Tropical disease

A tropical disease is an infectious disease that is prevalent or unique in tropical and subtropical regions, lying within tropical medicine, parasitology and global health and covering malaria, dengue, schistosomiasis, lymphatic filariasis, onchocerciasis, leishmaniasis, Chagas disease, African trypanosomiasis, leprosy and tuberculosis. Such diseases are less common in temperate climates, partly because a cold season forces insects into dormancy and so limits vector numbers. Insect vectors are the most common transmission link, with pathogens entering through the skin during a bite. Neglected tropical diseases together affect more than a billion people, mostly poor and marginalised populations in low-income countries, and are a priority of the global health agenda.

Terminology and classification

• Tropical disease is a descriptive geographical concept for infectious diseases occurring mainly in tropical and subtropical regions rather than a taxonomic unit built on pathogen or organ system.^[1]
• The concept differs from neglected tropical disease, which the World Health Organization defines as a group of poverty-related conditions long neglected in research and development, its extent set by an institutional list rather than by geography.^[1]
• The World Health Organization lists neglected tropical diseases as a global public health priority, the list spanning bacterial, parasitic, viral and fungal causes and poisoning and emphasising their disproportionate effect on poor populations.^[1]
• The International Classification of Diseases has no dedicated tropical disease chapter, and the relevant conditions are listed by pathogen in the chapters on infectious and parasitic diseases, with malaria, tuberculosis and sexually transmitted infections coded separately.^[1]
• ICD-11 arranges neglected tropical disease-related parasitoses together, and cross-version and cross-system conversion must rely on the official mapping rather than a geographical concept.^[1]
• A distinction must be drawn between tropical disease and tropical medicine, the former being the diseases themselves and the latter the discipline and practice that studies and manages them, and the two are not coextensive.
• Leprosy and tuberculosis are not unique to the tropics but are included in the agenda because of their high incidence there, showing that the category aggregates by burden rather than by strict geography.^[1]
• Some neglected tropical diseases have regional names, such as river blindness for onchocerciasis and breakbone fever for dengue, and formal reporting should use the standard name with generic terms and abbreviations.
• Emerging and re-emerging infectious diseases overlap with tropical disease in extent, and Ebola haemorrhagic fever, Lassa fever and Marburg virus, though rare, can appear as outbreaks and carry major public health significance.
• In cross-database and cross-language retrieval, tropical disease may be filed under infectious disease or listed as a global health topic, and searches must cover both pathogen names and the geographical concept to avoid missing records.^[1]
• Historical layering leaves different institutions with different scopes, so the World Health Organization, national agencies and academic bodies may classify the same disease differently, and comparisons must state which list version is used.^[1]
• This group is not a single disease entity but a category aggregated by geography and poverty, and the entry should state that it covers many conditions with different causes, mechanisms and courses.^[1]
• Terminology disputes centre on the geographical definition of the tropics, and as the climate warms and tropical diseases appear in formerly temperate regions the stability of the geographical label is questioned.^[2]
• Semantically a distinction must be drawn between endemic transmission, meaning sustained local spread, and imported cases brought by travel or migration, which have different implications for control.^[2]

History

• In the seventeenth and eighteenth centuries, before modern ideas of causation formed, many tropical diseases were present in northern Europe and North America, showing that their historical range was not confined to the tropics.^[1]
• The original impetus for tropical medicine was to protect the health of colonial settlers, especially in British-ruled India, and colonial needs drove the establishment of dedicated research institutions.^[2]
• In the late nineteenth century Manson and Ross turned tropical medicine from an empirical into an experimental discipline through work on parasites and mosquito vectors, laying the foundation of vector transmission theory.^[2]
• In the mid-twentieth century the wide use of insecticides and antibiotics reduced several tropical diseases markedly, but resistance and ecological side effects soon appeared.^[3]
• In 1975 the Special Programme for Research and Training in Tropical Diseases was created within the World Health Organization and co-sponsored by the UNDP, the World Bank, the World Health Organization and UNICEF.^[1]
• The vision of the programme was to promote effective global research on poverty-related infectious diseases and to develop research and leadership capacity in the countries where they occur.^[1]
• Around 1999 and 2000 dengue and sexually transmitted infections were successively added to the agenda, and the disease scope expanded with changing global health priorities.^[1]
• Through the late twentieth and early twenty-first centuries understanding of neglected tropical diseases deepened, and academic and policy documents redefined them as preventable and controllable public health problems.^[1]
• In 2012 several institutions and pharmaceutical companies signed the London Declaration, committing to expanded drug donation and control targets and marking a milestone in global cooperation on neglected tropical diseases.^[1]
• Around 2015 neglected tropical diseases were incorporated into the United Nations Sustainable Development Goals, making control and elimination progress a global health indicator.^[1]
• In 2021 the World Health Organization issued a new neglected tropical disease roadmap setting quantified control and elimination targets for 2030.^[1]
• From the late twentieth century large-scale drug donation, such as that of ivermectin, markedly advanced control of onchocerciasis and lymphatic filariasis and became a model of public health intervention.^[1]
• Recent years have brought genetic engineering and Wolbachia techniques into vector control, changing the traditional insecticide-centred approach.^[4]
• A hypothesis that has been overturned is the attribution of tropical disease simply to climate and miasma, since modern research emphasises the combined role of pathogen, vector, ecology and socio-economics.^[1]

