A vessel is harpooned and infiltrated by an enemy, then robbed of its precious cargo – this imagery may call to mind Long John Silver or other fictional marauders, but these events happen every day, all around us, on a scale so small we cannot see it. I’m referring to microsporidia, a group of real-life, present-day pirates closely related to fungi. Microsporidia are parasites, meaning they live inside other organisms (their hosts) and harm them by plundering their resources without offering anything in return. These parasites are tiny, smaller than some bacteria, and must live inside the cells of a host to complete their life cycle. Yet in great enough numbers, they can topple large animals, including humans. However, some microsporidia can positively impact our health by infecting deadly pests.
Animals acquire microsporidia when they eat food contaminated with microsporidial spores. Each spore contains the parasite and a spring-loaded structure called the polar tubule. When the spore reaches the intestine, it fires the tubule into one of the cells lining the gut, “harpooning” it (fig. 1). The parasite squeezes through the tube, injecting itself into the host cell (click the link and scroll down to “Supplementary Information” for cool videos showing this process). Inside the cell, the parasite steals nutrients from the host and releases factors that interfere with the cell’s immune machinery. With the cell’s immune system disabled and its resources being channeled into the parasite, the cell becomes little more than the parasite’s nursery. The parasite divides until its offspring burst the cell (fig. 2) and infect neighboring cells. This process continues, and if the parasite continues to proliferate unchecked, the intestine is seriously damaged. New spores are released into the environment when they are shed in the feces, and the cycle repeats.
The microsporidian life cycle is certainly unusual and may seem exotic, but these parasites have enormous implications for human health. Microsporidia are specialists, meaning each species of the parasite can only infect one or a few specific animal species, and opportunists, meaning they take advantage of hosts that are already weakened. Humans can contract the species Encephalitozoon and Enterocytozoon from contaminated food or water, causing the disease microsporidiosis, which afflicts over 10% of the world population. Microsporidiosis is especially prevalent and can be lethal in HIV patients and other individuals with a compromised immune system.
Scientists are trying to find ways to thwart microsporidia not only in humans, but also in other animals that impact human life. Most microsporidian species specialize on insects. For example, Nosema microsporidia that infect honeybees are particularly devastating and can contribute to colony collapse disorder (CCD), which occurs when most of the worker bees in a colony die or never return to the hive. CCD is a major threat to our crops because bees are important pollinators, meaning they transfer pollen from the male to the female part of the plant and allow it to produce seeds and fruit. If honeybees continue to die off, squash, berries, and other fruits would decline or be wiped out. Scientists believe insecticides in the environment make bees more susceptible to Nosema and are advocating for better pesticide stewardship.
While some microsporidia negatively impact humans, others may benefit human health. Malaria, like microsporidiosis, is caused by a parasite. Malaria is transmitted by mosquitoes and causes over half the human deaths associated with mosquito-borne disease. The most promising approach to eliminating malaria is to target the mosquito. However, since mosquitoes are abundant in many regions, many people worry about how wiping out mosquitoes would impact other organisms, such as animals that eat mosquitoes. A recently discovered microsporidian species that specializes on mosquitoes was shown to cause little or no disease in the insect but prevents it from transmitting malaria. Introducing this parasite into mosquito populations would theoretically reduce malaria transmission without negatively affecting mosquitoes.
Most people have never heard of microsporidia, but these examples demonstrate how unseen forces can meaningfully influence our everyday lives. Like malaria and COVID-19, they show us that what we can’t see can hurt us. This may be an unsettling epiphany, but as the famous chemist Marie Curie said, “nothing in life is to be feared, it is only to be understood.” A greater appreciation for organisms like microsporidia helps us find new ways to improve our quality of life.