Greg's Reviews > Parasites: Tales of Humanity's Most Unwelcome Guests

Parasites by Rosemary Drisdelle
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's review
Mar 03, 11

bookshelves: medical, science
Read in March, 2011

A truly comprehensive and unsettling book. Amoebas, protozoans, and insects capable of bringing whole nations to their knees. I'm always fascinated to hear about the ways natural organisms find to survive and reproduce. It is difficult to keep the risks in proportion, but it is important to know about the risks of drinking unfiltered water or traveling far away places. I would rather know about the guinea worm that may be eradicated by changing human behavior. I was troubled to hear about the round worm carried by raccoons. Much to think about as you scratch that itch.

I just finished reading "Parasites: Tales of Humanity's Most Unwelcome Guests". It had lots of interesting stories like how the hook worm was imported with slaves from Africa. The hookworms crawl no more than five feet away from the bushes used as a latrine and burrow in through bare feet. It turns out that they don't have much effect on people from Africa, but caused white southerners to become anemic. You might say that the hook worm is the cause of the stereotype of "po' white trash". For hundreds of years, no one could guess the cause. You might also say that the hook worm, imported because of slavery, contributed to its downfall by weakening the troops whose job it was to defend it.

Another story was about an endosymbiont bacteria called Wolbachia that produces certain nutrients needed by nematodes to survive and reproduce. There is a terrible disease called Onchocerciasis or river blindness. A nematode roundworm called O. volvulus is transferred to humans via the bite of a blackfly in the genus Simulium, and has infected thirty-seven million people in Africa. Numerous methods have been used to eradicate this disease which is the second highest infectious cause of blindness. Merck came up with an anti-parasitic drug called ivermectin. It doesn't kill the worms, but causes temporary infertility. Unfortunately the adult worms can live for 10 to 15 years, so the treatments need to be treated annually. Not only that, the black flies can spread the disease to new hosts. Resistance to ivermectin has appeared, with some females able to reproduce a few months after treatment. It turns out that the bacteria Wolbachia spp. live inside the cells and embryos of O.volvulus and other nematodes. The human disease is actually caused by the immune response to Wobachia which are exposed when the nematode dies. One useful approach may be to go after the Wolbachia with antibiotics. So far research hasn't revealed an antibiotic that will clear Wolbachia bacteria in human cases of onchocerciasis with less than three weeks of treatment.

Another round worm, B. procyonis, is picked up from the feces of raccoons. Studies reveal that raccoon infection with B. procyonis is rare, but the following is a cautionary note implying that raccoon presence around small children should be avoided. Adult worms live in raccoon intestines and may generate a hundred thousand eggs a day. Eggs enter the environment in raccoon droppings and mature in moist soil. Raccoons defecate in communal latrines such as the tops of stumps, large horizontal branches, and fallen trees. Small animals such as mice, rabbits, and birds may forage a raccoon latrine and incidentally swallow eggs of B procyonis. In these animals, the infection is deadly. Mature eggs hatch in the intestines and the larvae migrate through tissues into the bloodstream. The majority encyst near the head. A small percentage of the larvae invade the brain causing mice to "jump, run, and spin". Infected animals may fall prey to a hungry raccoon and thus pass the larvae on. Once swallowed, larvae develop into adult worms inside the raccoon intestine.

The people who get caught in the life cycle of B. procyonis are almost invariably small children, those most likely to stumble across a raccoon latrine and transfer eggs from hands to mouths. Immediate preventive treatment with antiparasitic drugs can avert disaster, but by the time the diagnosis is made, it is usually too late. Many victims die and the remainder are left with permanent brain damage. Migrating larvae may cause a rash on the face and trunk, respiratory symptoms, and enlarged liver. The patient develops a fever, with loss of coordination, sleepiness, and irritability. The illness progresses to seizures and coma.

The tough eggs of B. proconis remain infective for years, making a contaminated lawn almost impossible to clean up.

The book is full of similar stories, including a chapter on imaginary infections, so don't get too spooked by these samples stories.

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