Human Tropical Diseases: How Are They Invading New Areas and What Are the Steps to Minimize Their Impacts?

Where did malaria originate? Although the etymology is Italian in origin, this million years old disease is believed to originate in Africa. Its transmission across continents was probably facilitated by human movement, given that many malaria-infected farmers from North Africa migrated to the Italian island of Sardinia in 502 B.C.E. (Packard, 2007).

However, the parasite importation was not the only contributing factor to malaria invasion in that new area. Three elements are crucial for infectious disease causation: agent, host, and environmental factors (Seventer, 2016). Therefore, deforestation that occurred in Sardinia during agricultural land preparation should also be considered as it led to floods and standing waters in which the Anopheles mosquitos — carriers of the malaria pathogens — favour to breed.

This essay will discuss the roles of poverty, human mobility, and environmental changes on the emergence and persistence of tropical diseases in new areas. Minimizing the impacts of tropical diseases expansion is of interest because the Earth has been experiencing a rising temperature due to climate changes. This may render the disease vectors, such as mosquitos and sand flies, more capable of surviving in previously cooler regions. In addition, global air travel is becoming more accessible and affordable to a wider range of populations, which poses the risk of a pathogen being carried from one corner of the Earth to another one within a short period of time.

Tropical diseases, as the name implies, normally thrive in warmer and wetter parts of the globe. The diseases, in particular the neglected ones, are predicted to affect approximately two billion individuals at the turn of the millennium (Engels and Zhou, 2020). Tropical disease pathogens include viruses (e.g. dengue fever and Ebola), bacteria (e.g. leprosy and cholera), and parasites (e.g. sleeping sickness and lymphatic filariasis). These agents can be transmitted directly or indirectly. For instance, an individual with leprosy can directly infect others via mouth and nose droplets. However, indirect transmission is seemingly more frequent as many tropical disease agents depend on insect vectors — both of which are highly affected by climatic factors, including temperature, humidity, and precipitation — to spread to the next susceptible hosts.

Socioeconomic Drivers in Tropical Diseases

Intriguingly, tropical diseases tend to persist in the poorest areas of the world that deserve improved access to clean water, sanitation, suitable housing, and healthcare. For example, in rural areas of Central and South America, individuals living in a mud and thatched house are at a higher risk of contracting American trypanosomiasis, considering their building materials offer triatomine bugs — Chagas disease vectors capable of transmitting Trypanosoma cruzi — an ideal place to reproduce (Gurevitz et al., 2011). In addition, many people living in economically developing nations still depend on wells, cisterns, and sumps as their sources of water, granting opportunities for mosquitos to multiply. In Europe, tropical diseases are also the most prevalent in the developing areas of the continent such as Southern and Eastern Europe (Hotez, 2009), one example being Spain’s growing public health challenge of Chagas disease (Navarro et al., 2012). It is undeniable that poverty is a major risk factor for transmission and persistence of tropical diseases in one area, thus public health officials must consider the socioeconomic context when planning intervention strategies to predict, prevent, and mitigate tropical diseases.

Imported Tropical Diseases

Global Air Transportation

In recent years, Chagas disease — an endemic of Latin America — has spread not only to Spain, but also to the United States, Canada, Australia, and Japan (Navarro et al., 2012). How does this disease overcome international borders? Undetected infected newcomers. Recent advances in international air transportation means tropical disease agents can be carried by ill passengers or insect vectors from one far away place to another in the blink of an eye. As expected, Spain is the European country with the highest number of immigrants from Chagas disease-endemic countries (Navarro et al., 2012). Similarly, Montgomery et al. (2016) estimated that out of millions of Latin American immigrants in the United States, up to 300,000 of them could carry Trypanosoma cruzi.

