Tropical disease

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]

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.

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.

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:[5]

Historical TDR disease portfolio
Disease When added Pathogen Primary vector Primary endemic areas Frequency Annual deaths Symptoms Complications
Malaria 1975 Plasmodium falciparum and four other Plasmodium species of protazoa Anopheles mosquitoes throughout the tropics 228 million (2018) 405,000 (2018) fever, tiredness, vomiting, headache yellow skin, seizures, coma, death
Schistosomiasis /ˌʃɪstəsəˈmaɪəsɪs/[6][7] (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-/[8][9] (river blindness) 1975 Onchocerca volvulus filarial worms[10] 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,[11] 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:[12]

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.

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.[13][14] 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.[15] 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.

Prevention and treatment
Disability-adjusted life year for tropical diseases per 100,000 inhabitants. These include trypanosomiasis, chagas disease, schistosomiasis, leishmaniasis, lymphatic filariasis, onchocerciasis.
  no data
  ≤100
  100-200
  200-300
  300-400
  400-500
  500-600
  600-700
  700-800
  800-900
  900-1000
  1000-1500
  ≥1500

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.[16] 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.[17] 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.[18]

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.

Sexually transmitted diseases

Both pharmacologic pre-exposure prophylaxis (to prevent disease before exposure to the environment and/or vector) and pharmacologic post-exposure prophylaxis (to prevent disease after exposure to the environment and/or vector) are used to prevent and treat HIV

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.[19]

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.[20] Educational campaigns can yield significant benefits at low costs.

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).
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See also

References
  1. 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.
  2. Farley, John (2003). Bilharzia : a history of imperial tropical medicine (1. paperback ed.). [S.l.]: Cambridge Univ Press. ISBN 0521530601.
  3. "Deforestation Boosts Malaria Rates, Study Finds". npr.org. Archived from the original on 3 January 2018. Retrieved 4 May 2018.
  4. UK 'faces tropical disease threat' Archived 2006-06-15 at the Wayback Machine, BBC News
  5. "Disease portfolio". Special Programme for Research and Training in Tropical Diseases. Archived from the original on 2008-01-13. Retrieved 2009-08-01.
  6. "Schistosomiasis". Oxford Dictionaries UK Dictionary. Oxford University Press. Retrieved 2016-01-21.
  7. "Schistosomiasis". Merriam-Webster Dictionary. Retrieved 2016-01-21.
  8. "Onchocerciasis". Merriam-Webster Dictionary. Retrieved 2016-01-21.
  9. "Onchocerciasis". Oxford Dictionaries UK Dictionary. Oxford University Press. Retrieved 2016-01-21.
  10. "Archived copy". Archived from the original on 2008-03-24. Retrieved 2008-03-24.CS1 maint: archived copy as title (link) The World Bank | Global Partnership to Eliminate Riverblindness. Retrieved 2007-11-04.
  11. Kenneth J. Ryan and C. George Ray, Sherris Medical Microbiology Fourth Edition McGraw Hill 2004.
  12. 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.
  13. Climate change brings malaria back to Italy Archived 2016-03-05 at the Wayback Machine The Guardian 6 January 2007
  14. BBC Climate link to African malaria Archived 2006-06-16 at the Wayback Machine 20 March 2006.
  15. Pounds, J. Alan et al. "Widespread Amphibian Extinctions from Epidemic Deisease Driven by Global Warming." Nature 439.12 (2006) 161-67
  16. "Vector-borne diseases". www.who.int. Retrieved 2020-05-04.
  17. "Engineering malaria resistance in mosquitoes". National Institutes of Health (NIH). 2017-10-23. Retrieved 2020-05-04.
  18. 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). doi:10.1186/s13071-015-0996-x. ISSN 1756-3305.
  19. "WHO | Linking health to microfinance to reduce poverty". WHO. Retrieved 2020-05-04.
  20. 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.

Further reading

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