Climate Change, Vector Expansion, and the Shifting Geography of Zoonotic Disease: Implications for Clinical Practice

Climate change, land-use change, and globalization are all factors to consider in the changing geographic distributions of zoonotic and vector-borne diseases, namely that their relevant clinical presentations are now occurring in regions where physicians historically did not have to consider them. These shifts are driven largely by vector expansion, which can be defined as the movement of disease-carrying organisms such as ticks, mosquitoes, and triatomine bugs into new ecological ranges due to changes in temperature, precipitation, host distribution, and human behavior. Vector-borne diseases are especially sensitive to environmental conditions, as their arthropod vectors are ectothermic, which means that their continuing survival, reproduction, and feeding behavior are sensitive to changes in climate. As a result, even modest changes in temperature or precipitation alter their respective vector-borne disease transmission dynamics, introducing their corresponding pathogens into previously non-endemic regions.

For the practicing internist, the clinical consequence is clear: geography is no longer a reliable exclusion criterion. Diseases considered tropical, coastal, or southern in our medical school and continuing education curricula are now increasingly presenting themselves in the Midwest, Great Lakes region, and northern states. Recognition of these zoonotic and vector-borne diseases is thus essential for diagnostic accuracy, appropriate testing, and anticipatory counseling when encountering patients with these conditions in clinical practice.

Vector Expansion and the Ecology of Zoonotic Disease

Vector-borne and zoonotic diseases spread due to various interactions between pathogens and their relevant vectors, animal reservoirs, and environments in which they are found. Climate change can alter this balance by changing the geographic range of vectors, modifying host migration patterns, and lengthening transmission seasons. Warmer temperatures, for example, allow ticks and mosquitoes to survive winters in regions that were previously too cold, while increased precipitation creates new breeding habitats for mosquito larvae in regions that were previously too arid and hospitable to allow them to complete their larval life cycles. These changes have already been associated with the rising incidence and northward expansion of multiple vector-borne diseases in North America and Europe.

In the United States, reported vector-borne disease cases have increased substantially over the past two decades, with Lyme disease, West Nile virus, dengue, and other arboviral infections accounting for the majority of cases. Modeling studies support climate-driven changes being implicated in expanding vector distributions, which shift disease risk to areas of higher latitudes and elevations.

Lyme Disease in the Upper Midwest and Northern Michigan

Lyme disease is an example of a zoonotic, vector-borne disease that I see in my clinical rotations and effectively illustrates the clinical impact of vector expansion. The disease is transmitted by the black-legged tick (Ixodes scapularis), which also carries other zoonotic diseases, namely those caused by Babesia, Anaplasma, and other Borrelia species, which creates a possibility of co-infection between two or more of these pathogens.

Historically concentrated in the Northeast, Lyme disease is now well established across the Great Lakes region, including Wisconsin, Minnesota, and Michigan. Expansion into northern states is linked to warmer winters, increased deer populations, and other environmental changes that have favored tick survival. Lyme disease is now the most common vector-borne disease in the United States, with hundreds of thousands of estimated diagnoses annually.  

For clinicians in northern Michigan, the Great Lakes, and similar regions in the United States, this geographic shift means that symptoms such as erythema migrans, unexplained heart block, migratory arthritis, and cranial neuropathies should prompt consideration of tick-borne illness even in patients without travel history to historical Lyme disease endemic regions.

Expansion of Mosquito-Borne Illnesses

Mosquito-borne diseases are another example of zoonotic, vector borne disease with shifting geographical distributions as temperatures increase, breeding seasons lengthen, and mosquito survival increases as a result of these factors. Research has shown that temperature, rainfall, and humidity influence mosquito population density and allow for increased intra-vector viral replication, suggesting increased chance for transmission risk.

Mosquito-borne diseases relevant to the American internist include West Nile virus, Eastern equine encephalitis, dengue, chikungunya, and La Crosse encephalitis, all of which have demonstrated regional variability linked to climate patterns. Outbreaks in temperate regions are increasingly associated with periods of heavy rainfall followed by warmer weather, both of which are conditions that favor mosquito habitation.  

These changes thus require physicians to be aware of local public health alerts and to consider arboviral infection in patients presenting with febrile illness, meningitis, or encephalitis during summer and early fall, even in regions previously considered low-risk.

Autochthonous Transmission in the United States

Autochthonous transmission refers to infection acquired locally rather than through travel to an endemic region. The appearance of autochthonous cases is evidence that a full transmission cycle (i.e., vector, reservoir, and pathogen) has become endemically established in a new geographic area.

Chagas disease, caused by Trypanosoma cruzi and transmitted by triatomine (“kissing bug”) insects, is a notable current example in infectious disease literature. Multiple triatomine species have become native to the United States, and locally acquired cases have been documented, leading some investigators to classify the disease as hypoendemic in parts of the country that were not previously home to this typically considered “tropical disease,” now endemic in areas as far north as Tennessee and Southern Missouri.

Other examples of emerging or locally transmitted zoonotic infections in the United States include the previously mentioned:

• West Nile virus transmission throughout most states
• Eastern equine encephalitis in Great Lakes and northeastern regions
• Tick-borne illnesses expanding into Canada and northern U.S.
• Increasing reports of locally acquired dengue and chikungunya in southern states

As has been the current theme in this article, the presence of autochthonous transmission means that clinicians must consider these diagnoses even in patients without travel history.

Clinical Implications for Internal Medicine

Diagnostic Considerations

Published geographic distribution risk maps often lag behind current reality. Physicians should consider zoonotic and vector-borne diseases based on exposure history, seasonality, and symptoms rather than relying solely on traditional endemic boundaries.

Medication and Comorbidity Interactions

Patients with cardiovascular disease, immunosuppression, diabetes, or chronic kidney disease may experience more severe outcomes from zoonotic infections. Delayed diagnosis can lead to complications such as:

• Lyme carditis
• Neuroinvasive arboviral disease
• Chagas cardiomyopathy
• Severe babesiosis in asplenic patients

Public Health Awareness

Internists play a vital role in recognizing unusual presentations and reporting suspected cases to public health authorities, especially when suspected zoonotic disease occurs outside expected geographic ranges in patients without documented travel to known endemic areas.

Preventive Counseling

Patients living in newly affected regions may not recognize risk. Counseling should include:

• Tick avoidance and removal
• Mosquito control and repellents
• Safe outdoor practices
• Pet and livestock exposure history
• Screening of immigrants from endemic regions when appropriate

Clinical Recommendations

The following recommendations are considerations for the osteopathic internist to have in their daily clinical practice:

• Consider vector-borne disease even in non-traditional geographic regions.
• Obtain detailed exposure history including outdoor activity, animal contact, and travel within the United States.
• Evaluate unexplained cardiac, neurologic, or rheumatologic symptoms for tick-borne illness.
• Monitor regional public health alerts for arboviral activity.
• Counsel patients in northern and rural areas about tick and mosquito prevention.
• Screen high-risk patients for Chagas disease when epidemiologically appropriate.
• Coordinate with infectious disease and public health specialists for unusual cases.
• Maintain awareness of contemporary climate-related shifts in disease distribution.
• Educate patients that disease risk may change over time in their region.

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