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Prevalence of Trematode Larval Infections in Some Red Sea Snails

Article history: Received 12 October 2017 Accepted 22 December 2017 Available online 31 December 2017


INTRODUCTION
Mollusks are important to humans as food, decoration and materials. Gastropods are one of the most diverse and largest group of mollusks, with more than 62,000 described living species and they comprise about 80% of living mollusks. Most snails can become intermediate hosts for trematode cercariae, which may be transmitted to humans and animals. The life cycle of trematodes is very complex, as they require an intermediate host such as snails or fish for the maturation to the infective stage. Sporocysts and redia are formed inside the first intermediate host and feeds through diffusion across the tegument and develop into cercariae inside the snail. The cercariae are adapted for dispersal outside the snail and exhibit a large variety in morphology to recognize and penetrate the second intermediate host. They have behavioral and physiological adaptations, which are not found in earlier stages. To determine the ability of a snail species or a snail population to act as an intermediate host of a given species of digenea, it is necessary to measure several parameters such as the prevalence and intensity of snail infection (Graczyk & Fried 1999). Until present, only few studies have been carried out on the diversity and abundance of cercarial infection in the Red Sea snails. There is an urgent need to obtain information regarding environmental influences acting on emergence of cercariae in marine system (Koprivnikar & Poulin 2009). Thus, the aim of the present study was to elucidate the monthly and seasonal changes in the rate of cercarial infections of marine Nerita snails, as well as the effect of snails' length and temperature on the infection prevalence in the Red Sea, Jeddah coast. Saudi Arabia.

Measurement of shell length:
Maximum shell length was measured from the apex to the end of the shell using a Vernier Calipers Series 530-Standard Model. Snails species were classified according to their length into five classes, 2-3.9mm, 4-5.9mm, 6-7.9mm,8-9.9mm,10-11.9mm and 12-14mm.

Shedding and harvesting of cercariae:
Shedding and harvesting of cercariae were done according to Aboelhadid et al. (2016). Different snail species were separated in plastic specimen cups, cleaned and each snail was put in petri dish containing seawater and were kept in the laboratory at 23°C. They were exposed to artificial light for two hours. Water in each dish was checked at short intervals for emerging cercariae under a binocular dissecting microscope. The infected snails were separated and put in 250 ml beaker to facilitate emerging of cercariae. The non-shedding snails were crushed and examined under the binocular dissecting microscope searching for early larval trematode stages. Living cercariae were stained with diluted vital stain, neutral red or methylene blue. The stained cercariae were examined and photographed under a research microscope. They were identified according to their morphological characters using taxonomic keys and recent articles.

Statistical analysis:
The data were analyzed using Chi-square analysis to compare the infection rate capacity between infected and non-infected snails. The differences in snail's species between non-infected and infected during all months and different lengths were tested separately. All statistical analyses were performed using SPSS software program (version 22). Probability of P < 0.05 was regarded as significant at 95% confidence limit.

Monthly changes in infection of Nerita spp:
Monthly changes in the infection prevalence in each snail species was studied in table (3). Infection of N. albicilla, N. quadricolor and N. histrio were the highest in November with percentages of 86.36%, 77.78% and 71.42%, respectively. On the other hand, in N. grayana the highest infection prevalence was in June (71.42%). N. polita was infected during April and May only with infection rate of 50% for each month. The highest rate of infection in N. orbignyana was recorded in May (75%). Seasonal changes in the infection prevalence: As shown in Table (4) and Figure (3), the results showed significantly different prevalence of infection percentages, with no correlation between seasonal prevalence and mean temperature. The highest prevalence of cercarial infection in snails was recorded during autumn (September, October and November) with 38% prevalence, whilst it was lowest during winter (December, January and February) with 9.67% prevalence. Mean temperature was highest in summer (29°C) and lowest in winter (23°C). In autumn, the mean temperature was (28°C) and in spring (25°C).

Correlation between snail's length and infection prevalence:
As shown in Table (5), there is a positive correlation between infection prevalence and snail's length. The infection of the snail species, N. albicilla with the length of 12-14 mm was the highest (78.26%). The length 10-11.9 mm had the highest infection prevalence (71.42%) in N. grayana snails. In the snail species N. polita, infection was found only in the length 8-9.9 mm (22.22%). In N.quadricolor snails, the infection percentage was the highest in the length 8-9.9 mm (37.5%). As for N. orbignyana the highest percentage of infection was recorded in the snail length 10-11.9 mm (50%). The percentage of infection in N. histrio with length 10-11.9 mm was the highest (80%). The lowest rate of infection was recorded from snails of length 2-3.9mm.

