Eradicating Schistosomiasis at a Snail’s Pace
by Gloria Lorenz
BIOL 490, Fall 2014
Australorbis (= Biomphalaria) glabratus,
Schistosomiasis is an immunological disease that results from a parasitic infection and has vast effects on the people of Puerto Rico where it is prevalent. Schistosomiasis spreads via Australorbis glabratus (= Biomphalaria glabrata), an intermediate snail host for the parasitic worm Schistosoma mansoni. Various methods of control have been researched, particularly through chemical molluscacides as well as biological control using Marisa cornuarietis, another snail species. By researching schistosomiasis and its implications, as well as the role freshwater snails play in the spread of infection, the applied use of M. cornuarietis to control A. glabratus can be better understood. Since the spread of schistosomiasis is dependent on the distribution of A. glabratus, efforts have been researched that can eradicate A. glabratus to reduce the occurrences of infection.
Schistosomiasis is an infection that affects more than 200 million people across Africa, Asia, South America, and the Caribbean. Transmission of the infection may occur when a person is exposed to water contaminated by the snail host that has the parasite (Gray et al. 2011). Contact typically occurs through swimming, bathing, and washing clothes, and the longer a person is in the water, the higher the likelihood of becoming infected. The parasite causing this disease, Schistosoma mansoni, causes infection when cercariae (larvae) burrow into the skin, enter the vascular system, and then mature into adult worms (Pearce & MacDonald 2002). The infection continues as the worms continue to lay eggs inside the body. The liver is the main organ affected because of the high quantities of blood flowing through it. The infection advances in acute, chronic, and advanced stages, presenting a wide range of symptoms. When people are first infected, they may experience acute schistosomiasis, or Katayama syndrome (Gray et al. 2011). This stage includes fever, cough, and fatigue. As the infection progresses and the immune system responds, the person may develop intestinal disease, inflammation of the urinary system, and liver fibrosis. Cognitive impairment may occur in affected children. The most severe condition is neuroschistosomiasis, which results in high intracranial pressure due to thousands of eggs and granulomas (inflammations) in the brain and spinal cord. These inflammations form around new eggs deposited in the body (Pearce & MacDonald 2002). Schistosomiasis can be treated with drugs that cause the parasites to detach from veins and die. Corticosteroids are used to reduce the effects of allergic reactions, and anticonvulsants are used to treat seizures (Gray et al. 2011). Intracranial pressure can be treated through the surgical placement of a shunt in the brain. Surgery is reserved for those whose condition is declining despite the use of corticosteroids, which reduce inflammation, and other drugs. In many cases schistosomiasis can be prevented through environmental modifications, improved sanitation, and health education. However, in many places where schistosomiasis is prevalent, people cannot afford to live in better conditions. Therefore, the disease affects many and can potentially kill anyone who contracts it. In sub-Saharan Africa alone, approximately 280,000 people die each year from schistosomiasis (Pearce & MacDonald 2002). The link between schistosomiasis and freshwater snails provides the foundation for the research on control of the disease and control of the intermediate hosts for the parasite responsible for the infection.
An understanding of Australorbis is vital in researching its role in schistosomiasis. Australorbis glabratus is a freshwater snail that serves as an intermediate host for the parasite responsible for schistosomiasis, Schistosoma mansoni. The snail is the only species in Puerto Rico known to be an intermediate host, and it is widely distributed across the country (Berry et al. 1950). Retired Cornell University professor David Pimentel (1957) collected more than 1,000 Australorbis snails from Puerto Rico to study its life history. Snails were placed in aquaria according to 4 size groups. Observations provided much data about Australorbis. High fertility and year-round reproduction allows for rapidly increasing numbers, making it one of the most abundant species in Puerto Rico (Pimentel 1957). High numbers may increase the spread of schistosomiasis if more of the snails become infected and become intermediate hosts for the parasites, which can then be transferred to humans.
