Resting in plain sight: Dormancy ecology of the intermediate snail host of Schistosoma haematobium

Abstract

In rural northwestern Tanzania, land-use change to increase agricultural water availability has resulted in networks of rain catchment ponds teeming with snails that transmit Schistosoma haematobium, a parasitic worm causing urinary schistosomiasis in humans. These aquatic snails (Bulinus nasutus), however, must endure seasonal droughts that transform lush, nutrient-filled habitat to barren, parched earth for up to seven months yearly. The return of rain in the wet season is followed rapidly by the reappearance of abundant snail populations. The mystery of how snails endure the persistent harsh elements of long dry seasons is well described by severely decreased metabolism and other physiological adaptations that underlie the behavior of aestivation (periodic dormancy due to decreased moisture in a habitat, i.e., “dry season hibernation”), which is common in snails (Brown, 1994). The lack of knowledge of where snails aestivate has largely halted the study of schistosomiasis-transmitting snail dormancy in the last half century, even though successful dormancy could drive snail population dynamics and consequently, schistosome disease outcomes for humans.

Only a single publication, to our knowledge, has identified aestivating B. nasutus in the field, describing the typical locality as <2 cm below the soil and in the pond outer periphery (Webbe, 1962), but they did not provide standard survey protocols. Following the standardized methods for another Bulinus species (Betterton et al., 1988), we dug 2–5 cm transects or quadrats into nine dry agricultural ponds outside of Mwanza, Tanzania, searching for the locality of dormant snails in September and October 2022. We opportunistically searched 10 additional dry and nearly dry ponds for aestivating snails in drying vegetation and mud. But across these 19 different ponds, we observed only a single snail which then survived in the lab for over 30 days. We also found many desiccated shells in the landscape. We realized why scientists had largely ignored this behavior for the last half century: Dormant snails seemed well shielded from the elements, including curious scientists. Considering the duration of the dry period and its potential disease implications, however, we were determined to better characterize the dormancy ecology of B. nasutus.

In mid-2023, we began a small-scale dormancy experiment at the National Institute for Medical Research, Mwanza. We measured survival rates in four basins (two for one month of dormancy, two for two months of dormancy), each containing soil and 20 B. nasutus snails collected from active agricultural ponds. Within each dormancy period, basins contained either schistosome-infected or -uninfected snails. What we did not expect was that most snails (66.25%) did not burrow below the soil but instead remained on the surface throughout dormancy (Figure 1). We assumed that these were failed aestivators and would perish. When we rehydrated the basins, however, we found that the locality of dormancy (on or below the soil surface) had no significant effect on dormancy survival (binomial generalized linear mixed model [GLMM], p > 0.05): 16.98% of surface aestivators and 22.22% of burrowing aestivators survived dormancy. Future studies should assess aestivation locality (above or below the soil) and survival rates of B. nasutus in different moisture and temperature conditions.

If snails could survive dormancy exposed to the elements in a laboratory setting, could they also do so in the field? As the dry season returned, we tested our new hypothesis of potential for desiccation persistence of snails in September 2023. Our first true observation of B. nasutus aestivating in a field setting was in the small cracks and crevices of the rocky walls of a small waterbody dug by villagers for rainwater collection (Longo Site 2) in the shade of a small tree but otherwise exposed to the elements. These snails seemed completely desiccated, empty shells with the apparent absence of snail tissue. But this time, we did not assume they had perished. We collected 26 B. nasutus shells—or perhaps even live, dormant snails—from this site and returned to the laboratory. Within 48 h of hydration, 22 of the 26 snails were indeed alive, with the live tissue gaining color and volume with hydration. Of these 22 snails, only three did not survive through 14 days of laboratory observation. We collected 323 shells from another eight dry pond sites with highly variable rehydration and laboratory survival rates (Table 1).

