Population growth drives infectious disease rate upward

Demand-driven agriculture and livestock production predicted to increase spread and dangers of zoonotic diseases.

Issue: August 2007

BETHESDA, Md. — As the world population grows, the rise of mega agriculture to meet increased demand for food could aid the spread and virulence of infectious diseases worldwide.

Developing nations, where most of the population growth is expected to occur, will race to meet the demands, but poor public health infrastructures could put the global food supply at risk.

In the next decade, Livestock 2020, a consortium of food and agriculture industry experts, predicts significant global, societal and agricultural shifts and has dubbed the coming years “The Next Food Revolution.” The revolution will include numerous opportunities for already well-adaptable microbes.

“This is a perfect microbial storm,” said Lonnie J. King, DVM, MS, MPA, director of the CDC’s National Center for Zoonotic, Vector-Borne and Enteric Diseases. “The winners here are the ecological gymnasts: the microbes who change, move and cross species lines and acquire resistance.”

King presented an outline of the phenomenon and concerns rising from the convergence of animal and human health from 10,000 years ago to more than 20 years in the future at the 2007 Annual Conference on Antimicrobial Resistance, held here recently.

Zoonotic eras

Three eras should be considered when examining public health solutions to the spread of zoonotic diseases.

The advent of agriculture was the first contributor to the problem. When humans started to grow their own food, they remained stable and built communities. Infectious diseases were able to spread and grow in these stable human populations.

Animal domestication marks a second zoonotic era. The first zoonotic seasons likely began as humans began living with animals raised as livestock.

“If you look at smallpox, measles and tuberculosis, there is pretty good evidence that these were zoonotic diseases that transferred at that time,” King said.

A third transition occurred in the last 25 to 35 years, characterized by newly emerging infectious diseases and conditions that created a new world in which 75% of major infectious diseases were zoonotic.

Factors contributing to the perfect storm include microbial adaption to change and susceptibility, climate, weather and changing ecosystems.

Technology and industry also facilitated the spread of microbes to more populations in less time.

“About 1.5 million people travelled internationally last year, and those 747s are just like Petri dishes,” King said.

Of the 14 significant diseases of the past 15 years only one – Norovirus – is not zoonotic.

“The microbes are the winners, changing, adapting, acquiring resistance and moving worldwide. They are doing well in ways that are unprecedented,” King said.

Population growth trends

Trends in population — growth and the shift from rural to urban farming — are a driving force behind the need for food and the need to closely monitor production. The demand to feed the planet’s 6.3 billion people is driving food production from industrialized to developing nations.

It is estimated that by 2015, more than 90% of the world’s population growth will be in developing nations, which may lack the infrastructure to thwart zoonotic and foodborne diseases.

“These nations are potential hot spots for new emerging diseases,” King said.

Shifts from rural farming to urban farming are also of concern. In 2030, an estimated 60% of people will live in urban areas. Infectious and zoonotic diseases will be harder to control because livestock farming is predicted to move to the edge of these urban areas. Animal production will boom in places that may lack in public health.

“To feed 6.5 billion people, you have to concentrate the animals in a way that there are environments for the animals themselves,” King said. “Animal farming is already to the point where an egg can grow to a full-grown bird in 41 days for production purposes.”

In the United States, the annual burden of foodborne illness is 76 million illnesses, 25,000 hospitalizations, and 5,000 deaths. The bulk of common foodborne pathogens are zoonotic and can be communicable.

New food revolution

Another factor of the phenomenon some experts are calling the largest agricultural evolution in history is a demand-driven population outsourcing more crops and vegetable production to developing nations. Crop production can increase the population’s wealth; in turn, people’s lifestyles will change, as well, shifting many from poverty to a new type of middle class and Westernization.

“When I go abroad and see this, I see the locals buying things like mobile phones eating fast food,” King said. “But what we will really see is perhaps 22 million people shifting economic classes in the next decade or less.”

When the population progresses above the poverty level, the increased demand for animal protein will be massive. A current prediction is that there will be a 50% increase globally for animal protein by the year 2020.

Last year, 21 billion animals fed 6.25 billion people, and trillions of pounds of produce were distributed worldwide. If the global food system projections double in 2020, microbes will flourish in the sheer magnitude of distribution, which will likely come from developing countries with inadequate animal and public health infrastructures, according to King.

Understanding how societal changes and the changes in animals and their environments effect microbes are important for initiating measures for safe food supplies.

“It could be a spinach field, a nosocomial infection in a hospital or a microparasite on a contact lens, but understanding the ecological impact is critical,” King said.

The impact of mega agricultural production on populations as they relate to infectious disease also must be considered. For example, Salinas Valley, Calif., considered the “salad bowl of the world,” is capable of producing 6 million prepackaged leafy greens per day.

“Because of this, small errors in a large scale can mean big problems,” King said. He described the 2006 spinach outbreak, during which isolates from cows and wild boars were matched to water in systems around the spinach fields.

As the developed world moves to importing more food from developing countries, problems with infectious disease related to food are anticipated.

Animal, human health

Good animal health strategies are public health strategies, although they are often considered separate entities, according to King. Zoonotic diseases are “equal-opportunity microbes,” but they do not infect people equally, he said. A major concern is health disparities in resource-limited countries.

“Think about the world where millions of poor people produce livestock and are dually burdened because if they get a zoonotic disease, it will wipe out their livestock and their savings,” King said.

Women and children will get infected at the highest levels because they traditionally care for livestock in developing nations, according to King.

Shifts from rural livestock to farms closer to urban areas or the fringe of forests also have major implications for infectious disease. Fruit bats, which can have 4-foot wing spans and may cover 150 miles in 10 days, flourish in Southeast Asia. Working in large groups, they descend on the fruit at the edge of the jungle, where the swine are raised to increase production. Fruit bat saliva dropping on the hogs leads to entire herds being killed from infectious disease.

“The people taking care of the swine start getting sick and die, then the people that slaughter the hogs get sick and die, and low and behold, we have Nipah virus,” King said.

Three outbreaks of Nipah virus were reported in Bangladesh in the past five years.

Livestock production is moving closer into urban areas, creating another perfect scenario for infectious disease endemics. In Bangkok, the growth of poultry and livestock has already moved into the city.

“The idea is that our worlds are colliding, and water- and foodborne diseases are going to be of greater concern,” King said. – by Kirsten H. Ellis

For more information:

King LJ. The convergence of animal and human health. #21. Presented at: The 2007 Annual Conference on Antimicrobial Resistance; June 25-27, 2007; Bethesda, Md.