In the 1300s, flea-infested rats sailed on ships and transported devastating plague ‘Black Death’ across continents. Today, disease-causing insect vectors can hitch a ride on an airplane to travel rapidly from endemic to non-endemic regions, as in the case of ‘airport malaria’. This phenomenon threatens individuals in the vicinity of an airport and could lead to death, with a mortality rate ranging from 16.9% to 26% over a decade ago (Manguin et al., 2008). Multiple studies have reported cases where Anopheles mosquitos are hidden in suitcases or cargo, and later bite local residents in the destination areas (Siala et al., 2015; Wieters et al., 2019). Subsequently, the imported Plasmodium parasites and their insect vectors may lead to autochthonous transmission should local Anopheles mosquitos acquire the pathogens from ill passengers or residents that have been previously bitten by the imported mosquitos. The exportation and importation of products also facilitate tropical diseases spread, including Salmonella infections, which are commonly transmitted through contaminated food and water. For example, in the summer of 2017, the importation of Maradol papayas from Mexican farms caused a multistate Salmonella outbreak in the United States (Hassan et al., 2019).

Displaced People

Enormous human displacement raises the possibility of tropical diseases invasion in new areas. For decades, cutaneous leishmaniasis has been endemic in Syria (Haddad et al., 2015). The ongoing Syrian war has forced millions of civilians to flee to more stable countries. Consequently, the incidence of cutaneous leishmaniasis in countries with a high population of Syrian refugees has significantly increased in the past years. Neighbouring countries, such as Turkey, Jordan, and Lebanon, exhibit a higher risk of disease invasion upon hosting the asylum seekers given that these regions already have an ideal climate and vector potential (Salman et al., 2014). For instance, over one thousand new leishmaniasis cases associated with Syrian refugees were identified in Lebanon in 2013. This was in contrast with the preceding years where the annual number ranged between zero to six individuals (Alawieh et al., 2014). Furthermore, it has been suggested that leishmaniasis carrying sand flies from Syria were also brought into Lebanon along with the incoming refugees (Saroufim et al., 2014). Considering the poor living conditions and arduous journeys that the refugees must endure, they may be prone to tropical diseases that are endemic in their new destinations. If infected refugees later travel toward other countries or return to their homeland once the war is over, this may help the diseases expand their geographic range.

Transmission in Vector-Free Regions

Tropical disease pathogens are able to thrive in new areas in the absence of their natural vector species by adapting to indigenous vectors or taking advantage of person-to-person transmission. The pathogen Leishmania infantum — or Leishmania chagasi in South America — was brought to the Americas by European settlers during the colonialism era (Steverding, 2017). Although Phlebotomus sandflies — the natural carriers of the visceral leishmaniasis pathogen — were not present in South America, Leishmania infantum was able to adapt to a different species: Lutzomyia longipalpis. Considering that approximately 56 phlebotomine sand flies are able to host varying Leishmania protozoans, it is no surprise that Leishmania spp. could invade new regions with relative ease (Maroli et al., 2012).

Tropical disease pathogens are able to thrive in new areas in the absence of their natural vector species by adapting to indigenous vectors or taking advantage of person-to-person transmission. The pathogen Leishmania infantum — or Leishmania chagasi in South America — was brought to the Americas by European settlers during the colonialism era (Steverding, 2017). Although Phlebotomus sandflies — the natural carriers of the visceral leishmaniasis pathogen — were not present in South America, Leishmania infantum was able to adapt to a different species: Lutzomyia longipalpis. Considering that approximately 56 phlebotomine sand flies are able to host varying Leishmania protozoans, it is no surprise that Leishmania spp. could invade new regions with relative ease (Maroli et al., 2012).

Environmental Changes

Climatic conditions tend to restrict the habitat ranges of tropical disease vectors. For example, the risk of malaria transmission peaks at 25°C and significantly decreases if the temperature climbs beyond 28°C (Mordecai et al., 2012). It is evident that climate change has altered the global temperature and rainfall patterns. This may eventually lead to the unprecedented survival of insect vectors in previously cooler regions where the populations may have no prior immunity toward tropical diseases. It has been predicted that due to climate change, Chagas disease will potentially move northwards (Garza et al., 2014). Similarly, mosquito-borne tropical diseases spread by the bite of Aedes aegypti and Aedes albopictus will proliferate in North America and Europe as the temperature rises (Ryan et al., 2019).

