Becoming infected with one parasite could change your chances of becoming infected with another according to new research from Cardiff University. The new study analyses data from school aged children in Tanzania infected with the most common forms of worms.
The new study analyses data from school aged children in Tanzania infected with the most common forms of worms. It was found that infection by one parasitic species actually changes the risk of catching another, over and above other risk factors.
The study is the first to look at the significance of infection with one disease as a risk for further infections (i.e. co-infection). The findings could help us better understand the importance of co-infection as a risk, and could help inform disease control strategies.
Traditionally, co-infection, where the person or population of people has more than one parasite species in it, has been seen as a result of other risks. For example, if people in a population have two different parasites more often than expected by chance, then it is often thought that these two parasites must have been picked up at the same time because of where they are in the environment (co-exposure) and that the place the person lives or plays therefore is the cause of this co-infection (in other words a person's environment would be the risk factor).
Dr Joanne Lello, Cardiff University School of Biosciences and lead author on the study said: "Infectious diseases are among the most important causes of childhood mortality and morbidity in the developing world. Understanding what drives an individual's infections is essential if effective disease-control strategies are to be developed. We know there are many possible risk factors associated with an individual's infections, including their physical environment, genetics, behaviour and demographic factors, but a comparatively understudied risk factor for infection with one organism is co-infection with a second species."
By studying the associations between the four most prevalent infections among school-aged children in Tanzania - Ascaris lumbricoides, Trichuris trichiura, hookworm and self-reported fever - Dr Lello and colleagues (from the Universities of Bristol and Basel (Switzerland) and from the Ministry of Health in Tanzania), found that co-infection cannot be explained away as a simple coincidence that is due, for example by co-exposure in the environment.
They found that co-infection is itself a very important risk factor, and sometimes more important than any of the other risks considered, such as a child's living conditions, behaviour and sex. However, the risk does not always increase. They also find that infection with one parasite species can sometimes be associated with a lower risk of infection with a second parasite species.
Explaining the significance of these findings, Dr Lello, said: "We were able to include in our statistical model many different things which have been identified as risks before, for example a child's living conditions, behaviour, sex and so on. As such we could tell that the co-infections were not just caused by these 'other' risks and importantly we could also tell how much difference every risk made compared to one another. This showed us that co-infection was important for all the diseases we looked at and often it was more important than most of these 'other' risks.
"When medics and government design programmes to control diseases they use their knowledge of what things change the probability of becoming infected, called risk factors. If we miss out important risk factors then control programmes are unlikely to work effectively and resources can be wasted targeting the wrong things. Knowing that co-infection is a major risk factor is therefore really important and our study demonstrates this for the first time.
"Although our study was conducted on children in Tanzania, co-infection with different parasites occurs everywhere and so the principle that one parasite can change the probability of being infected with another has wide implications."
More information: 'The Relative contribution of co-infections to focal infection risk in children' is published in the Proceedings of the Royal Society B.
More than 1.5 billion people or 24% of the world's population, are infected with soil-transmitted helminth infections worldwide. Soil-transmitted helminth infections are widely distributed in tropical and subtropical areas, with the greatest numbers occurring in sub-Saharan Africa, the Americas, China and east Asia. Over 270 million preschool-age children and over 600 million school-age children [the age group we studied] live in areas where these parasites are intensively transmitted, and are in need of treatment and preventive interventions.
There is no good data for the whole of Zanzibar but in most regions tested the prevalence with at least one of the three worm species is over 50%. These worms are not just a problem in Zanzibar though, according to the World Health Organisation A. lumbricoides infects over 1 billion people, T. trichiura 795 million, and hookworms 740 million worldwide. In our particular study prevalence with T. trichiura was 40%, A. lumbricoides 49%, hookworm 26% and self-reported fever 43%. It is not possible to talk about the general prevalence of fever as we don't know the specific disease that is causing this symptom - only that it is microparasite related rather than worms.