Humans and Their Parasites

Toxoplasma gondii
Rendering of microscopic Toxoplasma gondii parasites

Last month’s video footage of a North Korean soldier defecting was as dramatic as it gets: a jeep speeds across a bridge, and its frantic occupant emerges and runs for his life, pursued by armed soldiers. The man was a North Korean soldier attempting to defect to South Korea. He was shot and wounded, but made it across the border and was admitted to a South Korean hospital, where doctors found something unexpected: His body was crawling with large parasitic worms. The man survived, but the worms complicated his recovery and came as a surprise, in part because a soldier should be among the healthiest of the North Korean population. The New York Times reported that parasite experts were not as surprised, however; “Defectors to the South have cited the existence of parasites and abysmal nutrition. Because it lacks chemical fertilizers, North Korea still relies on human excrement to fertilize its fields, helping parasites to spread, the experts said.”

Human parasites, however, are by no means exclusive to North Korea. There are six major classes of human parasites, ranging from unicellular Protozoa to the uncommon but terrifying-looking Acanthocephala (Spiny Headed Worms) or gross-looking Cestoda (Tapeworms). Of those, three groups, the Protozoa, Trematoda (Flukes and certain flatworms), and Nematoda (Roundworms, like those found in the North Korean defector) are the most prevalent.

The presence of parasites may contribute to poverty, as productivity declines in a chronically ill population.

Human parasites are found on every continent. Some are widely distributed across Africa, Asia, and the Americas, while others are more localized. The greatest diversity of human parasites is found on the Eurasian continent, but when controlled for population the greatest concentration of human parasites is found in Africa. As people evolved in Africa and dispersed, our worst parasites stayed in Africa where they have evolved in concert with our species. We found new ones wherever we dispersed, but many were less lethal.

Humans are usually the final stop for parasites, rather than an intermediate host, but poor sewage treatment and disposal can spread the organisms to other humans. In many cases, humans acquire parasites through dirty water or infected food. Fish are a frequent source of tapeworms, for example. Other parasites seek out human hosts during certain life cycles and may enter through infected soil. Others are transmitted by vectors, typically insects—malaria and river blindness are transmitted in this manner.

Particularly insidious are parasites that alter the behavior of a host and ease transmission. Toxoplasmosis, common in many house cats, often enters the cats by causing mice to become more active and thus more likely to attract a cat’s attention. The cat can then transfer the parasite to its human companions, and there is some evidence that human behavior is affected as well. These behavior-altering parasites are hard to fully control and thus persist even in developed countries where public health systems have otherwise eliminated most parasitic diseases.

The good news is that parasitic diseases can be effectively eliminated. The presence of so many parasites in a North Korean soldier, who ought to be among the healthiest citizens, is a testament to poverty and undeveloped health systems in the isolated land. The presence of so many parasites may even contribute to poverty, as productivity declines in a chronically ill population. But there is nothing special about the North Korean environment that breeds parasites. With better investment in public health infrastructure, such as sanitation, parasites can be dramatically reduced.

JSTOR Citations


By: Armand M. Kuris

The Journal of Parasitology , Vol. 98, No. 6 (DECEMBER 2012), pp. 1056-1064

Allen Press on behalf of The American Society of Parasitologists

James MacDonald

James MacDonald received a BS in Environmental Biology from Columbia and a PhD in Ecology and Evolution from Rutgers University, spending 4 years in Central America collecting data on fish in mangrove forests. His research has been published in scholarly journals such as Estuaries and Coasts and Biological Invasions. He currently works in fisheries management and outreach in New York.

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