Freshwater snail-borne parasitic diseases in Africa

Abstract

Background: Freshwater snails are the first obligatory intermediate hosts in the trematode life cycle. Several parasitic diseases transmitted by these snails are endemic in Africa, and their distribution closely follows that of the intermediate hosts. These diseases represent a major public health problem and cause significant socio-economic losses in Africa, particularly schistosomiasis and fascioliasis. In this review, we will describe the main roles of freshwater snails in the life cycle of trematode parasites, and the geographical distribution of these diseases in Africa. We will also discuss the different techniques for detecting parasitic infections in snails, as well as the various methods of controlling snails and the larval stages of parasites.

Methods: We carried out a literature search for articles dealing with parasitic diseases transmitted by freshwater snail hosts in Africa. The search was conducted in databases such as PubMed, Web of Science and Google Scholar using various search terms combined by Boolean operators. Our search was limited to peer-reviewed articles less than 10 years old. Articles published to date in the fields of control of parasitic diseases transmitted by freshwater snails were included. Results were presented in narrative and in table format.

Results: The results of the database search identified 1007 records. We included 84 studies in this review. These studies generally focused on freshwater snails and the diseases they transmit. We described the geographical distribution of 43 freshwater species belonging to nine snail families, as well as the parasites that infect them. Several methods for diagnosing parasites in their snail hosts have been described, including microscopic and molecular methods, as well as antibody and protein barcode-based techniques. Molluscicides have been described as the main strategy for snail control.

Conclusion: This study highlights several elements of knowledge about diseases transmitted by freshwater snails and their distribution. A good understanding of snail infection detection techniques and existing control methods is an essential component in adapting control strategies for these diseases.

Keywords: Africa; Diseases; Fasciolasis; Freshwater; Parasite; Schistosomiasis; Snail.

Fig. 1

Flow diagram for the literature review

Fig. 2

Illustration of the lifecycles of two genera of snail-borne parasites [credit: https://www.cdc.gov/dpdx/schistosomiasis/modules/Schistomes_LifeCycle_lg.jpg and https://www.cdc.gov/dpdx/fascioliasis/modules/Fasciola_LifeCycle_lg.jpg]: A Lifecycle of Schistosoma spp.: (1) Adult worms reproduce sexually in the venous system of the bladder (S. hæmatobium) or intestine (S. mansoni, S. intercalatum, S. guineensis), producing eggs which are excreted in the urine or faeces. (2) The eggs hatch upon contact with water, releasing miracidia which then enter a specific intermediate snail host. (3) Within the snail host, the miracidia develop into sporocysts and asexually reproduce daughter sporocysts which in turn produce cercariae. (4) The cercariae emerge from the snail and directly penetrate the skin of the human host. (5) After penetrating the skin of the human host, they transform into schistosomules. The schistosomules migrate via the circulatory system to the lungs and then the heart before arriving in the liver where they mature. Once mature, the adult worms emerge from the liver and mate in the mesenteric vessels of the intestine or bladder. B Lifecycle of Fasciola spp.: (1) immature eggs are evacuated into the bile ducts and passed in the faeces. (2) The eggs embryonate in freshwater for around 2 weeks. (3) The embryonated eggs release miracidia. (4) The miracidia invade a suitable intermediate host, a snail. In the snail, the parasites pass through several stages of development (sporocysts (4a), redia (4b) and cercariae (4c)). (5) The cercariae are released by the snail and encyst as metacercariae on aquatic vegetation or other substrates. (6) Humans and other mammals are infected by ingesting vegetation contaminated with metacercariae (e.g. watercress). (7) After ingestion, metacercariae excyst in the duodenum and penetrate through the intestinal wall into the peritoneal cavity. (8) Immature flukes then migrate through the liver parenchyma to the bile tract, where they become adult flukes and produce eggs. In humans, the maturation of metacercariae into adult flukes generally takes around three to 4 months. The development of F. gigantica may take slightly longer than that of F. hepatica [92]

Fig. 3

Distribution of schistosome species in Africa [93]

Fig. 4

A map showing the geographical distribution and occurrence of Fasciola spp. and their intermediate hosts, snails, in Africa. The taxa reported are symbolised next to the number of studies in each country [94]

Fig. 5

Methods for detecting parasitic infections in snails: A cercarial shedding test, B PCR and sequencing, and C MALDI-TOF MS

Fig. 6

A Use of a chemical molluscicide [95] and B the predator prawn Macrobrachium vollenhovenii [86] against intermediate snail hosts