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Tropical diseases are diseases that are prevalent in or unique to tropical and subtropical regions.^[1] The diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, which controls the insect population by forcing hibernation. However, many were present in northern Europe and northern America in the 17th and 18th centuries before modern understanding of disease causation. The initial impetus for tropical medicine was to protect the health of colonial settlers, notably in India under the British Raj.^[2] Insects such as mosquitoes and flies are by far the most common disease carrier, or vector. These insects may carry a parasite, bacterium or virus that is infectious to humans and animals. Most often disease is transmitted by an insect bite, which causes transmission of the infectious agent through subcutaneous blood exchange. Vaccines are not available for most of the diseases listed here, and many do not have cures.

Human exploration of tropical rainforests, deforestation, rising immigration and increased international air travel and other tourism to tropical regions has led to an increased incidence of such diseases to non-tropical countries.^[3]^[4] Of particular concern is the habitat loss of reservoir host species.^[5]

Health programmes

In 1975 the Special Programme for Research and Training in Tropical Diseases (TDR) was established to focus on neglected infectious diseases which disproportionately affect poor and marginalized populations in developing regions of Africa, Asia, Central America and North South America. It was established at the World Health Organization, which is the executing agency, and is co-sponsored by the United Nations Children's Fund, United Nations Development Programme, the World Bank and the World Health Organization.^{[citation needed]}

TDR's vision is to foster an effective global research effort on infectious diseases of poverty in which disease endemic countries play a pivotal role. It has a dual mission of developing new tools and strategies against these diseases, and to develop the research and leadership capacity in the countries where the diseases occur. The TDR secretariat is based in Geneva, Switzerland, but the work is conducted throughout the world through many partners and funded grants.^{[citation needed]}

Some examples of work include helping to develop new treatments for diseases, such as ivermectin for onchocerciasis (river blindness); showing how packaging can improve use of artemesinin-combination treatment (ACT) for malaria; demonstrating the effectiveness of bednets to prevent mosquito bites and malaria; and documenting how community-based and community-led programmes increases distribution of multiple treatments. TDR history

The current TDR disease portfolio includes the following entries:^[6]

Historical TDR disease portfolio
Disease	When added	Pathogen	Primary vector	Primary endemic areas	Frequency	Annual deaths	Complications
Malaria	1975	Plasmodium falciparum and four other Plasmodium species of protozoa	Anopheles mosquitoes	throughout the tropics	228 million (2018)	405,000 (2018)	fever, tiredness, vomiting, headache	yellow skin, seizures, coma, death
Schistosomiasis /ˌʃɪstəsəˈmaɪəsɪs/^[7]^[8] (snail fever, bilharzia, "schisto")	1975	Schistosoma flatworms (blood flukes)	freshwater snails	throughout the tropics	252 million (2015)	4,400–200,000	abdominal pain, diarrhea, bloody stool, blood in the urine. In children, it may cause poor growth and learning difficulty.	Liver damage, kidney failure, infertility, bladder cancer
Lymphatic filariasis	1975	Wuchereria bancrofti, Brugia malayi, and Brugia timori filarial worms	mosquitoes	throughout the tropics	38.5 million (2015)	few		lymphoedema, elephantiasis, hydrocele
Onchocerciasis /ˌɒŋkoʊsɜːrˈkaɪəsɪs, -ˈsaɪ-/^[9]^[10] (river blindness)	1975	Onchocerca volvulus filarial worms^[11]	Simuliidae black flies	sub-Saharan Africa	15.5 million (2015)	0	itching, papules	edema, lymphadenopathy, visual impairment, blindness
Chagas disease (American trypanosomiasis)	1975	Trypanosoma cruzi protozoa	Triatominae kissing bugs	South America	6.2 million (2017)	7,900 (2017)	fever, swollen lymph nodes, headache	heart failure, enlarged esophagus, enlarged colon
African trypanosomiasis (sleeping sickness)	1975	Trypanosoma brucei gambiense and T. b. rhodesiense protozoa	Glossina tsetse flies	sub-Saharan Africa	11,000 (2015)	3,500 (2015)	first stage: fever, headache, itchiness, joint pain second stage: insomnia, confusion, ataxia, hemiparesis, paralysis	anemia, endocrine disfunction, cardiac disfunction, kidney dysfunction, coma, death
Leishmaniasis	1975	Leishmania protozoa	Phlebotominae sandflies	throughout the tropics	4–12 million	24,200 (2015)	skin ulcers	fever, anemia, enlarged liver, enlarged spleen, death
Leprosy^† (Hansen's disease)	1975	Mycobacterium leprae and M. lepromatosis mycobacteria	extensive contact (probably airborne disease)	throughout the tropics	209,000 (2018)	few	skin lesions,^[12] numbness	permanent damage to the skin, nerves, limbs, and eyes
Dengue fever	1999	dengue virus	Aedes aegypti and other Aedes mosquitoes	tropical Asia	390 million (2020)	40,000	fever, headache, muscle and joint pain, rash, vomiting, diarrhea	low levels of blood platelets, hypotension, hemorrhage, shock
Tuberculosis^† (TB, consumption)	1999	Mycobacterium tuberculosis mycobacteria	airborne disease	worldwide	10 million (active, 2018), 2 billion (latent, 2018)	1.5 million (2018)	chronic cough, fever, cough with bloody mucus, weight loss	death
TB-HIV coinfection^‡	1999	HIV + Mycobacterium tuberculosis	sexual contact + airborne disease	Africa	1.2 million (2015)	251,000 (2018)
Sexually transmitted infections (notably syphilis, gonorrhoea, chlamydia, trichomoniasis, hepatitis B, HSV, HIV, and HPV)	2000	bacteria, parasite, viruses	sexual contact	worldwide			various	various

