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Agent Factors for Hookworm Infection
Agent
Adult worms live in the small intestine, mainly the jejunum where they attach themselves to the villi. Males, measure about eight to 10 mm, and females about 10 to 13 mm in length. The eggs are passed on in the feces in their thousands, one female A duodenale produces about 30,000 eggs and one female N americanus about 9,000 of them per day. High egg production ensures constant infection.

When deposited on warm, moist soil, a larva rapidly develops in the egg and hatches after one or two days. The newly hatched larva (rhabditiform larva) moulds twice in the soil and becomes a skin–penetrating third stage infective larva within five to 10 days. These lie and wait in the soil to pierce the skin of the human host. They move very little horizontally, but migrate upwards on blades of grass. They can survive in shaded, moist soil for up to one month. Infection occurs when the larva enters the body through the skin, most commonly through the feet. Larvae of A duodenale are also infective by mouth. Once inside the body, they migrate via lymphatics and the blood stream to the lungs. They break into the alveoli, ascend the bronchi and trachea and are coughed up and swallowed to reach the small intestine, where they become sexually mature. Adult A duodenale and N americanus are believed to be capable of surviving for an average of about one and four years, respectively.
Reservoir
Man is the only important reservoir of human hookworm infection.

Infective Material
Feces containing the ova of hookworms. However, the immediate source of infection is the soil contaminated with infective larvae.

Period of Infectivity
As long as the person harbors the parasite.

Host Factors for Hookworm Infection
Age and Sex
All ages and both sexes are susceptible to infection in endemic areas. The highest incidence is found in the age group 15 to 25 years.

Nutrition
Studies indicate that malnutrition is a predisposing factor. The chronic disabling disease does not occur in the otherwise healthy individual, who is well nourished and whose body gets enough iron.

Host Parasite Balance
In endemic areas, the inhabitants develop a host–parasite balance in which the worm load is limited. They harbor the parasite without manifesting clinical signs and symptoms. In some areas, the infection rate may be a 100 per cent, but most infections are light and only a small proportion of the people are heavily infected. The delicate balance may be upset by malnutrition and inter–current infections. Little is known about host immunity.

Occupation
It is to be expected that hookworm infection will have a higher prevalence in agriculture than in town workers and in many tropical countries, it is an occupational disease of the farming community.

Environmental Factors for Hookworm Infection
Hookworm larvae live in the upper half–inch of the soil. Favorable environmental conditions are therefore crucial for the survival of the hookworm larvae in the soil. These are:
Soil
The soil must be suitable for the eggs and larvae. The type of soil that favors the survival of hookworm larvae is a damp, sandy and friable soil with clay.

Temperature
A temperature of about 24 to 32°C is considered favorable for the survival of the larvae. The eggs fail to develop at temperatures below 13°C. Larvae are killed at 45 to 50°C.

Oxygen
This is required for the growth and development of the larvae.

Moisture
Moisture is necessary for survival of the larvae. Dryness is fatal.

Rainfall
A rainfall of 40 inches and above is considered a favorable environmental factor. More important than the total annual rainfall is the number of rainy days spread out evenly throughout the year to keep up the moisture content of the soil. Flooding is an unfavorable factor.

Shade
Direct sunlight kills the larvae whereas shade protects them.

Human Habits
The habits of the human host not only determine the mode and extent of soil pollution, but also the extent of contact between infected soil and skin or mouth. These include indiscriminate defecation using the same place for defecation, going barefoot, farming practices using untreated sewage, children wading in infected mud with bare feet and hands. These habits are compounded by social factors such as illiteracy, ignorance and a low standard of living.