Hookworm larvae cannot be distinguished from one another hence, the general term "hookworm infection" is used to connote infections caused by the different species of hookworm, notably Necator americanus and Ancylostoma duodenale, in humans. Like chapters 3 and 4, this chapter will touch on the epidemiology, mode of transmission, life cycle, pathology, clinical manifestations, and complications of the 2 most common hookworms in men.

Hookworms are blood-sucking nematodes that attach themselves to the mucosa of the small intestine (jejunum) by means of cutting plates or ventral teeth which they use to bite the mucosa.

The 2 most common hookworms are:

1. Necator americanus
2. Ancylostoma duodenale

• Prevalence rate is 5-45%, depending on specific demographic conditions
• 96% of infections is caused by N. americanus
• 2% of infections is caused by A. duodenale
• 2% are mixed infections
• Prevalence rate increases with age
• Man is the only important source of human hookworm infection
• Hookworm infection is higher in agricultural areas, where farmers are more prone to infection because they are often in contact with the soil
• Hookworm infection is dependent on 3 environmental factors:

     1.      The suitability of environment for eggs and larvae

     2.      The mode and extent of fecal pollution in the soil

     3.      The mode and extent of contact between infected soil and skin or mouth

• Skin penetration by filariform larva (N. americanus and A. duodenale)
• Ingestion of filariform larva (A. duodenale)
• Among breast feeding women, incidence of infection by ingestion of filariform larva through breast milk is increased. Circumstantial evidence in China indicates that trans-mammary transmission of A. duodenale is possible (WHO/CTD/SIP/96.1 Report of the WHO Informal Consultation on HW infection and anemia in girls and women. Geneva, Dec 1994).

The EGG

• The hookworm egg is ovoidal, thin-shelled, and colorless
• It measures 56-60 x 34-40 mm
• It is in 4-8 cell stage in the feces
• It is in delayed transit time, embryo may already develop inside the shell
• It is difficult to differentiate necator egg from ancylostoma egg

The Larva

1. The 1st stage rhabditidiform larva or feeding stage

• It hatches in 24-48 hours in the soil
• It is short and stout with long and narrow buccal chamber, a flask-shaped muscular or bulbous easophagus, and a very small genital primordium
• It feeds on bacteria and organic matter present in the feces or soil
• It molts in 2-3 days to become 2nd stage larva

2. The 2nd stage larva or transitional stage

• It is less constriction of the easophagus

3. The 3rd stage filariform larva or non feeding stage

• The larva is longer and slender with pointed posterior end
• The mouth closes and easophagus elongates
• The molting cuticle is retained as a protective sheath covering the larva
• It is the infective stage

The Adult Worm

1. Necator americanus

• The relatively stout with cervical curvature that appears like a hook, hence the name hookworm has well developed buccal capsules characterized by the presence of a pair of semi-lunar cutting plates
• The life span is up to 5 years Source: Product monograph (Zentel)
• The eggs/day/worm is 5000-10,000
• blood intake/day/worm is 0.01-0.02 ml
• The female worm is longer than the male and measures 9-11 x 0.4 mm and blunt posterior end
• The male worm measures 7-10 x 0.3 mm and it expanded posterior end that is fan-like bursa copulatrix used for copulation

2. Ancylostoma duodenale

• It is slightly reddish-white worm
• The big mouth located on the dorsal side of its anterior end
• The buccal capsules have 2 pairs of ventral teeth and a pair of accessory teeth
• The body contour tends to follow the general curvature of the body, hence it looks like the letter "C" Photo courtesy of Dr. Peter W. Pappas
• The life span is up to 8 years
• The eggs/day/worm is 20000-30000
• blood intake/day/worm is 0.1-0.2 ml
• The female worm measures 10-13 x 0.6 mm and the posterior end is straight
• The male worm measures 8-11 x 0.4 mm and the posterior end has a copulatory
  bursa

The Larva

1. Ground itch or Coolie itch is a localized dermatitis located at the site of entrance of the filariform larvae on the skin.
2. Creeping eruption or cutaneous larva migrans occurs during larval migration in which tunnels are created underneath the skin, not beyond the stratum germinativum. It is usually due to exposure to filariform larva of the animal hookworm A. brazilienze and A. caninum.
3. Pulmonary lesions are produced by migrating larvae and take the form of petechial hemorrhages with eosinophilic and leukocytic infiltration.

The Adult Worm

• The hookworm anemia is caused by continuous mechanical suction of blood from the intestinal mucosa.
• The raw mucosa left by the adult worm when it transfers to a new site bleeds.
• The development of anemia is dependent on the iron reserve of the host and on the intensity of infection.
• The microcytic, hypochromic anemia

Pathology due to hookworm infection is related to the intensity of infection

I. N. Amercianus

• 25 worms = There is no symptom
• 25 to 100 worms = There are mild symptoms, anemia
• 100 to 500 worms = There are considerable damage and moderate symptoms.
• 500 to 1000 worms = There would be severe damage and severe pathology and often fatal consequences.

II. A. Duodenale

• There are more pathogenic, as few as 100 adult worms can result in severe symptoms.

Complications of hookworm infection are mild and respond to treatment.

The A. Duodenale larvae may become dormant in human as per observation and it has important implications for the epidemiology, control, and treatment of the infection.

• Larval latency complicates the treatment and control of A. duodenale because dormant nematode larvae resist most anthelminthics and resume development at a later date.
• Larval latency also complicates the estimation of the life span of adult worms.
• Larval dormancy also occurs in the musculature of other mammals, suggesting that meat-borne ancylostomiasis may occur.