**3.2 Current status of Schistosoma zoonosis in Africa**

Zoonotic diseases (also known as zoonoses) are those diseases caused by viruses, bacteria, fungi or parasites that are naturally transmitted between humans and other vertebrate animals [18, 34]. Currently, six main species of *Schistosoma* infect humans: *S. mansoni, S. haematobium, S. intercalatum, S. guineensis, S. mekongi* and *S. japonicum.*

#### **Figure 3.**

*Distribution of* Schistosoma *hybrids across Africa. Notes: S.h. =* S. haematobium*; S.m. =* Schistosoma mansoni*; S.g. =* S. guineensis*; S.i. =* S. intercalatum*; S.b. =* S. bovis*; S.c. =* S. curassoni*; S.r. =* Sirthenea rodhaini*; S.ma. =* S. mattheei*. Figure adapted from Panzner and Boissier [16].*

Schistosoma *Hybridizations and Risk of Emerging Zoonosis in Africa: Time to Think of a One… DOI: http://dx.doi.org/10.5772/intechopen.103680*

The latter two species are acknowledged zoonoses, as they are capable of naturally infecting multiple species of mammalian hosts (human, livestock and wildlife) [18]. *S. japonicum* and *S. mekongi* are the major *Schistosoma* species in Asia, geographically distributed across the central and middle areas of the continent. Unlike other animal schistosomes, *S. japonicum* and *S. mekongi* are unique among zoonotic helminths in that they can be transmitted between humans and other animals and maintained by all host species [35]. In Africa, only human *Schistosoma* species are considered to be of public health significance [9]. In addition, wild animals such as rodents are known to be the main reservoir hosts of *S. mansoni* in the Caribbean and South America. Even though *S. mansoni* is one of the two *Schistosoma* species of public health importance in Africa, its magnitude in animals and the contribution of animals to the perpetuation of *S. mansoni* transmission in the African continent is not well established [36], because very few studies have been conducted on livestock or wildlife schistosomiasis [9].

Recent studies have reported evidence of some unique schistosomiasis transmission events in Africa. It had been believed that *S. haematobium* as a single pure species was solely capable of infecting humans. However, a study conducted in Benin showed that pure *S. haematobium* may infect livestock (i.e., cattle) as well [7]. Moreover, the female *S. haematobium-bovis* hybrid previously detected in humans and snail hosts, though never in animal (livestock and wildlife) hosts, has been found in a pair with pure *S. mansoni* in a mouse [33]. In addition, *S. bovis*, *S. curassoni* and *S. mattheei* are known to infect a wide variety of animals including cattle, sheep and goats. Though *S. mattheei* has been detected at high rates of prevalence in humans in one area in South Africa [37], the other two species have never been detected in human as a single pure species. Detection of hybrids of *S. haematobium* with either one of these species was thought by scientists to be evidence of possible human infection with the two animal (livestock) *Schistosoma* species. Human infections with *S. bovis* and *S. curassoni* were suggested to occur through zoonotic spillover; hence, it was believed that the infection could not persist, as these parasites cannot be maintained in the human body [38]. However, some researchers suggest that *S. haematobium-bovis* hybrids could be a result of an ancient introgression event between *S. haematobium* and *S. bovis* that resulted in the introgression of some *S. bovis* genomic tracts into several *S. haematobium* lineages [39].

#### **3.3 Risk of emerging** *Schistosoma* **zoonosis in Africa**

The magnitude of *Schistosoma* zoonotic transmission in which both livestock and wildlife are active participants is yet to be determined in endemic countries across Africa [36]. It has been explained that natural and anthropogenic changes (**Figure 4**) have created opportunities for mixing of and subsequent exposure to both human and animal (livestock and wildlife) schistosomes. The coexistence of multiple *Schistosoma* species and their hosts—both vertebrates (human and animals) and snails—has increased the potential for the emergence and establishment of novel zoonotic *Schistosoma* hybrids [21]. Sporadic studies have revealed several hybrids with potential zoonotic effects that naturally infect humans and animals (livestock and wildlife) across several African countries. Examples of natural *Schistosoma* hybrids with potential zoonotic effects identified in Africa include *S. haematobium-bovis*, *S. haematobiumcurassoni*, *S. bovis-curassoni*, *S. haematobium-mattheei* and *S. mansoni*-*rodhaini* [16].

The ongoing emergence (or discovery) of potential zoonotic *Schistosoma* hybrids has caught the attention of many researchers and scientists, due to possible implications for schistosomiasis transmission and control. Zoonotic *Schistosoma* hybrids are thought to have a wide definitive host range and an increased range of intermediate snail hosts

**Figure 4.**

*Schematic presentation of the causes and consequences of schistosome hybridization.*

relative to their pure "parent" single species, which may also enable a wider geographic range for hybrid schistosome infections. In addition, zoonotic *Schistosoma* hybrids are capable of establishing themselves in areas where their parental single/pure species are absent (e.g., *S. haematobium-bovis* hybrids on the French island of Corsica). Moreover, experimental studies (on *S. haematobium-bovis* and *S. haematobium-mattheei* hybrids) have revealed that these hybrids have greater virulence than the two parental species, as well as increased adult worm fecundity and increased cercarial shedding rates in snails [9, 39]. Field research (on *S. haematobium-bovis* hybrids) has revealed some indicators of altered patient morbidity patterns and reduced treatment response with praziquantel [4]. Also, researchers have expressed concern that hybridization could accelerate the evolution of drug resistance by allowing drug-resistance genes to be introgressed into new populations. On the other hand, hybridization may lead to the development of refugia for drug-susceptible genotypes and thus potentially help maintain drug susceptibility [9]. Therefore, it is important to understand the transmission dynamics of potential zoonotic *Schistosoma* hybrids [21].