^† Although leprosy and tuberculosis are not exclusively tropical diseases, their high incidence in the tropics justifies their inclusion.

^‡ People living with HIV are 19 (15–22) times more likely to develop active TB disease than people without HIV.

Other neglected tropical diseases

Additional neglected tropical diseases include:^[13]

Disease	Causative Agent	Comments
Hookworm	Ancylostoma duodenale and Necator americanus
Trichuriasis	Trichuris trichiura
Treponematoses	Treponema pallidum pertenue, Treponema pallidum endemicum, Treponema pallidum carateum, Treponema pallidum pallidum
Buruli ulcer	Mycobacterium ulcerans
Dracunculiasis	Dracunculus medinensis
Leptospirosis	Leptospira
Strongyloidiasis	Strongyloides stercoralis
Foodborne trematodiases	Trematoda
Neurocysticercosis	Taenia solium
Scabies	Sarcoptes scabiei
Flavivirus Infections	Yellow fever virus, West Nile virus, dengue virus, Tick-borne encephalitis virus, Zika virus

Some tropical diseases are very rare, but may occur in sudden epidemics, such as the Ebola hemorrhagic fever, Lassa fever and the Marburg virus. There are hundreds of different tropical diseases which are less known or rarer, but that, nonetheless, have importance for public health.

Relation of climate to tropical diseases

The so-called "exotic" diseases in the tropics have long been noted both by travelers, explorers, etc., as well as by physicians. One obvious reason is that the hot climate present during all the year and the larger volume of rains directly affect the formation of breeding grounds, the larger number and variety of natural reservoirs and animal diseases that can be transmitted to humans (zoonosis), the largest number of possible insect vectors of diseases. It is possible also that higher temperatures may favor the replication of pathogenic agents both inside and outside biological organisms. Socio-economic factors may be also in operation, since most of the poorest nations of the world are in the tropics. Tropical countries like Brazil, which have improved their socio-economic situation and invested in hygiene, public health and the combat of transmissible diseases have achieved dramatic results in relation to the elimination or decrease of many endemic tropical diseases in their territory.^{[citation needed]}

Climate change, global warming caused by the greenhouse effect, and the resulting increase in global temperatures, are possibly causing tropical diseases and vectors to spread to higher altitudes in mountainous regions, and to higher latitudes that were previously spared, such as the Southern United States, the Mediterranean area, etc.^[14]^[15] For example, in the Monteverde cloud forest of Costa Rica, global warming enabled Chytridiomycosis, a tropical disease, to flourish and thus force into decline amphibian populations of the Monteverde Harlequin frog.^[16] Here, global warming raised the heights of orographic cloud formation, and thus produced cloud cover that would facilitate optimum growth conditions for the implicated pathogen, B. dendrobatidis.^{[citation needed]}

Prevention and treatment

Vector-borne diseases

Vectors are living organisms that pass disease between humans or from animal to human. The vector carrying the highest number of diseases is the mosquito, which is responsible for the tropical diseases dengue and malaria.^[17] Many different approaches have been taken to treat and prevent these diseases. NIH-funded research has produced genetically modify mosquitoes that are unable to spread diseases such as malaria.^[18] An issue with this approach is global accessibility to genetic engineering technology; Approximately 50% of scientists in the field do not have access to information on genetically modified mosquito trials being conducted.^[19]

Other prevention methods include:

Draining wetlands to reduce populations of insects and other vectors, or introducing natural predators of the vectors.
The application of insecticides and/or insect repellents to strategic surfaces such as clothing, skin, buildings, insect habitats, and bed nets.
The use of a mosquito net over a bed (also known as a "bed net") to reduce nighttime transmission, since certain species of tropical mosquitoes feed mainly at night.

