Introduction

Jalpaiguri, one of the northern most districts of West Bengal, lies at the center of vast fertile plains of Terai at southern Himalayan foothills. It is watered by innumerable rivers and rivulets like Mahananda, Neora, Jaldhaka, Teesta, Torsa, etc.; and hilly jhoras rising from and flowing down the Himalayas and enrich the district both economically and naturally. The climate has undergone a remarkable change since the past few years, with the soaring temperature and the decreasing rainfall each year [1]. The water qualities of the river Mahananda and Teesta, the lifelines of north Bengal, are also degrading due to developmental pressure like deforestation, indiscriminate constructions at the riversides of Teesta, overpopulation on the riversides of Mahananda and Teesta, series of dams on Teesta, dumping of sewage via open drains into Mahananda and stagnation of water [2]. All these factors have a great impact on ecological, social, cultural, religious, aesthetic, tourism-related and economic aspects of these rivers [3]. In the near past, the region was well known for its rich fish diversity [4] and there has been no record of serious disease infestation. In the changing scenario, a preliminary survey on prevalence of myxozoan (Myxozoa: Myxosporea: Bivalvulida) parasites was conducted during the 2014–2017 season from two major rivers of Jalpaiguri district to evaluate the threat imposed by the developmental activities on fishes.

Material and Methods

The fish specimens were collected from the two sample collection sites of Jalpaiguri district for three consecutive years; i.e., from March 2014 to February 2017. Sample collection sites of Mahananda and Teesta rivers were located at Mahananda Bridge (Latitude 26°71′N, Longitude 88°41′E), Siliguri and Teesta barrage, Gajoldoba (Latitude 26°75′N, Longitude 88°58′E), respectively. The fishes were examined thoroughly for myxozoan parasites and were examined at least twice in every season during the study period to record the seasonal fluctuations of infection. Plasmodia, when found, were carefully removed from the host body parts with the help of a sterile forceps and smeared on clean grease-free slides with drops of 0.5% NaCl solution. Some of the fresh spores were treated with 5% of KOH solution for the extrusion of polar filaments. The spores were fixed in absolute methanol. For examination of the spores, the Giemsa-stained slides were placed under the oil immersion lens (Leica DM 2500 Light microscope). Mucus envelopes of spores were observed by following the method of Lom and Vavrá [5]. Measurements (based on fifty fresh spores treated with Lugol’s iodine) were taken with the aid of a calibrated ocular micrometer. Myxozoans are identified from standard literature [6,7,8]. The fish samples were identified after Dey [9] and Talwar and Jhingran [10]. Classification of fishes was followed on lines of Jayaram [11]. All the ecological terms used in this communication are in conformity with the Margolis [12]. The prevalence is expressed in percentage and is calculated by total number of infected fishes divided by total number of host fishes examined [13].

$${\text{Prevalence of infection}}\, = \,\left( {{\text{Total number of infected fishes }}{/}{\text{ Total number of host fishes examined}}} \right)\, \times \,100$$

Results and Discussion

During the course of the survey from March 2014 to February 2017, 20 species of fishes under 8 different families were examined, out of which 6 species of fishes under 4 different families, viz., Cyprinidae, Ophiocephalidae, Ambassidae and Bagridae were found infected with 6 different myxozoan species (Table 1). However, 3 host fishes out of 6 belonged to family Cyprinidae.

Table 1 Fishes examined for myxozoan infection from the rivers of Jalpaiguri district during the survey period 2014–17
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Altogether, 108 fishes out of 1874 fishes examined were found to be infected with myxozoan parasites, i.e., 5.76% of fishes were found infected during the survey period. The average percentage of infection recorded highest in Rasbora daniconius (21.6%) and lowest in Channa punctatus (16.7%). Infections were recorded only from gills and tail fins (Table 2). The percentage of infection of the host fishes in different seasons was also calculated during the three years survey period (Table 2). The overall rate of infections was recorded high during summer (March to June) and winter (November to February) and comparatively low in Monsoon (July–October) throughout the study period (Fig. 1). These kinds of observations could be due to apparent scarcity and abundance of water at the respective seasons. The highest seasonal incidences were recorded in Chanda nama (33%) and Channa punctatus (33%) during summer of 2014–15 and winter of 2014–15, respectively (Table 2).

