Vector-Borne Diseases: Understanding Their Transmission and Prevention, and the Role of Infection Preventionists

Vector-borne Diseases

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Vector-borne diseases are medical conditions in humans caused by pathogens transmitted by host organisms, which facilitate the transmission of diseases between individuals, from animals to humans, or through vectors. Vectors are often insects or other parasites that feed on the blood of animals and spread pathogens through that blood contact. Vector-borne illnesses account for more than 17% of all infectious diseases.¹ Some of the most common vector-borne diseases include malaria, dengue virus, Zika virus, West Nile fever, and Lyme disease. However, numerous other illnesses are transmitted by vectors worldwide. Some common vectors include mosquitoes, fleas, ticks, and select species of flies.¹

Vector-borne pathogen hosts can be divided into 3 main types: reservoir, incompetent, and dead-end hosts. Reservoirs are animals or environments where the pathogen can survive and reproduce without causing illness or symptoms. Incompetent hosts are organisms that provide nutrition for the vector but do not develop disease. Dead-end hosts are the final link in the chain of infection, where the pathogen causes illness but is not directly transmitted to other susceptible hosts. Humans are often the dead-end hosts for many vector-borne diseases.

According to World Health Organization (WHO) data, up to 80% of the world’s population is at risk for 1 or more vector-borne diseases.² However, the most significant burden of vector-borne diseases is currently in tropical and subtropical areas.³

Climate change is having a direct impact on the epidemiology of vector-borne illnesses. Travel-associated cases of vector-borne diseases do occur. However, there had been no local spread because the vector could not survive in the new environment. We now see favorable climate changes for the expansion of vector habitats. An example of how climate-impacted vector conditions is the Zika virus outbreak in South America in 2015. The weather patterns associated with El Niño in that same time frame were very conducive to the mosquito species Aedes aegypti and Aedes albopictus, resulting in an expanded vector population and a regional outbreak of Zika virus in the human population.³

Human interactions with the environment also impact vector-borne diseases. Building dams, crop irrigation, deforestation, farming and livestock practices, and urban expansion impact the vector habitats that transmit pathogens to newly susceptible hosts. For example, the expansion of irrigation for farming purposes has been shown to significantly impact the spread of emerging infectious and zoonotic diseases in the local population.⁴

Dengue virus is the most widespread vector-borne illness. A study of cities in India found that increased green spaces with open water and densely populated areas with poor waste management practices had increased the Aedes mosquito populations.⁵ Traditional approaches to removing the vectors in these urban areas are no longer effective, as the vectors’ breeding grounds evolve with changes in city design.

Travel from endemic areas often introduces vector-borne diseases to nonendemic regions. For example, mosquitoes transmitted on flights have been known to spread malaria, dengue virus, and chikungunya to France.⁵ Typically, the vector would not have been available in France to continue the transmission cycle, but with adaptation, the local mosquito population is now capable of transmitting these pathogens, and numerous local cases are identified annually.

Additionally, antimicrobial use in livestock and humans can cause mutations in the pathogen’s genetic code, leading to drug-resistant variants. Like other multidrug-resistant organisms, antimicrobial stewardship is essential for vector-borne diseases.

Prevention practices for vector-borne diseases have a hierarchical level of effectiveness. The most effective control types are elimination, engineering, and behavioral controls. Active surveillance and understanding a disease’s epidemiology are the foundations of prevention efforts. As with many prevention programs, a combination of all types is necessary to reduce the transmission risk effectively.

Elimination controls in these cases include the removal of the vector. Chemical insecticides are used widely in this instance. Insecticides have proven effectiveness; however, there are concerns with resistance and adverse effects of the chemicals, which limit their use.⁴ Removal of the breeding ground for the vectors is another approach to elimination controls. Water and sanitation programs are crucial public health practices that can reduce risk. Zoonotic transmission and reservoirs must also be included in control efforts. Trends over the past decades have shown that most emerging infectious diseases have a zoonotic transmission origin. For example, the West Nile virus emergence in the US was seen as one of the most significant zoonotic diseases transmitted via vectors to humans.⁴

Engineering controls help to isolate people from a hazard. Some examples of engineering controls include mosquito nets, screens in windows and doors, and, more recently, genetic modification of vectors to limit their ability to spread pathogens. Research on arboviruses can lead to possible genetic solutions to disease transmission. For example, a 2022 study examined why yellow fever is not endemic in Asia, even though the vector is prevalent. Researchers found that the mosquitoes infected with Japanese encephalitis could not transmit yellow fever.⁵ Understanding these natural occurrences could lead to further research on potential genetic modification to mimic the same controls.

Vaccination is available for some vector-borne diseases, such as yellow fever and dengue virus. However, further research and development are needed to increase vaccine availability and effectiveness. Additionally, trust in national public health and vaccine programs has decreased over time, so much education and culture-specific dialogue are needed to regain the acceptance of vaccines internationally.

Behavioral modification is often the least effective intervention, as human factors cannot be controlled in a standard way to ensure compliance. However, education and personal practices are still essential in prevention practices. Examples include using bug sprays or lotions, wearing long sleeves and pants, emptying standing water around one’s home, and engaging in behaviors to minimize vector exposure. The use of prophylactic medication is another example of behavioral control.

With all this information, what does a typical infection preventionist need to know about vector-borne diseases? As stated above, the prevention aspects are typically in public health, which aligns with infection prevention but has a slightly different lens to how disease prevention is approached. Infection preventionists must collaborate with local public health agencies. Knowing your local epidemiology of vector-borne diseases, concerns, and whether we have known vectors in our area should be shared with your frontline staff, including emergency room providers and infectious disease clinicians. I worked with local public health staff members when the first locally transmitted case of dengue virus was identified in an assisted living community under our health care organization’s umbrella. Infection prevention acted as the liaison between public health and the administrators of the subacute facility to help with case identification, implementation of mitigation efforts, and staff and resident education. As vector-borne diseases spread into novel areas, infection preventionists can be at the forefront of preparedness efforts to ensure their facilities and communities are proactively engaged.

One last thing to consider, as many of these diseases are preventable with basic public health strategies, is why they still have such an impact. Understanding the impacts of climate change, focusing efforts on vaccine development, and ensuring access to prophylactic medications are needed from international perspectives to reverse the trend of increasing spread and suitable reservoirs for vectors. Support of legislation and regional and national public health efforts that address sanitation, urban development, and mitigation factors is essential to decrease the global impact of vector-borne diseases.

References

1. Vector-borne diseases. World Health Organization. September 26, 2024. Accessed November 17, 2024. https://www.who.int/news-room/fact-sheets/detail/vector-borne-diseases

2. Updated WHO guidance for controlling vector-borne diseases through indoor residual spraying. World Health Organization. February 15, 2024. Accessed November 5, 2024. https://www.who.int/news/item/15-02-2024-updated-who-guidance-for-controlling-vector-borne-diseases-through-indoor-residual-spraying

3. Rocklöv J, Dubrow R. Climate change: an enduring challenge for vector-borne disease prevention and control. Nat Immunol. 2020;21:479-483. doi:10.1038/s41590-020-0648-y

4. Chala B, Hamde F. Emerging and re-emerging vector-borne infectious diseases and the challenges for control: a review. Front Public Health. 2021;9:715759. doi:10.3389/fpubh.2021.715759

5. Vector-borne diseases: mosquitoes, ticks, biting flies… how far will they go? Institut Pasteur. June 17, 2024. Accessed November 18, 2024. https://www.pasteur.fr/en/research-journal/reports/vector-borne-diseases-mosquitoes-ticks-biting-flies-how-far-will-they-go