384 specimens examined 364 of them were infected by at least one of the parasites (Table 5), and multiple infection accounts for 285 (74%; Table 4). The prevalence of infection by Diplostomum spp. accounts for the highest (93%) and leeches the lowest (1.6%) (Table 1). Similarly, Diplostomum spp. had the highest infection intensity followed by Myxosporea (Myxobolus cysts), Contracaecum spp., Clinostomum spp., Eustrongylides and leeches, respectively (Table 1; Figure 3).

Table 4. The prevalence of parasites in relation to host sex.

Sex	Parasite genera
	Diplostomum spp.	Clinostomum spp.	Contracaecum spp.	Myxobolus cysts	Eustrongylides	Leech	Mixed
Male (n = 227)	216(95.2)	5(2.2)	140(61.7)	98(43.2)	4(1.8)	5(2.2)	183(81)
Female (n = 157)	141(90)	3(1.9)	99(63.1)	53(33.8)	3(1.9)	1(0.6)	102(65)
Total (n = 384)	357(93)	8(2.1)	239(62.2)	151(39.3)	7(1.8)	6(1.6)	285(74)

3.4. Physico-chemical condition of the dam and suitability for parasite survival and multiplication

The mean temperature of the dam during the study periods was found to be 24.4^oC. It fluctuated between a low of 22.8°C and a high of 26.1°C. The dissolved oxygen ranged from 6.26 to 8.61 mg/L with a mean of 7.55 mg/L. Table 7 shows the mean and standard deviation of the measured physicochemicals of the dam during January and April 2016. The measurements of the water quality parameters were found to be close to satisfactory levels and have no influence on the parasite diversity and species richness. The dam was conducive and suitable for different fish parasites. Because the majority of the specimens examined were infected with parasites (Table 2).

3.5. Relationship between fish condition factor and number of parasites

Figures 4–8 indicate the relationship between the fish condition factor and the number of parasites of four representative and one mixed genera. The health of the fingerlings in the dam is affected by the parasite burden. Most of the fish in the dam are infected by different parasites. The optimum size and weight of a healthy fingerling in the dam are roughly 12 cm and 30.5 g, respectively. However, these measurements are lower in those fingerlings affected by parasites. Overall, males were slightly in better condition (K = 1.62 ±0.27) than females (K = 1.61 ± 0.32). In general, the mean condition factor of the fish varied slightly between sexes. However, there was no significant difference in condition factors between males and females. The two groups show evidence of having similar mean values (t = 0.723, d.f. = 382, p < 0.05). In addition, there was no significant difference between males and females in parasite burden or intensity of infection (t = 1.008, d.f. = 382, p > 0.05) because their mean difference is low.

Figure 4. Relationship between condition factor and the number of Eye flukes (Diplostomum spp.) Parasites (y = −0.004x + 0.014; P = 0.031; df  = 1).

Figure 5. Relationship between condition factor and the number of Contracaecum parasites (Y = 0.003x + 0.002; P = 0.456; df  = 1).

Figure 6. Relationship between condition factor and the number of Myxosporea parasites (y = 0; P = 0.894; df  = 1).

Figure 7. Relationship between condition factor and the number of Leech parasites (y = 0.49x + 0.003; P = 0.338; df  = 1).

Figure 8. Relationships between condition factor and the number of mixed parasites (y = 0.002; P = 0.373; df  = 1).

4. Discussion

The study reveals six types of parasites and infested O. niloticus fingerlings with an overall prevalence of 94.8% (Table 5). Among these two of them were ectoparasites and there was a profound variability in the prevalence and intensity. One is Myxosporea (Myxobolus cysts) which is found and distributed beneath scales or on the surface of the skin and the leeches were other ectoparasites found in gills of the Nile tilapia fingerlings. In the current investigation, the Myxosporea was more prevalent and greater in the intensity of infection than the leeches (Figure 3). In Africa, in agreement with this study, histozoic myxosporidians (Myxobolus spp.) occur as skin infections in fishes particularly in juvenile cichlids, with cysts formed in the dermis, under the scales, extending to the surface of the head (face, lips) or onto the fins (Paperna 1973; Abolarin 1974; Fomena et al. 1984/1985; Landsberg 1986; Paperna 1996). Regarding the leeches, a report by Wangatia (2012) from Ziwa Dam (Kenya) indicates that leeches had a prevalence rate of 3% which is twice the results (Tables 1 and 2).

