Hainan Island locates in the very south of China, a tropical region. The climate is warm throughout the year, with plentiful rainfalls, proper humidity, and abundant crops in the farmland, which is very suitable for the growth and reproduction of a large number of frogs. It also provides optimal natural conditions for the reproduction and spread of sparganums.
The investigation showed that Sparganum mansoni were isolated in 1,556 wild frogs, and a total of 612 parasites were detected. Among them, up to 22 sparganums were detected in a single frog. The natural infection rate of Sparganum mansoni in wild frogs in Hainan Island is 12.92%, and the average infection intensity was 3.04 per frog, indicating that the sparganums are widely distributed in various regions of the island, and the phenomenon of sparganamu infection in wild frogs is common (Table 1). Although the infection rate in the wild frogs in Hainan Island is lower than many other areas in China, as indicated by the survey in Henan Island by Deng et al. (26.6%) , the survey in Guangzhou by Hong et al. (51.9%) , and the survey in Yulin by Xie et al. (77.6%) , there are still many people who are not aware of sparganum infection in wild frogs, and perform insufficient protection of sparganosis. Especially in recent years, with the changes of eating habits, more and more people start to eat frog meat. Some local people in Hainan Island have the habit of preying on wild frogs. Although there is no confident evidence showing that locals eat raw frog meat, insufficient heating or incomplete cooking process may still cause sparganum infection.
In this research, we found that wild frogs in different areas are infected by Sparganum mansoni in different situations(Fig. 1)related to factors such as ecological environment, geographical location, climate, local customs and habits. A total of 341 sparganums were colletected from the central region, which had high rate of sparganum infection in wild frogs compared with other regions ( 19.53%) (Table 2). The infection rate in Baoting and Ding’an counties located in the central region are higher (32.93%) and (27.48%) than other places, suggesting that people of these two regions have a higher risk of sparganosis than in other regions. The central area of Hainan Island is the birthplace of main rivers on the island, with an average annual rainfall of more than 2,200 mm, rich freshwater resources and good water quality. There are many ponds, reservoirs and rice fields suitable for the living and reproduction of frogs. In Baoting county and Wuzhishan city, the original ecological environment is relatively well preserved, and there are more free-range cats and dogs in the area. Free-range cats and dogs have more chances to take wild frogs as foods, and be infected by sparganums. The eggs in their feces can easily enter the water after being washed by rain, thereby forming a natural circulation and reiterative transmission, which provides a better chance for completion of the life cycle of Spirometra mansoni . The infection rate of sparganum in wild frogs in Qionghai city and Dongfang city is zero, and the infection rate in Changjiang county and Lin'gao county is not high (Fig. 1). This may be related to the fact that the collection point is close to the sea. The sea is relatively far from highly crowded living areas of people, so there are less cats and dogs, which are also important sources of infection of sparganum, and actions should be taken to prevent the feces of cats and dogs to enter the water bodies closely related to human’s daily life . It has been reported in literatures that no sparganum infection was found in manually farmed frogs, which may be related to the lack of intermediate hosts required for a complete life cycle of Spirometra mansoni in the farms during manual breeding process, such as cats, dogs, and Cyclopes [26–27]. Frogs manually bred is fed with artificial foods, the frogs have less chance to contact with the outside world and have less chance of preying on cyclopes. In addition, manual breeding environments are generally sterilized and disinfested, which reduces the infection rate of sparganum .
The frogs collected in the study are mainly Rana tigrina and Rana plancyi. The Rana tigrina is distributed in various regions and inhabits near water plants that are not far from the land. The body length of Rana tigrina is 60–120 mm, and the largest can weigh up to 200 g. The Gold-stripped frogs are mainly distributed in Baoting county and Wuzhishan city, they inhabits on aquatic plants in the ditch and is smaller in size. The results of the study showed that no significant difference in the infection rate of sparganum between different frog species (Table 3), however the tendency of difference in the infection rate was found among frogs of different weights, though it is not statistically significant (Table 4). It was shown that the infection of sparganum is random, and they parasitize in frogs of different species and weights. Therefore, the infection rate is not related to them, but to the growth environments of the frogs. However, the infection intensity of sparganum in large frogs is relatively high (Fig. 3), indicating that the flesh of the hypertrophied frogs, which is rich in protein, calcium and phosphorus, especially the muscles of the hindlegs, provides abundant nutrients and plentiful space for the growth and development of sparganums. In addition, large frogs have lived for a longer time, and prolonged exposure to the wild environment will increase the probability of recurrent infection, leading to large scale infection. Sparganum mansoni can be widely parasitized in various locations of the frog's body, and it is more common in muscle tissue. There will be edema in the area infected by sparganums, and some may have congestion or bleeding. The current results showed that the frog’s hindleg muscles have the most parasites of sparganums, with 447 (Fig. 5), and the infection rate is also the highest (72.14%) compared to other tissues, and the average infection intensity was 3.08 per frog (Table 5). The observation that sparganum is mainly parasitized in the leg muscles of frogs is consistent with the findings made by other domestic scholars [14,15,17,23,24]. It further indicates that sparganums have strong contractility and movtility, and it mostly extends under the skin or curls to live in the cavities in leg muscles, since frogs’ leg muscles are more nutritious and provide better condition for the survival of sparganums.
On Hainan Island, wild frogs are infected by Sparganum mansoni commonly, which is a potential threat to local people. In order to prevent the infection of Sparganum mansoni, it is necessary to raise public awareness of sparganum infection in frogs and the associated risk of sparganosis, advocate healthy diet concepts and habits, and abandon capture, sale and purchase of wild frogs, so as to prevent and decrease the incidence of sparganosis .