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Vector-Borne Diseases

As the Earth becomes warmer as a result to climate change, diseases that traditionally have been found in tropical or subtropical areas will now expand into new geographical areas where these diseases were not normally found. 

 

As average global temperatures increase, the Intergovernmental Panel on Climate Change reported with high confidence that the incidence of vector borne diseases has increased in recent years. 

 

Climate change from rising levels of CO2 and other greenhouse gasses leads to warming in the global atmosphere. As a result, this sets off a myriad of other events with increases in extreme storms, increase in droughts and increase in temperatures and heat waves. The resulting consequences include creating or destroying potential vector breeding sites, longer disease transmission seasons and also our inability to prevent, properly diagnose and treat these diseases. 

 

This can have devastating impacts for the new geographical areas affected for many reasons. Since these diseases have been rare to occur in these new areas, people may not be familiar with how these diseases are transmitted, how to protect themselves and how to further prevent spread of disease.

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What are vector-borne diseases?

Image by Victor Grabarczyk

A vector borne disease is defined as a disease that results from an infection transmitted to humans and other animals by blood feeding arthropods, including mosquitoes, ticks and fleas.

 

Climate change is one of several factors that can drive the distribution and prevalence of vector borne diseases. 

 

Last year in 2023, various vector-borne diseases such as West Nile virus, dengue, malaria and Lyme disease were of public health concern in the United States.  The US Department of Health and Human Services reviews the current state of vector-borne diseases in the US in the past year here: https://www.hhs.gov/climate-change-health-equity-environmental-justice/climate-change-health-equity/climate-health-outlook/west-nile/index.html

 

In Feb 2024, the US Department of Health and Human Services released the National Public Health Strategy to Prevent and Control Vector-Borne Disease in People. Link here: https://www.cdc.gov/ncezid/dvbd/pdf/VBD-National-Strategy-508.pdf


 

(Expand on as human come in more contact with nature through expansion of housing, also climate impacts increasing life cycles of mosquitos, etc)

What are some of the most common vector-borne diseases? And which diseases are present in Arizona?

Close-up of a Bug

West Nile virus (WNV) infection

 

WNV is the most common mosquito-borne disease in the United States, with 2,406 cases reported across 47 jurisdictions in 2023, and close to 1,600 of those cases to be neuroinvasive. 

 

People typically become infected following a bite from an infected mosquito. Rare forms of WNV transmission include mother to baby during pregnancy, delivery, breastfeeding and via blood transfusions and organ transplantation. 

 

Eight out of 10 people infected with WNV are asymptomatic. The other 20% will experience a fever and other flu-like symptoms and <1% of these cases will go on to develop severe West Nile neuroinvasive disease (WNND). In severe cases, persons may need to be hospitalized to receive supportive treatment. This condition can lead to death or long-term disability. Older persons and people with immunocompromising conditions are at higher risk of WNND. There currently is no available treatment of vaccine for WNV infection.

West Nile virus infections was introduced in the US in 1999. Since then, most US counties have reported WNV infection. However, the distribution of infection varies greatly across the US. The western states and the Great Plains are more likely to have higher incidence of WNV. 

Six US counties including Maricopa County, AZ reported high incidence of WNV infection and accounts for close to 25% of all WNND cases during 2009-2018. Because reporting of WNV cases vary year to year, accurately predicting the number of WNV cases that will occur each year and in each county is challenging. WNV infection follows seasonality and occurs during the mosquito season, typically in the summer months and continues through fall. WNV cases are most commonly reported between June and October.

Climate change has resulted in milder winters, earlier springs, longer and warmer summers. And changes in regional precipitation. These factors can potentially affect WNV transmission. Currently, we still do not fully understand how climate change impacts WVN transmission in the US, however, seasonal weather patterns can affect transmission.

Local Context of West Nile virus

In 2021 in Maricopa County, AZ, the country experienced the largest ever WNV outbreak with 1,4807 WNV cases reported, over 1000+ hospitalizations and over 100 deaths. It is believed that the wetter-than-average monsoon season in 2021 contributed to this outbreak. The wetter summer months led to longer duration and increased amount of moisture that was a perfect environment to support mosquito larval habitat sites, leading to more mosquitoes and greater WNV transmission. See MMWR here: https://www.cdc.gov/mmwr/volumes/72/wr/mm7217a1.html

Dengue infection

 

Dengue viruses are spread to people through bites of infected Aedes species mosquitoes. Globally, there are an estimated  400 million dengue infections annually. One-quarter of people infected with dengue become sick. Symptoms include fever with aches and pains, rash and nausea and vomiting. Less than 5% of dengue infections progress to severe disease, which can lead to hospitalizations and death. Early diagnosis and supported treatment are vital as the disease can become life threatening quickly. There currently are no medications available to treat dengue. 