Epidemiology

• Neglected tropical diseases together affect more than a billion people, mainly in low-income and middle-income countries of Africa, Asia and the Americas.^[1]
• In 2024 malaria accounted for about 241 million cases and 627,000 deaths, children under five making up most deaths and sub-Saharan Africa carrying the heaviest burden.^[2]
• Dengue is estimated at 390 million infections a year, a substantial proportion asymptomatic, and reported cases keep rising with urbanisation and vector spread.^[4]
• In 2021 schistosomiasis was estimated to require treatment in 252 million people, with sub-Saharan Africa the main endemic region and school-age children especially affected.^[5]
• In 2015 lymphatic filariasis was estimated to affect 38.5 million people and can cause lymphoedema, elephantiasis and hydrocele.^[1]
• Chagas disease is estimated at 6.2 million infections in the Americas and African trypanosomiasis at about 11,000 reported cases in 2015, both concentrated in poor rural areas.^[1]
• Leishmaniasis is estimated at 4 to 12 million prevalent cases, the visceral form being fatal and the cutaneous form widely distributed across tropical and subtropical regions.^[1]
• The distribution of these diseases tracks climate, vector abundance and sanitation, and year-round heat and abundant rain shape breeding sites and vector numbers.^[2]
• Age and sex affect exposure, with school-age children having the highest prevalence of soil-transmitted helminths and schistosomiasis and adults exposed occupationally to vectors and infected water.^[5]
• Risk factors include at least lack of safe water and sanitation, housing without bed nets or screens, contact with infected water or contaminated soil, poverty and malnutrition and population movement and urbanisation.^[1]
• Socio-economic factors matter greatly, since the world's poorest countries lie mostly in the tropics, poverty limits access to care and protection and disease and poverty reinforce each other.^[1]
• Urbanisation and deforestation alter vector habitat and human-animal contact, and urban dengue and imported parasitic disease rise with population movement.^[2]
• Climate change is spreading tropical diseases and vectors to higher altitudes and latitudes, and formerly unaffected temperate regions report local transmission.^[2]
• Economic and social effects include loss of labour, interrupted schooling and deepening poverty, making neglected tropical diseases both a health and a development problem.^[1]
• Data sources are mainly the World Health Organization, the Special Programme for Research and Training in Tropical Diseases and national surveillance, and prevalence and incidence under different definitions cannot be compared directly.^[1]

Pathophysiology

• The causes of tropical disease include parasites, bacteria, viruses and fungi, parasites and vector-borne viruses being the majority, and transmission often depends on intermediate hosts or insect vectors.^[1]
• A vector is an organism that transmits disease between people or from animals to people, and mosquitoes are the most common vector, carrying dengue and malaria among tropical diseases.^[4]
• Pathogens enter through the skin during a vector bite, parasites undergo development or multiplication in the vector, and temperature and humidity affect the rate of development.^[2]
• The schistosome life cycle depends on freshwater snails as intermediate hosts, cercariae penetrate the skin and eggs retained in tissue provoke granulomatous inflammation and fibrosis.^[5]
• Lymphatic filariae and Onchocerca are transmitted by mosquitoes and blackflies, adults live in the lymphatic system or subcutaneous tissue and cause lymphatic obstruction and skin and eye lesions.^[1]
• The pathogenesis of dengue is closely tied to vascular endothelial function, and immune injury to the endothelium during viraemia causes plasma leakage and a bleeding tendency.^[6]
• Climate directly shapes transmission ecology, since higher temperatures may favour pathogen replication inside and outside the vector and rainfall alters breeding sites and vector generations.^[2]
• Human activity intensifies transmission, as fossil fuel burning, deforestation, industrial agriculture and urbanisation release greenhouse gases, alter the climate and expand vector habitat.^[2]
• Habitat destruction disturbs natural hosts and increases human contact with wildlife, raising the risk of zoonotic introduction into human populations.^[2]
• Reservoir hosts and animal disease form a transmission pool, and zoonoses cycling between vectors and reservoir animals complicate elimination.^[1]
• Immune status affects outcome, since malnutrition and human immunodeficiency virus infection weaken resistance and co-infection aggravates the course and raises mortality.^[1]
• The pathology of chronic infection is dominated by fibrosis and organ dysfunction, such as schistosomal liver fibrosis and bladder disease and cardiac damage in Chagas disease.^[5]
• Co-evolution of pathogen and vector poses adaptive challenges, and insecticide resistance and drug resistance emerge with prolonged intervention.^[3]
• Environmental and host factors together determine the disease spectrum, and the same pathogen can produce different clinical types and transmission intensity in different ecological zones.^[2]

