Trans R Soc Trop Med Hyg 2014; 108: 179–180 doi:10.1093/trstmh/tru020
EDITORIAL
How important are Dipteran vectors of disease in Africa? Maureen Coetzee Wits Research Institute for Malaria, Faculty of Health Sciences, University of the Witwatersrand, 7 York Avenue, Parktown, Johannesburg 2001, South Africa; E-mail: [email protected] Received 20 January 2014; accepted 20 January 2014
The short answer to the above question is ‘very’. Of all the arthropods, the Order Diptera contains the most species that transmit pathogens or are harmful to humans. Yet, of the 106 Families included in this Order, only 10 are of medical importance and, within these, relatively few species are implicated in disease transmission. For example, in the Afrotropical Region, the Culicidae (mosquitoes), widely regarded as the most important of the Diptera because of their involvement in malaria and filariasis transmission, is represented by over 780 species but only around 20 of these are important vectors of disease, representing around 2.5% of this diverse Family. So just how important are these few medically important insects? Arthropods have been known to be harmful to humans for thousands of years and references can be found to this effect in the Talmud, Koran and Bible. Service1 provides a succinct account of this history going back several hundred years BC to the time of Aristophanes, Aristotle and Pliny. In the late nineteenth century, definitive proof was provided that many pathogens undergo obligatory development in insects before transmitting them back to humans. The most influential discovery was that of Patrick Manson from the London School of Hygiene & Tropical Medicine showing that Culex quinquefasciatus transmitted Bancroftian filariasis. Subsequently, Ross and others showed that mosquitoes transmitted malaria parasites, and a number of ticks, mites and insects were incriminated in quick succession as vectors of disease-causing organisms.1
Malaria Malaria is confined mainly to the tropics and subtropics, having been eradicated from most of Europe and North America. WHO, in its latest malaria report,2 states that globally, approximately 3.4 billion people are at risk of contracting malaria and over 200 million cases are estimated to have occurred in 2012. Over 620 000 (upper estimate of around 790 000) malaria deaths occurred worldwide in 2012, 90% of them in the African Region and 77% in children under 5 years old. When one considers the negative impact of malaria on the economy of African countries, with loss of growth in GDP ranging from under 0.5% (e.g., Ghana) to almost 9% (Chad), the figures are staggering. An estimated US$1.5–2.2 billion is needed
annually to combat the disease in malaria endemic countries.3 Any setback to current control operations would be disastrous. Although malaria is a preventable and curable disease, control programmes have to confront parasite resistance to the drugs used for treatment and resistance by the mosquitoes to the insecticides approved by WHO for use in vector control. Currently, emerging parasite resistance to artemisinin derivatives in South-east Asia is ringing alarm bells in Africa.4 Insecticide resistance is an even more urgent problem as there is now almost no country in Africa that does not have some level of resistance in the mosquitoes to the insecticides approved for vector control. Publication by WHO of the Global Plan for Insecticide Resistance Management5 goes some way to assisting control programmes to manage the problem, but the options are limited and the threat to malaria control and elimination is huge. While malaria poses the greatest challenge to African countries, the following diseases transmitted by Dipteran flies also play a significant role in the health of the African people.
Filariasis Bancroftian filariasis WHO estimates that over 120 million people are infected with lymphatic filariasis in 73 countries globally, with around 40 million disfigured or incapacitated by the disease. Of these cases, 30% occur in Africa.6 Control of the disease is by mass drug administration that clears microfilariae from the blood stream. The mosquito vectors are controlled by the same interventions that target malaria vectors.
Onchocerciasis Ninety percent of the disease ‘river blindness’ is restricted to Africa. Blindness is the result of dead microfilariae releasing symbiotic bacteria resulting in severe inflammatory responses. Treatment is with ivermectin for the microfilariae and the antibiotic doxycycline for the Wolbachia symbiotic bacteria.7 The vectors are Simulium blackflies that breed in fast-running rivers and streams. The WHO African Programme for Onchocerciasis Control operates in 19 participating countries.
# The Author 2014. Published by Oxford University Press on behalf of Royal Society of Tropical Medicine and Hygiene. All rights reserved. For permissions, please e-mail: [email protected].