Across the nine dry ponds, we recovered snails from a diversity of microhabitat types typically characterized as textured, shaded, or protected surfaces, that is, areas that maintain moisture and/or buffer temperature. More often, snails were found on the substrate surface exposed to the elements, rather than submerged fully below soil. Snails were also frequently found within dry roots and vegetation that they typically feed on during the wet season, and not necessarily at the pond periphery. The locations of aestivation suggest that snails may simply rest in the muddy surfaces where the last ripples of water leave them as the pond shrinks with desiccation, rather than actively seeking refuge as the pond dries. However, future research is necessary to better characterize this behavioral response. For example, either in situ enclosures or laboratory experimental designs simulating the dry season drop in water could provide insights into the aestivation behavior of B. nasutus.

Interestingly, none of the 108 snails that survived aestivation on the sediment surface or in association with vegetation in the field were shedding schistosome parasites in the 14 days of observation, despite infections being previously identified in four ponds (Table 1). Bulinus snails are infected when infected humans contaminate water with urine harboring schistosome eggs. Free-living miracidia hatch out of the eggs and these parasites pierce into the snails, then multiplying and growing into larger free-living larval forms (cercariae) that return to the water and seek out humans. Seasonality constrains this cycle as 6–18 weeks are needed for infections to develop in the snail (Sturrock, 1967) while ponds contain water. If adequate time allows for an infection to develop before the snail enters aestivation, they are forced to endure—and typically succumb to—the physiological demands of both desiccation and infection (Badger & Oyerinde, 1996). This is supported by findings from our laboratory experiments simulating aestivation. None of the 40 schistosome-infected snails survived dormancy regardless of aestivation length (1 or 2 months), whereas nine (45%) and six (30%) of the uninfected snails survived one and two months of aestivation, respectively (binomial GLMM, p < 0.001, Figure 2).

Aestivation may indeed serve to cull infected snails, resulting in the disruption of transmission yearly. Drying waterbodies in this Tanzanian landscape have four times lower schistosome infection rates than non-drying waterbodies (Starkloff, Angelo, et al., 2024; Starkloff, Mahalila, et al., 2024). Additionally, Badger and Oyerinde (1996) showed that in the intermediate host (Biomphalaria species) of Schistosoma mansoni survival rates of snails with mature infections (shedding parasites) entering aestivation are almost 0%. However, Barbosa and Barbosa (1958) found that early in infections, S. mansoni can co-aestivate successfully with snails, developing into mature infections with the return to aquatic habitats. Such studies are still scant in this snail–schistosome system. One of our field aestivators shed non-schistosomes (likely Xiphidiocercariae) in its beaker on Day 13 and there has been evidence of a few B. nasutus aestivators shedding schistosome and non-schistosome infections promptly following aestivation in past field studies (Starkloff, Angelo, et al., 2024; Starkloff, Mahalila, et al., 2024; Webbe, 1962). If we observed snails for longer than 14 days or for immature infections, we may have found more infected aestivators. Future work should identify the survival rates of B. nasutus with immature infections though aestivation and the transmission risk this provides following dormancy.

In addition to immature infections enduring aestivation, schistosome transmission risk can also be characterized by the repopulation of ponds by snails susceptible to new infections following aestivation. Of the 101 snails that survived aestivation across our laboratory and field studies, none produced eggs within 14 days, even though they were provided with a 5 mm × 5 mm piece of water lettuce (Pistia species) and herbivory by the snails was observed. Previous studies suggest that post-aestivation snails tend to delay egg laying for about 2 weeks following dormancy (Oyeyi & Ndifon, 1990). Tracking snail reproductive rates and offspring success following aestivation is vital information to understanding the transmission risk as different aestivation conditions are likely to impact the timing and intensity of population rebounding.

The continued transmission of S. haematobium relies on the maintenance of infections in its definitive hosts, humans, as well as the yearly revival of enough aestivating snails to repopulate habitats and transmit new infections. To understand the potential of snails to repopulate—and provide the next generation of potential intermediate hosts—across different waterbody conditions, we need to better understand the ecological and physiological determinants of parasite persistence through aestivation as well as snail aestivation survival rates and post-aestivation reproductive rates. Understanding these determinants can have key mitigative impacts on disease outcomes for humans if we can characterize and create waterbodies less hospitable to the repopulation of these intermediate hosts.

The authors declare no conflicts of interest.