Sickness-induced lethargy can increase host contact rates and pathogen spread in water-limited landscapes

Abstract

Severe infections in vertebrates commonly elicit sickness behaviour that includes anorexia and lethargy. Intuitively, sickness-induced lethargy (SIL) should reduce contact among hosts. Therefore, for directly transmitted pathogens, sickness behaviour should reduce pathogen spread. However, there are indications that the relationship between SIL and host contact rates can be reversed under specific social conditions. Here, we used an agent-based model to investigate the possibility that the nonsocial environment can also impact the relationship between SIL and host contact rates. Our results demonstrate that in water-limited landscapes, SIL can increase host contact rates and associated pathogen spread. Based on our results, we hypothesize that a sickness-induced increase in contact rates should be particularly likely and most pronounced in animals that are highly water dependent such as African buffaloes living in savanna environments. In the context of virulence evolution, our findings contradict the expectation that the direct transmission of pathogens generally favours the evolution of reduced pathogen virulence compared to vector- and water-borne transmission. Instead, our findings suggest that the opposite effect is possible in water-limited landscapes: compared to vector- and water-borne transmission, direct transmission can favour the evolution of increased virulence. Our findings could be relevant in contexts other than direct transmission in water-limited landscapes. For example, in addition to aggregating around limited water sources, sick individuals might aggregate at, or around, limited food sources, which could facilitate the spread of pathogens with different transmission modes. Therefore, sickness-induced behavioural changes could critically affect the transmission of many pathogens in different environmental contexts. A plain language summary is available for this article.