179
Downloaded from http://trstmh.oxfordjournals.org/ at University of Hawaii - Manoa on June 13, 2015
Keywords: Africa, Arthropods, Diptera
M. Coetzee
Arboviruses
Transmitted by horse flies of the genus Chrysops, this disease occurs in the rain forests of mainly central tropical Africa. Loiasis is a relatively benign disease but treatment is over a long period and can result in complications. Preventive measures against fly bites, such as wearing protective clothing and using repellents, are advisable.
Within the Diptera, only mosquitoes are of any importance in the transmission of arboviruses and within the Culicidae, the genera Aedes, Culex and Mansonia contain the most important vector species, although the malaria vectors Anopheles gambiae and An. funestus can play a minor role. In the Afrotropical Region, these viruses include yellow fever, chikungunya, Sindbis, West Nile, Wesselsbron, o’nyong-nyong and Rift Valley fever. Increasing numbers of dengue virus cases have been recorded from Africa but the problem is not yet widespread on the continent. Yellow fever virus is the only arbovirus for which there is a vaccine and this is mandatory for travellers in most of the Afrotropical Region. Perhaps it is not surprising that Africa should be plagued by so many insect-borne diseases, given the origin of Homo sapiens and the co-adaptation that has taken place over millennia between insects, parasites and humans. The cost to the people of Africa, however, is enormous.
Mansonella There are two species occurring in the Afrotropical region and both are transmitted by Culicoides biting midges.7 They are considered to be less important than the other filarias with infections often asymptomatic. The biting midges are notoriously difficult to control, breeding in dung, leaf rot and compost heaps.
Trypanosomiasis Human African Trypanosomiasis, better known as sleeping sickness, is widespread in Africa covering much of the tropics and subtropics. Currently, an estimated 30 000 people are infected in 36 countries. The population at risk is around 70 million people.8 Treatment is difficult and there are no vaccines available. The control of tsetse flies is aimed at the adults and ranges from insecticidal spraying of vegetation, to targets and traps of various sorts. The fly’s natural attraction to large, dark objects led to the development of traps baited with odour or colour attractants and using insecticides to kill the flies.9
Competing interests. None declared.
References 1 Service MW. A short history of early medical entomology. J Med Entomol 1978;14:603–26. 2 WHO. World Malaria Report 2013. Geneva: World Health Organization; 2013. 3 Okorosobo T, Okorosobo F, Mwabu G et al. Economic burden of malaria in six countries of Africa. European J Bus Management 2011;3:42–62. 4 WHO. Global Report on Antimalarial Drug Efficacy and Drug Resistance: 2000–2010. Geneva: World Health Organization; 2010.
Leishmaniasis
5 WHO. Global Plan for Insecticide Resistance Management in Malaria Vectors. Geneva: World Health Organization; 2012.
Leishmaniasis is found in 88 countries around the globe with approximately 1.5–2 million new cases reported annually. There are an estimated 12 million people currently infected with the parasite.10 Cutaneous leishmaniasis is the most common African form, found mainly in North Africa and localized in West, East and southern Africa. Phlebotomus sand flies transmit the parasites from reservoir hosts to humans. The visceral form of leishmaniasis (kala-azar) is found in Sudan, Ethiopia and East Africa and is fatal if left untreated. An increase in co-infections with HIV has resulted in visceral leishmaniasis becoming an important opportunistic disease.10 Where sand flies are associated with humans and domestic animals, indoor house spraying and the use of insecticide treated bed nets for malaria control have seen a decrease in leishmaniasis cases.
6 WHO. Lymphatic filariasis. Geneva: World Health Organization. Fact Sheet No. 102; 2013. http://www.who.int/mediacentre/factsheets/ fs102/en/ [accessed 20 January 2014].
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7 Russell RC, Otranto D, Wall RL. The Encyclopedia of Medical and Veterinary Entomology. Boston: CABI; 2013. 8 WHO. Trypanosomiasis, Human African (sleeping sickness). Geneva: World Health Organization. Fact Sheet No. 259; 2013. http://www. who.int/mediacentre/factsheets/fs259/en/ [accessed 20 January 2014]. 9 Torr SJ, Hargrove JW, Vale GA. Towards a rational policy for dealing with tsetse. Trends Parasitol 2005;21:537–41. 10 WHO. Report on global surveillance of epidemic-prone infectious diseases – leishmaniasis. Geneva: World Health Organization; 2014. http://www.who.int/csr/resources/publications/CSR_ISR_2000_1leish/ en/index.html [accessed 20 January 2014].
