Document Type : Opinions, Commentaries
Author
Department of Biological Sciences, Federal University Dutse, P.M.B. 7156, Jigawa State, Nigeria.
Abstract
Highlights
Keywords
Main Subjects
1. Introduction
Schistosomiasis or bilharziasis, is a neglected tropical disease (NTD) caused by the trematode worms of the Schistosoma genus. There were an estimated 240 million people with Schistosomiasis infection in the world, with 700 million living in endemic areas [1]. Schistosomiasis is the most prevalent NTD in tropical and subtropical countries [2] and has the highest morbidity after malaria in Africa [3]. In 2019, about 24 000 deaths and 2.5 million disability-adjusted life-years were recorded due to the disease, with about 236 million people requiring mass drug administration [4]. The species that were known to infect humans include Schistosoma haematobium, Schistosoma mansoni, Schistosoma intercalatum, Schistosoma mekongi, Schistosoma guineensis, and Schistosoma japonicum [5, 6]. The common species that infect livestock and wildlife are Schistosoma bovis, Schistosoma curassoni, and Schistosoma matheei [7].
Sub-Saharan Africa has the highest burden of the infection globally, with a 90% prevalence [8]. S. haematobium and S. mansoni, which are the causative agents of urogenital schistosomiasis and intestinal schistosomiasis, respectively are the major species infecting humans in sub-Saharan Africa [9]. Nigeria has the highest burden of schistosomiasis in Africa [9].it is endemic in 35 of Nigeria’s 36 states [10], with a severe transmission rate [4] and there is evidently an increase in the transmission of the disease in Nigeria [11].
The parasite’s life cycle alternates between two hosts. The intermediate hosts are snails, and the definitive hosts are mammals. Asexual amplification occurs in the intermediate host, snails [5]. It occurs with the development of miracidia into sporocysts; the sporocysts multiply and develop into the infective stage called the cercaria[5]. In the mammalian host, the male and female parasites fuse, mature, and produce eggs. The definitive hosts (mammals) excrete eggs in urine or faeces into waterbodies. In water, the worm eggs hatch and released miracidia that penetrate aquatic snails [5]. The common snails, which are the intermediate hosts in Africa, are of the genera, Biomphalaria and Bulinus, which are responsible for the transmission of S. mansoni and S. haematobium, respectively [12]. People get infected with schistosomiasis when they come into contact with infested water that habours the infective larval stage of the parasite, the cercaria, when it penetrates their skin. Upon penetration, the cercaria lost its tail and became schistosomulae. The schistosomulae migrate through the circulatory system, mature in the mesenteric veins or venous plexuses of the bladder in terms of S. haematobium infection or the rectal venules in terms of S. mansoni infection, and produce eggs [5].
One of the most devastating and common natural disasters in the world is flooding [13]. Flooding occurs when excess water flows into a dryland or when too much rainfall exceeds the absorption capacity of the soil, which in turn causes great environmental consequences [14]. The most occurring natural disaster in the last 20 years was flooding, which affected 2.3 billion people worldwide and constituted 47% of all natural disasters recorded [15]. Of all natural disasters, flooding is the most deadly; in 2019, it caused 43.5% of all death as a result of natural disasters with more than three quarter of the deaths in low-income countries [16].
In developing countries, flooding results from blocking waterways [17], too much rainfall [18], climate change, a rise in sea-level, population growth, the operation of dams [19,20], and a lack of adequate preparation [21]. In the last ten years, the most frequent natural hazard in Africa was flooding [22]. Flooding in Nigeria occurs annually, but the most recent devastating floods were in 2012, 2018, and 2022 [23, 24]. Because of anthropogenic activities and climate change, there is no doubt that the intensity and occurrence of extreme weather conditions such as excessive rainfall, storms, floods, and high temperatures, among others, will increase globally [25].
Flooding is one of the major causes of the spread of snails that aid schistosomiasis transmission in areas without the infection, causing a shift in transmission dynamics [26]. In Nigeria, studies on monitoring the spread of infective; snails, livestock, wildlife, and humans after flooding were inadequate. This review highlights the potential threat of flooding to schistosomiasis control in Nigeria.
2. Effect of flooding on the spread of Biomphalaria and Bulinus Snails
The control of snails, especially those of the genera, Biomphalaria and Bulinus, is pivotal for schistosomiasis control, especially in Africa [27]. Although the number of snails could decrease due to intense flooding and heavy rainfall at transmission sites, there is an increased possibility for the snails to establish new colonies in areas where schistosomiasis is eliminated or in places without schistosomiasis transmission [28]. In China, a retrospective analysis that was conducted to determine the dispersal of the snail intermediate host Onchomelania hupensi found an increased spread of O. hupensi snails as a result of floods, the habitats of the snails were 2.6-2.7 times bigger than in years without floods. Although there was a decrease in the density and infection rate of the snails two years after the flood, there was, however, a significant increase in the third year [29].
Studies in Nigeria have shown a decrease in the number of Biomphalaria and Bulinus snails in the rainy season and an increase in the dry season in infested water bodies [30,31,32,33]. Although there was little available study on the dispersal of snails, especially Biomphalaria and Bulinus, or the modeling of snails dispersal patterns after flooding in Nigeria, due to the incessant floods that Nigeria is experiencing, there is a high possibility for the infective snails (intermediate hosts) to reach areas that are without prior schistosomiasis transmission. This is important in determining the extent of the spread and in marking possible transmission areas for effective control interventions.
