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ScienceWeek
MEDICAL BIOLOGY: ON HOOKWORM INFECTION
The following points are made by P.J. Hotez et al (New Engl. J. Med. 2004 351:799):
1) Hookworm infection in humans is caused by an infection with the helminth nematode parasites Necator americanus and Ancylostoma duodenale and is transmitted through contact with contaminated soil. It is one of the most common chronic infections, with an estimated 740 million cases in areas of rural poverty in the tropics and subtropics.(1)
2) Because hookworm infection occurs predominantly among the world's most impoverished people,(1) it holds a unique place in modern history. For example, the reputation of pre-1949 China as the "sick man of Asia" was partly a result of the high prevalence and intensity of infection with hookworm.(2) Mohandas Gandhi had hookworm infection in the last years of his life.(3) Hookworm was also a contributing factor in the slowing of economic development during the early part of the 20th century in the southern US.(4) Today, hookworm infection is among the most important tropical diseases in humans; the use of disability-adjusted life years as a quantitative measure of the burden of disease reveals that this infection outranks African trypanosomiasis, dengue, Chagas' disease, schistosomiasis, and leprosy.(5)
3) The greatest number of hookworm cases occur in Asia, followed by sub-Saharan Africa.(1) In China alone, approximately 190 million people are infected, an estimate based on a nationwide study involving the examination of fecal specimens obtained from almost 1.5 million people between 1988 and 1992.(2) N. americanus is the most common hookworm worldwide, whereas A. duodenale is more geographically restricted. In contrast to these major anthropophilic species, three species of zoonotic hookworm are minor causes of disease in humans. A. ceylanicum infects dogs and cats and can also infect humans but is not considered an important pathogen. The dog hookworm A. caninum causes human eosinophilic enteritis in northeastern Australia, and A. braziliense causes cutaneous larva migrans.
4) Some of the highest rates of hookworm transmission occur in the world's coastal regions, where infective third-stage larvae can migrate freely in sandy soils and where temperatures and moisture are optimal for viability of larvae. In these areas, repeated exposure to third-stage larvae of N. americanus or A. duodenale results in a local pruritic, erythematous, papular rash known as "ground itch." Although the entire body surface is vulnerable, ground itch appears most frequently on the hands and feet, which are the major sites of entry for third-stage larvae. In contrast to ground itch, skin invasion by zoonotic A. braziliense third-stage larvae results in cutaneous larva migrans, or "creeping eruption," a self-limited dermatologic condition characterized by serpiginous burrows, 1 to 5 cm long. Created by third-stage larvae migrating in the epidermis, the burrows appear on the feet in 39 percent of cases, the buttocks in 18 percent, and the abdomen in 16 percent; in the remainder of cases, the burrows appear predominantly in the lower legs, arms, and face. In the US, cutaneous larva migrans is seen commonly in military personnel, in travelers returning from resorts that have sandy beaches, and in residents of Florida and the Gulf Coast; it is treated successfully with short oral courses of either albendazole or ivermectin.
References (abridged):
1. de Silva NR, Brooker S, Hotez PJ, Montresor A, Engels D, Savioli L. Soil-transmitted helminth infections: updating the global picture. Trends Parasitol 2003;19:547-551
2. Hotez PJ. China's hookworms. China Q 2002;172:1029-1041
3. Wolpert S. Gandhi's passion: the life and legacy of Mahatma Gandhi. New York: Oxford University Press, 2001:214
4. Bleakley H. Disease and development: evidence from the American South. J Eur Econ Assoc 2003;1:376-86
5. Hotez PJ, Zhan B, Bethony JM, et al. Progress in the development of a recombinant vaccine for human hookworm disease: the Human Hookworm Vaccine Initiative. Int J Parasitol 2003;33:1245-1258
New Engl. J. Med. http://www.nejm.org
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SCIENCE POLICY: ON THE SOCIAL IMPACT OF CHRONIC DISEASE
The following points are made by C.G. Mascie-Taylor and E. Karim (Science 2004 302:1921):
1) The burden of disease and injury attributable to undernutrition, poor water supply, poor sanitation, and inadequate personal and domestic hygiene accounts for almost 23% of the disability-adjusted life years (DALY) from all causes worldwide and for 26% of DALY in developing regions (1). International initiatives are targeted primarily at conditions that cause higher mortality (such as AIDS, tuberculosis, malaria, and vaccine-preventable diseases), but there is also a need to focus attention on controlling conditions such as soil-transmitted helminths and schistosomiasis that lead to considerable morbidity.
2) Until recently, it was thought that human populations were experiencing a simple epidemiological transition. This idea, first put forward by Omran (2), envisaged three stages -- the age of pestilence and famine, the age of receding pandemics, and the age of degenerative and man-made diseases -- and assumed that as infectious diseases are eliminated, chronic diseases will increase as the population ages. However, chronic diseases are emerging as a major epidemic in many nonindustrialized countries because of their association with overweight and obesity. In addition, the upsurge of infectious diseases and the emergence of new ones also casts doubt on this simple, unidirectional epidemiological process.
3) A recent review (3) suggested that 175 human pathogens (12% of those known) were emerging or reemerging and that 37 pathogens have been recognized since 1973, including rotavirus, Ebola virus, HIV-1 and HIV-2, and most recently, Nipah virus. Among the infectious vector-borne diseases, dengue, dengue hemorrhagic fever, yellow fever, plague, malaria, leishmaniasis, rodent-borne viruses, and arboviruses are persisting, and sometimes reemerging, with serious threats to human health. For example, malaria, which is the foremost vector-borne disease worldwide, continues to worsen in many areas, and there are now an estimated 300 million to 500 million cases of malaria worldwide each year with 2 million to 4 million deaths. Since 1975, dengue fever has surfaced in huge outbreaks in more than 100 countries, with as many as 100 million cases each year. These increases reflect societal changes arising from population growth, ecological and environmental changes, and especially suburbanization, together with widespread and frequent air travel.