Downloaded from http://trstmh.oxfordjournals.org/ at University of Hawaii - Manoa on June 13, 2015
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EDITORIAL
How important are Dipteran vectors of disease in Africa? Maureen Coetzee Wits Research Institute for Malaria, Faculty of Health Sciences, University of the Witwatersrand, 7 York Avenue, Parktown, Johannesburg 2001, South Africa; E-mail: [email protected] Received 20 January 2014; accepted 20 January 2014
The short answer to the above question is ‘very’. Of all the arthropods, the Order Diptera contains the most species that transmit pathogens or are harmful to humans. Yet, of the 106 Families included in this Order, only 10 are of medical importance and, within these, relatively few species are implicated in disease transmission. For example, in the Afrotropical Region, the Culicidae (mosquitoes), widely regarded as the most important of the Diptera because of their involvement in malaria and filariasis transmission, is represented by over 780 species but only around 20 of these are important vectors of disease, representing around 2.5% of this diverse Family. So just how important are these few medically important insects? Arthropods have been known to be harmful to humans for thousands of years and references can be found to this effect in the Talmud, Koran and Bible. Service1 provides a succinct account of this history going back several hundred years BC to the time of Aristophanes, Aristotle and Pliny. In the late nineteenth century, definitive proof was provided that many pathogens undergo obligatory development in insects before transmitting them back to humans. The most influential discovery was that of Patrick Manson from the London School of Hygiene & Tropical Medicine showing that Culex quinquefasciatus transmitted Bancroftian filariasis. Subsequently, Ross and others showed that mosquitoes transmitted malaria parasites, and a number of ticks, mites and insects were incriminated in quick succession as vectors of disease-causing organisms.1
Malaria Malaria is confined mainly to the tropics and subtropics, having been eradicated from most of Europe and North America. WHO, in its latest malaria report,2 states that globally, approximately 3.4 billion people are at risk of contracting malaria and over 200 million cases are estimated to have occurred in 2012. Over 620 000 (upper estimate of around 790 000) malaria deaths occurred worldwide in 2012, 90% of them in the African Region and 77% in children under 5 years old. When one considers the negative impact of malaria on the economy of African countries, with loss of growth in GDP ranging from under 0.5% (e.g., Ghana) to almost 9% (Chad), the figures are staggering. An estimated US$1.5–2.2 billion is needed
annually to combat the disease in malaria endemic countries.3 Any setback to current control operations would be disastrous. Although malaria is a preventable and curable disease, control programmes have to confront parasite resistance to the drugs used for treatment and resistance by the mosquitoes to the insecticides approved by WHO for use in vector control. Currently, emerging parasite resistance to artemisinin derivatives in South-east Asia is ringing alarm bells in Africa.4 Insecticide resistance is an even more urgent problem as there is now almost no country in Africa that does not have some level of resistance in the mosquitoes to the insecticides approved for vector control. Publication by WHO of the Global Plan for Insecticide Resistance Management5 goes some way to assisting control programmes to manage the problem, but the options are limited and the threat to malaria control and elimination is huge. While malaria poses the greatest challenge to African countries, the following diseases transmitted by Dipteran flies also play a significant role in the health of the African people.
Filariasis Bancroftian filariasis WHO estimates that over 120 million people are infected with lymphatic filariasis in 73 countries globally, with around 40 million disfigured or incapacitated by the disease. Of these cases, 30% occur in Africa.6 Control of the disease is by mass drug administration that clears microfilariae from the blood stream. The mosquito vectors are controlled by the same interventions that target malaria vectors.
Onchocerciasis Ninety percent of the disease ‘river blindness’ is restricted to Africa. Blindness is the result of dead microfilariae releasing symbiotic bacteria resulting in severe inflammatory responses. Treatment is with ivermectin for the microfilariae and the antibiotic doxycycline for the Wolbachia symbiotic bacteria.7 The vectors are Simulium blackflies that breed in fast-running rivers and streams. The WHO African Programme for Onchocerciasis Control operates in 19 participating countries.
# The Author 2014. Published by Oxford University Press on behalf of Royal Society of Tropical Medicine and Hygiene. All rights reserved. For permissions, please e-mail: [email protected].
179
Downloaded from http://trstmh.oxfordjournals.org/ at University of Hawaii - Manoa on June 13, 2015
Keywords: Africa, Arthropods, Diptera
M. Coetzee
Arboviruses
Transmitted by horse flies of the genus Chrysops, this disease occurs in the rain forests of mainly central tropical Africa. Loiasis is a relatively benign disease but treatment is over a long period and can result in complications. Preventive measures against fly bites, such as wearing protective clothing and using repellents, are advisable.