 

Several dengue vaccines are available, however, only one dengue vaccine has been approved for use in the US territory of Puerto Rico, where dengue is endemic. 

 

Climate change impact on dengue transmission is complex. The Aedes species mosquitoes require warm temperatures for virus transmission, with optimal transmission occurring between 77-86F. As the Earth warms due to climate change, it seems this is a good recipe for increased dengue transmission.

Local Context of Dengue infection


In 2022 in Maricopa County, AZ the county experienced its first evidence of locally acquired human dengue infection. The case was symptomatic requiring hospital admission. Mosquitoes in the vicinity of the case were sampled and one mosquito from these samples tested positive with the same genotype as in the case. This genotype DENV-3 strain was not known to circulate where this case traveled to in Mexico, thus suggesting local dengue transmission. See MMWR on this case here: https://www.cdc.gov/mmwr/volumes/72/wr/mm7211a5.html

Malaria infection

 

For the first in 20 years, the US has seen locally acquired cases of malaria in Florida, Texas and Maryland. 

 

Malaria is a serious mosquito borne disease caused by different species of Plasmodium parasites that infect Anopheles species mosquitoes. People become infected when bitten by a mosquito carrying the parasite. 

 

Most cases of malaria reported in the US are acquired through travel to endemic areas, however, in 2023, the US has seen locally acquired malaria cases. 

 

The Anopheles species mosquitoes can be found in much of the continental US, making local spread possible if people infected with malaria are bitten by local Anopheles mosquitoes. These mosquitoes are now infected and can go on to spread the parasite to humans. The risk of local transmission will be highest in areas where the local climate conditions allow for Anopheles mosquitoes to survive for all or most of the year. Temperatures need to be warm enough for malaria parasite to thrive in mosquitoes and with warmer and longer summers, this could potentially increase local spread. 

 

While local malaria transmission in the US is rare, it is possible with warmer temperatures and increased humidity and precipitation, there could be increased local malarial spread under these new climate patterns. 

 

See MMWR on local acquired malaria in Florida and Texas here: https://www.cdc.gov/mmwr/volumes/72/wr/mm7236a1.htm#:~:text=Eight%20cases%20of%20locally%20acquired,18%E2%80%93July%2017%2C%202023

 

See Notes from the Field on local acquired malaria in Maryland here: 

https://www.cdc.gov/mmwr/volumes/72/wr/mm7241a3.html

What can patients do to protect themselves?

Image by Online Marketing

West Nile virus 

 

The best way to protect against WNV infection is to present yourself from mosquito bites. When outdoors, use and EPI-registered insect repellent. Wear loose fitting, long sleeve shirts and pants. Other preventive measures including having screens in windows and doors, and remove standing water. 

 

Dengue infection

 

Mosquito control methods are one of the important ways in preventing the spread of dengue. This includes trapping of adult mosquitoes, using insecticides and larvicides and removing standing water. Improved sanitation services can also decrease mosquito breeding sites. Use EPA-registered insect repellants and wear loose-fitting, long sleeve shirts and pants. 

 

Dengue vaccination is available for children ages 9 through 16 years with laboratory-confirmed dengue virus infection and living in areas where dengue is endemic. 

 

Malaria infection

 

As with other mosquito-borne infections, prevention of malaria infection includes protecting yourself from mosquito bites. Use EPA-registered insect repellants and wear loose-fitting, long sleeve shirts and pants. 

 

Malaria chemoprophylaxis is available for travelers going to a malaria endemic region. Check the CDC website for these recommendations here: https://www.cdc.gov/malaria/travelers/drugs.html

 

Malaria infection has treatment regimens available depending on the type of malarial infection. See CDC page here on guidelines to treat malaria infection: https://www.cdc.gov/malaria/diagnosis_treatment/clinicians1.html


Globally, there currently are two approved malaria vaccines for use in children in the Africa Region. See WHO page on approved malaria vaccines for use in children in the Africa Region here: https://www.who.int/news/item/02-10-2023-who-recommends-r21-matrix-m-vaccine-for-malaria-prevention-in-updated-advice-on-immunization#:~:text=WHO%20also%20issued%20key%20immunization,a%20WHO%20recommendation%20in%202021

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