Clinical features

• Malaria produces periodic chills, high fever and sweating with anaemia and hepatosplenomegaly, and falciparum malaria can progress rapidly to cerebral malaria and multi-organ failure.^[2]
• Dengue presents with fever, headache, muscle and joint pain, rash, vomiting and diarrhoea, and severe cases show thrombocytopenia, hypotension, bleeding and shock.^[4]
• Acute schistosomiasis may cause fever and rash, while chronic disease causes abdominal pain, diarrhoea, rectal bleeding and haematuria, and children may show growth retardation and learning difficulty.^[5]
• Lymphatic filariasis presents with lymphoedema, elephantiasis and hydrocele, and recurrent attacks cause irreversible disfigurement and loss of function of limbs and genitalia.^[1]
• Onchocerciasis begins with pruritus and subcutaneous nodules and progresses to corneal and optic nerve damage and blindness, hence the name river blindness.^[1]
• Cutaneous leishmaniasis produces crusted ulcers, while the visceral form causes fever, anaemia and hepatosplenomegaly and, untreated, death within months to two years.^[1]
• Chagas disease is usually occult in the acute phase and in the chronic phase causes heart failure, oesophageal dilation and colonic dilation, a leading cause of heart failure in South America.^[1]
• African trypanosomiasis has two stages, the first with fever, headache and arthralgia and the second with insomnia, confusion, ataxia and coma.^[1]
• Leprosy mainly affects skin and peripheral nerves, causing sensory loss and skin lesions, and late disease brings permanent damage and deformity of hands, feet and eyes.^[1]
• Complications include at least blindness and visual impairment, lymphoedema and elephantiasis, liver fibrosis and portal hypertension, heart failure and renal impairment and the social exclusion and poverty caused by long-term infection.^[1]
• Onset and course differ markedly, as vector-borne viral disease is usually acute and self-limiting while helminthic disease and schistosomiasis are chronic and often leave irreversible change.^[5]
• Among special populations, pregnant women and children are more prone to severe disease, infection in pregnancy can cause miscarriage, preterm birth and congenital infection and immunosuppressed people may develop disseminated disease.^[1]
• Coexisting disease is common in endemic regions, where one population may carry several neglected tropical diseases at once, compounding the presentation and the burden.^[1]
• Early signs are often non-specific, with fever, fatigue and rash overlapping many febrile illnesses, and diagnosis must combine exposure history with epidemiological clues.^[6]

Diagnosis

• Diagnosis depends on the pathogen, with malaria relying on thick and thin films and rapid antigen tests and dengue on nucleic acid and antigen serology.^[4]
• Schistosomiasis is diagnosed by egg detection in stool or urine and by serology, and imaging can reveal liver fibrosis and portal changes.^[5]
• Lymphatic filariasis and onchocerciasis can be confirmed by microscopy for microfilariae in blood and by skin biopsy, and immunochromatographic rapid tests have improved community screening.^[1]
• Leishmaniasis and trypanosomiasis are diagnosed by combining microscopy, serology and molecular tests, with bone marrow or splenic aspirate to detect the pathogen in visceral disease.^[1]
• Molecular tests are highly sensitive for low-load infection and for judging cure and relapse, but cost and grassroots availability remain bottlenecks.^[1]
• Differential diagnosis includes at least imported non-tropical parasitic disease, traveller diarrhoea, influenza and other febrile illness, viral hepatitis and drug rash.^[6]
• Dengue must be distinguished from influenza, measles and other febrile exanthems, and epidemiological history with thrombocytopenia and haemoconcentration points to dengue.^[6]
• Diagnostic delay is common where resources are limited, because primary care lacks equipment and reagents and patients are often first treated as ordinary fever, missing the window for early intervention.^[1]
• Screening differs by population and disease, with schoolchildren screened for soil-transmitted helminths and schistosomiasis and pregnant women and blood donors screened for malaria and Chagas disease.^[5]
• Vector surveillance is part of diagnosis and control, and monitoring mosquito density and resistance guides the intensity and timing of intervention.^[3]
• Diagnostic criteria differ between institutions, with varying thresholds and positivity rules, and multicentre comparisons must report the standard and version used.^[1]
• Population diagnosis should combine syndromic surveillance and laboratory data, since case reports alone cannot gauge epidemic trends or intervention effect.^[1]