Within the Diptera, only mosquitoes are of any importance in the transmission of arboviruses and within the Culicidae, the genera Aedes, Culex and Mansonia contain the most important vector species, although the malaria vectors Anopheles gambiae and An. funestus can play a minor role. In the Afrotropical Region, these viruses include yellow fever, chikungunya, Sindbis, West Nile, Wesselsbron, o’nyong-nyong and Rift Valley fever. Increasing numbers of dengue virus cases have been recorded from Africa but the problem is not yet widespread on the continent. Yellow fever virus is the only arbovirus for which there is a vaccine and this is mandatory for travellers in most of the Afrotropical Region. Perhaps it is not surprising that Africa should be plagued by so many insect-borne diseases, given the origin of Homo sapiens and the co-adaptation that has taken place over millennia between insects, parasites and humans. The cost to the people of Africa, however, is enormous.
Mansonella There are two species occurring in the Afrotropical region and both are transmitted by Culicoides biting midges.7 They are considered to be less important than the other filarias with infections often asymptomatic. The biting midges are notoriously difficult to control, breeding in dung, leaf rot and compost heaps.
Trypanosomiasis Human African Trypanosomiasis, better known as sleeping sickness, is widespread in Africa covering much of the tropics and subtropics. Currently, an estimated 30 000 people are infected in 36 countries. The population at risk is around 70 million people.8 Treatment is difficult and there are no vaccines available. The control of tsetse flies is aimed at the adults and ranges from insecticidal spraying of vegetation, to targets and traps of various sorts. The fly’s natural attraction to large, dark objects led to the development of traps baited with odour or colour attractants and using insecticides to kill the flies.9
Competing interests. None declared.
References 1 Service MW. A short history of early medical entomology. J Med Entomol 1978;14:603–26. 2 WHO. World Malaria Report 2013. Geneva: World Health Organization; 2013. 3 Okorosobo T, Okorosobo F, Mwabu G et al. Economic burden of malaria in six countries of Africa. European J Bus Management 2011;3:42–62. 4 WHO. Global Report on Antimalarial Drug Efficacy and Drug Resistance: 2000–2010. Geneva: World Health Organization; 2010.
Leishmaniasis
5 WHO. Global Plan for Insecticide Resistance Management in Malaria Vectors. Geneva: World Health Organization; 2012.
Leishmaniasis is found in 88 countries around the globe with approximately 1.5–2 million new cases reported annually. There are an estimated 12 million people currently infected with the parasite.10 Cutaneous leishmaniasis is the most common African form, found mainly in North Africa and localized in West, East and southern Africa. Phlebotomus sand flies transmit the parasites from reservoir hosts to humans. The visceral form of leishmaniasis (kala-azar) is found in Sudan, Ethiopia and East Africa and is fatal if left untreated. An increase in co-infections with HIV has resulted in visceral leishmaniasis becoming an important opportunistic disease.10 Where sand flies are associated with humans and domestic animals, indoor house spraying and the use of insecticide treated bed nets for malaria control have seen a decrease in leishmaniasis cases.
6 WHO. Lymphatic filariasis. Geneva: World Health Organization. Fact Sheet No. 102; 2013. http://www.who.int/mediacentre/factsheets/ fs102/en/ [accessed 20 January 2014].
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7 Russell RC, Otranto D, Wall RL. The Encyclopedia of Medical and Veterinary Entomology. Boston: CABI; 2013. 8 WHO. Trypanosomiasis, Human African (sleeping sickness). Geneva: World Health Organization. Fact Sheet No. 259; 2013. http://www. who.int/mediacentre/factsheets/fs259/en/ [accessed 20 January 2014]. 9 Torr SJ, Hargrove JW, Vale GA. Towards a rational policy for dealing with tsetse. Trends Parasitol 2005;21:537–41. 10 WHO. Report on global surveillance of epidemic-prone infectious diseases – leishmaniasis. Geneva: World Health Organization; 2014. http://www.who.int/csr/resources/publications/CSR_ISR_2000_1leish/ en/index.html [accessed 20 January 2014].
Downloaded from http://trstmh.oxfordjournals.org/ at University of Hawaii - Manoa on June 13, 2015
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