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Around the world, even in 2016, about 450 million people are infected with hookworms. Before 1980, up to 15 percent of the population of the southeastern United States was also infected with the worm. Hookworms are common wherever sanitary conditions are suboptimal and people often go barefoot.
The Creepy Life Cycle of One of the World's Most Common Bloodsucking Parasites
Female hookworms release their eggs inside the human small intestine. These eggs are swept away in feces, and if the fecal material lands on open ground, they hatch into infectious larvae, feeding on the feces in which they were laid. They rapidly from blob-like hatchling to something that looks like a tiny string of vermicelli capable of infecting any human with which it comes in contact for five minutes or more.
Hookworm larvae burrow into the skin, sometimes causing a local skin inflammation known as ground itch. Ground itch can be intensely itchy, but many people feel nothing at all. The larvae enter the bloodstream and after about 10 days find their way into the lungs, where they burst through the linings of the alveoli. At this stage there can be cough and fever, a severe complication known as rLöffler syndrome with shortness of breath and pneumonia-like symptoms, or, again, no symptoms at all.
From the lungs hookworms climb into the throat, waiting to be swallowed, so they can swim down to the small intestine. Once in the gut, they molt again and develop teeth (or, in the case of North American hookworms, cutting plates) that allow them to cling to the sides of the intestine. They release an enzyme called hyalouronidase, which eats through the lining of the gut so the worms can feed on human blood. This can result in obvious symptoms such as diarrhea, vague abdominal pain, colic, flatulence, nausea, or loss of appetite, or it might just result in poor absorption so that the infected person (especially if a child) is malnourished despite being provided a good diet. The hookworm then usually dies of "old age." Most of the world's varieties of hookworms live about a year, although North American hookworms live 5 years.
A mature female hookworm living in the human intestine releases from 5,000 to 30,000 eggs every day. When infected people relieve themselves outdoors, hookworm eggs lie in wait for their next human host. Hookworms in dogs and cats can also infect people through the skin, but they do not usually mature from larvae into reproducing adults. Hookworm symptoms in dogs and cats are analogous to those in people, but most people who catch hookworm from their pets will only develop a rash, not the lung or intestinal symptoms.
Defeating Hookworm Symptoms Seems to Result in Autoimmune Symptoms
Getting rid of hookworms with simple sanitary measures and anthelminthic drugs such as albendazole and mebendazole would seem to be an unquestionably good thing, but for many years scientists have observed that when hookworms are wiped out, immune system conditions like asthma and celiac disease become more common. Scientists have even develop a "hygiene hypothesis" that explains the growing frequency of these diseases with "too much" sanitation. But why should getting hookworms make some people sicker and other people healthier?