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.