Factors in the Increase of Emerging Infectious Disease

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Infectious diseases are threats to everyone. We come into contact with all types of pathogens (organisms that cause disease) every day. We can reduce the incidence of disease through strategies such as immunizations or through environmental controls such as providing safe water supplies. Still, we cannot completely eliminate the risk of disease. Despite enormous efforts in battling infectious disease, only one has been eradicated; smallpox was eradicated in 1977 through multiple worldwide interventions that took years.

Infectious diseases continue to be one of the leading causes of death worldwide.  Emerging and re-emerging diseases have become even more of a concern in recent years.

An emerging disease is any disease that appears in a population for the first time. Infectious diseases that have existed in the past but suddenly start to rapidly increase in incidence or in the number of geographic locations are called re-emerging diseases. In almost all cases of emerging or re-emerging diseases, a probable factor for their occurrence can be identified.

Why are there emerging and re-emerging diseases?

Many changes in our world account for the increase in emerging and re-emerging diseases. These include:

woman getting water out of a hole in the sand and putting it in a bucket.
Many people in the world get their drinking water from unreliable sources. Image: https://en.wikipedia.org/wiki/

► Poor public health measures—Inadequate sanitation, contaminated and untreated water supplies, poor vector control programs, and mediocre or absent public health care contribute to the transmission of infectious disease pathogens.

Cholera in Haiti

Years after a magnitude 7.0 earthquake hit Haiti in 2010, the country is still plagued with cholera. The earthquake damaged infrastructure essential to providing a clean water supply and the sanitation equipment needed to safely dispose of human wastes. Feces contaminated with cholera bacteria entered the water supply and thousands became ill. The epidemic spread to other Caribbean countries, but more developed countries stopped the outbreak while Haiti continues to suffer.

Not all scientists are convinced that the earthquake was the main instigator of the
bacteria entering the water supply. Haiti’s water systems were not that good to begin
with and many areas of the country do not have good sanitation systems. If people are
getting their water from the local river and also defecating in the river because they don’t have proper sanitation facilities, bacteria can make their way into a human host. The disease spreads.

This issue is magnified in poor countries of the world which do not have good sanitation systems; contaminated human wastes pollute the soil and water supplies. In other countries, a lack of vector control programs allows insects, rodents, and other pathogen-carrying species to thrive and transmit disease.

►Changes in the ecology of an area

Deforested land with burned trees in part of a forest.
Deforestation causes major ecological changes. Image: https://commons.wikimedia.org/wiki/File:

Changes in land use, dams, agriculture, and deforestation cause pathogens to emerge in new areas. Clearing forests destroys habitats of animals that carry disease; these animals move to new locations and carry the pathogens into new host populations. As developing countries continue to deforest areas for agriculture, people living near the formerly forested regions are more exposed to vectors carrying disease.


Malaria in Peru

  • Malaria is caused by the protozoan Plasmodium sp. and is spread through the bite of an infected Anopheles mosquito. Although malaria has been present in Peru for decades, it had been dramatically reduced due to mosquito eradication programs. Then, an increase in population led to deforestation to convert the forests to farmlands and malaria cases sky-rocketed. Anopheles mosquitos breed in standing water and when the forests were cleared, standing water environments were much more prevalent. Farming activities created pools of water ideal for Anopheles mosquito breeding.

Hundreds of thousands of cases of malaria occurred in Peru due to the deforestation. To determine whether or not deforestation actually caused the increase in malaria cases, scientists tracked the cases and breeding sites of mosquitos in Peru. It was found that 83% of breeding sites with malaria-carrying Anopheles mosquitos occurred within 500 meters of human villages, towns, or farms. The mosquitos had an abundance of humans to feed on and they thrived.

Similar situations have occurred worldwide. Malaria and other vector-borne diseases have spread to countries which had not previously seen such high incidences of those diseases.

Leishmaniasis worldwide

  • A large Leishmaniasis ulcer on a lower leg.
    Leishmaniasis ulcer. Content Provider: CDC/Dr. Mae Melvin. Image: https://phil.cdc.gov/phil/details.asp

    American cutaneous leishmaniasis is a vector-borne disease caused by the parasitic protozoan Leishmania sp. It is classified by the World Health Organization (WHO) as a neglected tropical disease. Leishmaniasis is transmitted by a sand fly, usually infected when they feed on an infected animal reservoir. If bitten by an infected sand fly, a person develops multiple painful lesions or ulcers which can take over a year to heal. Relapses occur in about 10% of cases and can be more severe than the initial infection. There are an estimated 1.5 million new cases each year with more than 12 million people infected worldwide (WHO).

The re-emergence of leishmaniasis is also linked with deforestation with the greatest numbers of cases occurring in formerly forested areas that have been cleared for agriculture or for urbanization.