The rest four groups of parasites were endoparasites. The two most common parasite genera encountered in the present study were Diplostomum spp. and Contracaecum spp. The infection prevalence of these parasites was 93% and 62.2%, respectively (Table 1). Eye flukes of the species Diplostomum are well known for their strong impact on freshwater fish populations. Diplostomum metacercariae are the major fish parasites with complex three-host lifecycles that involve piscivorous birds, snails, and fish as the definitive host, first intermediate host, and second intermediate host, respectively (Ndeda et al. 2013). In the fish, the metacercariae move to the lens, retina, and aqueous humour of fish eyes as well as the brain, spinal cord, and nasal spaces causing substantial losses. They cause diplostomosis (eye fluke disease) which is characterized by severe ocular involvement, lens opacity, and blindness (Ibrahim et al. 2016). In Nile tilapia, Diplostomum in beginning periods caused exophthalmia without eye murkiness and in interminable stages caused total eye waterfall and white specks around the eye cornea (Abd El-Monem et al. 2005). Mass infections with Diplostomum cercariae have been reported to kill young/small fish or fingerlings (Brassard et al. 1982) and by reduction of visual capacity of their second intermediate hosts, the parasite interferes with feeding efficiency and predator avoidance (Crowden and Broom 1980; Owen et al. 1993; Seppälä et al. 2004). In addition, since some fish's physiological activities like swimming, feeding, and mating depend on vision, the presence of this parasite affects its ability to perform and compete well with others. Poor feeding affects fish growth, and consequently the fish farming business. It could also expose the fish to predation increasing mortality. Diplostomum spp. metacercariae have been recovered from eyes at various prevalences. In the study O.niloticus fingerlings specimens we obtained 95.2% prevalence in males and 89% in females (Table 4). In a study by Ibrahim et al. (2016) in Oyo state Nigeria, 33.18% of the cat samples had diplostomum parasites in the eye lens with higher occurrence in males (23.5%) than the females (9.7%).

The genus Contracaecum was the second most prevalent parasite affecting both sexes of the study fish species in Midmar dam. Contracaecum spp, which was the most prevalent nematode larvae, was found in the pericardial cavity of the heart coiled in a circular form with a mean of five parasites having different sizes. This result is in agreement with Yimer (2000) from Lake Ziway (Ethiopia) and Wangatia (2012) from Ziwa Dam (Kenya). These coiled parasites were observed in fish specimens both alive and cooled ones. Parasites that were observed from live specimens were moving actively in a wriggling manner (Wangatia 2012). There are about nine species of Contracaecum identified from African birds (Canaries and Gardner 1967), with at least seven species present in South Africa (Mashego 1989). It has not been possible yet to differentiate the species from larvae in fish (Mashego 1982).

In the present investigation, the explanation behind the high predominance of trematodes and nematodes pervasion may be because of the low host particularity of the grown-up phases of these parasites that makes these parasites conceivably tainting distinctive fish genera and species. It may likewise be a direct result of the accessibility of the various hosts required for the culmination of the existence cycle of these parasites. These hosts incorporate the piscivorous, savage, and man as definite hosts and gastropod molluscs and fishes as transitional hosts (Yanong 2002). The fish may go about as a moderate host in certain parasites while in others it is the last host.

The helminths larval parasites belonging to the genera Clinostomum and Contraecum species are known to occur in most African freshwater fishes (Tedla and Tadesse 1979; Paperna 1980). Adult stages are found in birds and most of the freshwater lakes in Ethiopia support a large population of aquatic birds (Tedla and Tadesse 1979). It is therefore likely that the major aquatic birds in Tigray particularly in Midmar reservoir support the adult stages of these parasites.

