Can ruminants become infected with trematode parasites through irrigation?

Introduction

Irrigation is necessary in agriculture to produce various food grains for humans and fodder for domestic animals. When rainfall is less or water is not available in sufficient quantity, water is supplied through canals from dams, large ponds, and lakes for irrigation of crops. Most of these reservoirs also harbor a wide variety of species of snails belonging to the families Lymnaeid, Planorbid, Thiazide (Melanidae), and Viviparid of the class Gastropod a of the phylum Mollusca. The most common snail species found in various freshwater bodies are Lymnaea acuminate f. patula, L. acuminate f. chlamys, L. acuminate f. typica, L. acuminate f. rufescens, L. luteola f. australis, L. luteola f. typica, L. luteola f. impura, Gyraulus convexiusculus, Planorbis (Indoplanorbis) exustus, Faunus ater, Melania (Plotia) scabra, Thiara (Tarebia) lineata, Melanoide striatella tuberculata, Vivipara bengalensis race gigantica, V. bengalensis race mandiensis, etc. [1-8]. Though these snails look simply, they cause or spread a number of dangerous diseases called trematodiases in domestic ruminants like cattle (Bos taurus), water buffaloes (Bubalus bubalis), sheep (Ovis aries), goats (Capra hircus), and camels (Camelus dromedarius). These diseases are caused by infection with digenetic trematode parasites in these animals [9,10]. The most common trematodiases in diverse species of ruminants are fascioliasis, amphistomiasis, paramphistomiasis, and schistosomiasis caused by infection with Fasciola spp., Amphistomum spp., Paramphistomum spp, and Schistosoma spp., respectively (Figure 1). Thousands of domestic animals are killed every year due to the outbreak of these parasitic diseases caused by trematode infection, which has a profound adverse effect on livestock farmers and the country's economy. It is well known that snails usually live in various water bodies throughout their lives and they keep spreading various types of trematodiasis diseases among the animals grazing in their surrounding areas. But it is also possible that these snails from water bodies also reach various agricultural fields through irrigation water and spread various trematodiases among the ruminant animals grazing there. That is, through irrigation, these snails can spread diseases from one area to another. This has also been seen in a recent study conducted in northern Tanzania [11]. The present editorial focuses on the fact that irrigation plays an important role in trematode infection in most of the ruminant animals grazing in irrigated areas.

Trematode Infection in Ruminants Through Irrigation

Trematode infection in ruminants in any region is possible only if infected snail species with trematode larvae are found in the water bodies of that region. During the rainy season most snails become infected with the miracidium larvae of trematode parasites. In fact, snails are the intermediate hosts of these digenetic trematodes and complete their life cycle. In snails, different larval stages of these trematodes such as sporocyst, radial, and cercarial develop by multiplication through asexual reproduction. Interestingly, these intra-molluscan stages, the sporocysts, radial, and cercarial larvae, are also highly pathogenic to their snail hosts and cause extensive damage to their liver and gonads [12-20]. Among these trematode larvae, cercarial larvae are active and free swimmers [21]. Among the various species of snails, the most common species of cercariae reported are Monostomes, Amphistomes, Echinostomes, Xiphidiao, Farcocercos, and Gymnocephalous [22-36]. Most of the cercarial species are released from their snail hosts in post monsoon and pre-winter [37-41]. After release from the snail, these larvae either penetrate the skin of ruminants on contact with water (in case of schistosome parasites) or get deposited in the form of cysts on the leaves of aquatic plants. These encysted cercarial larvae, commonly known as metacercariae, are highly infective. While grazing aquatic plants contaminated with metaceracariae, these trematode larvae enter the body of ruminant animals along with food (aquatic fodder) and reach their specific organs where they start growing rapidly day by day by eating the tissues of these organs, causing severe damage to these organs. Due to which trematodiasis develops [42]. This is a normal and natural process of the life cycle of trematodes and their infection in animals. But water from various reservoirs is also used for irrigation for agriculture and from these reservoirs various species of snails also reach large areas of various agriculture through water. In irrigation areas or agricultural fields, these snail species also release trematode cercariae into the irrigation water and these cercariae are then deposited as infectious metacercariae on the leaves of crops and grasses. When herds of ruminants consume crop feed or agricultural forage and grass contaminated with metacercariae, these animals get trematode infections. Thus, ruminants living or grazing in irrigated areas are infected with trematode parasites in higher numbers than those living in non-irrigated areas. This was also observed in a recent study conducted in northern Tanzania [11]. This study looked at the prevalence of F. gigantica, Paramphistomes, and Schistosoma bovis in cattle from randomly selected villages in (i) zero grazing (ZZ) (ii) community grazing without irrigation (ZC), and (iii) community grazing with irrigation (ZCI) areas. Based on faecal examination of cattle in these villages (n=241), the prevalence of these trematode parasite infections was 29.7%, 36.0% and 0% in ZZ, 6.3%, 15.0% and 3.8% in ZC and 57.7%, 56.7% and 1.0% in ZCI, respectively. This suggests that irrigation has an important role in the high prevalence and spreading of trematode infection or trematodiasis in large numbers of domestic animals. However, more such studies are needed in different species of ruminants living in irrigated and non-irrigated areas to confirm and understand the mode of infection of trematodes or trematodiasis. Infection of trematodes in this manner is also possible in humans and different species of wild ruminants living in irrigated areas. Hence, special attention needs to be paid to prevent infection of these parasites in animals of irrigated areas. Although prevention and control of trematode infections in ruminants is possible, it is indeed a very difficult goal, as most of these parasites have more than one primary and secondary host.

Figure 1: The most common digenetic trematode parasites of ruminants, (a) Fasciola gigantica, (b) F. hepatica, (c) Amphistome sp., and (d) Schistosome sp.

Conclusion

It is well known that most of the animals grazing and living near various water bodies containing snails of various species are infected with trematode parasites. However, compared to non-irrigated areas, animals living and grazing in irrigated areas are infected with trematode parasites in comparatively higher numbers. This indicates that irrigation has a potential role in trematode infection in ruminants. People and wild animals living in irrigated areas are also likely to be infected with these parasites. Therefore, it is necessary to take precautions so that these parasites do not cause fatal infection in animals and humans.

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