Table 2 Percentage and site of infection of myxozoans, in different host fishes of Mahananda and Teesta River during the survey period 2014–2017
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Fig. 1
figure 1

Seasonal fluctuations of myxozoan infections of six host fishes of Jalpaiguri district during the survey period 2014–17 (a – Rasbora daniconius, b—Amblypharyngodon mola, c – Puntius sophore, d—Channa punctatus, e—Chanda nama and f—Mystus vittatus)

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Microscopical observations of myxozoan species obtained from six different host fishes revealed that all the myxozoan species are different. Three of them are identified as Myxobolus shantipuri, Myxobolus koli [6] and Thelohanellus qadrii [6] (Figs. 2, 3, 4). As per literature available, rest of the species are identified up to generic level only and they are described here as Thelohanellus sp. I, Henneguya sp. I and Henneguya sp. II (Figs. 5, 6, 7). Detail taxonomic information is compiled in Table 3.

Fig. 2
figure 2

Giemsa-stained spores of Myxobolus shantipuri Basu and Haldar, 2002 (Bar = 10 µm)

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Fig. 3
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Giemsa-stained spores of Myxobolus koli Lalitha Kumari, 1969 (Bar = 10 µm)

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Fig. 4
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Giemsa-stained spores of Thelohanellus qadrii Lalitha Kumari, 1969 [PC-Polar Capsule, PF-Polar Filament, SP-Sporoplasm] (Bar = 10 µm)

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Fig. 5
figure 5

Giemsa-stained spores of Thelohanellus sp. I [PC-Polar Capsule] (Bar = 10 µm)

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Table 3 Comparative characters of six myxozoans collected in Jalpaiguri district during the survey period 2014–2017
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Fig. 6
figure 6

Giemsa-stained spores of Henneguya sp. I [CA- Caudal Appendages, PC-Polar Capsule] (Bar = 10 µm)

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Fig. 7
figure 7

Giemsa-stained spores of Henneguya sp. II [IV- Iodinophilous Vacuole, SN- Sporoplasmic Nuclei] (Bar = 10 µm)

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Conclusion

Incidences of heavy fish mortality or irreparable loss in fish industries due to protozoan infestation are very common [14]. The myxozoans are parasitic protozoans (recently Fiala et al.[15] placed it under phylum Cnidaria) that inhabit primarily in the tissues and organ cavities of cold-blooded (exothermic / poikilothermic) vertebrates, especially culturable fishes and as such they have an importance in ichthyopathology [14, 16, 17].

The present study records a considerable percentage of myxozoan infection and diversity in the fishes collected from Mahananda and Teesta River. Though, only 5.76% fishes are found infected, but in some host species, it may reach up to 33% in a particular season. Significant infection rate of fishes of these two rivers indicates depletion of water qualities in this region. North Bengal is well known for its icthyofaunal diversity with 141 fish species in the three districts of north Bengal, viz. Cooch Behar, Alipurduar and Jalpaiguri [4]. However, in the past few years, freshwater habitats of this region are drastically modified due to various anthropogenic activities, which have profound effect on fish health [18]. In the changing environment, fishes are stressed with suppressed immune system and they are easily getting infected [19]. So, there is an urgent need to plan comprehensively for management of these aquatic bodies for sustainable utilization of aquatic resources. Proper riverine environment is crucial to preserve the endemic and endangered species from becoming extinct due to anthropogenic stresses. However, before coming into final conclusion, further investigations are required in search of myxozoan as well as other fish diseases of other aquatic bodies of north Bengal and it should be correlated with other parts of West Bengal.