An Introduction to Nematology, Helminthology, and a Look at One Parasitic Nematode Dubbed the Eye Worm

Nematodes are worms, but not the sort you’d think about upon hearing the word. Worm is a broad description given to invertebrates with soft, slender, elongated bodies. You’re quintessential worm is from the phylum Annelida—they have the familiar segmented bodies and are much larger than Nematodes. On average nematodes are 2.5mm in length. They have non-segmented bodies that are tapered at each end, and while there are free-living varieties, a good number are parasites. They are ubiquitous, and humans are grossly outnumbered by these creatures. In fact, everything on the planet is outnumbered by nematodes as they account for 80% of all the individual animals on earth.

“In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.”—Nathan Augustus Cobb in Nematodes and Their Relationships, 1915

Cobb (1859-1932) is known as the father of nematology in the United States, specifically gaining recognition for his work with the United States Department of Agriculture. He laid the foundation for nematode taxonomy, and contributed major discoveries in morphology and methodology as well. His numerous papers on the subject were complimented with thorough illustrations. Cobb himself was a gifted self-taught artist and illustrator, but his collaboration with William Chambers distinguished his work among his peers. Chambers’ art is said to have had a profound influence on the quality of illustrations throughout the world of nematology. This lead to a more organized classification identification system, furthering their study.

Years later, across the pond Robert Thomson Leiper (1881-1969) was referred to as the father of modern helminthology. Helminthologists, like Leiper, focus on the study of parasitic worms spanning a variety of phyla, including Nematoda. Leiper was devoted to his work, and his scientific attainments fell into two categories—taxonomy and life cycles. The journey to modern helminthology was filled with expeditions to the tropics, though not confined to them. The conditions for these explorers were less than ideal. They were filled with mosquitos, flies and, ticks—vectors of many horrible diseases, and of course the inevitable dysentery. However, Leiper’s discoveries disentangled many important taxonomic puzzles including the vector of the filarial worm Loa loa. His blunderbuss methodology for solving this mystery involved letting various insects feed on an infected patient. He’d then grind up the insect and inspect it for the worm’s early microscopic form. A deer fly common to the area of Calabar turned out to be the culprit. He was able to repeat the experiment, but due a seasonal shortage of the fly he had to abort his study. It was confirmed a decade later by a husband and wife duo (Connal) who detailed the development of the larvae within the fly.

The diurnal deerfly Chrysops silacea, Chrysops dimidiata, and consequently L. loa, are indigenous to rainforests in parts of Western and Central Africa. There have been documented cases of Loiasis with serious symptoms, but often patients experience only minor discomforts. Itchy swellings that came to be named for the region they were first documented in, Calabar, can come and go. The most dramatic and alarming clinical manifestation of Loiasis happens when the adult worm wriggles under the skin or across the eye. The first documented account of the worm was published in 1770 by a French surgeon named Mongin:

“Saw a worm which seemed to crawl superficially on the eye, but when I tried to catch it with forceps, I realized that it was between the conjunctiva and the cornea. When it approached the transparent cornea, the pains became more severe…it was one and a half inches long, the width of a violin string, and dark colored.”

Diagnosis is simple when such obvious outward symptoms present themselves, but that’s not always the case. Blood collection and examination is a practical procedure for the identification of L. loa’s microfilariae, the early larval form present in human stages of development. However these microfilariae have a diurnal periodicity, so sample collection must be completed between 10 AM and 2 PM to detect them.

Another complication arises in treatment. There is an overlap in the habitat of L. loa and another parasitic worm Onchocerca volvulus. This parasite causes a more serious disease than Loiasis, known as Onchocerciasis or River Blindness—the second most common cause of infectious blindness. The good news is that there is a drug called Ivermectin that can eliminate it. The complication I mentioned? It can prove deadly if the patient has a co-infection of Loiasis. In these cases, it is crucial to investigate the levels of L. loa microfilariae in the patient. Due to this, in L. loa endemic areas, mass drug administration (MDA) programs have been suspended leaving the local population at risk for River Blindness.

A rapid and point-of-care solution is needed to re-establish the MDA program for onchocerciasis. The current methodology is not practical for field settings, and the output is too low. A research team lead by UC Berkley engineers have recently developed a phone-based video microscope that can automatically quantify L. loa microfilariae. This eliminates the need for slide examination by trained technicians, other time-intensive methods, and cumbersome equipment. They have completed a pilot study of the Cellscope Loa with great success—getting the same results on the phone as with the current gold standard of diagnosis. According the researchers, this “Test and not Treat” Cellscope Loa strategy is “an extremely promising and practical approach to the safe implementation of large-scale treatment for Loaisis and onchocerciasis in L. loa co-endemic areas.”




Loa loa endemic areas (light gray).

Life-cycle of Loa loa. Chrysops fly bites the human host and third stage L. loa enter, and mature into adults (A) within about one year. Adult worms (left/female, right/male) live for about 4 to 12 years. Worms mate, eggs mature within the female (B) and produce microfilariae (C) which circulate in the blood stream, and are then picked up by another fly. The microfilarie shed their sheath (D) and enter fat bodies within the fly. They mature into sausage-like forms (E), and then into infective third stage larvae (F). These larvae migrate into the fly’s proboscis and are deposited into another human host when they take a blood meal. Repeat.


Photograph of microfilariae (630X).

Resources

American Society of Tropical Medicine and Hygiene Abstract Book. Vol. 95. N.p.: n.p., 2016. ASTMH. Web. Abstract No. 1115, Cell Scope-Loa, Sebastien Pion, et al. Web.

Cobb, Nathan Augustus. Nematodes and their relationships. Washington: G.P.O., 1915. Print.

Cox, F. E. G. “History of Human Parasitology.” Clinical Microbiology Reviews. American Society for Microbiology, Oct. 2002. Web.

D’Ambrosio, Michael V. “Point-of-care quantification of blood-borne filarial parasites with a mobile phone microscope.” Science Translational Medicine 7.286 (2015): 286-90. Web.

Garnham, P. C. C. “Robert Thomson Leiper. 1881-1969.” Biographical Memoirs of Fellows of the Royal Society 16.0 (1970): 385-404. Royal Society Publishing. Web.

Grove, David I. A history of human helminthology. Wallingford, Oxon, UK: C.A.B. International, 1990.

Harrison, Richard. “Smartphones Can Be Smart Enough To Find A Parasitic Worm.” All Things Considered. NPR, 6 May 2015. Web.

Maggenti, A.R. General nematology. New York a.o: Springer, 1981. Print.

“Parasites – Loiasis.” Centers for Disease Control and Prevention. N.p., 20 Jan. 2015. Web.

“Parasites – Onchocerciasis (also known as River Blindness).” Centers for Disease Control and Prevention. N.p., 10 Aug. 2015. Web.

Smith, D. Scott. “Loiasis.” Life Cycle and Morphology. Stanford University, n.d. Web.

Yang, Sarah. “Smartphone video microscope automates detection of parasites in blood.” Berkley News. UC Berkley, 5 May 2015. Web.