Compensatory density feedback of Oncomelania hupensis populations in two different environmental settings in China
Guo-Jing Yang
1
Xiao-Nong Zhou
xiaonongzhou1962@gmail
0
Le-Ping Sun
1
Feng Wu
1
Bo Zhong
Dong-Chuan Qiu
Jrg Utzinger
Corey JA Bradshaw
0
National Institute of Parasitic Diseases, Chinese Center for Disease Control and Prevention
,
Shanghai 200025
,
People's Republic of China
1
Jiangsu Institute of Parasitic Diseases
,
Meiyuan Yangxiang 117, Wuxi 214064
,
People's Republic of China
Background: The most recent strategy for schistosomiasis control in the People's Republic of China aims to reduce the likelihood of environmental contamination of schistosome eggs. Despite considerable progress, it is believed that achievements would be further consolidated with additional intermediate host snail control measures. We provide an empirical framework for discerning the relative contribution of intrinsic effects (density feedback) from other extrinsic drivers of snail population dynamics. Methods: We set up experiments in two study locations to collect reproduction data of Oncomelania hupensis, the intermediate host snail of Schistosoma japonicum. We applied a set of four population dynamic models that have been widely used to study phenomenological time-series data to examine the properties of demographic density feedback patterns from abundance data. We also contrasted the obtained results with the component feedback of density on survival rate to determine whether adult survival was the principal driver of the demographic feedback observed. Results: Demographic density feedback models (Ricker- and Gompertz-logistic) accounted for > 99% of Akaike's information criterion model weight, with the Gompertz ranking highest in all O. hupensis population groups. We found some evidence for stronger compensatory feedback in the O. hupensis population from Sichuan compared to a Jiangsu population. Survival rates revealed strong component feedback, but the log-linear relationships (i.e. Gompertz) had less support in the demographic feedback analysis. Conclusions: Our findings indicate that integrated schistosomiasis control measures must continue to reduce parasite abundance further because intermediate host snail populations tend to grow exponentially at low densities, especially O. hupensis populations in mountainous regions. We conclude that density feedback in adult survival is the principal component contribution to the demographic phenomenon observed in the population fitness (r)-abundance relationship.
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Background
In the Peoples Republic of China (P.R. China),
schistosomiasis caused by the blood fluke Schistosoma
japonicum has a documented history of more than 2,000 years
[1,2]. The first large-scale surveys done in the
mid1950s suggested that the disease was endemic in 12
provinces located along and south of the Yangtze River.
More than 10 million people were infected, causing
considerable morbidity and even mortality [3,4]. Hence, a
national schistosomiasis control programme was
launched, placing particular emphasis on the control of
the intermediate host snail Oncomelania hupensis,
including environmental management and chemical
mollusciciding [2-6]. As a result, snail-infested areas
have been reduced from approximately 14,320 km2 in
the mid-1950s to 3,720 km2 in 2008 [6,7].
Recently, a comprehensive strategy was proposed with
the ultimate aim to reduce further the likelihood of
contamination of the environment with schistosome eggs.
This integrated control strategy consists of health
education, access to clean water and adequate sanitation,
mechanization of agriculture and fencing of domesticated
bovines, along with preventive chemotherapy [8,9]. The
main rationale for implementing this new strategy is that
schistosomiasis is an environmentally mediated disease,
and that it is difficult to eliminate all snail habitats,
particularly in lake and marshland regions [10]. However, this
new strategy alone does not succeed in eliminating or
substantially reducing the incidence of schistosomiasis
[11], especially in mountainous regions where suitable
snail habitats persist. Additional control measures are
needed, such as mollusciciding, which is a
time-consuming and costly strategy, because large fluctuations in snail
abundance [12,13] can arise from flooding [14,15].
Hence, measures for increasing the effectiveness of
mollusciciding, which in turn reduce intermediate host snail
abundance and limit the likelihood of re-emergence of
schistosomiasis, are required [16].
For the effective control of O. hupensis populations, a
fundamental step is a deeper understanding of the snails
intrinsic population dynamics because these properties
influence the rate of recovery after withdrawal of snail
control [1]. There is, however, a paucity of information
describing even basic population dynamics for this species,
which severely limits our understanding of the processes
of schistosomiasis transmission, and hence hampers the
development of effective control approaches. here is a
general consensus among ecologists that account must (...truncated)