Population Growth: Density and Crowding

A virus emerges when it reaches a new host population and then disseminates to different locations. The dramatic increase in the human population on Earth is correlated with higher human densities in given areas. The global human population growth curve is shown in FIGURE 15-2A. Increases in human populations lead to urbanization, the migration of people from the countryside to cities and suburbs. It is often associated with jobs in new industries and technology. As of 2010, the world had 26 megacities, each of which had a population of at least 10 million inhabitants. FIGURE 15-2B shows the global distribution of the human population in 2010. China, India, and other parts of Asia and Africa are the most populated areas. The largest megacity is the greater Tokyo-Yokohama area, which has a population of 37.8 million. As of 2015, the nine largest megacities after Tokyo are (in decreasing order of population):

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  Jakarta, Indonesia: 30,500,000

  
  
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  Delhi, India: 24,900,000

  
  
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  Manila, Philippines: 24,100,000

  
  
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  Seoul-Incheon, South Korea: 23,480,000

  
  
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  Shanghai, China: 23,410,000

  
  
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  Karachi, Pakistan: 22,100,000

  
  
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  Beijing, China: 21,000,000

  
  
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  New York City, United States: 20,600,000

  
  
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  Guangzhou-Foshan, China: 20,500,000

Higher population densities favor the spread of viral diseases; for example, dense populations result in the combined problems of:

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  Crowding

  
  
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  Improper sanitation

  
  
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  Inadequate drinking water

  
  
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  Pollution

  
  
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  Overwhelmed healthcare facilities

When sanitary conditions are poor, people are more susceptible to respiratory and gastrointestinal infections.

Human Movement (Travel and Migration): Viruses Live in a World Without Borders

Humans are constantly on the move either for planned migration (e.g., for leisure, work, or commerce) or unplanned migration (e.g., because of war or civil unrest or natural disasters). Commercial air travel allows infected travelers to reach any part of the world in less than 24 hours, possibly starting a global pandemic. The spread of viral diseases to new areas can happen at any time. Viruses present in rural populations in Africa or Asia may show up in a developed part of the world, such as Europe or the United States. The severe acute respiratory syndrome (SARS) pandemic of 2002–2003 demonstrated the rapid spread of a viral disease by international air travel (FIGURE 15-3). Within a month, almost 20 countries had reported cases of SARS.

It is difficult to predict when the next influenza A pandemic will occur. However, avian influenza A (H5N1) virus has raised concerns about a potential human pandemic for several reasons:

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  It is highly virulent.

  
  
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  It is spread by migratory birds.

  
  
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  It can be transmitted from birds to mammals (e.g., from birds to domestic cats) and, in limited circumstances, to humans.

  
  
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  It continues to evolve due to antigenic shift and drift.

Since 2003, 15 countries have reported human cases of avian influenza A (H5N1) to the WHO (TABLE 15-2 and FIGURE 15-4). Avian influenza A (H5N1) occurs naturally among wild waterfowl, which usually do not get sick from it. Avian influenza A (H5N1) virus is shed in bird feces, saliva, and nasal secretions. Avian influenza A (H5N1) is highly contagious and nearly 100% fatal in domesticated birds such as chickens, turkeys, and ducks, often within 48 hours. People in direct contact with sick birds or surfaces contaminated with secretions and/or excretions from infected birds on farms or live animal markets are most susceptible to infection by the virus. At the time of this writing, the virus does not efficiently spread from person to person. Approximately 53% of people infected with the influenza A (H5N1) strain died. To date, the risk of avian influenza A (H5N1) virus infection to travelers is very low.

The U.S. Geological Survey’s National Wildlife Health Center located in Madison, Wisconsin, monitors samples colected from migratory waterfowl for avian influenza A viruses. They speculate that influenza virus A (H5N1) will enter North America through migratory birds in the Pacific flyway, which follows the eastern Pacific coastline from the western Arctic, including Alaska and the Aleutian Islands, down the Rocky Mountain and Pacific coastal regions of Canada, the United States, and Mexico, to where it blends with other flyways in Central and South America. If avian influenza A virus (H5N1) is detected among migratory birds, the monitoring can provide an early warning to agricultural communities, public health officials, and experts of wildlife agencies. Highly pathogenic avian influenza A viruses can also enter North America through other routes, such as through illegal importation of infected domestic birds or by an infected traveler.

Cruise vacations are an increasingly popular leisure activity. From 1993 to 1998, the number of cruise ship passengers in the United States increased by 50%. In North America alone, about 6.8 million passengers board cruise ships each year. In recent years, there has been an increase in gastrointestinal illness caused by norovirus on cruise ships, which in essence are floating mini-cities with a constantly changing population of hundreds of thousands of people in a confined space. Between 2001 and 2004, the number of norovirus outbreaks on cruise ships increased 10-fold. In 2002, a new norovirus variant emerged in hospitals in the United Kingdom and Ireland and on U.S. cruise ships, and two new variants emerged in 2006 in the Netherlands. In March 2012, a new norovirus strain, Genogroup 2, Genotype 4 II, originating in Sydney, Australia, became the leading cause of norovirus outbreaks in Australia, the United States, and the United Kingdom. The norovirus name is abbreviated GII.4, Sydney noro-virus. GII.4 Sydney norovirus is the predominant cause of norovirus outbreaks today, replacing the GII.4 New Orleans strain. GII.4 noroviruses are associated with increased hospitalizations and deaths during outbreaks. Norovirus outbreaks are reported through the CDC’s CaliciNet Laboratories Outbreak Support Centers (http://www.cdc.gov/norovirus/reporting/calicinet/participants.html).

