Changing patterns of parasitic disease: old enemies and new invaders--the battle wages on.

Abstract 1. A. The classic parasitic diseases—malaria, schistosomiasis, the filariases, and many other familiar “old enemies”—continue to be the major parasitic causes of morbidity and morality and the continuing objects of our research and control efforts. Although all have been receptive to control interventions under certain circumstances, most of these infections continue to be transmitted without apparent restrictions in the rural, developing regions where nearly one-half of all the world's people live. New technologie achievements will improve the chances for treatment and control of these infections, but more important, may be improved administrative capacity and the commitment of political will and economic resources toward these goals. 2. B. The emergence of resistance to drugs and chemical control agents as a result of selection of resistant strains in the face of exposure to the chemical is a growing phenomenon among parasites with very serious public health implications. Strains of Plasmodium falciparum resistant to most drugs formerly effective for prophylaxis and treatment have appeared in Africa, Asia and parts of South and Central America at a faster pace than new compounds can be made available. Similar problems have been reported with gastrointestinal nematodes of livestock animals and many insect vectors. 3. C. Cryptosporidiosis emerged dramatically as a human disease when it appeared as one of the many opportunistic infections suffered by patients with AIDS. Increased clinical awareness and improved diagnostic procedures have now implicated Cryptosporidium as an important worldwide cause of diarrheal illness in persons with normal immunologic function. Zoonotic and various forms of person-to-person transmission are now recognized as important modes of spread for this protozoan. 4. D. Development and widespread use of specific serodiagnostic tests for human toxocariasis has revealed that zoonotic transmission of the common dog and cat ascarids is widespread in populations in industrialized societies. Serologic studies suggest that human Toxocara infection is even more common in tropical, developing areas but the health impact of this has not yet been measured. New nematode agents of larva migrans have been identified recently. One of them, the racoon ascarid, Baylisascaris procyonis, may increase in importance as its host extends its range as a commensal resident. 5. E. In developed countries where social and environmental conditions limit opportunities for transmission of most protozoal and helminthic disease agents, parasitic diseases are seen often as imported infections in returning tourists or immigrants who became infected in foreign endemic areas. One of these, imported malaria, has been exacerbated in the past decade by the resurgence of malaria transmission in Asia, the resettlement, in many countries, of refugees from Southeast Asia, and the spread of chloroquine-resistant P. falciparum in Africa. 6. F. Other special problems of parasitic diseases in developed countries include the increase in opportunistic parasitic pathogens (Pneumocystis, Toxoplasma, Strongyloides and others) in patients with AIDS and iatrogenically-caused immunodepression and the occurrence of infections resulting from changing dietary preferences and “fads”, e.g. diphyllobothriasis from “Shushi” and “Sashimi” (Japan, United States and Canada) and gnathostomiasis from imported fish (Japan).