The Life and Times of Parasites: Rhythms in Strategies for Within-host Survival and Between-host Transmission

Abstract

Biological rhythms are thought to have evolved to enable organisms to organize their activities according to the earth’s predictable cycles, but quantifying the fitness advantages of rhythms is challenging and data revealing their costs and benefits are scarce. More difficult still is explaining why parasites that live exclusively within the bodies of other organisms have biological rhythms. Rhythms exist in the development and traits of parasites, in host immune responses, and in disease susceptibility. This raises the possibility that timing matters for how hosts and parasites interact and, consequently, for the severity and transmission of diseases. Here, we take an evolutionary ecological perspective to examine why parasites exhibit biological rhythms and how their rhythms are regulated. Specifically, we examine the adaptive significance (evolutionary costs and benefits) of rhythms for parasites and explore to what extent interactions between hosts and parasites can drive rhythms in infections. That parasites with altered rhythms can evade the effects of control interventions underscores the urgent need to understand how and why parasites exhibit biological rhythms. Thus, we contend that examining the roles of biological rhythms in disease offers innovative approaches to improve health and opens up a new arena for studying host-parasite (and host-parasite-vector) coevolution.

The Complex Interplay of Parasites, Their Hosts, and Circadian Clocks

“…For instance, high nightly counts of Dirofilaria repens microfilaria were observed in dogs (Di Cesare et al, 2013). A similar pattern was observed with Wuchereria bancrofti (Hawking, 1967), presumably an adaptation to the night-biting Anopheles and Culex insect vectors (Reece et al, 2017). In contrast, the Pacific-type W. bancrofti has a greater number of parasites in circulation in the afternoon (Hawking, 1967), consistent with the phase of biting behavior of its diurnal vector, Aedes.…”

“…show oscillations in fever and chills with periods that are multiples of 24 h (Hawking et al, 1968;Karunaweera et al, 1992). This is paralleled by a 24, 48, or 72 h developmental cycle within red blood cells (RBCs), depending on the parasite species (Reece et al, 2017). Indications of rhythms also exist in leishmaniasis: blood samples collected from patients in the daytime contained fewer Leishmania amastigotes than night time-collected samples (Saran et al, 1997).…”

Infection by the castrating parasitic nematode Sphaerularia bombi changes gene expression in Bombus terrestris bumblebee queens

“…Here we identified an enrichment of DEGs associated with circadian rhythm. Within host-parasite interactions, parasites must contend with and can affect host circadian rhythms (Reece et al, 2017;Westwood et al, 2019). This is best documented in parasites of medical importance, where symptoms of infection include irregular circadian rhythms, such as in the case of human infections by parasitic species, such as Plasmodium (Kwiatkowski and Greenwood, 1989) and Trypanosoma species (Rijo-Ferreira et al, 2018).…”

Host circadian clocks do not set the schedule for the within-host replication of malaria parasites

“…At each sampling point, blood was collected from the tail vein and parasites at each IDC stage quantified from thin blood smears. Stages were characterised by morphology, based on parasite size, the size and number of nuclei and the appearance of haemozoin (as per [11] and [2]). Red blood cell (RBC) densities were measured at each sampling time by flow cytometry (Z2 Coulter Counter, Beckman Coulter).…”