4) In summary: The shift from acute infectious and deficiency diseases to chronic noncommunicable diseases is not a simple transition but a complex and dynamic epidemiological process, with some diseases disappearing and others appearing or reemerging. The unabated pandemic of childhood and adulthood obesity and concomitant comorbidities are affecting both rich and poor nations, while infectious diseases remain an important public health problem, particularly in developing countries. The authors suggest more attention should be given to the high burden of disease associated with soil-transmitted helminths and schistosomiasis, which until recently was not considered a priority even though regular drug treatment is obtainable at relatively little cost. In developing countries, the pressing requirement is to provide an accessible and good quality health-care system, whereas industrialized countries have a major need for greater public health education and the promotion of healthy life-styles.(3-5)
References (abridged):
1. C. J. L. Murray, A. D. Lopez, The Global Burden of Disease (Harvard University Press, Cambridge, MA, 1996)
2. A. R. Omran, Milbank Mem. Fund Q. 49, 509 (1971)
3. D. J. Gubler, Emerg. Infect. Dis. 4, 442 (1998)
4. M.-S. Chan, Parasitol. Today 13, 438 (1997)
5. E. Cooper, Trans. R. Soc. Trop. Med. Hyg. 85, 168 (1991)
Science http://www.sciencemag.org
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PUBLIC HEALTH: EXPECTED CONSEQUENCES OF GLOBAL WARMING
Climate change produced by global warming is expected to result in melting ice caps, rising sea levels, torrential floods, devastating droughts, and severe harvest failures. What are often not considered in discussions of global warming are the effects of global warming on public health.
The following points are made by Pim Martens (American Scientist 1999 87:534):
1) Concerning heat stress: The author suggests that perhaps the most immediate consequence of increasing global temperatures will be a rise in the number of heat waves and heat-related illnesses. Such temperature extremes can, for example, increase the sensitivity of asthmatics to their condition. There will also be an increasing number of deaths from heat stress brought about by high ambient temperatures lasting days on end. On the other hand, the milder winters associated with global warming will offer a better chance of survival for at-risk groups such as the elderly during the coldest months. Research into the effect of a gradual temperature increase has revealed that we can expect a decline in mortality from cardiovascular and pulmonary disease in the winter. Whether the milder winters could offset the mortality during the summer heat waves is not clear.
2) Concerning malaria: The spread of this disease is limited by conditions that favor the disease vector (the malarial mosquito Anopheles) and the protozoan parasite (Plasmodium). The malarial mosquito is most comfortable at temperatures of approximately 20 to 30 degrees centigrade and at a relative humidity of at least 60 percent. Also, the malaria parasite develops more rapidly inside the mosquito as the temperature rises, and the development ceases entirely below approximately 15 degrees centigrade. Increased rainfall and increased surface water, expected to result from global warming, will produce more breeding grounds for the mosquito. Malaria currently kills 1 to 2 million people each year.
3) Concerning schistosomiasis (bilharzia): The enormous expanse of irrigation systems in many tropical countries has doubled the incidence of this disease in the past 50 years. There are some estimates that nearly 200 million people are infected worldwide. The disease is caused by a parasitic worm (a trematode; also called a "fluke"; a type of flatworm) whose eggs enter the water supply by way of human urine or feces. Infected water snails serve as hosts for the parasites while they develop into free-swimming "mini-worms" (larvae; cercaria). The circle closes when a larva penetrates the skin of a human who comes in contact with the contaminated water. The development of the parasite and the population of the host snails are both governed by the ambient temperature, with warm waters favoring their growth. Also, the warmer the ambient temperature, the more often people come into contact with water. In places where the disease is endemic, it is known that the number of infected snails declines sharply during the winter months. A temperature rise of only a few degrees will ensure that this disease is transmitted throughout the year. It is estimated that currently worldwide approximately 500 million people are at risk of infection by this pathogen.
4) Concerning dengue: Like malaria, this disease is transmitted by mosquitoes (Aedes aegypti, which also transmits yellow fever), but the pathogen is a virus (dengue virus, a flavivirus). The dengue virus is currently restricted to the tropics, approximately between latitudes 30 degrees south and 20 degrees north. Temperature affects the development of both the mosquito and the virus as well as the frequency of mosquito bites. A warmer climate may increase not only the elevations above sea level at which the disease occurs, but also its northern and southern ranges. Dengue hemorrhagic fever, a severe form of the disease, has a mortality of 6 to 30 percent, with most deaths occurring in infants less than 1 year old.
5) Concerning various water-borne diseases: Changes in the amount of precipitation will accompany the temperature changes to a warmer Earth. Many disease-causing organisms require water for survival, and increases in rainfall and flooding will encourage the wider distribution of such pathogens, with higher temperatures increasing the chances of pathogen survival. Various bacteria (e.g., Salmonella and Shigella), viruses (e.g., rotavirus), protozoa (e.g., Giardia and Cryptosporidium) can cause diarrhea, which kills more than 3 million children every year.
6) In general, many factors will interact with a changing climate in a nonlinear way, so their effects on human health are extremely difficult to quantify. Despite the uncertainties, there are increasing indications that a changed global climate may be a major factor in the global distribution of many diseases.
American Scientist http://www.americanscientist.org
ScienceWeek http://scienceweek.com
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