Hookworm refers to two different organisms: Necator americanus and Ancylostoma duodenale. There are two major distinctions between the two organisms. First, the geographic distribution of each organism varies slightly. Second, and more importantly for identification, the adult worms of each species differ slightly in morphology. The egg and larva stages, on the other hand, are virtually indistinguishable.
Necator americanus and Ancylostoma duodenale are commonly known as New and old World hookworm respectively causing diseases or condition such as Hookworm infection, ancylostomiasis, necatoriasis.
Hookworm infects nearly a quarter of the world’s population, according to estimates. Hookworm infection is common in warm areas where people practice poor sanitation, particularly when it comes to proper fecal treatment and disposal. Because all three organisms require the same soil conditions to survive, mixed infections with any combination of hookworm, Trichuris, and Ascaris are possible. The specific geographic locations of each hookworm species will be presented later. Those who walk barefoot in feces-contaminated soil are at risk of contracting hookworm in these areas.
North and South America are the primary habitats for N. americanus. This species is also known to exist in China, India, and Africa as a result of international travel.
Although A. duodenale was historically a parasite of the Old World, it has been transported to other parts of the world via modern world travel. A. duodenale can currently be found in Europe, China, Africa, South America, and the Caribbean.
Morphology of Hookworms
Egg of Hookworms
Although the only difference between the eggs of N. americanus and those of A. duodenale is size, the two genera are not usually distinguished solely on this basis.
- The average N. americanus egg is 60 to 75 m long, whereas the average A. duodenale egg is 55 to 60 m long.
- The width of both organisms ranges from 35 to 40 µm.
- Eggs recovered in freshly passed stool may be unsegmented or show a visible embryonic cleavage, usually at the two-, four-, or eight-cell stage.
- A thin, smooth, colorless shell provides protection for the developing worm.
- Because the size ranges of these two organisms are so close and the other characteristics are identical, recovered eggs are considered as indistinguishable and are usually reported as hookworm eggs.
The average immature, newly hatched hookworm rhabditiform larva measures approximately 15 by 270 µm. The actively feeding larva will, at a minimum, double in length, ranging from 540 to 700 µm, when it is only 5 days old. The presence of a long oral cavity known as a buccal cavity or buccal capsule, as well as a small genital primordium, distinguishes this morphologic form (i.e., a precursor structure to a reproductive system consisting of a clump of cells in an ovoid formation).
After the rhabditiform larva has completed its second molt, the infective, nonfeeding filariform larva emerges. There are two distinguishing features that aid in the identification of this morphologic form. First, the esophagus of this slender larva is shorter than that of Strongyloides stercoralis, a similar intestinal nematode. The hookworm filariform larva, on the other hand, has a distinct pointed tail.
The small adult hookworms, which are rarely seen, have a grayish-white to pink cuticle and a somewhat thick cuticle. The anterior end usually has a noticeable bend, which is referred to as a hook—hence the name hookworm. The hook is usually much more pronounced in adults of N. americanus than in those of A. duodenale, and it can be used to tell the difference between the two species.
Adult female hookworms typically measure 9 to 12 mm in length and 0.25 to 0.5 mm in width. Males are typically smaller, measuring 5 to 10 mm by 0.2 to 0.4 mm. A copulatory bursa is a prominent, posterior, umbrella-like structure that aids in copulation in males.
Adult N. americanus and A. duodenale worms are distinguished by differences in the composition of their buccal capsules. The buccal capsule of N. americanus contains two cutting plates, whereas the buccal capsule of A. duodenale is made up of actual teeth.
Life Cycle of Hookworms
Hookworm infects humans when third-stage filariform larvae penetrate the skin, particularly in unprotected areas such as the feet. The filariform larvae migrate to the lymphatic and blood systems once inside the body. The larvae are carried to the lungs by the blood, where they penetrate the capillaries and enter the alveoli. The larvae continue to migrate into the bronchioles, where they are coughed up to the pharynx, swallowed, and deposited in the intestine.
The larvae develop into adult hookworms in the intestine. The adults that result live and multiply in the small intestine. Adult females produce 10,000 to 20,000 eggs per day. Many of the resulting eggs are passed into the environment through the feces. First-stage rhabditiform larvae emerge from the eggs in 24 to 48 hours under ideal conditions (warm, moist soil). The larvae develop further by molting twice. Third-stage infective filariform larvae emerge, ready to start a new cycle.
Asymptomatic Hookworm Infection
Some people with a light hookworm burden do not show clinical symptoms. A well-balanced diet high in iron, protein, and other vitamins aids in the maintenance of this asymptomatic state.
Hookworm Disease: Ancylostomiasis, Necatoriasis
Patients who are repeatedly infected with hookworms may experience severe allergic itching at the site of hookworm penetration, a condition known as ground itch. A number of symptoms associated with larvae migration into the lungs are experienced by infected people, including sore throat, bloody sputum, wheezing, headache, and mild pneumonia with cough.
The symptoms of hookworm disease in the intestinal phase are determined by the number of worms present. Chronic infections with a light worm burden (defined as 500 eggs/g of feces) are the most common. These patients may have vague mild gastrointestinal symptoms, anemia, and weight loss or weakness.
Patients with acute infections (>5000 eggs/g of feces) may experience diarrhea, anorexia, edema, pain, enteritis, and epigastric discomfort. Furthermore, because adult hookworms compete with the human host for nutrients as they feed, infected patients may develop microcytic hypochromic iron deficiency, weakness, and hypoproteinemia. The massive loss of blood could lead to death.
Laboratory Diagnosis of Hookworm
The recovery of hookworm eggs in stool samples is the primary method for laboratory diagnosis. Larvae can develop and hatch from eggs in stool that has been allowed to sit at room temperature without the addition of a fixative. It is critical to distinguish these larvae from those of S. stercoralis in order to ensure proper diagnosis and subsequent treatment. The buccal capsule must be recovered and examined in order to identify the specific hookworm organism (i.e., whether it is A. duodenale or N. americanus).
Treatment of Hookworm Disease
Mebendazole and pyrantel pamoate are the drugs of choice for treating hookworm disease. Only iron replacement and/or other dietary therapy (including proteins, iron, and other vitamins) may be administered when indicated, particularly in people with asymptomatic hookworm infection.
Prevention and Control of Hookworm Disease
The prevention and control measures for hookworm are similar to those for A. lumbricoides. Proper sanitation practices, particularly appropriate fecal disposal, prompt and thorough treatment of infected people, and personal protection of people entering known endemic areas, such as covering bare feet, are all measures aimed at interrupting the hookworm life cycle.
Clinical Parasitology: A PRACTICAL APPROACH. Elizabeth A. Gockel-Blessing (formerly Zeibig) P. 217 – 221