Effects of Snail Density on Growth, Reproduction and Survival of Biomphalaria alexandrina Exposed to Schistosoma mansoni

The effects of snail density on Biomphalaria alexandrina parasitized with Schistosoma mansoni were investigated. Laboratory experiments were used to quantify the impact of high density on snail growth, fecundity, and survival. Density-dependent birth rates of snails were determined to inform mathematical models, which, until now, have assumed a linear relationship between density and fecundity. The experiments show that the rate of egg-laying followed a negative exponential distribution with increasing density and this was significantly affected by exposure to parasitic infection. High density also affected the weight of snails and survival to a greater degree than exposure to parasitic infection. Although snail growth rates were initially constrained by high density, they retained the potential for growth suggesting a reversible density-dependent mechanism. These experimental data can be used to parameterise models and confirm that snail populations are regulated by nonlinear density-dependent mechanisms. 1. Introduction The density of conspecific individuals in the environment is a critical ecological factor that can affect the growth, survivorship, and fecundity of individuals and the consequent dynamics of populations [1–3]. Such factors become important for controlling host-parasite systems where the presence and magnitude of population regulatory factors are crucial for disease dynamics and the design of effective long-term control strategies [4]. In particular, control efforts may be negated if there is strong density dependence acting within the host-parasite system [4]. Schistosomiasis is a parasitic disease caused by digenetic trematodes, belonging to the Schistosomatidae family. It is currently endemic in 74 developing countries, infecting an estimated 200 million people [5, 6]. Schistosomiasis caused by Schistosoma mansoni is a major public health problem and repeated exposure can lead to liver damage and anaemia, particularly in children [6]. It is primarily found in rural areas in tropical and subtropical countries and infects humans and other vertebrates, using freshwater snails of the genera Biomphalaria as intermediate hosts. Transmission relies upon natural water polluted with human excreta, often found in areas of poverty or low income, where a lack of facilities forces people to use natural water bodies for domestic, recreational, occupational, or religious purposes. One proposed method of controlling schistosomiasis is to treat water bodies with a molluscicide to reduce the number of snail intermediate hosts, thereby breaking the