Necator americanus
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| Necator americanus |
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Necator americanus is a species of hookworm (a type of helminth) commonly known as the New World hookworm. Like other hookworms, it is a member of the phylum Nematoda. It is a parasitic nematode that lives in the small intestine of hosts such as humans, dogs, and cats. Necatoriasis - a type of helminthiasis - is the term for the condition of being host to an infestation of a species of Necator. Since N. americanus and Ancylostoma duodenale (also known as Old World hookworm) are the two species of hookworms that most commonly infest humans, they are usually dealt with under the collective heading of "hookworm infection". They differ most obviously in geographical distribution, structure of mouthparts, and relative size.[1]
Necator americanus has been proposed as an alternative to Trichuris suis in helminthic therapy.[2]
Morphology
This parasite has two dorsal and two ventral cutting plates around the anterior margin of the buccal capsule. It also has a pair of subdorsal and a pair of subventral teeth located close to the rear. Males are usually 7–9 mm long, whereas females are about 9–11 mm long. The typical lifespan of these parasites is three to five years. They can produce between 5000 and 10,000 eggs per day.[3]
Life cycle
This worm starts out as an unembryonated egg in the soil. After 24–48 hours under favorable conditions, the eggs become embryonated and hatch. This first juvenile stage 1 is known as 'rhabditiform'. The rhabditiform larvae grow and molt in the soil, transforming into a juvenile stage 2. The juvenile stage 2 molts once more until reaching the juvenile 3 stage, which is also called 'filariform'; this is also the infective form. The transformation from rhabditiform to the filariform usually takes five to 10 days.[4] This larval form is able to penetrate human skin, travel through the blood vessels and heart, and reach the lungs. Once there, they burrow through the pulmonary alveoli and travel up the trachea, where they are swallowed and are carried to the small intestine, where they mature into adults and reproduce by attaching themselves to the intestinal wall, causing an increase of blood loss by the host. The eggs end up on the soil after leaving the body through the feces.[5] On average, most adult worms are eliminated in one to two years. The N. americanus life cycle only differs slightly from that of A. duodenale. N. americanus has no development arrest in immune hosts and it must migrate through the lungs.
Epidemiology
In the United States, 95% of human hookworm cases are caused by N. americanus, primarily in young school children in economically deprived rural areas. Juveniles cannot survive freezing temperatures so the highest prevalence occurs in areas with warmer temperatures and greater rainfall. The greatest incidence of infections occurs in Asia and sub-Saharan Africa especially in poverty-stricken areas with poor sanitation.[6] A. duodenale infections occur at a lesser rate and are seen primarily in Europe and the Mediterranean.[7]
Genome
A draft assembly of the genome of Necator americanus has been sequenced and analyzed.[8] It comprises 244 Mbp with 19,151 predicted protein-coding genes; these include genes whose products mediate the hookworm's invasion of the human host, genes involved in blood feeding and development, genes encoding proteins that represent new potential drug targets against hookworms, and expanded gene families encoding likely immunomodulator proteins, whose products may be beneficial in treating inflammatory diseases.
Treatment
Antimicrobial Regimen
- Necator americanus
- Preferred regimen: Albendazole 400 mg PO single dose[9]
- Alternative regimen (1): Mebendazole 100 mg PO bid or 500 mg daily for 3 days
- Alternative regimen (2): Pyrantel pamoate 11 mg/kg PO qd (maximum 1 g/day) for 3 days[10]
References
- ↑ Georgiev VS (May 2000). "Necatoriasis: treatment and developmental therapeutics". Expert Opin Investig Drugs. 9 (5): 1065–78. doi:10.1517/13543784.9.5.1065. PMID 11060728.
- ↑ Croese J, O'Neil J, Masson J; et al. (January 2006). "A proof of concept study establishing Necator americanus in Crohn's patients and reservoir donors". Gut. 55 (1): 136–7. doi:10.1136/gut.2005.079129. PMC 1856386. PMID 16344586.
- ↑ Roberts, Larry S., and John Janovy, Jr. Foundations of Parasitology. Seventh ed. Singapore: McGraw-Hill, 2006. Print.
- ↑ "Hookworm." CDC. 2009. CDC Online. 4 Dec. 2009. <http://www.dpd.cdc.gov/dpdx/HTML/Hookworm.htm>.
- ↑ "Hookworm disease." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 15 May. 2009 <http://www.britannica.com/EBchecked/topic/271350/hookworm-disease>.
- ↑ Hotez P, Bethony J, Bottazzi ME, Brooker S, Buss P (2005) Hookworm: “The Great Infection of Mankind”. PLoS Med 2(3): e67
- ↑ John, David T. and William A. Petri, Jr. Markell and Voge’s Medical Parasitology: Ninth Edition. St. Louis: Saunders Elsevier, 2006.
- ↑ Tang YT, Gao X, Rosa BA, Abubucker S, Hallsworth-Pepin K, Martin J, Tyagi R, Heizer E, Zhang X, Bhonagiri-Palsikar V, Minx P, Warren WC, Wang Q, Zhan B, Hotez PJ, Sternberg PW, Dougall A, Gaze ST, Mulvenna J, Sotillo J, Ranganathan S, Rabelo EM, Wilson RK, Felgner PL, Bethony J, Hawdon JM, Gasser RB, Loukas A, Mitreva M (Mar 2014). "Genome of the human hookworm Necator americanus". Nat Genet. 46 (3): 261–9. doi:10.1038/ng.2875. PMID 24441737.
- ↑ Keiser J, Utzinger J (2008). "Efficacy of current drugs against soil-transmitted helminth infections: systematic review and meta-analysis". JAMA. 299 (16): 1937–48. doi:10.1001/jama.299.16.1937. PMID 18430913.
- ↑ Bennett, John (2015). Mandell, Douglas, and Bennett's principles and practice of infectious diseases. Philadelphia, PA: Elsevier/Saunders. ISBN 978-1455748013.
Further reading
- Hotez, P.J., and D.I. Pritchard. 1995. Hookworm infection. Sci. Am. 272:68-74.
- Murray, P.R., K.S. Rosenthal, and M.A. Pfaller. 2009. Medical Microbiology, 6th ed. Elsevier/Mosby Publishing Company, Philadelphia, U.S.A.,865 p.
- Schmidt, G.D., and L.S. Roberts. 2009. Foundations of parasitology, 8th ed. McGraw-Hill Companies, New York, p. 472-473.