Climate change is often characterized by extreme weather events (e.g. drought, heavy rainfalls, melting of snow, and typhoon). It is possible for such events to facilitate the invasion of tropical diseases in new areas. Take for example powerful winds that may carry mosquitoes to places hundreds of kilometres away, as seen in the long-range migration of African malaria mosquitos (Besansky, 2019). Likewise, in 2011, a post-flood leptospirosis outbreak occurred in non-endemic dry areas in Sri Lanka (Agampodi et al., 2014).

Minimizing Their Impacts

Early prevention of tropical disease expansion by implementing appropriate control measures will reduce the burden of illness and death. To date, at least six neglected tropical diseases have been associated with drug resistance: leishmaniasis, African trypanosomiasis, onchocerciasis, trachoma, soil-transmitted helminths, and schistosomiasis (Akinsolu et al., 2019). The emergence of drug-resistant tropical disease agents in new locations can be fatal if therapeutic development fails to balance the disease spread. In addition, low- and middle-income nations, which are particularly more prone to the invasion of tropical diseases, may experience a huge financial burden. Therefore, it is urgent to prevent transmission in the first place.

To minimize the importation risk of tropical diseases, non-endemic countries should establish medical assessment standards for individuals travelling from or through endemic regions, such as mandatory screening for malaria and Ebola. Furthermore, quarantine can be implemented for incoming travellers from high-risk regions who may potentially have been exposed to dangerous pathogens. This will help filter asymptomatic individuals with initial false-negative medical examination results. Imported livestock from endemic countries should also be subjected to quarantine and testing, whereas for other imported consumables, a small portion of the products ought to be tested for the presence of specific pathogens. In the same way, when planning to visit tropical disease endemic countries, travellers should take precautions (e.g. vaccination, applying insect repellent, and consuming only hygienic food) to avoid getting infected and bringing the infection back to their home countries.

The risk of disease importation via “incidental” transportation of insect vectors as in the case of airport malaria can be reduced by disinfecting the aircraft. However, human travellers who act as pathogen reservoirs likely present a greater danger, thus decontamination may not be an effective route for impending the transmission of this vector-borne disease. For transmission through blood transfusion and organ transplant, accepting donors whose antibody test results return negative for the pathogen of concern is essential. This screening strategy has been adopted to prevent Chagas disease person-to-person transmission in countries with a large population of Latin Americans, including the United States and Spain.

Lastly, climate change is likely inevitable, thus it is essential to predict the next outbreaks using modelling approaches. If disease-transmitting vectors manage to shift to new areas, various strategies can be employed to combat them, including utilizing sterilizing compounds or enhancing the immune system of the insect vectors to inhibit the pathogen lifecycle. At present, the Wolbachia-sterilizing method is on the rise to suppress the wild population of disease-transmitting vectors. For example, World Mosquito Program has been using the Wolbachia-carrying mosquitoes to limit the spread of dengue in the Indonesian city of Yogyakarta where the intervention’s protective efficacy was reported to reach 77% (Utarini et al., 2021). In addition, a surveillance and response system such as the 1–3–7 strategy implemented by China to reduce the malaria burden can be adopted to manage other tropical diseases. The strategy requires new cases to be reported within one day post-diagnosis, investigated within three days, and responded within seven days, allowing prompt public health action to prevent further transmission.

To conclude, poverty, human mobility, and climate change drive the invasion of tropical diseases in new areas. Since tropical diseases tend to persist in poor living conditions, progress against tropical diseases expansion ought to be supported by poverty alleviation efforts through international collaboration among countries. Giving aid to countries in need of better clean water infrastructure, housing, sanitation, and healthcare will eventually reduce the risk of the global spread of tropical diseases.

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