Community approaches

Assisting with economic development in endemic regions can contribute to prevention and treatment of tropical diseases. For example, microloans enable communities to invest in health programs that lead to more effective disease treatment and prevention technology.^[20]

Educational campaigns can aid in the prevention of various diseases. Educating children about how diseases spread and how they can be prevented has proven to be effective in practicing preventative measures.^[21] Educational campaigns can yield significant benefits at low costs.^{[citation needed]}

Other approaches

Use of water wells, and/or water filtration, water filters, or water treatment with water tablets to produce drinking water free of parasites.
Sanitation to prevent transmission through human waste.
Development and use of vaccines to promote disease immunity.
Pharmacologic treatment (to treat disease after infection or infestation).

References

^ Farrar, Jeremy; Hotez, Peter J; Junghanss, Thomas; Kang, Gagandeep; Lalloo, David; White, Nicholas (2013). Manson's tropical diseases (New ed.). Philadelphia: Saunders [Imprint]. ISBN 9780702051012.
^ Farley, John (2003). Bilharzia : a history of imperial tropical medicine (1. paperback ed.). [S.l.]: Cambridge Univ Press. ISBN 0521530601.
^ "Deforestation Boosts Malaria Rates, Study Finds". npr.org. Archived from the original on 3 January 2018. Retrieved 4 May 2018.
^ UK 'faces tropical disease threat' Archived 2006-06-15 at the Wayback Machine, BBC News
^ von Csefalvay, Chris (2023), "Host-vector and multihost systems", Computational Modeling of Infectious Disease, Elsevier, pp. 121–149, doi:10.1016/b978-0-32-395389-4.00013-x, ISBN 978-0-323-95389-4, retrieved 2023-03-05
^ "Disease portfolio". Special Programme for Research and Training in Tropical Diseases. Archived from the original on 2008-01-13. Retrieved 2009-08-01.
^ "Schistosomiasis". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 2020-04-12.
^ "Schistosomiasis". Merriam-Webster.com Dictionary. Merriam-Webster. Retrieved 2016-01-21.
^ "Onchocerciasis". Merriam-Webster.com Dictionary. Merriam-Webster. Retrieved 2016-01-21.
^ "Onchocerciasis". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 2022-08-29.
^ "Global Partnership to Eliminate Riverblindness". Archived from the original on 2008-03-24. Retrieved 2008-03-24. The World Bank | Global Partnership to Eliminate Riverblindness. Retrieved 2007-11-04.
^ Kenneth J. Ryan and C. George Ray, Sherris Medical Microbiology Fourth Edition McGraw Hill 2004.
^ Hotez, P. J.; Molyneux, DH; Fenwick, A; Kumaresan, J; Sachs, SE; Sachs, JD; Savioli, L (September 2007). "Control of Neglected Tropical Diseases". The New England Journal of Medicine. 357 (10): 1018–1027. doi:10.1056/NEJMra064142. ISSN 0028-4793. PMID 17804846. 17804846.
^ Climate change brings malaria back to Italy Archived 2016-03-05 at the Wayback Machine The Guardian 6 January 2007
^ BBC Climate link to African malaria Archived 2006-06-16 at the Wayback Machine 20 March 2006.
^ Pounds, J. Alan et al. "Widespread Amphibian Extinctions from Epidemic Deisease Driven by Global Warming." Nature 439.12 (2006) 161-67
^ "Vector-borne diseases". www.who.int. Retrieved 2020-05-04.
^ "Engineering malaria resistance in mosquitoes". National Institutes of Health (NIH). 2017-10-23. Retrieved 2020-05-04.
^ Boëte, Christophe; Beisel, Uli; Reis Castro, Luísa; Césard, Nicolas; Reeves, R. Guy (2015-08-10). "Engaging scientists: An online survey exploring the experience of innovative biotechnological approaches to controlling vector-borne diseases". Parasites & Vectors. 8 (1): 414. doi:10.1186/s13071-015-0996-x. ISSN 1756-3305. PMC 4530488. PMID 26259589.
^ "WHO | Linking health to microfinance to reduce poverty". WHO. Archived from the original on June 17, 2013. Retrieved 2020-05-04.
^ LaBeaud, A. Desiree; Glinka, Allison; Kippes, Christopher; King, Charles Harding (October 2009). "School-Based Health Promotion for Mosquito-Borne Disease Prevention in Children". The Journal of Pediatrics. 155 (4): 590–592.e1. doi:10.1016/j.jpeds.2009.03.009. ISSN 0022-3476. PMC 3104726. PMID 19773005.