Construction of dams and agriculture

A dam on a river.
The Nam Ngum Dam in Laos. The dam has negatively impacted the ecology of the region. Contributor: Chaoborus. Source: https://en.wikipedia.org/wiki/Nam_Ngum_Dam

The construction of dams dramatically changes river flows, creates floodplains, destroys fish habitats, and alters sediment nutrients. Ecosystems and their biodiversity are impacted and increases in infectious disease result. Increases in dengue fever, malaria, Cryptospoidium, Giardia lamblia, Entamoeba sp. and Schistosoma sp. and liver, lung, and intestinal fluke infections have occurred after construction of dams in Southeast Asia along the Mekong River basin. Similar increases in diseases have been recorded after dam construction in other parts of the world.

Disease Examples:

  • Korean hemorrhagic fever outbreaks occur annually when fields are flooded for rice production. People are more exposed to infected field mice which live in rice fields.
  • Japanese encephalitis cases spike to over 30,000 annually with 7,000 deaths when agricultural fields are flooded. Outbreaks occur regularly during rice harvests.
  • Junin virus outbreaks, causing Argentine hemorrhagic fever, have increased due to the conversion of grassland for maize production. The disease is carried by a rodent vector which lives in the fields.

Change in land use for agricultural or economic development is a leading factor for the emergence of infectious disease. As forests are cleared and fields are flooded for crop production, animal vectors such as disease-carrying rodents thrive.

Large scale changes to existing ecosystems can drastically increase infectious disease. Loss of biodiversity, change in inter-species relationships, and increases in vectors and animal reservoirs can result in changes in disease transmission dynamics resulting in critical public health emergencies.

Climate change

Changes to climate variables, such as temperature, precipitation, wind, and severe weather events can have significant impacts on infectious disease. Why? Because more frequent periods of warmer temperatures, periods of drought and flooding, and impacts of many types of severe weather events expand the areas where infectious disease pathogens and their hosts can survive. More favorable environmental conditions cause pathogens and hosts to thrive with devastating results.

Lyme Disease

Large bulls-eye rash from Lyme's Disease on a shoulder and back.
Typical “bull’s eye” rash of Lyme disease. Image: CDC http://phil.cdc.gov/Phil/details.asp

Warmer temperatures change the feeding patterns of the ticks that carry Lyme disease, according to Yale University scientists. Tick season in some regions has been extended, allowing many more people to become infected with the Lyme disease virus through the bite of the infected tick.

The disease is treatable with antibiotics but diagnosis is difficult. If left untreated, the infection spreads to joints, the heart and the nervous system.The number of new Lyme disease cases continues to rise dramatically. According to the CDC, 30,000 cases of Lyme disease are reported to state health departments each year; there are an estimated 329,000 cases annually in the U.S.

 Malaria and Climate Change

Areas in yellow represent the current distribution of malaria. Areas in red indicate possible extended distribution by 2050 as predicted by the Hadley Centre model’s high scenario.
Image: Hugo Ahlenius, UNEP/GRID-Arendal. https://scied.ucar.edu/longcontent/climate-change-and-vector-borne-disease

Worldwide, 3.2 billion people live in high risk areas of malaria transmission. (CDC) The World Health Organization estimates that malaria caused 214 million malarial episodes and 438,000 deaths in 2015.

Malaria is caused by the protozoan parasite Plasmodium falciparum and is spread by the bite of an infected female Anopheles mosquito. Warming temperatures allow the parasite to spread into new areas. Although malaria cases are reported every year in the U.S., they are almost all in recent travelers. Other cases occur in people who have been bitten by local mosquitos who have become infected by biting people already carrying the parasite. If the Anopheles mosquito can spread into new areas, the cases of malaria will increase.

►Changes in Human Demographics

As people move into areas that were previously uninhabited, pathogens find increased numbers of hosts and victims to infect and transmission occurs rapidly by different transmission methods.

A crowded airport.
Crowded airports can spread pathogens around the world. Image Credit: https://www.flickr.com/photos/lunchtimemama/110765169
  • Increased travel, trade, and markets have provided the means for pathogens to travel around the globe. Dengue fever and the more severe dengue hemorrhagic fever (DHF) were once diseases of Southeast Asia. Dengue has since become a major worldwide public health concern. Worldwide incidence has increased 30-fold and epidemiologists predict that from one-third to one-half of the world’s population is at risk from dengue infection. Moreover, cases of the more severe type of dengue (DHF) are increasing. According to the World Health Organization (WHO) there are 50 million dengue infections each year with 500,000 cases of DHF and 22,000 deaths, mostly children.
A mosquito feeding on a human host.
An Asian tiger female mosquito feeding on a human host. Image: CDC/James Gathany. https://phil.cdc.gov/phil/details.asp

After dedicated efforts to rid areas of the dengue (mosquito) vector, Aedes aegypti, incidences of the disease dramatically decreased in the Americas. Now, due to greatly increased global movement of people and goods by air travel, all four serotypes of dengue are causing outbreaks and becoming endemic (regularly found) in some areas.

Another example is the introduction of Aedes albopictus, the Asian tiger mosquito, into the U.S. in tires imported from Asia. The mosquito has aggressive daytime human-biting behavior and can serve as a vector host for many different viruses, including the West Nile virus and Easter equine encephalomyelitis (EEE), a severe disease. About one-third of patients with EEE die and those who survive have mild to severe brain damage. There is no treatment for EEE other than supportive care.