Fish samples with weights ranging between 24 and 33 g recorded the highest percentage of infection (36.3%) while fish with weights ranging from 74 to 83 g recorded a very low level of infection (0.5%). The total number of parasites increased with fish length, with the highest number of parasites observed between the sizes of 10.5–14.0 cm. Fish samples within this range showed about 96.1% of infection which is the highest in terms of the total fish examined. Although the number of fish with lengths ranging from 14.5 to 18 cm was less than the other ranges, 100% of them were infected with parasites (Tables 5 and 6). This is in view with Goselle et al. (2008) who reported a low level of infection in larger sizes of fish in the Lamingo reservoir, Jos, Nigeria. Another study in Nigeria by Uneke Bilikis Iyabo (2015) showed an almost similar result to this study.

Table 5. Prevalence of ecto and intestinal parasite in O. niloticus fingerlings collected in relation to their total length in Midmar reservoir.

Total length (cm)	No. (%) examined fishes	No.(%) infected fishes	No. (%) of parasites Recovered
6.5–10	66(17.2)	57(86.3)	860(13.9)
10.5–14	279(72.6)	268(96.1)	4192(67.6)
14.5–18	39(10.2)	39(100)	1145(18.5)
Total	384(100)	364(94.8)	6197(100)

Table 6. Prevalence of ecto and endoparasite in O. niloticus fingerlings collected by their body weight in Midmar reservoir.

Bodyweight (g)	No. (%) examined fishes	No.(%) infected fishes	Total No. (%) of parasites recovered
4–13	30(8)	26(86.7)	482(7.8)
14–23	108(28)	100(92.6)	1612(26)
24–33	139(36)	132(94.9)	2007(32.4)
34–43	65(17)	64(98.5)	1149(18.5)
44–53	21(5.5)	21(100)	248(4)
54–63	7(2)	7(100)	158(2.5)
64–73	12(3)	12(100)	218(3.5)
74–83	2(0.5)	2(100)	324(5.2)
Total	384(100)	364(94.8)	6197(100)

Knowing the physicochemical characteristics of the dam is important to associate with the intensity and prevalence of the parasite. For example, at temperatures above 22°C tilapias rarely show disease signs. However, tilapias are prone to diseases due to stress from low temperature, handling, overcrowding, or poor water quality. At temperatures between 16 and 18°C and in the absence of severe stress, Nile tilapia (O. niloticus) rarely becomes diseased (Cheng 1986). The mean dissolved oxygen was 7.55 mg/l, fluctuating between a low of 6.26 mg/l and a high of 8.61 mg/l. Mean Conductivity was 849.5 µs/cm ranging between 730 μs/cm and 1020μs/cm and the mean pH of the dam was 7.5 with the highest at 7.69 and lowest at 7.23 as indicated in Table 7. The current physio-chemical parameter of the dam was suitable for parasites’ growth and fish survival. The fact that the parasite in the fish was low despite the favourable condition recorded, suggests the fish may have a high tolerance and/or resistance to parasitic infection (Bichi and Bizi 2002 as cited in Bichi and Ibrahim 2009).

Table 7. Water quality parameters of Midmar dam during January and April 2016.

Parameters	Range	Mean ± S.D
Temperature(^oC)	22.8–26.1	24.4 ± 1.4
DO(mg/L)	6.26–8.61	7.55 ± 0.95
pH	7.23–7.69	7.5 ± 0.24
Conductivity (µS/cm)	730–1020	849.5 ± 131.5
Turbidity (NTU)	2.76–12.64	9.3 ± 3.5

The inlet stream channel had emergent semi-marshes, waterweeds (macrophytes) and sparsely grown papyrus reeds. The dam had flocks of birds (water ducks) that lived within the wetland. The birds feed on fish and invertebrates of the dam. The papyrus reeds and macrophytes observed in the stream are a possible habitation of molluscs that act as intermediate hosts for some parasites (Wangatia 2012). A lot of grazing activities go on around this dam and the water is used by cattle for drinking. The area around the dam and inlet stream was densely populated by people who grow different crops and irrigational activities that could contribute to eutrophication through fertilizers washed into the dam. Besides this, the surrounding rural dwellers use the water for washing clothes, swimming and drinking.

