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Geographic Distribution

Fig. 12.1 Geographic distribution of Ancylostoma duodenale.

Fig. 12.2 Geographic distribution of Necator americanus

The two hookworms, Ancylostoma and Necator, are spread through two broad geographical bands which overlap in certain areas to produce mixed infections. In the north, Ancylostoma extends through southern and eastern Europe, the Mediterranean and North Africa, the Middle East, Pakistan and northern India, with a few foci in northern Australia and South America (Peru, Chile, Paraguay) (Fig. 12.1). In the south, Necator extends through Central America and northern South America, the West Indies, Africa south of the Sahara, the Iture forest (in Pygmies), India and Sri Lanka, Melanesia and Polynesia (Fig. 12.2). Mixed infections are found in Asia, particularly southern India, Myanmar (Burma), Malaysia, Vietnam, Laos, Cambodia, Taiwan, China and Japan, the Philippines, Indonesia, Polynesia, Micronesia, and Angola and neighboring African countries. Necator is occasionally seen in the southern United States and Puerto Rico, but with decreasing frequency. In fact, successful eradication programs in recent decades have almost eliminated or greatly limited prior endemic areas in the United States, Europe, Korea, Japan, Taiwan, Australia, and several Pacific and Caribbean islands. Ancylostoma ceylanicum, the cat hookworm, occurs rarely in humans in India, Taiwan, the Philippines, and Papua New Guinea.

From the above it can be seen that N. americanus is primarily a parasite of the tropical regions of the world, whereas A. duodenale is usually the lone species in temperate climates and has a spotty distribution in the tropics. The variations in distribution of these parasites are in large part due to the way the tropics were colonized by different groups of people who brought their parasites with them, as well as the fact that Necator tolerates higher temperatures better than Ancylostoma. It should be noted also that Ancylostoma is a natural parasite of carnivores, whereas Necator parasitizes herbivores. Man has become the unwitting host of both species through his association with both types of animals.

Although hookworms have been successfully controlled or eradicated in many temperate countries, they remain prevalent in many diverse parts of the world, principally in the tropics, despite concerted efforts at eradication. The prevalence in Puerto Rico at the turn of the 20th century was as high as 80% and has fallen to less than 15% today, but stubborn pockets persist. In the southern United States, intensive efforts during the last five decades have reduced the overall incidence to less than 1%, with scattered foci primarily in the Appalachian area. However, in the tropics the prevalence remains disturbingly high, ranging from 40-60% in parts of Brazil to 70% in Colombia and as high as 90% in the rural areas of Paraguay; 95% of Peruvian Amazon school girls in Iquitos are infected, 75% of whom have medium to heavy worm loads. The incidence in many countries of Asia and Africa is, likewise, alarmingly high.

The highest incidence of infection is in men in their second and third decades of life. In recent decades, immigration of people from rural areas to the cities has produced urban pockets of infection in some areas. Agriculture has also been important in the spread of hookworm infection. For example, in the Americas, the coffee and cocoa plantations with their high humidity and shade and abundant soil fertilizer serve as endemic areas. In the Orient, the use of human night soil as a major means of fertilization maintains the disease there. As De Leon and Maldonado state in their excellent treatise on the subject: "All of these data point to man as the primary factor in the epidemiology of uncinariasis. Through man, by promiscuous defecation, ignorance, malnutrition and overcrowding, hookworm disease still maintains a place in pathology."

It is common in workers in mines and tunnels, because of the moisture and warmth underground and inadequate fecal sanitation. Where it is endemic, everyone who walks unshod is at risk of contracting the disease. Hookworms are also found in gorillas, monkeys, rhinoceros, pangolin, rodents and puppies. To what extent such infections are caused by "human" parasites remains undetermined because different species of hookworms may be involved.

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