Population genetics of schistosomiasis

Schistosome surveys are conducted in local schools

Schistosome surveys are conducted in local schools. © Aidan Emery, SCAN

Principal Investigator

Dr David Rollinson

Project summary

  • Focus: To determine how varying schistosomiasis treatment programmes are influencing the population genetics of schistosome species
  • Funding: SCORE
  • Start date: 2014
  • End date: 2017

We are researching the impact of human schistosomiasis treatment programmes on the population genetics of Schistosoma species, and on the infection rate of the snail intermediate hosts.

Schistosomiasis is a neglected tropical disease that affects over 200 million people worldwide.

Currently, disease control programmes rely heavily on the administration of Praziquantel, the only drug effective against all species of schistosome parasites.

This brings up questions about the impact of repeated human treatment on the parasite population, and raises concerns about treatment failure and schistosome drug tolerance.

Aims

We are using population genetics to monitor changes in the human schistosome parasite population. In particular, we are looking for changes after different rounds of drug treatment in endemic areas of Niger, Tanzania and Zanzibar.

Our research aims to:

  • assess the impact of different treatment strategies on the schistosome parasite population
  • identify the treatment strategy that is most effective at reducing parasite populations
  • evaluate the risk of Praziquantel tolerance or resistance occurring during disease control programmes

Treatment strategies

Tanzania

In Tanzania, six differential treatment arms are being tested for efficacy and cost effectiveness. We are monitoring parasite population genetics in the two most extreme treatment pressure arms:

  • Arm 1: the drug is given to the whole community (community-wide treatment, or CWT) every year. This group is therefore under the highest treatment pressure.
  • Arm 6: the drug is given in schools (school-based treatment, or SBT) every other year, representing the lowest treatment pressure.

As well as monitoring the parasite population in humans, we are collecting the intermediate host snail (Biomphalaria spp.) from water contact sites around the villages in Arm 1 and Arm 6. We are testing the snails for active schistosome infections to determine the number of infected snails at each site and how that changes over time and with the seasons.

Niger

In Niger, six treatment arms are being tested, and we are monitoring parasite population genetics in the four most extreme treatment pressure arms. As in Tanzania, we are also collecting and testing the local intermediate host snails.

Zanzibar Elimination of Schistosomiasis Transmission (ZEST)

We also aim to determine the impact of a disease elimination project in Zanzibar called ZEST (Zanzibar Elimination of Schistosomiasis Transmission), or Tokomeza Kichocho.

There we are researching three differential intervention strategies: 

  • MDA: mass drug administration of Praziquantel to the villagers twice a year
  • MDA + snail: alongside treating humans with Praziquantel, local water contact sites are sprayed with the molluscicide Niclosamide to kill the intermediate host snail species (Bulinus globosus)
  • MDA + behaviour: alongside treatment with Praziquantel, this arm includes community-led behavioural change intervention to stop behaviour that leads to transmission of schistosomes

Schistosomiasis: Parasites in motion

A video explaining the three intervention strategies of the ZEST programme in relation to the lifecycle of schistosomiasis.

French version

The stop-motion video about the life cycle of Schistosoma is now available in French.

Watch the Schistosomiasis: Parasites in motion video in French

Parasite survey sampling

Sampling infected children

In selected schools in each country (Tanzania, Niger and Zanzibar) we are collecting parasite samples (miracidia stage) from infected children. We are genotyping these individual miracidia and using this data to determine genetic diversity and population structure within the same species.

We plan to compare the genetic diversity between pre- and post-treatment samples and between the different treatment arms, using indices such as:

  • allelic richness
  • number of alleles per locus
  • number of private alleles
  • observed and expected heterozygosity

Sampling infected snails 

We are also collecting cercariae from infected snails for molecular species identification. We then genotype the cercariae to see how many different genotypes emerge from infected snails, and determine whether those snails are involved in the infection found in humans.

Preliminary results

We have completed two years of miracidia collections from our study sites in Tanzania and Niger. All samples have been entered into the Schistosomiasis Collection at the Natural History Museum (SCAN) database.

We have optimised species-specific microsatellite multiplex panels for S. haematobium (18 loci) and S. mansoni (20 loci) and have started genotyping baseline miracidia.

External collaborators

Funded by

Sustainability

We are exploring new sources of food, predicting the spread of disease and finding scarce elements

Parasites and vectors

Understanding parasites and the organisms that transmit them to humans and animals

Zoology collections

Our zoology collection has 29 million animal specimens and is rich in voucher, type and historical specimens