The Hidden World of Parasitic Control and Host Defense
The world of parasites is rife with manipulation, extending beyond simple nourishment to outright behavioral control of their hosts. Recent research highlights the amazing ways parasites influence creatures ranging from snails to fruit flies, and the ingenious defenses hosts have evolved in response.
One striking example involves a parasitic hairworm that infects snails. As described by Hutter and Mazhar,the worm transforms the snail’s behavior,compelling it to venture into brightly lit areas – “into grotesquely pulsating green-yellow neon tubes,” – a perilous move for a creature typically seeking twilight’s protection.This exposes the snail to predation by birds, who inadvertently become part of the parasite’s lifecycle. A bird consuming a snail infected with the worm will ingest the parasite, which then continues its progress, while the snail’s feeler, lost to the bird, simply regrows, restarting the cycle.Birds themselves frequently enough remain unaware of the infestation, only experiencing weight loss during heavy infections.
However, hosts aren’t passive victims. Japanese stink bugs (megymenum gracilicorne) have developed a remarkable symbiotic relationship with fungi to defend against parasitic wasps.A research group lead by Takanori Nishino (Japan) discovered that female stink bugs cultivate fungi in an organ previously thought to be used for sound perception. They then coat their eggs with fungal threads, creating a protective barrier against the wasps, as reported in science 390, p. 279.
This defensive strategy echoes the behavior of Cordyceps mushrooms, which are known for manipulating host behavior to optimize fungal spore dispersal. Similarly, caterpillars of the moth Pyrrharctia isabella increase their consumption of poisonous plant substances like coltsfoot and butterbur, leveraging the plants’ antiparasitic properties. Monarch butterflies also exhibit this behavior, laying their eggs on milkweed species whose metabolic products inhibit parasite growth in their offspring. Self-medication has also been observed in honeybees, birds, and chimpanzees.
The mechanisms behind these behavioral shifts are beginning to be understood at a molecular level.Greg Seong-bae Suh (South Korea) studied fruit flies (Drosophila melanogaster) and found that increased levels of reactive oxygen species (ROS) in the gut, triggered by factors like sleep deprivation, heat stress, and infection, lead to increased vitamin C consumption. Vitamin C, in turn, improves intestinal health and extends lifespan, mitigating the negative effects of the infection (Pnas 122, e2512852122).
a meta-analysis of 59 studies by British epidemiologist Hannah S. Wolmuth-Gordon examined the relationship between climate change and parasite prevalence. Published in PNAS 122, e2508970122, her research concluded that climate had no overall influence on parasite infestation rates, whether the parasites were endoparasites (living inside the host) or ectoparasites (living on the host’s surface). However, the study suggests that climate change may lead to a redistribution of parasites rather than a widespread increase in their numbers.
