Vector-Borne Diseases and �Climate Change

Roxanne Connelly

Chief Entomologist/Arboviral Diseases Branch

Division of Vector-Borne Diseases

Centers for Disease Control and Prevention

National Center for Emerging and Zoonotic Infectious Diseases

Drivers for Disease Emergence

• Climate and weather
• Changing ecosystems
• Economic development and land use
• Microbial adaptation and change
• Human susceptibility to infection
• Human demographics and behavior
• Technology and industry
• International travel and commerce
• Breakdown of public health measures
• Poverty and social inequality
• War and famine
• Lack of political will
• Intent to harm

Source: Institute of Medicine 2003 report – Microbial Threats to Health

The key challenge is in determining how much of the observed and anticipated trends can be attributed to climate change versus other factors.

Outline

• Climate change observations and projections relevant to disease emergence
• Vector-borne (VBD) burden and trends and the influence of weather and climate change
• Case Studies
• Mosquito Introductions and Changing Distributions
• Conclusions

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Climate change – Summary of Observations and Trends…

• Longer and warmer summers
• Shorter and milder winters
• Increased frequency of severe and unpredictable weather events (e.g., storms, heat waves, droughts)
• Regional variations
• Influence on global weather patterns

Source: The frost-free season length, defined as the period between the last occurrence of 32°F in the spring and the first occurrence of 32°F in the fall, has increased in each U.S. region during 1991-2012 relative to 1901-1960. Increases in frost-free season length correspond to similar increases in growing season length. (Figure source: NOAA NCDC /CICS-NC).

Climate, Weather, and Infectious Diseases: �the Big Picture

• Climatic variables (temperature and precipitation) affect disease transmission by impacting the replication, interaction, and survival of disease agents in animals, disease vectors, and the environment.
• Annual and seasonal variation in weather patterns, such as severe storms, droughts, winds, and El Niño Southern Oscillation (ENSO) events affect disease patterns and outbreaks.

Climate Change and Emerging Infectious Diseases Through a One Health Lens

Changes in climate lead to changes in the environment, which result in changes in the incidence and distribution of diseases that have strong environmental linkages

Trends in VBDs in the U.S.

• Between 2004 and 2019, more than 800,000 cases of VBDs were reported in the U.S.
• The number of reported cases of disease from mosquito, tick, and flea bites more than doubled over this period.
• The reported data substantially underestimate actual disease occurrence.
• Tickborne diseases (TBDs) now account for over 80% of all reported VBD cases.
• Mosquito-borne disease epidemics happen more frequently.

Top Ten Notifiable VBDs, U.S. States and Territories, 2019

* RMSF = Rocky Mountain Spotted; † Includes travel-associated cases

Source: https://wonder.cdc.gov/nndss/static/2019/annual/2019-table1.html

West Nile Virus Neuroinvasive Disease Incidence Reported to CDC, 1999 – 2022�

Source: https://www.cdc.gov/westnile/statsmaps/cumMapsData.html#one

Case Studies

• West Nile Virus Infection
• Rocky Mountain Spotted Fever

Case Study: West Nile Virus Infection

• Member of the Flavivirus genus in the Japanese Encephalitis virus subcomplex
• Transmitted primarily by Culex species mosquitoes
• Amplified by birds
• Humans and other mammals are “dead end” hosts
• Not essential for pathogen life cycle
• Clinical syndromes:
• West Nile fever (about 25% of cases)
• Neuroinvasive disease (<1% of infections)

West Nile Virus Outbreak, Arizona, 2021

• Largest local outbreak in U.S. history, since WNV introduction in 1999, with 2,695 cases and 191 deaths
• Unprecedented Outbreak of West Nile Virus — Maricopa County, Arizona, 2021 | MMWR (cdc.gov)
• 1,487 Human cases (May – December 2021)
• 956 Neuroinvasive; 101 deaths (6.8%)
• Analyses are still underway looking at how regional weather patterns, including a wetter than average monsoonal season, may have interacted with other factors in causing this outbreak

Climate, Weather and West Nile Virus

• Temperature (e.g. milder winters, earlier onset of spring, warmer summers) influences mosquito life cycle and rate of viral replication
• Precipitation has a significant effect, but is more complicated and varies regionally…
• Mosquito vector species vary in the eastern and western US
• Rainfall has different effects on the breeding habitat of these different vector species
• Rising temperatures, changing precipitation patterns, higher frequency of extreme weather events influences distribution, abundance, and prevalence of infection in mosquito vectors by altering habitat availability and reproduction rates (of both mosquitoes and virus)
• Alterations in distribution, abundance, and infection rate of mosquitoes will likely influence human exposure to bites from infected mosquitoes, changing the risk for human disease.

