First parasite changes risk of a second

CARDIFF U. (UK) — Infection by one type of parasite actually affects your risk of catching another, over and above other risk factors, say researchers who studied data from children in Tanzania. 

“Infectious diseases are among the most important causes of childhood mortality and morbidity in the developing world,” says lead author Joanne Lello of Cardiff University’s School of Biosciences.

“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, behavior, and demographic factors, but a comparatively understudied risk factor for infection with one organism is co-infection with a second species,” explains Lello, whose findings appear in Proceedings of the Royal Society B.

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The new study analyses data from school-aged children in Tanzania infected with the most common forms of worms. It 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).

By studying the associations between the most prevalent infections among school-aged children in Tanzania—Ascaris lumbricoides, Trichuris trichiura, hookworm, and self-reported fever—Lello and her colleagues found that co-infection cannot be explained away as a simple coincidence that is due, for example by co-exposure in the environment.

A primary risk factor

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, behavior, 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, Lello says: “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 doctors and government design programs to control diseases, they use their knowledge of what things change the probability of becoming infected, called risk factors,” Lello explains. “If we miss important risk factors then control programs 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.”

Source: Cardiff University