Hookworm - Ancylostoma duodenale

·       Widely distributed in tropical and sub-tropical countries

·       One of the small intestine nematodes

·       Cause intestinal hookworm disease in humans

·       Important cause of Iron deficiency anaemia

 Belongs to family Ancylostomatidae- 2    important human Spp:

·     Ancylostoma duodenale -named so, since anterior end of adult worm is bent -(Ankylos (Greek)=hooked, stoma=mouth) /Old world hookworm

·       Necator americanus (American Murderer)/New World hookworm

 Ancylostoma duodenale

·       Causes ancylostomiasis

·       Dubini in 1843 described the parasite;  Arthur Loss in 1898 described pathogenesis and mode of infection

Epidemiology

·       Globally 900 million people infected. 

·      Hookworm infection is prevalent in the tropics and subtropical countries – Asia, Africa, America, China and Southern Europe

·       In India – prevalent in Punjab, Uttar Pradesh and Bihar

·       Males & young adults commonly affected

·       Anaemia severe in children & pregnant women

 

Morphology

 Adult Worm:

·       small, stout, greyish white and cylindrical

·       Body is curved, dorsal surface concave and ventral surface convex,

·       Anterior end- bent dorsally - hence name hookworm

·       Oral aperture- directed towards dorsal surface

·       Mouth located dorsally-prominent Buccal capsule- has 6 teeth, 4 hook-like on the ventral surface and 2 knob-like on the dorsal surface. 

 Male worm differs from female worm

 

Adult male worm

Smaller- 8 to 11 mm long x 0.4 mm wide. Posterior end expanded into a copulatory bursa. Situated within the copulatory bursa, is the cloaca into which the rectum and genital canal open. There are two long retractile bristle-like copulatory spicules, which project from the bursa.

Copulatory bursa -Present in male, For attachment with female during copulation. Consists of 3 lobes, one dorsal and two lateral lobes. Each lobe: supported by 13 chitinous rays.

Adult female worm

Larger /longer than male 10 to 13 mm long x 0.6 mm wide. Posterior end is conical, with a subterminal anus situated ventrally. Vulva opens ventrally at the junction of the middle and posterior thirds of the body. Vagina leads to two intricately coiled ovarian tubes which occupy the hind and middle parts of the worm.

During copulation, the male attached its copulatory bursa to the vulva- copulating pair appears Y-shaped.

Life span of adult worm in human intestine: 3 to 4 years.

Differences between Adult male and female A. duodenale

Characteristic	Male	Female
Size	Smaller- 8 to 11 mm long x 0.4 mm wide	larger /longer than male 10 to 13 mm long x 0.6 mm wide
Posterior end	Expanded in an umbrella like fashion - as copulatory bursa	Tapering and no expanded bursa
Genital opening	Posterior and opens with cloacae	Present at junction of posterior and middle third of body

 

Egg

·       Oval/elliptical; 65 µm long x 40 µm wide

·       Colourless- non-bile stained

·       Surrounded by a thin transparent hyaline shell-membrane

·       Contains a segmented ovum with four (4) blastomeres

·       Clear space between egg shell and segmented ovum.

·       Egg floats in saturated salt solution

·       Single female worm- lays about 25 000 to 30 000 eggs/day

·       Eggs are excreted in faeces 4-7 weeks after infection

 

Life cycle of Hookworm

Definite host: man

Habitat: Small intestine of infected persons, mostly in jejunum- less often in duodenum and ileum

Infective Stage: Filariform larva

·      

 Eggs are passed in the stool   , and under favourable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days and become free-living in contaminated soil.

These released rhabditiform larvae (250 µm in length- feeds on bacteria and other organic matter in the soil) grow in the feces and/or the soil, and after 5 to 10 days (and two moults) they become filariform (third-stage) larvae that are infective. 

 Filariform (third-stage) larvae (500-600 µm long- non-feeding) live in the soil for 5-6 weeks. These infective larvae can survive 3 to 4 weeks in favourable environmental conditions.

On contact with the human host, typically bare feet, the larvae penetrate the skin and are carried through the blood vessels to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed. They undergo third moulting during migration or on reaching the oesophagus. The final fourth moulting occurs in small intestine, they develop the buccal capsule, attaches to small intestine, where they reside and mature into adults.