Discussion:
One of the most important factors that influence the occurrence of trematode infection in marine water is the availability of suitable snail host. Snails play a very important role in the transmission of trematode species as they act as intermediate hosts in which three larval stages of the trematodes life cycle are completed (Sprocysts, rediae and cercariae). Several snail species of marine snails were found in the Red Sea that act as intermediate hosts for different trematodes that affect our life stock and birds.
In the present study, cercariae of nine species of digenetic trematodes belonging to six morphotypes were collected from marine Nerita snails. Total prevalence of larval trematodes in all Nerita species was 21.45%. The most prevalent infected snail species was N. histrio (31.94%) while, N. polita snails had the lowest infection prevalence (3.77%). Few studies concerned with the prevalence in marine snails' cercarial infection such as Byers et al. (2008). On the other hand, many studies interested in the study of infection rate of fresh water snails were done all over the world such as, Hasanat et al. (2017) who recorded infection rate of 11.07 % for Lymnaea snails in Pakistan. Jayawardena et al. (2011) also found 16% infection rate for the snails, Thiara scabra, T. tuberculate, Paludomous sherica and Gyrulus saigonensis in Srilanka. In addition, Ahmed et al. (2006) recorded 14.9% infection rate in the snail, Bulinus truncates in Sudan. Differences in the overall prevalence of trematode infections may involve many factors, including different collection time, locality, season, type and number of parasite and intermediate host, availability of infected definitive or reservoir hosts. The prevalence of snails in different locations of the country was different. This was due to difference in the environmental and managemental conditions (Kulsantiwong et al. 2015& Pfukenyi et al. 2005. The most prevalent month of cercarial infection in Iran is May to September (Sharif et al. 2010).
Seasonality that is mirrored by changes in environmental variables can intervene in snail's ecology and influence the larval development of a trematode inside its host snail. It may also affect cercarial shedding (the release of cercariae from the host snail in nature). However, the influence of environmental elements on cercarial shedding is trematode-specific (Abrous et al. 1999).
In the present study, the highest cercarial infection rates in snails were observed during autumn (38%), while it was lowest during winter. Infection rate of snails in different seasons are fluctuated. Hasanat et al. (2017) recorded seasonal data with high prevalence rate during autumn (19.6%) and low in spring (4.2%). Aboelhadid (2004) detected that, the prevalence of Schistosoma mansoni cercariae was high in spring (8.3 %) and Imani- Baran et al. (2011) found high infection rate in early summer (June-July) in Iran. Seasonal data of Tanveer & Khan (1989), Maqbool et al. (1998), Khan et al. (2008) showed significantly higher number of snails in summer. Almost all snails' genera had highest prevalence in summer season. Different results for seasonally infection rate might be due to low levels of water, slow-flowing in the stream during hot non-rainy season, also it could be due to high contamination by the faecal matter of livestock, birds and other animals. It is probably that snail abundance is low in the summer and miracidia enter most available snails. Thus, it can be anticipated that, both snail's propagation and their infection with trematodes are correlated with seasonal variations. various ecological factors such as season and water temperature, pH and dissolved oxygen influence the emergence of cercariae from the snails and their release inside the water resources. snails can reach their maximum incidence during summer and autumn providing optimum temperature required for their breeding and reproduction (Farahnak et al. 2011).
Environmental conditions in different habitats, in different climatic zones may support the trematode life cycle differently. Trematodes show distinct and direct relationship with the temperature in their transmission process. Cercarial output is directly influenced by the temperature due to both stimulating effect of temperature increasing the emergence from the snail and the acceleration of cercarial production within the snail host. However, under high temperature conditions some cercariae encyst within snails. Even though, snail populations grant higher trematode diversity (Smyth 1962, Poulin 2006. The cercarial shedding is related to temperature, humidity and rainfall; temperature between 25-30 °C is favorable for shedding of cercariae (Alim 1997, Rahman et al. 1997. These conditions favor the growth and development of vector snails and hasten the development and shedding of cercariae from snails by rapid development of miracidium to sporocyst, redia and finally cercariae (Alim 1997).
Transmission of digenetic trematodes from the snail to the next host in the life cycle depends largely on the proportion of snails that release cercariae, as well as the number of cercariae released from each infected snail (Anderson & May 1991). These factors may be quantified, respectively, in terms of the prevalence and intensity of patent infection (Margolis et al. 1982, Bush et al.1997. In field populations, both these factors tend to increase with increasing snail size. In the present study, infection prevalence of Nerita spp is positively related to their length. Among freshwater snails, for example, a positive correlation between snail size and the prevalence of infection was reported in73% of the field studies reviewed by Sorensen & Minchella (2001). Size may also be positively correlated with intensity of infection (Smith, 1984). For both mammalian and avian schistosomes, the prevalence and intensity of infection tend to be highest among the largest snails (Sturrock 1973, Kulesa et al. 1982, Loker 1983, Woolhouse 1989, Niemann & Lewis 1990. Large snails are older, on average, than small snails, within a given population (Minchella et al. 1985). This age variation translates into differential exposure to and duration of infection among snails of differing sizes because larger-older snails may have been exposed to more miracidia or may have been infected for a longer period and thus have patent, highly productive infections. A complete coupling between age and size would require further experimental work. Trematode induced gigantism can disrupt the correlation between age and size of snails by accelerating the