Noroviruses cannot be cultured in the laboratory, and there is no animal model available to study them, making it difficult to determine what genetic changes in a norovirus trigger outbreaks or cause more severe symptoms. The CDC started the Vessel Sanitation Program (VSP) in the early 1970s as a result of several disease outbreaks on cruise ships. The mission of the VSP was to protect the health of passengers and crew by minimizing the risk of gastrointestinal illness on cruise ships. Routine inspections are now performed on the following in cruise ships:

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  Water supply (storage, distribution, protection, and disinfection)

  
  
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  Spas and pools (filtration and disinfection)

  
  
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  Food (protection during storage, preparation, and service)

  
  
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  Employee hygiene practices

  
  
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  General cleanliness and physical condition of the ship (absence of insects and rodents)

  
  
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  Training programs (environmental and public health practices)

Inspections scores are posted on the CDC’s VSP web-site (http://www.cdc.gov/nceh/vsp/).

War, famine, and natural disasters have played a large part in introducing infectious diseases into humans. Refugee camps and temporary shelters are often characterized by crowded living conditions, poor sanitation, a lack of access to clean water, and populations with poor nutritional status and limited access to medical care. Under these conditions, people are highly susceptible to infectious agents; for example, in 2005 Hurricane Katrina evacuees in the New Orleans Superdome and the Houston Astrodome and Reliant Center experienced a norovirus outbreak. For some time, there were no working toilets and little clean water in those facilities (FIGURE 15-5).

Aircraft are a potential means of unintentionally moving virus-infected animals or arthropod vectors such as mosquitoes from country to country and continent to continent. Researchers demonstrated that mosquitoes, houseflies, and flour beetles can survive in the wheel bays of commercial Boeing 747 airplanes. The insects were able to withstand the extreme temperatures of one-way and round-trip flights. The leading theory is that the 1999 West Nile virus (WNV) outbreak in New York was attributed to airplanes carrying WNV– infected Culex pipiens mosquitoes when they flew across the Atlantic Ocean from Israel. At the time, aircraft cabin spaces were not routinely sprayed with insecticides on intercontinental flights entering the United States.

Prior to 1999, West Nile encephalitis had never been reported in the Western hemisphere and no one had considered that it could be introduced. The outbreak was originally thought to be caused by St. Louis encephalitis virus, an endemic disease transmitted by mosquitoes in the United States. A correct diagnosis was made after a veterinarian at the Bronx Zoo correlated the death of native birds with human cases of encephalitis. (St. Louis encephalitis virus commonly infects birds but does not cause disease outbreaks.) WNV quickly spread across the contiguous states of the United States and into Canada, the Caribbean, and Mexico (FIGURES 15-6A and 15-6B). The main reservoir of WNV is birds. Mosquitoes pick up WNV while taking a blood meal from infected birds. From there, WNV moves to horses and humans through a mosquito bite. Humans and horses are “dead-end hosts” that cannot transmit WNV to other species (FIGURE 15-6C). Between 1999 and 2010, 30,702 human cases of encephalitis and 1,220 (4%) human deaths from encephalitis were reported in the United States. Between 1999 and 2010, 25,889 cases of equine encephalitis were reported. By 2004, 15,000 horses had died of West Nile encephalitis.

Investigation of a new reservoir of WNV began in 2007. Stable flies were suspected of being involved in the WNV deaths of hundreds of pelican chicks at the Medicine Lake National Wildlife Refuge in northeast Montana. Montana State University entomologist Greg Johnson observed stable flies feeding on the blood of sick and dying pelicans. Stable flies look like house flies, but they have a painful bite. Johnson captured 1,300 stable flies and divided them into 60 groups. Eighteen of those groups tested positive for WNV. This was the first evidence to suggest that stable flies might transmit WNV at the pelican colony.

In 2002, four novel routes of West Nile virus transmission to humans were reported:

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  Blood transfusion

  
  
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  Organ transplantation

  
  
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  Transplacental transfer

  
  
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  Breastfeeding

In the United States, 23 people were reported to have acquired WNV infection after receiving blood transfusions and 500 donations were identified as being given by donors infected with WNV in 2002. Beginning in 2003, per the recommendation of the Food and Drug Administration (FDA), blood-collection agencies implemented nucleic acid amplification tests (NATs) to screen all blood donations for WNV in order to prevent transmission of WNV through contaminated blood products. All blood units that test positive for WNV are discarded appropriately.

At the time of this writing, Zika virus is an emerging viral threat that is the focus of an ongoing pandemic and public health emergency. Zika virus is transmitted by day-biting Aedes spp. mosquitoes. Before 2007, Zika virus infections were limited to sporadic or small clusters of patients in Africa and Asia. The first major outbreak of Zika virus infections occurred in Yap (Federated States of Micronesia) in which 78% of the population was infected and 18% of infected individuals developed symptoms. Since then, Zika virus infection spread rapidly. Outbreaks have occurred in French Polynesia, Cook Islands, Easter Island, New Caldonia, and most recently, South America. Zika virus was first reported in May 2015, in Brazil where 440,000–1,300,000 people were subsequently infected by February 2016. Furthermore, cases originating in 29 additional countries in the Americas were reported. The ease of air travel and international trade caused further spread into regions where Zika virus is not endemic (VIRUS FILE 15-1).