3. Effect of flooding on Schistosomiasis Transmission
3.1 Effect of flooding on the transmission of schistosomiasis in humans and animals
Floods destroyed sanitary facilities and infrastructure leading to the contamination of water from sewage and chemicals, damage to health facilities, disruption of hospital accessibility, damage to schools infrastructure and residential areas [34], and the spread of diseases such as cholera and typhoid among the displaced flood victims [16].
Nigeria is considered to be at an extremely high risk of the impact of climate change, according to UNICEF’s children’s climate risk index [34]. In the recent 2022 flood in Nigeria, more than 1.3 million people were displaced, about 600 people lost their lives, and more than 2 000 residential areas were damaged [24]. The displacement of people and the lack of sanitary facilities would increase water contact activities there by increasing the risk of communicable and non-communicable diseases, including schistosomiasis infection.
Flooding leads to the destruction of lavatories that harbor Schistosoma eggs, this increases the chances of the flood victims to have contact with cercaria-infested water [35]. Water contact increased the frequency of schistosomiasis transmission; for example, in China, the number of acute cases of schistosomiasis at the Yangtze River was 2.8% higher in years with floods when compared to years without floods [29]. In Nigeria, a significantly higher prevalence of schistosomiasis was recorded in flood-displaced persons when compared to the population not affected by floods in Delta state, after the devastating 2012 flood [36]. Additionally, the migration of people from flooded areas to non-flooded areas will increase the risk of schistosomiasis transmission from endemic areas to non-endemic areas.
Moreover, there were more than 40 mammalian reservoir hosts of schistosomiasis [37]. There was an increasing recognition of the prevalence and morbidity of animal schistosomiasis in both wild and domestic animals in Africa [38]. In sub-Saharan Africa infections due to Schistosoma bovis in Cattle, Schistosoma matthei and Schistosoma curassoni in small ruminants were prevalent and were known to cause mortality and morbidity in livestock. Primates and rodents were known reservoir host of Schistosoma mansoni, a causative agent of human intestinal schistosomiasis [38]. Heavy rainfall, which causes flooding, give rise to lush vegetation, which attracts livestock for grazing. The flooded vegetation may harbor infective washed-away snails, increasing the risk of infection with schistosomiasis in livestock [39]. For instance, after the devastating flood in 1998 in China, there was a 1.68-fold higher increase in the prevalence of schistosomiasis in cattle in Hubei province than in 1997 [40].
Flooding also causes the migration of wild animal populations, such as rodents, to live in close proximity to human settlements. The potentially infective wild animals will transmit eggs to areas without the infection or to non-endemic areas, thus increasing schistosomiasis transmission [39]. Although not much study was conducted on livestock and wildlife schistosomiasis in Nigeria, there is a need for research to determine the prevalence of wildlife and livestock schistosomiasis in Nigeria and the extent of animal schistosomiasis spread after flooding.
3.2 Effect of Flooding on the Spread of Hybrid Schistosomes
The sharing of water bodies by humans and animals (especially livestock) increases the tendency for exposure to different species of Schistosoma that infect both humans and animals, thereby increasing the establishment of hybrid schistosomes [42]. There were concerns that the Schistosoma hybrids pose a major challenge in the control of schistosomiasis because of their genetic diversity, adaptability, greater infectivity, wide reservoir hosts [44,43] and the ability to lower the potency of praziquantel [45,43].Although information about hybrid schistosome epidemiology in Nigeria was sparse, there were reports on hybrid Schistosomes based on egg morphology [46] and S. haematobium with S. bovis hybrid infections in some parts of Nigeria [47].
Research in several West African States neighboring Nigeria has recorded human infections with several Schistosoma hybrids, including S. haematobium with S. guineensis in Cameroon [48] and Benin [44], S. haematobium with S. bovis and S. haematobium with S. curassoni in Senegal [49,50] and Mali [51], and S. bovis with S. curassoni in the Niger republic [42]. Moreover, all these countries share the Niger River Basin (Figure 1).
There were floods in the Niger River Basin countries, and there was a projected increase in flooding in the NRB countries because of climate change and land use changes [52].The basin traverses the territory of ten countries, including Nigeria. There is a high chance that the hybrid schistosomes that are not present in Nigeria will be established. This is because the hybrid schistosome infective snails, livestock, wildlife, and humans may disperse into Nigeria from the neighboring infected countries as a result of flooding. Research is therefore needed in Nigeria to know the status of hybrid schistosomes in animals and humans. This is important for effective control intervention.
4. Conclusion
In this article, I gave an overview of the possible impact of flooding on schistosomiasis transmission in Nigeria, which may hinder its control. The World Health Organisation (WHO) set a target to eliminate schistosomiasis and disrupt its transmission around the world by 2030 [4]. There is more at stake in schistosomiasis endemic countries like Nigeria because of an increasing climate change consequence, flooding. Flooding causes the dispersal of infective intermediate hosts (snails) from areas of active schistosomiasis transmission to areas without the infection. The destruction of sanitary facilities and the spreading of infested water bodies by floods cause the migration of humans, livestock, and wildlife, which in turn increases the odds of contact with the cercaria-infested water bodies, thereby increasing schistosomiasis transmission. There was also the probable diffusion of hybrid schistomes from NRB countries into Nigeria. Research is therefore needed to know the extent and modeling of snails (Biomphalaria and Bulinus) spread after flooding and the status of livestock, wildlife, and hybrid schistosomes before and after flooding in Nigeria. This would help in providing effective intervention for the successful elimination of schistosomiasis by 2030.
Abbreviations:
WHO- World Health Organisation
NRB- Niger River Basin
NTD- Neglected Tropical Diseases
UNICEF- United Nations Children’s Fund
Funding: None. Author funded
Conflict interest: None.
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