Mosquito-Borne Diseases Pose Growing Threat to United States

A recent historical analysis highlights the need for experts to be ready to respond to outbreaks of emerging mosquito-borne diseases in the United States.

The article emphasizes the potential threat for the introduction of new mosquito-borne pathogens from tropical regions, discusses the impact of globalization on disease emergence, and highlights the importance of keeping track of public health issues in developing countries.

Max J. Moreno-Madriñán, PhD, from Indiana University—Purdue University Indianapolis, Indianapolis, Indiana, and Michael Turell, PhD, from VectorID LLC, Frederick, Maryland, published their article online in the May 2018 issue of Emerging Infectious Diseases, the Centers for Disease Control and Prevention’s (CDC) monthly peer-reviewed public health journal.

Although diseases caused by mosquito-borne pathogens are typically considered to arise in tropical regions, Dr. Moreno-Madriñán and Dr. Turell advise researchers and health experts against taking this for granted.

“In fact, cases of disease caused by mosquito-transmitted pathogens such as West Nile virus (WNV) occur readily in North America, and several encephalitides occasionally occur in the United States,” they write.

Adding to the burden of WNV infections in North America, the recent spread of chikungunya and Zika viruses through the Americas has also sounded the alarm about the potential for tropical pathogens to become established in North America, the authors note.

Yellow fever and malaria were once very common in the United States. The Aedes aegypti mosquito, which is the vector for yellow fever, and the plasmodia parasites of malaria (Plasmodium vivax and P. falciparum) probably arrived here during the 17th century on ships of the slave trade, according to the authors.

During the 18th and 19th centuries, yellow fever claimed thousands of lives in the United States. Furthermore, from 1861 to 1865, during the American Civil War, malaria is reported to have caused approximately 1,300,000 cases of illness and about 10,000 deaths among soldiers.

However, the combination of the introduction of mosquito control methods and improved living standards in the United States, particularly since World War 2, has reduced the opportunity for contact between mosquitoes and humans. As a consequence, diseases such as malaria, yellow fever, and dengue have largely disappeared from the United States.

Similarly, because the viruses that cause chikungunya and Zika arrived in the United States after living conditions had improved, they have not become established in the continental United States.

The link between socioeconomic status and prevalence of mosquito-borne diseases applies more to diseases caused by viruses such as chikungunya and Zika, the authors say. These viruses are anthroponotic, spreading from human to human through the bite of an infected mosquito.

In contrast, WNV, St. Louis encephalitis virus, eastern equine encephalitis virus, western equine encephalitis virus, and La Crosse encephalitis virus are zoonotic. “These viruses are maintained in natural transmission cycles involving various mosquito and bird or rodent species and therefore are not usually greatly affected by improved housing for humans,” the authors explain. “Because humans are not involved in the transmission cycle of these viruses, the viruses persist in the United States.”

Indeed, the arrival and spread of WNV highlight the potential for an exotic virus to establish in North America. Since WNV entered the United States in 1999, it has spread not only throughout the continental United States but also to southern Canada and most of Central and South America. Approximately 40,000 cases of encephalitis and about 2,000 deaths have been reported in association with WNV infection in the United States alone.

With respect to novel viruses recently introduced into the United States, the authors say that about 10,000 imported cases of Zika or chikungunya virus infections have occurred since 2014. However, relatively few local cases have been reported, they say, and all of them occurred in areas where their Aedes aegypti mosquito vectors were present. Travelers have also played a role in introducing these two viruses into the United States, they add.

“Given that the decreased exposure of humans to mosquitoes in the United States is primarily driven by changes in socioeconomic conditions, it is important to note that these very conditions could be threatened by massive natural disasters or any other similarly disruptive event,” Dr. Moreno-Madriñán and Dr. Turell conclude. “Consequently, appropriate disaster preparedness plans need to be in place to address this potential threat.”

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.