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The Parasite That Crosses Divides

An Excerpt from 'False Positive'
By Theodore Dalrymple | July 18, 2019

November 23, 2017

Are smartphones a blessing or a curse? When you are in a restaurant and observe four people around a table not speaking to one another, their eyes glued to the little screens, you think the phones are a curse, destructive of true social life and real human contact. Indeed, schools around the world are trying to reduce their distracting effects by prohibiting their use during the school day.

But that is not the whole story, of course, and in this week’s Journal one can read of an ingenious, unexpected and unequivocally beneficial use of smartphones. Specifically, they are proving to be a highly useful and efficient tool in the campaign to eradicate river blindness in Africa. First, a little background is needed.

Onchocerciasis, or river blindness, is caused by a parasitic worm that goes by the charming name of Onchocerca volvulus. It is spread by the bite of little black flies, one of which goes by the equally grandiloquent name of Simulium damnosum. The flies breed in fast-flowing water.

A fertilized adult female Onchocerca worm, which lives up to fifteen years in its human host, produces tiny offspring, a thousand a day, called microfilariae. These pass into the Simulium fly when it—or rather she (the male of the species being vegetarian)—bites a person infected with the parasite. The microfilariae undergo development in the fly and eventually find their way into its saliva, by which they are transmitted back to a human host, where they migrate from the bloodstream to the skin tissue, where they mature into adult worms, and the whole process starts again. In a way, it is an admirable contrivance, though one wishes it could all have been to a better purpose.

Most of the microfilariae are not involved in reinfection (nature being wasteful of her progeny) and die in the human host, setting up an inflammatory reaction in the skin. If it occurs in the eye, it causes blindness. Onchocerciasis does not kill, but it blinds; it is the second commonest infective blindness in the world, now almost entirely confined to Africa.

Onchocerciasis does not kill, but it blinds; it is the second commonest infective blindness in the world, now almost entirely confined to Africa.

There have been ongoing efforts to eradicate river blindness for more than forty years, and not even the most mystical of modern nature-worshipping pagans object to the attempted elimination of Onchocerca volvulus as a species.1 Eradicating the disease can be done in two ways: via the fly or via the worm. Flies are more irrepressible than worms, however, and while initial efforts directed at Simulium damnosum appeared to be successful, the fly soon made a comeback. Fortunately, a drug was discovered that kills the microfilariae for about a year after ingestion; it is called ivermectin, and the discoverers were awarded a Nobel Prize in 2015. Ivermectin does not, alas, kill the adult worm that produces the microfilariae, but if everybody in an endemic area were to be treated once a year with ivermectin for fifteen years—the maximum life span of the worm—transmission of Onchocerca would be interrupted once and for all, and the disease eliminated.

Attempts at mass treatment with ivermectin were started, but unfortunately there was . . . I nearly said a fly in the ointment. In some areas, people were infected not only with Onchocerca but with another kind of filarial worm called Loa loa, which in adult form can live even longer in the human host, seventeen years. Loa loa is spread by another genus of flies, including Chrysops silacea. The worm causes an inflammatory reaction in the skin and can lead to very gross swellings; the adult worms move about and have a predilection for the eye. Infected people sometimes see a worm moving across their field of vision, and if it gets stuck there it has to be removed surgically. Incidentally, Loa loa does not seem to have any host other than humans, so the worm and its life cycle must have evolved with man. If anyone ever tells you that man is no different from the other animals, you can reply, “Oh yes he is, he is susceptible to parasitization by Loa loa.”

There have been ongoing efforts to eradicate river blindness for more than forty years, and not even the most mystical of modern nature-worshipping pagans object to the attempted elimination of Onchocerca volvulus as a species.

