What factors drive persistent schistosomiasis infections in hotspot communities.

Researchers in Kampala and Glasgow identified key factors driving persistant S. mansoni infections in hotspot communities on the banks of Lake Victoria. These findings will help schistosomiasis elimination programmes design targeted interventions to bring down infections in these high transmission areas.

Schistosomiasis in Uganda

Schistosomiasis is a debilitating, and highly prevalent, parasitic disease, transmitted through fresh water contact and maintained through aquatic snails. It is one of a group of diseases termed Neglected Tropical Diseases (NTDs), and treatment programmes, using the drug of choice, praziquantel, have succeeded in greatly reducing the burden of disease across many countries in sub–Saharan Africa. However, disease control programmes can struggle with reduced efficacy of praziquantel in certain settings, transmission bounce backs, high reinfection rates and low treatment coverage. In Uganda 91 of the 134 districts are endemic for intestinal schistosomiasis caused by Schistosoma mansoni, with more than more than 7 million people infected, and the region around Lake Victoria has one of the highest S. mansoni burdens. This is despite over a decade of Mass Drug Administration (MDA) of praziquantel. Studies have revealed that particular villages have had persistently high schistosomiasis intensity and/or prevalences despite repeated MDA campaigns, and are referred to as transmission hotspots.

With the launch of the new World Health Organization (WHO) 2030 Roadmap for NTDs, where the goal is to eliminate schistosomiasis as a public health problem in all endemic countries, there is a renewed need to understand drivers of disease transmission in communities that are not responding to regular MDA campaigns. This will help ministries of health and programme managers design and implement effective targeted interventions for these transmission hotspot communities.

Such a community is Bugoto in eastern Uganda, on Lake Victoria [see previous blog here] which has maintained a high prevalence of schistosomiasis despite more than 14 years of MDA and remains a highly endemic community according to the WHO endemicity criteria. This study carried out in Bugoto in 2017 aimed to determine the drivers of infection among individuals as well as quantify risk factors in this hotspot setting in eastern Uganda which has undergone regular praziquantel MDA since 2003.

What are the drivers of transmission?

To answer the question of what the drivers of infection are in a hotspot community, researchers sampled 384 individuals from Bugoto. Individuals were randomly selected from the community register to represent equal sex and age ratios. Individuals were proportionally sampled from these three age groups:

  1. Pre-school aged children (PSAC) nine months to five years of age
  2. School aged children (SAC) 6 to 14 years of age and
  3. Adults 15 years and more.

Participants were given a semi-structured questionnaire designed to determine specific individual demographic, behavioural, environmental, and socio-economic characteristics related to schistosomiasis infections. Demographic variables included age, sex and religion, and environmental variables included the number of years an individual has been resident in the community and the approximate distance of the house from the lakeshore.

The researchers also mapped out frequently visited water contact sites along the lakeshore and differentiated risk associated with characteristics of transmission sites where control interventions could be targeted. The lakeshore was partitioned into the different ecological zones:

  • Zone 1 – swampy shoreline where some community members go to get water as well as grow rice.
  • Zone 2 – rocky extreme end of the community extending into the lake and 10 minutes metres from the shoreline that is used by the community for fishing and transit to islands.
  • Zone 3 – open lake shoreline stretching more than 300 metres but not exceeding 10 metres from the shoreline with some floating vegetation that is used mostly for swimming gathering household water and transit.

Participants were asked about water contact at these sites including activity, frequency and duration, and about treatment access, such as whether they had received treatment in the previous MDA campaign in July 2017 and at anytime during their lifetime.

Researchers also assessed the socio-economic status of households using an established method which assigns values to different construction materials within the household and assessed latrine ownership, water source and level of education. Finally, an assessment of the participants knowledge of modes of transmission, effects of infection on individuals and types of control methods was undertaken in order to ascertain awareness and knowledge of schistosomiasis.

Each participant was assessed for schistosomiasis using stool samples and Kato katz filtration technique, carried out over three consecutive days. The researchers used multivariate logistical regression with all the significant univariate variables to determine the most important factors influencing the risk of getting intestinal schistosomiasis as measured by the presence of eggs in stool. Although several factors were important for S. mansoni infections when examined in isolation through logistic regression models, only a few of these were important when controlling for all possible combinations using a multivariate model. Key factors included:

  • How long have you lived in this community? The best fit model indicated that an age group, proportion of lifetime residence and duration of water contact were all significant predictors of S. mansoni infections. This best fit model indicated a significant interaction between age group and residence time, meaning that the impact on age is dependent on the proportion of lifetime spent locally. So, a school aged child who has lived a small proportion of their lifetime in Bugoto actually has a low risk of infection but someone living an entire lifetime in Bugoto had a 6725 times increase in risk of infection.
  • How long have you been in this water? Lake water contact for five minutes or more, regardless of frequency, was associated with an increased S. mansoni infection risk.
Fishing is a big part of the Bugoto community as can be seen from nets laid out and the fishing boats in the photo. Photo credit: Dr Poppy Lamberton

Knowledge and education does not necessarily lead to behaviour change.

Another interesting finding of this study was that knowledge of schistosomiasis disease, transmission and control among participants was associated with a higher risk of infection with S. mansoni. The researchers explained this by highlighting that a greater knowledge can only lead to reduced infections if people are also given feasible options to change their behaviour. The researchers suggested that the education and community sensitization undertaken by programmes may be working at reaching high risk people in the community, who are then able to recognise symptoms and know that they are at risk of transmission, however the absence of alternative water sources limits their ability to change their behaviour and protect themselves because they still need to use the same unsafe lake water. The researchers linked this with another interesting finding, that MDA compliance was also positively associated to infection, again potentially because people who knew that they were at risk of infection and also recognised symptoms, would take up any treatment offered for the disease, however they would still need to go back to the same water and thus get re-infected.

Dealing with focal areas of high transmission and disease burden

The study by Arinaitwe et al identified potential pathways to bring down infections and disease burden in these persistently high disease burden communities. Tailored interventions could include:

  • Expanding treatment coverage in hotspot areas by adding targeted treatment to residents who have lived more than 10% of their lifetime in the community.
  • Providing sustainably accessible, affordable, and safe water sources for all water source needs.
  • Designing and implementing safe structures and processes to reduce and/or protect individuals from water contact, such as lake barriers in the form of jetties, safe dry landing sites for fishing boats, protective clothing for farmers and safe water storage practice to avoid passive exposure to schistosome infections, e.g. storing lake water for 48 hours before bathing.

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