Emerging Microbes

by National Institute of Health

(Page 8 of 8)

Scientists usually define newly emerging microbes as those that have only recently appeared in a population or have existed but are rapidly increasing in incidence or geographic range. Recent examples of the infectious diseases they cause include West Nile virus (WNV) infection, SARS (severe acute respiratory syndrome), and avian influenza (bird flu).

West Nile virus infection

In the early summer of 1999, a mysterious cluster of encephalitis (inflammation of the brain) cases and related deaths appeared in New York City, raising the concern of public health officials. Within a short time, researchers identified the cause of the outbreak as WNV, which is common in Africa, West Asia, and the Middle East, but never before seen in North America.

Symptoms of WNV infection are usually mild and include fever, headache, body aches, skin rash, and swollen lymph glands. If WNV enters the brain, however, it can cause life-threatening encephalitis or meningitis. These more severe complications of the disease most often affect the elderly or people with weakened immune systems.

WNV is transmitted to humans by mosquitoes, which pick up the virus from infected birds. Although health experts don't know the route by which WNV entered the United States, they think the virus may been introduced by one of three possible ways an infected bird imported into the country, an infected mosquito that stowed away on a shipment or transport vehicle entering the country, an infected person returning from a country where the virus is common.

Since WNV first appeared in the United States, there have been annual outbreaks of the disease and it has spread across the country. Experts believe WNV has now become established as a seasonal epidemic in North America that flares up in the summer and continues into the fall.

SARS

In 2002, a deadly new human pneumonia emerged in southeastern China and caused an epidemic that spread across many Asian countries and even across the ocean to Canada and the United States in 2003. The new disease was named severe acute respiratory syndrome, or SARS. A global research effort resulted in rapidly identifying the virus that caused SARS - part of the human coronavirus family. Prior to the emergence of SARS, coronaviruses were best known for their role in causing up to 30 percent of common colds in humans and for causing a host of other diseases that affect birds and mammals. The SARS virus is thought to be a rare recombination between avian and mammalian coronaviruses that may have made the jump to humans through animals sold in Asian food markets.

Avian Influenza

Since 2003, yet another new microbe has emerged as a threat to human health: the H5N1 avian influenza virus. Although avian influenza usually infects only birds, there have been numerous instances of transmission of these viruses to humans, resulting in severe disease or death in those affected.

At the moment, this avian influenza virus is not easily transmitted from human to human. If the virus was to acquire the ability to spread from one human to another, however, it could result in a flu pandemic that would cause widespread illness, death, and social disruption. For this reason, the public health community continues surveillance of avian influenza and has made flu preparedness and prevention an urgent public health priority.

Re-Emerging Microbes

The reappearance of microbes that had been successfully conquered or controlled by medicines and vaccines is distressing to the scientific and medical communities, as well as to the public. One major cause of disease re-emergence is that microbes responsible for causing these diseases are becoming resistant to the drugs used to treat them. Also, the decrease in vaccine use for vaccine-preventable diseases is contributing to re-emergence of previously controlled diseases. Some examples of re-emerging infectious diseases that are of significant public health concern are TB, malaria, and polio.

TB

According to the World Health Organization (WHO), nearly 2 billion people, one-third of the world's population, are infected with Mycobacterium tuberculosis, the bacterium that causes TB. This includes between 10 and 15 million people in the United States.

TB is the world's leading cause of death from a single infectious organism, killing 2 million people each year. Failure to stop the spread of TB can be attributed to several factors, including the co-epidemic with HIV/AIDS which has led to more and more TB cases developing in people with weakened immune systems. The failure of infected people to complete the entire drug treatment needed to eliminate the disease (this treatment may take up to 9 months to complete). The emergence of multidrug-resistant TB (MDR-TB), which does not respond to available treatments.

Malaria

Malaria, the most deadly of all tropical parasitic diseases, has been resurging dramatically in recent years. Increasing resistance of Plasmodium protozoa (one of the microbes that causes malaria) to inexpensive and effective medicines presents problems for treating active infections. WHO estimates between 300 million and 500 million new cases of malaria occur worldwide each year, causing more than 1 million deaths annually. In the United States, approximately 1,300 cases are reported annually. Most of the U.S. cases occur in people who had been infected while traveling abroad. Other cases occur in people bitten by infected mosquitoes in the United States.

Polio

Polio is another disease that had come close to eradication (elimination), due to the widespread use of polio vaccines. Recently, however, polio has been re-emerging. According to the Global Polio Eradication Initiative, global polio eradication efforts have resulted in a reduction from 350,000 cases in 1988 to 1,111 cases worldwide by August 2005. Six countries, however, continue to see new polio cases (Nigeria, India, Pakistan, Afghanistan, Niger, and Egypt). In addition, poliovirus has been spreading to previously polio-free countries. Since mid-2003, eighteen previously polio-free countries have been re-infected. There are ongoing efforts to increase vaccine coverage in these areas.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.