Control of schistosomiasis in Puerto Rico has been attempted through the use of molluscacides on intermediate snail hosts (Berry et al. 1950). This method of chemical destruction has been in use for some time. However, other methods of control have also been investigated. Berry et al. (1950) conducted studies researching the effects of various chemicals as control agents on Australorbis glabratus. More than 750 chemicals were used to make varying compounds which were tested in laboratory settings. In a 24-hour test, 11 compounds were found to be 100% effective against Australorbis. These compounds, such as phenacyl chloride, diisobutyl phenol, and copper salts were then employed in field settings in a swamp in Vega Baja, Puerto Rico. In order to measure the effectiveness of the compounds, a comparison was made between the number of viable snails in various plot areas before and after application of the compounds. Observations taken at 24, 48, 60, and 72 hours showed lethal effects on the snails in a very short amount of time. Six of the compounds were at least 90% effective when used at a concentration of 10 parts per million (Berry et al. 1950). The use of chemical compounds at a test cite in 1955 in southeastern Puerto Rico cost approximately $7.80 to treat 100 cubic meters of water (Jobin et al. 1970). High costs of these compounds and toxic effects on fish may result in a decrease in the use of chemicals to control Australorbis. If chemicals are not a viable option, it is important to consider other methods of control.
Biological control of intermediate snail hosts has been researched as well. Control of schistosomiasis in Puerto Rico has been attempted by displacing the snail host, A. glabratus with another snail, Marisa cornuarietis. One study looked at the effects on A. glabratus in Puerto Rican farm ponds where M. cornuarietis had been introduced (Ferguson et al. 1958). Out of 10 ponds where Marisa was introduced, colonies were able to establish within a year. In the other 5 ponds, establishment took longer than a year. In 8 out of the 10 ponds, Marisa colonies were able to become very well established. In all 8 of those ponds, colonies of A. glabratus were eradicated in an average time of 8 months. Marisa was also introduced into a stream watershed in southern Puerto Rico (Ferguson et al. 1958). Despite storms, housing developments, and physical changes to the watershed, Marisa was able to establish itself and even flourish in several locations. Surveys of various plots with Australorbis taken before and after Marisa introduction showed a decline in the numbers of Australorbis. This decline was attributed to spatial competition between the two snail species. As a result of these findings, Puerto Rico began placing Marisa into several other waterways around the country where chemical control of Australorbis had been previously used.
It is important to understand the ecology of Marisa in order to know how it can be used as a biological control. Marisa cornuarietis, the giant ramshorn snail, is sexually dimorphic with a shell diameter ranging from 40 to 50 mm (Selck et al. 2006). This omnivorous, freshwater snail resides in a variety of habitats in the Caribbean, as well as in Central and South America. Selck et al. (2006) researched food preferences of Marisa and found that it may be able to withstand starvation depending on the nutritional content of what it is eating. The implication here is that Marisa may be able to survive better in varying habitats, thus allowing it to be used as a control agent across a larger range. Marisa was also studied as a weed control in Puerto Rico and Florida, in addition to being investigated as a biological control for the intermediate host of Schistosoma mansoni. Knowing a little bit about Marisa cornuarietis can provide a greater understanding of how it is used in different laboratory and field settings.
Under laboratory settings, studies have shown that Marisa consumes Australorbis (Chernin et al. 1956). Laboratory-reared samples of Australorbis snails were kept in aquaria, while Marisa samples were kept in separate tanks. In one container, Australorbis was observed to be laying egg masses on vegetation. Marisa that was also in that container was observed consuming the water cress on which the eggs had been laid, destroying the eggs in the process. In a few instances, it was seen that Marisa was actually consuming the egg masses. Another experiment examined the effects of Marisa on the growth of Australorbis. When in the same container, it was found that both species were able to reproduce effectively without great negative effects from the other species. The various studies completed indicate that Marisa does not purposefully consume Australorbis. Instead, destruction may be due to accidental consumption as Marisa searches for food (Chernin et al. 1956). However, given the nature of the feeding habits of Marisa, it may be useful in field settings as a biological control of Australorbis.
Although schistosomiasis is a highly infectious disease affecting millions of people, several methods for control of parasites have been investigated. As an intermediate host for the Schistosoma mansoni parasite, Australorbis glabratas has been researched in several studies to find a way to control the spread of the disease. Chemical and biological agents have been applied in laboratory as well as field studies. These studies have provided much data as to how intermediate snail hosts of the parasite can be destroyed either through molluscacides or another snail species, Marisa cornuarietis. Further research may provide additional insight as to how this disease can be controlled, either through chemical or biological agents, and may even reveal other species that can be used to decrease the spread of the disease. It is important to consider the options for control methods so that actions can be taken to reduce the number of illnesses and deaths resulting from schistosomiasis.
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