It is estimated that about 1.5–2.7 million people die from malaria each year which is about the same that die from HIV/AIDS (World Health Report, WHO).
Many devastating diseases in tropical areas of the world are the result of infection with parasites. In these regions, the combination of climate and poverty contribute to the transmission of parasitic infections. Some parasites, like malaria, are a common cause of death, while others, like parasitic nematodes can lead to disfigurement, blindness and severe economic hardship. A recent World Health Organization (WHO) report on the leading causes of death world-wide shows that one-third of all deaths are due to infectious and parasitic diseases. It is estimated that about 1.5–2.7 million people die from malaria each year which is about the same that die from HIV/AIDS (World Health Report 1998, WHO).
In many tropical and subtropical areas, the prevalence of parasitic infections is on the rise due to rapid and unplanned growth of cities, which creates additional breeding sites for the mosquitoes that transmit the parasites responsible for malaria and filariasis. Effective control of these diseases requires repeated administration of potent drugs and continuous efforts to reduce vector insect populations.
Malaria is probably the most prevalent and debilitating parasitic disease afflicting humans. It is normally caused by one of four species of Plasmodium. Infection with Entamoeba histolytica, resulting in amoebic colitis and liver abscesses, is the second leading cause of death resulting from a parasitic infection. E. histolytica is estimated to infect one tenth of the worlds' population or 500 million people (WHO Weekly Epidemiologic Record  72, 97–100).
Filariasis is caused by several different species of parasitic nematodes that are transmitted to humans by a variety of insect vectors. Collectively, the various species of filarial parasites are estimated to infect approximately 150 million people worldwide, and over 1 billion people live in areas where filariasis is common (WHO  Fact Sheets 102, 95). While filarial infection is rarely life-threatening, it causes chronic suffering, social stigma and disability. The degree of disability resulting from infection greatly hinders the ability to work and the economic impact of this disease is considerable.
Brugia malayi is a filarial nematode with widespread distribution in South and South East Asia. It is transmitted via the bite of blood-feeding mosquitoes. The parasite resides in the lymphatic system and is a cause of lymphatic filariasis. In the later stages of infection, the disease is characterized by a disfiguring condition known as elephantiasis.
Onchocerca volvulus is the causative agent of River Blindness and the second leading cause of blindness worldwide. This filarial parasite is transmitted by female blackflies that breed in fast flowing rivers. Over 20 million people are infected world-wide although approximately 99% of cases are found in Africa (WHO  Fact Sheet 95).
Currently available anti-parasitic drugs have their limitations and there is a need to focus on the discovery of new drugs to ameliorate the conditions associated with the various diseases and to control the parasites responsible.
The end of 2014 saw the release of the Global Burden of Disease Study 2013 (GBD 2013), in which 240 causes of death were studied through a systematic analysis. Among the important findings were that globally, parasitic diseases caused more than one million deaths in the year 2013.
To no one’s surprise, malaria was by far the major parasitic disease killer in 2013, causing over 850,000 deaths, with many of those deaths in African children under the age of five infected with Plasmodium falciparum. However, kinetoplastid infections – leishmaniasis, Chagas disease, and African trypanosomiasis – caused the deaths of over 80,000 people, while two intestinal protozoan infections – cryptosporidiosis and amoebiasis – resulted in over 50,000 deaths.
A breakdown of those deaths is shown in Table 1.
Table 1. Deaths caused by parasitic diseases in 2013
Global Deaths in 2013
Total Deaths from Parasitic Infections
According to the GBD 2013, the vast majority of parasite-related deaths resulted from protozoan infections. However, it is likely that deaths resulting from helminth infections were under estimated. For example, ascariasis and trichuriasis are important causes of protein-energy malnutrition in developing countries, and almost 500,000 deaths were attributed to this condition in 2013; similarly hookworm infection is an important cause of iron deficiency anemia, a condition leading to 200,000 deaths in 2013. However, none of those deaths were specifically attributed to intestinal helminth infections in GBD 2013. Similarly, urogenital schistosomiasis is an important cause of renal failure in Africa, and yet none of the world’s almost 400,000 deaths from chronic kidney disease due to “other” causes were specifically attributed to schistosomiasis. Indeed helminth infections and other neglected tropical diseases are important yet neglected causes of non-communicable diseases (NCDs), a point that policymakers often miss when debating the importance of infectious diseases versus NCDs as major global health threats. Additional findings suggest that schistosomiasis and other helminthiases may contribute to the transmission of malaria co-infections.
Still another major finding of GBD 2013 was the enormous impact of both new vaccines and expanded use of old vaccines in reducing the mortality of major childhood scourges such as diphtheria, tetanus, measles, rotavirus and pneumonoccal infections, and meningitis caused by Haemophilus influenzae type B. We have some promising new drugs for parasitic diseases in advanced development, but vaccines have previously lagged. However, the RTS,S malaria vaccine is completing advanced clinical trials and is moving towards licensure, while new vaccines for leishmaniasis, hookworm infection, and schistosomiasis are in phase 1 clinical trials and beyond. Such vaccines could go a long way towards reducing the one million deaths that are now attributed to parasitic diseases. In addition, we could also see a reduction in the additional 1.1 million deaths from malnutrition, anemia, and renal failure, conditions that can be caused by underlying parasitic infections.
Peter J Hotez MD PhD is Co-Editor-in-Chief of PLOS Neglected Tropical Diseases. Both Dr. Hotez and Dr. Jennifer Herricks are with the National School of Tropical Medicine at Baylor College of Medicine, and the James A Baker III Institute for Public Policy at Rice University.