Management

• Treatment aims to clear the pathogen, relieve symptoms, interrupt transmission and reduce complications, with the regimen set by pathogen, course and locally available resources.^[1]
• Malaria is treated first-line with artemisinin-based combination therapy, severe malaria with intravenous artesunate and vivax and ovale malaria with added primaquine to clear hepatic hypnozoites.^[2]
• Onchocerciasis is controlled by mass administration of ivermectin and lymphatic filariasis by mass treatment with albendazole combined with diethylcarbamazine or ivermectin, both named therapies.^[1]
• Schistosomiasis is treated with praziquantel, and mass chemotherapy of schoolchildren is an important strategy to reduce the burden of infection.^[5]
• Improving socio-economic conditions aids prevention and treatment, since microcredit allows communities to invest in health projects and so obtain more effective treatment and prevention technologies.^[1]
• Health and child education can deliver large benefit at low cost, and teaching children how diseases spread and how to prevent them has proved effective in practice.^[7]
• Vector control is central to prevention, including draining wetlands to reduce breeding sites, applying insecticides to clothing and nets and using bed nets to reduce night-time transmission.^[3]
• Indoor residual spraying is a named malaria control measure, and the use of dichlorodiphenyltrichloroethane and similar insecticides in Africa is constrained by resistance and environmental concern.^[3]
• Genetically modified mosquitoes are an emerging vector control technology that prevents female mosquitoes from surviving to adulthood and reduced mosquito populations by more than ninety percent in Brazilian field trials.^[8]
• The Wolbachia method makes mosquitoes resistant to dengue virus, and a trial in Yogyakarta, Indonesia, showed a seventy-seven percent reduction in symptomatic dengue.^[4]
• Integrated vector management combines biological control, insecticides and public education and has proved successful in reducing arboviral transmission, offering a more sustainable solution.^[9]
• Safe water and sanitation interrupt faecal-oral and water-contact transmission, and wells, filters and purification tablets remove parasites from drinking water.^[5]
• Vaccines are an important means of prevention, with products available for some diseases, but most neglected tropical diseases still lack an effective vaccine and require continued development.^[1]
• Prognosis varies markedly with disease and with how early care is sought, most patients recovering with timely treatment, chronic diseases producing organ damage decades later and elimination goals depending on sustained intervention.^[1]

Research and controversies

• Access to vector control technology is contested, since genetic engineering performs well but global access is uneven and about half of scientists could not obtain information on ongoing genetically modified mosquito trials.^[10]
• The role of climate change attracts wide attention, with most studies supporting warming-related expansion of vector habitat and longer transmission seasons but quantitative effects on specific diseases and regions remaining uncertain.^[2]
• The debate between elimination and control is long-standing, since some diseases can be eliminated locally while others, with animal reservoirs and vectors, can only be controlled in the long term.^[1]
• Drug donation and patent policy are contested, as mass donation improves access but long-term sustainability and incentives for new drug development remain questioned.^[1]
• Insecticide resistance threatens past gains, and balancing resistance management against highly effective but environmentally sensitive measures such as dichlorodiphenyltrichloroethane is an ongoing policy dispute.^[3]
• Diagnostic access is contested, since primary care lacks equipment and reagents, rapid tests lose sensitivity as pathogen load falls and both underdiagnosis and overdiagnosis occur.^[1]
• Overdiagnosis and underdiagnosis distort burden estimates, since counting or missing asymptomatic infection skews prevalence, making diagnostic thresholds a focus of dispute.^[1]
• Ecological and ethical disputes concern the impact of vector control on non-target organisms, and the release of genetically modified organisms requires risk assessment and community consent.^[10]
• Unmet needs include broad-spectrum antiparasitic drugs, vaccines covering several diseases, low-cost sensitive tests and sustainable services for poor populations.^[1]
• Data gaps leave global estimates unstable, because many endemic countries lack complete registration and death certification and the reliability of modelled estimates is limited by input data quality.^[1]
• Community participation and health education are considered indispensable, but how to implement them effectively across cultural settings still lacks high-quality evidence.^[7]
• Cross-disciplinary and cross-national cooperation is the research trend, and the intersection of parasitology, epidemiology, climate science and engineering can drive innovation in assessment and intervention tools.^[2]