By Roberta Attanasio
Global climate change noticeably impacts human health—safe drinking water, sufficient food, and secure shelter are threatened by rising sea levels and severe weather events. Heat waves dramatically increase death rates not only from heat strokes, but also from complications arising from cardiovascular, respiratory, and cerebrovascular diseases. Although global warming may bring some localized benefits, such as fewer winter deaths in temperate climates and increased food production in certain areas, the overall health effects of a changing climate are likely to be overwhelmingly negative. For example, climate warming is predicted to increase the transmission of parasite infections. Now, results from a recent study show that host immunity can influence the impact of warming on host–parasite interactions and mitigate its long-term effects.
For the study (Host immunity shapes the impact of climate changes on the dynamics of parasite infections), researchers focused on soil-transmitted gastrointestinal helminths, also known as parasitic worms. In humans, these worms cause some of the most common parasitic infections worldwide. According to the World Health Organization (WHO), approximately 2 billion people are infected with soil-transmitted helminths globally, mostly in the poorest and most deprived communities. They are transmitted by eggs present in human feces, which in turn contaminate soil in areas where sanitation is poor. However, the researchers focused on two parasitic worms of rabbits, Trichostrongylus retortaeformis and Graphidum strigosum.
In previous studies, the researchers found that, in rabbits, infections from one of the parasites are controlled by the immune response, whereas infections from the other parasite species are not controlled, even though the rabbits do mount an immune response to the parasite. Therefore, the researchers designed the new study to understand the contribution of climate change and immunity on the long-term and seasonal dynamics of infections caused by the two rabbit parasitic worms. They examined samples collected monthly between 1977 and 2002 in Scotland.
The study results show that climate warming—rising temperature and humidity—increases the availability in pastures of the infective stages of both intestinal worms. The intensity of infection increases for the worm not regulated by immunity. In contrast, there is no significant long-term positive trend in the intensity for the immune-controlled worm. Specifically, G. strigosum infection is not controlled by the rabbit immune response. Therefore, the intensity of the parasite infection increases with warming, leading to significant accumulation of G. strigosum in rabbits, mostly in adult rabbits. Why? The rabbits aren’t able to clear the infection caused by G. strigosum with their immune response; therefore, the rabbits accumulate more and more parasites as they age—the result is that older individuals carry most of the infection in the population. However, because T. retortaeformis infection is controlled by the rabbit immune system, the effects of climate on the intensity of infection caused by this worm are mitigated.
Isabella Cattadori, senior author of the study, said in a press releae: “Over the course of 23 years, we saw clear evidence of climate warming at our study site in Scotland. The warmer climate leads to increases in the number of soil-transmitted parasites in the pastures where the rabbits live because the parasites can survive longer in the soil. With more parasites, there is an increased risk of infection, but how this increased risk affects the severity of the infection in the long term depends on the ability of the host to mount an immune response.”
How do these results contribute to the treatment of parasitic worm infections in humans? Cattadori said: “”Our research shows that as climates continue to change, we will need to tailor our treatment of parasite infections based on whether or not the host can mount an effective immune response. When a host’s immune response cannot control the infection, treatment should be targeted at older individuals because they carry the most severe infections. When a host’s immune response can control the infection, treatment should be targeted at younger individuals because they are at the greatest risk.”
The video below provides an overview of the significance of the study.