Abstract
Zoonotic spillover and hybridization of parasites are major emerging public and veterinary health concerns at the interface of infectious disease biology, evolution, and control. Schistosomiasis is a neglected tropical disease of global importance caused by parasites of the Schistosoma genus, and the Schistosoma spp. system within Africa represents a key example of a system where spillover of animal parasites into human populations has enabled formation of hybrids. Combining model-based approaches and analyses of parasitological, molecular, and epidemiological data from northern Senegal, a region with a high prevalence of schistosome hybrids, we aimed to unravel the transmission dynamics of this complex multihost, multiparasite system. Using Bayesian methods and by estimating the basic reproduction number (R0 ), we evaluate the frequency of zoonotic spillover of Schistosoma bovis from livestock and the potential for onward transmission of hybrid S. bovis × S. haematobium offspring within human populations. We estimate R0 of hybrid schistosomes to be greater than the critical threshold of one (1.76; 95% CI 1.59 to 1.99), demonstrating the potential for hybridization to facilitate spread and establishment of schistosomiasis beyond its original geographical boundaries. We estimate R0 for S. bovis to be greater than one in cattle (1.43; 95% CI 1.24 to 1.85) but not in other ruminants, confirming cattle as the primary zoonotic reservoir. Through longitudinal simulations, we also show that where S. bovis and S. haematobium are coendemic (in livestock and humans respectively), the relative importance of zoonotic transmission is predicted to increase as the disease in humans nears elimination.
Highlights
Zoonotic spillover and hybridization of parasites are major emerging public and veterinary health concerns at the interface of infectious disease biology, evolution, and control
Livestock can be infected with S. bovis, and humans can be infected with five categories of worm genotypes: S. haematobium (Sh), S. bovis (Sb), two categories of first-generation or F1 hybrids (F1a being the product of a pairing between a male S. haematobium and a female S. bovis; F1b being the product of a pairing between a female S. haematobium and a male S. bovis), and later-generation/introgressed S. haematobium × S. bovis hybrids (Hyb)
In longitudinal simulations it is assumed that praziquantel is efficacious against all worm genotypes
Summary
Zoonotic spillover and hybridization of parasites are major emerging public and veterinary health concerns at the interface of infectious disease biology, evolution, and control. There is a growing acknowledgment of the profound threat that zoonoses (diseases transmitted between animals and humans) pose to human health worldwide, with animal reservoirs presenting diverse complications to elimination efforts for many existing diseases, as well as novel threats in the form of newly emerging diseases [1, 2] Global trends such as increased migration, altering agricultural practices, and a changing climate are all predicted to enhance the potential for human and animal populations to encounter new infectious agents, thereby increasing opportunities for coinfection by multiple pathogen species within the same host [3,4,5,6,7]. While molecular data from the study in Senegal indicated that S. bovis cannot be maintained by the human population ( fitting the definition for a spillover pathogen), onward transmission of hybrids was shown to occur within the human population via backcrossing and introgressions, leading to a complex array of observed miracidia genotypes (miracidia being the first larval schistosome stage which hatches from eggs shed by an infected host) from human specimens
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