Adult worms live in the lumen of the small intestine, typically the distal jejunum, where they attach to the intestinal wall with resultant blood loss by the host.

 Most adult worms are eliminated in 1 to 2 years, but the longevity may reach several years.

 Some A. duodenale larvae, following penetration of the host skin, can become dormant (hypobiosis in the intestine or muscle). These larvae are capable of re-activating and establishing patent, intestinal infections. 

Infection by A. duodenale may probably also occur by the oral and the trans mammary, transplacental route. 

 

    In small intestines: Adult worm -   inhabit small intestine of man-   Attachment to mucous membrane by help of mouth parts

·       Eggs contain segmented ova with 4 blastomeres passed out in faces of infected person (non-infective)

  In soil: Rhabditiform larva hatches from each egg (24- 48 hours) -moults twice on 3rd and 5th day-   develops into filariform larva-is the infective form- penetrates unbroken skin (of toes, dorsum of foot and medial aspect of sole) -Remains infective up to 6 weeks

  

N. americanus has similar morphology (though smaller in size) and life cycle. However, they do not appear to be infective via the oral or trans-mammary route.

People at risk

·       Walking barefoot on soil containing filariform larva

·       Farm workers: larva penetrates skin of hands -reaches subcutaneous tissues and enters lymphatics or small venules- heart - pulmonary capillaries- - migrates to bronchi, trachea and larynx- pharynx - gets swallowed. During migration through oesophagus, undergo 3rd moulting.

·       Settles in small intestines-undergo 4th moulting and develop into adult worms. -Attach to small intestine by their mouth parts -After 6 weeks- mature sexually -Fertilization occurs -Female begin to lay eggs in faeces -cycle repeats

      Pathogenesis 

The overall prevalence and intensity of hookworm infection are higher in males than in females, because males have greater exposure to infection. However, women and young children have the lowest iron stores and are most vulnerable to chronic blood loss as the result of hookworm infection.

The most serious effects of hookworm infection are the development of anemia and protein deficiency caused by blood loss at the site of the intestinal attachment of the adult worms. 

dyspnoea-difficult or laboured breathing

koilonychia- a nail disease that can be a sign of hypochromic anemia, especially iron-deficiency anemia

Loeffler syndrome - a rare, transient, self-limiting (less than one month), and benign pulmonary eosinophilia 

Adult hookworms cause morbidity in the host by producing intestinal hemorrhage. The adult hookworms then ingest the blood, rupture the erythrocytes, and degrade the hemoglobin. Therefore, the disease causes silent blood loss leading to iron deficiency anemia and protein malnutrition.

Diagnosis

The standard method for diagnosing the presence of hookworm is by identifying hookworm eggs in a stool sample using a microscope. Because eggs may be difficult to find in light infections, a concentration procedure is recommended.

The diagnostic approaches include:

Occult blood- not easily seen with the naked eye-due to  bleeding in the digestive tract

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. Infections are generally treated for 1-3 days. Morbidity can be reduced through anthelminthic drug treatment (also known as “deworming”).

Iron supplements may also be prescribed if the infected person has anemia.

Prophylaxis

The best way to avoid hookworm infection is not to walk barefoot in areas where hookworm is common and where there may be human faecal contamination of the soil. Also, avoid other skin contact with such soil and avoid ingesting it. Use footwear and gloves.

Infection can also be prevented by not defecating outdoors and by effective sewage disposal systems.

Treatment of patients and carriers, limit the source of infection.

The global disease burden from hookworm exceeds all other major tropical infectious diseases with the exception of malaria, leishmaniasis, and lymphatic filariasis. 

Hookworm infection is a major health threat to adolescent girls and women of reproductive age, with adverse effects on the outcome of pregnancy. Severe iron-deficiency anemia during pregnancy has been linked to increased maternal mortality, impaired lactation, and prematurity and low birth weight. Infants and preschool children are particularly vulnerable to the developmental and behavioural deficits caused by iron-deficiency anaemia. In addition, hookworm has been associated with impaired learning, increased absences from school, and decreased future economic productivity.