Reisen et al. 2006. J. Med. Entomol. 43: 309–317

Chuang et al, 2011. J. Med. Entomol. 48: 669–679

Morin & Comrie, 2013. PNAS; doi:10.1073/pnas.1307135110

Case Study : RMSF

• Caused by Rickettsia rickettsii
• Transmitted in the U.S. by various tick species
• Hosts for the ticks vary, but principal hosts tend to be deer, dogs, and livestock
• Is generally thought that vector ticks are also reservoirs due to efficient vertical transmission, both transstadial and transovarial
• Early signs and symptoms are non-specific, including fever and headache.
• Disease can rapidly progress to serious, life-threatening illness

The Brown Dog Tick Emerges as an Unexpected Vector of �Rocky Mountain Spotted Fever (RMSF) in Arizona

Source: Demma LJ, Traeger MS, Nicholson WL, et al. N Engl J Med. 2005 Aug 11;353(6):587-94.

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• Not considered important in the epidemiology of RMSF in the U.S. until 2004
• Cause of epidemic levels �of RMSF in several tribal�communities in Arizona
• Outbreaks precipitated �by large populations �of free-roaming, �tick-infested dogs

Climate, Weather and Tick-Borne Diseases

• Shorter and milder winters with increasing minimal temperatures allow for expanding tick geographic distribution
• Warmer annual temperatures lead to larger tick populations
• Longer summers result in extended seasonal activity
• As a result, more people at risk over a larger area for longer periods of time, leading to increasing disease burden

Vector Introductions and Changing Distributions

• Transport by humans
• Containers
• Ornamental plants
• Expanding range of aquatic plants
• Landscape alterations (natural or anthropogenic)

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Container Inhabiting Mosquitoes

= recent reports

= recent reports

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Vectors

• Education programs on how humans can prevent the movement of mosquitoes might be helpful

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• Exotic, invasive, and endemic mosquitoes are on the move
• Surveillance of mosquitoes is a critical component of public health surveillance and prevention programs

How Climate Change Potentially Affects VBD Emergence

• Will likely change geographic and seasonal occurrence of vectors and VBDs in the U.S.
• Rising temperatures, changing rainfall patterns, and increases in some extreme weather events will likely affect the distribution, abundance, and infection rates in the mosquitoes that spread West Nile virus and other pathogens
• Vector-borne pathogens will likely emerge or reemerge due to the interactions of climate factors with many other drivers (e.g., changing land-use patterns including reforestation, resurging deer populations, etc.)

^{Source: Beard, C.B., R.J. Eisen, C.M. Barker, J.F. Garofalo, M. Hahn, M. Hayden, A.J. Monaghan, N.H. Ogden, and P.J. Schramm, 2016: Ch. 5: Vectorborne Diseases. The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. U.S. Global Change Research Program, Washington, DC, 129–156. http://dx.doi.org/10.7930/J0765C7V}

Conclusions

• Climate change has wide-ranging health impacts.
• An integrated understanding of climate, ecology, and epidemiology is critical for predicting and averting epidemics of infectious diseases.
• Preparation to prevent and adapt to emerging infectious disease threats related to climate change include…
• Understanding of trends and drivers
• Enhanced surveillance
• Prevention, diagnosis, and treatment efforts
• State and local implementation of vector-borne disease prevention and control programs

CDC Activities and Priorities Related to VBDs and Climate Change

• We are making progress through a coordinated strategy
• New entomologic control methods, new insecticides and repellents, new vaccines, improving workforce capacities
• With a combined approach – government, academia, NGOs, vector control units, industry – considerable progress can be made
• Expanding human and ecological surveillance and research, including modeling and forecasting
• Identifying and validating effective prevention and control strategies that address environmental justice and health equity…and are tailored for communities that are disproportionately affected  

CDC Activities and Priorities Related to VBDs and Climate Change

• Developing and maintaining local, state, and federal capacity to respond to emerging disease threats
• Conducting outreach to the public and to clinical providers to increase awareness of changing disease patterns and/or exotic pathogens
• Combating West Nile Virus Disease — Time to Revisit Vaccination | NEJM

Roxanne Connelly, PhD

Division of Vector-borne Diseases

National Center for Emerging Zoonotic and Infectious Diseases

Arboviral Diseases Branch

csz5@cdc.gov

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Thank you

The findings and conclusion in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.