27 year monitoring of an alveolar echinococcosis hotspot in south-Gansu
Résumé
In the late 1980s, a “large” focus of human alveolar echinococcosis (AE) was described in Zhang and Min counties, south Gansu, western China, with 71% of the families keeping dogs and a 4-23% of E. multilocularis prevalence in dogs (Craig et al., 1992). Multidisciplinary studies were carried out there from 1994 to 1997 and found 4.1% human AE prevalence, with peaks of 15% in some villages (Craig et al., 2000). AE was nearly 3 times higher in villages situated where areas of shrub and grassland were larger. However, populations of dogs and probably red foxes had collapsed by the early 1990s, for example only one family was found to still own a dog in Cao Tan valley. Therefore, it was inferred that human AE cases detected had been infected before the definitive host populations collapsed. From those observations, a robust transmission model involving landscape characteristics, climate, small mammal and definitive host population dynamics, was proposed which proved to be generally applicable in endemic areas of Europe as well as in most parts of China and Central Asia (Giraudoux et al., 2003; Giraudoux et al., 2013a; Giraudoux et al., 2013b). The question of the continued lack of infected definitive hosts and thus continued low/no transmission, was however pending, since Zhang and Min counties are geographically situated close to the eastern edge of the Tibetan plateau, known to be a large stable zoonotic area for AE from where reinfection is possible (dog trade, fox movements, etc.).
Here we report the analysis of results obtained from a human ultrasound screening carried out in the same area in 2005-2006, and from a dog survey in May 2015 to investigate possible re-establishment of transmission after collapse of definitive host populations. No evidence was found of recent AE infection in humans in 2005-2006. Although the dog population had recovered and that forest protection and the reforestation of some areas may favor fox population growth, no evidence of infection in owned dogs was found in 2015. This suggests a sustained largely environmental elimination of E. multilocularis transmission in this area, despite the relative vicinity of the large active foci of the eastern Tibetan plateau. The history and the socio-ecological conditions for the sustainability of continued environmental control will be discussed.