Taxonomy of Hookworms

Taxonomic classification:
All hookworms infecting humans are Class Nematoda, Order Strongylidea, Family Ancylostomatidae, and Genera Ancylostoma and Necator.

Humans are the definitive host (ie, adult parasite reproduces in this host) for three species of hookworm:
Ancylostoma duodenale
Ancylostoma ceylanicum
Necator americanus

Humans can also become infected with the dog hookworm, Ancylostoma caninum. However, this species is infertile in humans.

Gross appearance:
Adult hookworms are white and about 1.2 cm long. When they are alive, they are semitransparent and if the gut is full of blood, they have a central red, longitudinal streak (Fig. 1). The anterior end is bent dorsally (ie, equivalent to, if you were a hookworm, of having your head bent right back so that you looked backwards). This gives the body a "hooked" or J shaped appearance. Females have a tapered narrow posterior end, while males have a "feathery" posterior end owing to their copulatory bursa. Hookworm species can not be determined from the gross appearance.

Fig. 1: N. americanus in small intestine. The larger worm on the right is a female with the amorous male on the left. A video of a N. americanus as seen with capsule endoscope can be viewed on the Hookworm page.

Significant microscopic features:
Species of adult hookworms can be identified by the appearance of their mouths. Structures are bilaterally symmetrical (mirror image on each side).
Ancylostoma duodenale has 2 prominent pointed ventral teeth and rarely a very tiny third tooth (Fig. 2)
Ancylostoma ceylanicum has a cutting plate with a sharp dorsal end that looks like a tooth and a less distinct sharp ventral end (Fig. 3).
Ancylostoma caninum has three prominent pointed ventral teeth (Fig. 4)
Necator americanus is completely different and has a rounded ventral cutting plate (Fig. 5).

If adult hookworms are collected either from the gut during endoscopy or from faeces after treatment, the species can be identified. Hsu and Lin (2012) described a case report of A. ceylanicum infection identified from a worm collected at endoscopy. John Croese identified many cases of infection with A. caninum in north Queensland by recovering adult parasites at colonoscopy (Croese et al 1994).

Fig. 2: Apical view of the mouth of A. duodenale showing the 2 prominent pointed ventral teeth on each side. Image from http://parasito.montpellier-wired.com/nemato/ancylostoma_duodenale.jpg.

Fig. 3: Apical view of the mouth of A. ceylanicum showing the lateral cutting plate with pointed ends dorsally and ventrally. Image from http://www.sciencephoto.com/media/366462/enlarge.

Fig. 4: Apical view of the mouth of A. caninum showing the 3 prominent pointed ventral teeth on each side.

Fig. 5: Apical view of the mouth of N. americanus showing the rounded ventral cutting plate. Image from http://www.infectionlandscapes.org/2012/02/hookworm.html. (The parasite named "A. duodenale" at this site has 3 prominent pointed ventral teeth and is more likely to be A. caninum).

Indentification from faecal stages:
All hookworm eggs look alike: oval, thin shelled, colourless, containing a morula of 8 cells when laid (Fig.6). Size is approximately 60 µm by 35 µm. Hence, the species a person is infected with can not be determined from the egg.

Fig. 6: Hookworm egg in 8 cell stage. This was an experimental infection with N. americanus. However, species can not be identified from the morphology of the egg.

A. duodenale and N. americanus infective larvae (iL3) have different morphologies and these species can be identified from the iL3 obtained after 7 days from a faecal culture (Fig.7). There appears to be no description of the morphology of the iL3 of A. ceylanicum.

Fig. 7: Infective larvae of N. americanus from Harada-Mori culture. Note the transparent sheath that protects the larvae.

Molecular biology:
Species can be determined by PCR and molecular techniques, using ITS gene (Gasser et al 1996, Monti et al 1998) or cytochrome oxidase gene (Zhan et al 2001). This can be done from adults, larvae or even eggs. Molecular biological techniques can add to the standard techniques in population surveys for hookworm and allows for more sophisticated field based studies that can determine prevalence, intensity and the species of hookworm. In a recent study (Sato et al 2010) the study population in Loa PDR was shown to be infected with A. duodenale, N. americanus, A. ceylanicum and the zoonotic hookworm A. caninum. Mixed infections with all three human hookworms have been described previously in Thailand using identification of adults collected from faeces after treatment. PCR is a much less demanding technique!

References:
Croese J, Loukas A, Opdebeeck, Fairley S, Prociv P. Human enteric infection with canine hookworm. Annals of Internal Medicine 1994;120(5):369-374.

Gasser RB, Stewart LE, Speare R. Genetic markers in ribosomal DNA for hookworm identification. Acta Tropica 1996;62(1):15-21.

Hsu Y, Lin J. Intestinal infection with Ancylostoma ceylanicum. New England Journal of Medicine 2012;366:e20.

Monti JR, Chilton NB, Qian BZ, Gasser RB. Specific amplification of Necator americanus or Ancylostoma duodenale DNA by PCR using markers in ITS-1 rDNA, and its implications. Molecular and Cellular Probes 1998;12(2):71-78.

Sato M, Sanguankiat S, Yoonuan T, Pongvongsa T, Keomoungkhoun M, Phimmayoi I, Boupa B, Moji K, Waikagul J. Copro-molecular identification of infections with hookworm eggs in rural Lao PDR. Transactions of the Royal Society of Tropical Medicine and Hygiene 2010;104(9):617-622.

Zhan B, Li T, Xiao S, Zheng F, Hawdon JM. Species-specific identification of human hookworms by PCR of the mitochondrial cytochrome oxidase I gene. Journal of Parasitology 2001;87(5):1227-1229.

Page by Rick Speare 15 July 2012