Water quality plays a vital role in unsettling the stability between the host and the pathogen (Jadhav 2009). Poor water quality parameters such as high organic matter, high ammonia, low dissolved oxygen and high bacterial load can create a suboptimal environment that is stressful for the fish leading to a higher incidence of parasitic outbreaks (Komar and Wendover 2007). According to, Gupta and Gupta (2006) the preferably appropriate water quality parameters necessary to maintain good growth and healthy conditions in cultured fish such as Tilapia species are greater than 5 ppm of dissolved oxygen, 6.5–8.5 and 25–35°C ranges of pH and temperatures, respectively. The minimum and maximum values of most of the water quality parameters in Midimar dam were within the literatures range as indicated in Table 7. Hence, these suitable ranges as required since they are favourable for the survival of the fish and their parasites (DWAF 1996).

In Midmar dam there was no previous study to check with this result, but as explained in most literatures, helminths are largely found in freshwater dwelling fishes wherever factors like parasite species and its biology, host and its feeding habitats, physical factors, hygiene of the water body and presence of intermediate hosts contribute to their prevalence and intensity (Chandra 2006; Hussen et al. 2012).

5. Conclusion and recommendations

The results of the study indicated that the majority of the fish in the study reservoir experienced a serious parasitic infestation. Fingerlings of O. niloticus from Midmar dam were infected with parasites of Diplostomum spp, Contracaecum spp, Myxosporea, Clinostomum spp., Eustrongylides and leeches. The first three parasites showed a high prevalence of infection with Contracaecum spp being the highest. The intensity of infection did not vary between sexes and was not statistically significant in all because the samples were from different sites of the same environment, subject to similar water quality parameters and feeding opportunities. Out of the four taxonomic groups, the females had lower infection intensity than the males. Therefore to overcome the above problems the following recommendations were forwarded. These are (1) the problem of fish infestation by parasites in the dam could be best handled through proper management procedures that will help eliminate the suitable conditions favouring the parasite infestation (2) regular surveillance of the water body for parasites and pollution control will go a long way in controlling parasitic infestation in the dam; this should include control and monitoring of possible sources of parasites that include predators such as fish-eating birds and molluscs to ensure effective control of parasites (3) public awareness creation activities should be conducted on zoonotic nature of fish parasites (4) to protect the dissemination of these parasites to other reservoirs, ponds and other water bodies stocking fingerlings of O. niloticus from this dam should be protected until the reservoir is treated (5) further study is needed to investigate the exact sources of parasites.

Authors' contributions

The corresponding author Mr. Solomon carried out the responsibilities of proposal drafting, data collection and write-up of the manuscript. Dr. Mekonen Teferi and Prof. Dr. Tadesse Djenie participated in write up and commenting of the manuscript. Mr. Tsegaluel Abay and Mr. Girmay Hiluf participated in the field sampling and laboratory analyzes. All authors read and approved the final manuscript.

Consent for publication

It is declared that the information given in the manuscript now can be published by the Publication House and the ‘Journal of Applied Animal Research’.

Availability of data and material

We declare that whatever data have been used in the manuscript will keep remain intact. These data can be made available to anyone who desires to see them from the corresponding author on request.

Acknowledgments

The authors would like to acknowledge the Department of Biology, College of Natural and Computational Sciences, Mekelle University, for laboratory facilities and financial support. We are grateful to Belay Gebreyohannes and to all Aquatic research team members for their help during field sample collection and technical support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was funded by the College of Natural and Computational Sciences (CNCS), Mekelle University, from its recurrent budgets.