By Karen Carvajal, M.D.
Over the course of human history we have come a long way in our fight against the threat of infectious diseases and, despite several close calls, we have survived. The 20th century featured a plethora of advancements in the field of infection control: Vaccine campaigns, improvements in sanitation and salubrity, and the discovery of antibiotics. These advances gave us a sense that the threats posed by infectious diseases were receding.
Clearly, they are not.
The current crisis posed by the worldwide spread of COVID-19 is just the latest to tell us that we are facing new threats we lack the means to control. In the years leading up to this pandemic we have seen the growing impacts of Ebola outbreaks, of the spread of Zika, as well as the continuing impacts of older challenges, including tuberculosis, HIV, measles, vector-borne diseases. According to the U.S. Centers for Disease Control and Prevention, mosquito and tick-borne diseases have tripled in the US from 2004 to 2016, and nine new germs spread by mosquitoes and ticks have been discovered or introduced since 2004.
A common denominator behind these developments is the environmental destruction that is a direct result of our actions. Air pollution, extreme heating, floods, deforestation, the devastation of wildlife, and intrusion into natural sanctuaries have all increased our risk of exposure to pathogen transmission along with the re-emergence of water, food, tick, and mosquito-borne diseases.
The greenhouse effect is a product of the heat caused by CO2 released in the combustion of fossil fuels (coal, oil, gas) and it is passing the point of no return. While this presents us with challenges, mosquitos are benefiting.
Humidity, warm temperatures, and shortened winters are creating conditions for the expanding geographic presence of the Culex and Aedes genera, the mosquitos responsible for transmitting arboviruses, of which five types have emerged in the western hemisphere: yellow fever virus, dengue virus, West Nile virus, Zika virus and chikungunya virus (alphavirus).
Dengue disease, or “breakbone fever,” was for many years a disease imported mainly by travelers from tropical areas. In the United States, the virus was first observed as locally transmitted in Hawaii in 2001, followed by an outbreak in Texas in 2005, and another in Florida in 2009. It is now a part of our epidemiological geography with 1203 people sick with dengue reported in the U.S. in 2019. The reestablishment of dengue can be explained by multiple complex factors that basically have facilitated its development and survival through the improvement of its ecological conditions. These conditions favor both the female Aedes aegypti, which bite and reproduce with increased speed and effectiveness in warmer temperatures, as well as the dengue virus itself, allowing it to develop and become infective more rapidly.
Another mosquito-borne disease now presenting domestic challenges is the West Nile virus. In the summer of 1999, fatal cases of encephalitis were found in horses, birds, and humans in New York City . Within just a few years, the virus has become endemic in the US, accounting for 958 cases in 2019, with 65% of cases being classified as neuroinvasive disease.
The latest emergent infections, Zika and chikungunya virus, were first identified in Africa, but are now expanding into new geographical areas. Zika outbreaks have tragic implications with the potential of the virus to cause obstetric complications including microcephaly, miscarriage, and stillbirths. In 2015, Zika was reported in 21 states, with nine cases acquired through presumed local transmission.
Tick getting thicker
The history of tick-borne disease outbreaks is similar, and is also explained by changes in environmental conditions. Ticks perish anywhere between -2 and 14F, need an 85% humidity level to thrive, and are able to withstand temperatures of up to 130F. With the continued expansion of agriculture into previously forested areas, we have brought the tick upon ourselves, in the process exposing ourselves to diseases such as Lyme disease and other bacterial, viral and even protozoan diseases. Within the past 20 to 30 years, we have observed a steady increase in the incidence of tick-borne diseases and their expansion into the Northeast and Midwest. According to CDC reports, domestic cases of Lyme disease increased by more than 300% between 1993 and 2012.
Opening Pandora’s Box
The destruction of the forests and other habitats causes species displacement, exposing humanity not just to animals previously contained within these previously intact ecosystems, but to the pathogens they carry. Antigenic shift, an abrupt and major change that can result in a new virus subtype, happens when a virus jumps from an animal to a human, adapting so as to better infect us. This major mutation is clearly more lethal, making our immune system unable to recognize the virus, and as we know: Little to No Prior Immunity + High Contagiousness = Pandemic.
The commercial wildlife trade, particularly in wet markets, sells all kinds of live animals from all over the world. In the close confines of a wet market, animals and humans are likely sharing more than just space. Wet markets are a living laboratory in which minimal sanitation conditions give rise to viral mutations. The 1929 (“Spanish Flu”) and the 2009 (Swine flu) H1N1 flu pandemics resulted from the reassortment in pigs of the genetic material from three different species: swine, birds and human. But even without wet markets, displacement, migration and urbanization stemming from environmental degradation put us at increased exposure to an interchange of pathogens between specie. Take for instance, the recent Ebola and coronavirus outbreaks — in both cases, it is believed that bats and perhaps other species served as a reservoir. In previous centuries, Eurasian civilization was brought to its knees by the flea-bearing rats that carried the bubonic plague from the central Asian steppes to the streets of Paris and Milan. Almost 700 years later, humanity faces an array of potential vectors as it continues its expansion into the earth’s ever-dwindling wild spaces.
Then, in recent decades, coronaviruses, which were previously known to cause mild upper respiratory infections, have come to international attention with the emergence of SARS in China in 2003, and of MERS in Saudi Arabia in 2012. Both coronaviruses are believed to have originated from animal hosts, civets and camels respectively. Both were deadly. Both were warnings of future calamity. Both went largely unheeded by the global community.
Those who do not learn history are doomed to repeat it
The COVID-19 pandemic has changed us in so many ways that it will probably mark a new era for humanity. This last event has not only shown us our vulnerability to pathogens but has also taught us what is basic and essential: Harmonious coexistence with nature is not only a goodwill matter, it is the ultimate public health problem and needs urgent attention. Now more than ever as a society we need to intensify our collective efforts to change the course of our future before the next pandemic arrives. The infectious diseases community is well placed to lead this discussion. We can tell the story of how we have contained threats to our health and our lives before, and need to do it again. It is time to start writing the next chapter in the book of humankind’s success over infectious disease threats.
Karen Carvajal, M.D. is an infectious diseases fellow at University of Rochester, NY. After studying philosophy at the University of Chile, she began her medical education in Cuba. Working in chronic disease programs in an underserved area of Santiago, Chile reinforced for her how living conditions directly affect health outcomes. Her interest in HIV and global health drew her to the study of infectious diseases and to seeing the urgency that neglected diseases pose. She believes that the need to understand the fragile relationship between the humans and the environment is urgent, calling it “a delicate balance that, if it breaks, will threaten our existence.”