Ancylostoma duodenale and Necator americanus
Background. The hookworms of humans are of great importance in the tropics and subtropics. Human hookworm disease is a common helminth infection that is predominantly caused by the nematode parasites Necator americanus and Ancylostoma duodenale; organisms that play a lesser role include Ancylostoma ceylonicum, Ancylostoma braziliense, and Ancylostoma caninum. The two species (Necator americanus and Ancylostoma duodenale) belong to the order Panagrolaimida and their second superfamily Strongyloidea (not to be confused with Strongyloidoidea) and the family Ancylostomatidae. Their members are called “bursa nematodes” because of the male copulatory apparatus, the conspicuous bursa copulatrix.
Morphology and Life cycle. The worms are curved at the anterior end to give them the shape of a hook. They have a large mouth cavity with a skewed opening armed with teeth or cutting plates. The eggs are thin-shelled as in all Strongyloides. The copulatory bursa has short ribs and is relatively small. The two spicules are long and thin. The adult females are about 12 x 0.6 mm and the males are 10 x 0.45 mm. An adult female produces about 28,000 eggs per day which pass out of the body via the faeces. Once the eggs make contact with warm moist soil they embryonic. Within 48 h the first stage larvae (rhabditiform) hatch out and feed on soil bacteria and debris. After two further molts while in the soil they develop into the infective stage, the filariform larvae. These larvae crawl into a position, such as on blades of grass or a point high enough to make contact with humans. The filariform larvae can actively penetrate skin through hair follicles or damaged skin. Once they reach the dermal layers of the skin they first migrate along through the dermal layers and eventually enter a blood vessel. After molting into the fourth stage larvae they enter into the bronchi of the lungs. From the lungs, they crawl up the throat and are swallowed and the fifth stage larvae emerge into the gut. Adult females produce eggs within about 40 days. The production of eggs is higher in A. duodenale (25,000 eggs/day) than in N. americanus (10,000 eggs/day).
Infection. Hookworm infection is acquired through skin exposure to larvae in soil contaminated by human feces. Soil becomes infectious about 9 days after contamination and remains so for weeks, depending on conditions. They inhabit the mucosa of the small intestine. They become embedded within the villi and use their biting mouthparts to feed off blood. They are able to secrete an anti-coagulant to keep the blood flowing. Infection with A. duodenale can also occur by oral uptake of larvae. In this case, a body migration does not occur.
Epidemiology. About 900 million humans are currently infected, 50,000–60,000 per year die of the disease they cause. A. duodenale, commonly known as the Old World hookworm, prevails in subtropical countries of the Old World (Middle East, North Africa, India, and formerly Southern Europe) and N. americanus predominates in tropical regions (the Americas, sub-Saharan Africa, Southeast Asia, China, and Indonesia).
CUTANEOUS LARVA MIGRANTS
Background. Cutaneous larva migrans or creeping eruption is a skin disease in humans caused by the larvae of the genus in dogs and cats, Ancylostoma braziliense, Ancylostoma ceylanicum, Ancylostoma tubaeforme, and (more rarely) Ancylostoma caninum. The larvae can invade the skin of humans, but cannot develop into adults. During the lung phase, these parasites have an effect upon the host similar to Ascaris. All ascarid larvae have a compulsory migratory route which starts where the adult parasite eventually ends up, ie the lumen of the ileum. It is interesting to speculate as to why the larvae appear to be compelled to undergo the migratory phase. The disease, also known as ground itch, is an intense itching eruption, which may become very painful and, if scratched, may develop a secondary bacterial infection. The itching ceases when the parasites die. The creeping eruption, especially by A. braziliense, is one of the most common tropically acquired dermatoses.
Epidemiology The L3 larvae need warm, humus-rich soil for their development. They do not tolerate direct sunlight, water, and urine. Ideal conditions exist where defecation takes place near dwellings and working environments, where fertilization with untreated excrement is undertaken and where people walk barefoot. Children in such areas are particularly exposed to infection. Interestingly, A. duodenale was prevalent in coal mines of Central Europe in the past, where the necessary warm temperatures were present in the mines. An explanation for the migration of ascarid larvae could be that each larval stage requires a different physiological and biochemical environment, and the different organs encountered en route represent intermediate hosts. Alternatively, migration could be an escape from the host’s immune system.
Treatment. Anthelminthic medications (drugs that rid the body of parasitic worms), such as albendazole and mebendazole, are the drugs of choice for treatment of hookworm infections. Most cases of classic hookworm disease can be managed on an outpatient basis with anthelmintic and iron therapy, complemented by appropriate diet. Infections are generally treated for 1-3 days. The recommended medications are effective and appear to have few side effects. For patients with cutaneous larva migrants who have minimal symptoms, specific anthelmintic treatment may be unnecessary.
References:
Centers for Disease Control and Prevention. Parasites – Hookworm. [Updated:January 10, 2013]. Available from: https://www.cdc.gov/parasites/hookworm/treatment.html
Chernin, J. Parasitology – Lifeline (Modules in life sciences). CRC Press, London: 2000.
Lucius R., Brigitte Loos-Frank, Richard P.L., Robert P., Craig W.R., and Richard K.G. The Biology of Parasites. Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, German: 2017.
