Daniel B. Blatt, M.D., Norton Children’s Infectious Diseases, presented “Hot Topics in Global Health” at the 2024 “Just for Kids” symposium.

The twenty-first century has seen a wave of severe infectious disease outbreaks, including the COVID-19 pandemic in 2020. These outbreaks have an enormous impact on morbidity, mortality and the livelihoods of people around the world. Regardless of specialty, every provider is navigating rapid evolution in new technologies, increased international travel and immigration, demographic changes and dramatic climate change – all of which influence the spread of disease to their patient populations.

How disease spreads

Transmission depends on many factors including life cycle and reproduction cycle of the pathogen, human behavior, and human response to the outbreak. When animal and human habitats change, climate shifts and other factors move into place, this triad is complete – agent, host, and environment – and disease spreads.

Pathogen biology also plays a role in the spread of disease. “R-naught is the reproduction number of a disease,” said Dr. Blatt. Researchers in infectious diseases continue to make advances in modeling the spread of pathogens. Often racing a spreading pathogen, researchers will use complex calculations, taking into account information like “serologic and epidemiologic data, theoretical model data, human behavior and other factors” to determine the likely progression of transmission.

All of these factors can influence each other, and often have a greater impact on historically marginalized populations. For example, a common disease in some parts of Africa is tick-born relapsing fever. “The burrows of small mammals become infested with ticks. The burrows are close to where humans live and work. This increases the chance of humans being bitten. As the Saharan desert expands, and as climate change warms the earth, more small mammals will burrow closer to human habitats, further increasing the potential of vector encounters,” said Dr. Blatt.

Immigration and air travel impact disease spread. Air travel has increased fourfold in the last 30 years, so the risk of human-to-human transmission has gone up as more humans interact. And as more people are moving around, providers will need to consider more possibilities, for example screening local patients for diseases that are common in tropical regions. From 2018 to 2022, nearly 16,000 immigrants and refugees came to Louisville from around the world.

Climate change has been linked to global increases in dengue fever, malaria and Lyme disease, which have traditionally been called tropical diseases.

Controlling disease spread with vaccines

In challenging health care environments, providers can deploy a combination of hotspot vaccination, ring vaccination, and priority immunization for high-risk individuals, health care workers and frontline workers.

While a consistent and rigorous preventive vaccination program helps control disease spread, it may not be viable for all populations. For example, in low-resource economies, and countries with high levels of political instability or a low physician population, it may not be possible. An Ebola outbreak in West Africa spread to Guinea, Liberia, Sierra Leone and then to Europe and North America. “This happened in three of the poorest countries in the world, with domestic turmoil and less than one physician per 100,000 people,” said Dr. Blatt.

“It’s ideal to vaccinate everyone, but there are other strategies to implement if that is not possible,” said Dr. Blatt. “One approach is to vaccinate in hotspots that are more likely to develop into outbreaks.” Likely hotspots will depend on the agent, host and environment.

A ring vaccination protocol immunizes first and second-degree contacts in the areas where cases are identified. “The trick with this technique is that all cases must be identified quickly, or the pathogen gets beyond that ring,” said Dr. Blatt.

Impact of inaction against disease spread

“Missed vaccinations have real-world consequences,” said Dr. Blatt. “In August 2020, Africa was declared free of indigenous wild polio, but in 2022, health officials in Malawi declared an outbreak of indigenous wild polio virus.” In the U.S., no endemic cases of wild polio have been observed since 1979, but in June 2022 poliovirus was confirmed in an unvaccinated immunocompetent adult in Rockland County, New York. “It can be spread in the community, and this is a disease we have the ability to prevent.”

Since the COVID-19 pandemic, there has been a lapse in vaccination programs in the U.S. There have been dramatic decreases in routine childhood vaccines. “This has led to a significant increase in vaccine-preventable disease outbreaks,” said Dr. Blatt. “In 2023, there were 58 cases of the measles in the U.S. and four outbreaks. As of July 2024, there were 167 cases and 13 outbreaks in 23 states. This is the direct result of poor vaccination rates.”

Slow response to an outbreak, slow communication out among countries where outbreaks occur, and undereducation about the impact of the disease can contribute to spread of a disease. It is obviously more difficult to control an outbreak than a few isolated cases.

Conclusion

“We as U.S. providers need to keep learning and surveilling tropical illnesses,” said Dr. Blatt. “We need to be at the forefront of medical information distribution, and our response to pathogens through surveillance, medical treatments and public health measures will shape the outcome of infections. A world of inequity is a world at risk.”