When ivermectin is given to people infected with Loa loa, a proportion of them suffer severe reactions and even die. Loa loa produces vastly greater numbers of microfilariae than does Onchocerca, and ivermectin duly kills them; but where they are extremely numerous, more than 30,000 per cubic millimeter of blood, their death may set up a severe and sometimes fatal inflammatory reaction around the brain. In a mass campaign of treatment with ivermectin in 1999 in an area of Cameroon, 500 people suffered from encephalitis, from which 60 died. Understandably, ivermectin was not used there again—until 2015, but with a new strategy.

The article in this week’s Journal relates how doctors and researchers returned to the same area of Cameroon to perform a new mass campaign with ivermectin. This time, before giving it to anyone, they checked the Loa loa filarial count in his blood, and they refrained from giving the drug to anyone with a count higher than 26,000 per cubic millimeter.

Participation in the campaign was impressive, given the persisting memory of the earlier one: 16,259 of 22,842 persons living in the area (71.2 percent) agreed to take part. Everyone who participated was treated except those who had too high a filarial count, or were in poor general health, or were pregnant or inebriated (what a humanizing picture that single word in the middle of a scientific paper conjures up!). All together, 737 people (4.4 percent of the population surveyed) were excluded, and of the remaining 15,552 people, 22 percent had onchocerciasis. No person treated had a serious reaction to the drug as in the previous campaign, and none died.

What really astonished me, however, was the manner in which the counts of Loa loa microfilaria in the blood were established. This is where smartphones enter the story: “The LoaScope, a mobile-telephone-based videomicrosope . . . was developed. With the use of a smartphone coupled to a simple optical device, the LoaScope automatically           counts L. loa microfilariae in peripheral blood collected in disposable rectangular capillary tubes without the need for sample processing.” How admirable—more than admirable—the ingenuity, and this time to what a worthy end! The people who used the LoaScope were trained in its use in an hour. This means it should be possible, in theory, to reduce the prevalence of onchocerciasis to very low levels and perhaps, with the addition of Simulium control, to eliminate it entirely. Of course, the price of onchocerciasis elimination is, if not eternal vigilance exactly, vigilance for fifteen years.

If anyone ever tells you that man is no different from the other animals, you can reply, “Oh yes he is, he is susceptible to parasitization by Loa loa.”

Even rural Africa now has telephone networks. (When I was in Africa, phoning anyone more than a few hundred yards away was a labor of Hercules, and often not possible at all, especially in the rainy season.) And notwithstanding the supposed legacy of colonialism so fondly emphasized in a thousand academic publications, the people of the area must have placed a considerable degree of trust in the interlopers who came to help them.

As we have just seen, not all the works of nature are benign from the human point of view. Not many of us are fond of slugs—least of all, gardeners; and while for some of us a rose may be a rose may be a rose, for most of us quite definitely a slug is a slug is a slug. Recently, however, I bought a book by a slug enthusiast in the hope of repairing my lamentable ignorance of these creatures. While most books about slugs teach you how to kill them, this one tells you how to preserve them. I have not yet gone this far.

Are slugs of medical interest? This week’s article in the NEJM series “Clinical Implications of Medical Research” discusses the mucusbased multicomponent glue that the dusky arion slug (Arion subfuscus) secretes when under threat. The secretion makes it difficult for predators to dislodge the slug.

Looking into the composition of this glue, researchers have developed a similar glue which they hope might serve as an adhesive in surgical operations, where sutures and metal clips have many disadvantages. This all strikes me again as admirably clever: first the observation of the secretion, then the leap of the imagination to think that it might have some useful application, then the elucidation of its structure, and finally the development of a synthetic analogue. What a piece of work is a man—and a slug, come to that.

1There is a highly amusing webpage satirizing modern nature-worship or mysticism, run by the shadowy Save the Guinea Worm Foundation, which is dedicated to opposing the eradication of another repellent parasite of humans that will soon be driven to extinction. The existence of such parasites is another example of the natural evil that theodicy is meant to explain, or (depending on your outlook) explain away.

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Theodore Dalrymple is a retired physician and psychiatrist. He is a contributing editor of City Journal and frequent contributor to the London Spectator, The New Criterion, and other leading magazines and newspapers.


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