Snail Fever: Schistosome-Parasitized Snails
Snail fever? That sounds cute and sad but it’s not a snail with a fever.
Snail fever is much worse than that. Snail fever, also known as Bilharzia and
Schistosomiasis, is a parasitic disease that affects over 200 million people
globally (70% in sub-Saharan Africa), that is equivalent to everyone in Brazil
being infected with this parasite! It is primarily transmitted between humans
and snails, making snails our enemy in Schistosomiasis.
Unlike many other waterborne disease, people become infected through
contact of parasites in contaminated water rather than ingestion of
contaminated water.
The life cycle of schistosomes is a bit complicated as it involves
different larval stages and hosts. Schistosome
cercariae penetrate the skin of the human host, detaching the tail with only
the head (known as a schistosomula) enter the human body.
The schistosomula migrate
through various organs in the body and mature into adult worms that live in the
blood vessels surrounding the intestines (S. mansoni and S. japonicum) or bladder (S. haematobium). Male
and female worms live together in copula and
female worm produces between 300 and 3000 eggs daily. The eggs pass into the
lumen of the intestine (S. mansoni and S. japonicum) or bladder (S. haematobium) and
pass out in faeces or urine. To hatch and release miracidia, eggs must reach
freshwater. Miracidia infect snail hosts, later transforming into sporocysts,
which multiply. Finally, the sporocysts transform into infectious cercariae
that emerge from the snail and the life cycle continuous.
So, what happens to a snail infected with schistosome?
Just like humans, snails have a central nervous system which controls
the snail’s organs and tissues by chemical transmitters ‘neuropeptides’. These
small protein-like peptides affect growth, immunity etc. By using mass
spectrometry and Nano-HPLC, differences in neuropeptides in infected and non-infected
snails could be compared. It was seen that neuropeptides involved in snail reproduction,
feeding and growth was downregulated, altering the behaviour of infected snails.
Schistosome-parasitized snails show slower movement, have reduced
refuge-seeking and anti-predator behaviour making them less likely to hide or
move out of water to avoid predators and seek and aggregate with other infected
snails. This to me is interesting as the parasitic infection affects the
predator-prey dynamics in a way that does not benefit transmission of the
parasite. Having infected snails being slower, not responding to predation cues
and aggregated leads to the infected parasitized population being an easy snack
for predators such as prawns; consuming both the snail host and Schistosome
parasite. As predators of snails do not get infected by schistosomes this certainly
is not a useful strategy. On the other hand, these behavioural changes could be
beneficial to the parasites! Remaining in the water even when predators are around
increases the dispersal of the schistosome cercariae in the environment needed
for transmission, aggregation allows for increased probability of both male and
female cercariae infecting the human host at the same time.
Maybe schistosomes are not as intelligent as I thought they would be?
The changes of behaviour
to anti-predator snails allow biological control of schistosomiasis through
predation, but maybe they know that humans are continuously going to defecate
in water with egg-containing excreta continuing the cycle endlessly.
This just shows how important public health is. Away from all the drug treatments, diagnostic tools etc, investing in water, sanitation and hygiene is soooooo
important!
By this point I usually have my mind set on if I like the parasite or
not, but not today. What do you think?
Extra reading:
https://www.usaid.gov/what-we-do/global-health/maternal-and-child-health/technical-areas/water-sanitation-hygiene-wash
WATER, SANITATION, AND HYGIENE (WASH)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4377019/
The roles of water, sanitation and hygiene
in reducing schistosomiasis: a review
https://edition.cnn.com/2016/02/09/health/snails-spread-schistosomiasis-in-africa/index.html
The snails spreading fever across Africa
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