The USDA (US Department of Agriculture) has designated A. albopicus a national invasive species.

  • Wars that cause people to flee conflict also spread disease. Large numbers of refugees flooding into other regions or countries tax the public health infrastructure and bring new pathogens and unvaccinated populations into areas unprepared for such over-whelming health emergencies. Pathogens flourish and spread.


Overcrowded conditions in a refugee tent city.
Overcrowded conditions and poor healthcare cause disease outbreaks in refugee camps. Image: Voice of America News: Scott Bobb https://commons.wikimedia.org/wiki/File:

The on-going war in Syria and other countries in the Mid-East have introduced epidemics of infections. Epidemics of polio, measles, leishmaniasis, hepatitis A, meningitis and scabies, to mention just a few, are occurring. In an environment of violence, millions living in refugee camps are vulnerable to infectious pathogens. Health care workers leave the countries, public health measures collapse, immunization programs are almost non-existent, and resources do not begin to meet the need for intervention. Hundreds of doctors have been killed and hundreds have been jailed. Understandably, over 80,000 more have fled the country.(WHO)

Pathogens, such as the poliomyelitis virus, which thrive in sewage, water, and contaminated food, infect susceptible people. Although a few immunization programs have been started by global health organizations, constantly migrating populations prevent an effective response. Polio has been declared a public health emergency in Syria.

These conditions also promote the increase of other vaccine-preventable diseases in the population. Measles has flourished in not only Syria, but in neighboring countries as well, due to its high infectivity. Other issues, such as the need for the measles vaccine to remain refrigerated, complicate the immunization program; in an area that does not have reliable power sources, refrigeration is not guaranteed.

►Microbial Adaptation and Change

The emergence of antibiotic resistant strains of bacteria is a result of microbial adaptation and natural selection. Viruses can show a high mutation rate; new variants are produced through changes in viral surface proteins, usually the H (hemagglutinin) protein. People have no resistance to the new strain since the new antigen (the surface protein) is not recognized by the person’s immune system. A disease outbreak occurs.

►Changes in Human Behavior

Some sexual behaviors, drug use, immigration, and increased use of child care cause an increase in infectious disease rates of emerging and re-emerging diseases.

Resurgence of TB

Chest x-ray of a TB patient.
Chest x-ray of a patient with advanced tuberculosis. Note the whitish areas of decreased lung function. Image: https://en.wikipedia.org/wiki/Tuberculosis
  • After decades of much lower rates of tuberculosis (TB), case rates in the United States began to increase several years ago. The U.S. was not alone in this increase; other developed countries, as well as third world countries were recording an upsurge in TB. In the U.S. the cases were centered primarily in highly populated urban areas. (Centers for Disease Control and Prevention (CDC), Morbidity and Mortality Weekly Reports (MMWRs), 2013, 2014.

Ironically, the previous low incident rates resulted in public health funding for TB education, awareness, tests, and treatment being cut. These cuts in funding, along with other factors, contributed to the increase in TB cases. Renewed efforts for controlling and reducing the incidences of TB have now slowed the numbers of new cases. The rate of decreasing cases, however, is the smallest in decades.

The issue is complicated. Disparities in incidence exist among racial and ethnic groups;
incidence is higher in foreign-born (non-U.S. born) persons and higher among Asians,
for example. For persons over 15 years of age with TB, 99% were homeless, in long-term care facilities, or in prison. The development of drug-resistant forms of Mycobacterium tuberculosis, the causative agent for TB, has made treatment difficult.

TB rates are higher for more susceptible HIV positive patients. These patients progress more rapidly to contagious TB and are therefore more likely to transmit the disease, resulting in an even larger spread of disease.

It is essential that the social issues surrounding the prevalence of tuberculosis be
addressed if the disease is going to be controlled. Increasing numbers of HIV positive individuals, drug resistant strains of the bacterium, increased immigration without adequate screening procedures, and reduced resources for identification, treatment, and surveillance of known TB positive people contribute to the complex problem.

Tuberculosis is an alarming emerging disease. While there is a vaccine used in some countries to prevent severe TB in children, it is not used in the U.S. due to its limited effectiveness.

A Combination of Factors

A sooty mangabey monkey sits in a tree.
A sooty mangabey. Image: By Year of the dragon – Own work, CC BY-SA 3.0, https://commons.wikimedia.org

Emergence of a disease is often a combination of factors. HIV is one such disease which is now at pandemic proportions. Ecological factors allowed human exposure to a host carrying a virus that was a precursor to HIV-1. The disease probably started in a rural area in Liberia with a sooty mangabey monkey infected with the HIV-2 virus. This monkey is widely hunted for food in rural areas. A person was infected and through human demographic changes, moved from the rural area to a city which allowed the virus to move along transportation routes in Liberia and then globally by air travel. Human behavior, such as sexual behavior and IV drug use, facilitates transmission. In the early days of HIV transmission, changing technology played a role in the spread through blood and blood product transfusions.