New and Old Microbes
By the mid-20th century, some scientists thought that medicine had conquered infectious diseases. With the arrival of antibiotics and modern vaccines, as well as improved sanitation and hygiene, many diseases that formerly posed an urgent threat to public health were brought under control or largely eliminated.
The emergence of new microbes and the re-emergence of old microbes has continued,however,as it has throughout history. Several pressures are contributing to
the emergence of new diseases such as
- Rapidly changing human demographics
- Rapid global travel Changes in land use patterns
- Ecological,environmental,and technological changes
Even public health practices such as widespread antibiotic use are contributing to this emergence. These pressures are both shaping the evolution of microbes and bringing
people into closer and more frequent contact with microbes.
Unsanitary conditions in animal agriculture and increasing commerce in exotic animals (for food and as pets)have also contributed to the rise in opportunity for animal microbes to jump from animals to humans.From time to time, with the right combination of selective pressures, a formerly harmless human or animal microbe can evolve into a pathogen that can cause a major outbreak of human disease.At times,changes in societal
and environmental factors can also lead to re-emergence of diseases that were previously under control.
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
This is common in Africa,West Asia,and the Middle East. 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.
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.
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.
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.
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. 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 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
SOME NEWLY EMERGING/ EMERGED PATHOGENS
- Ebola virus
- H5N1 avian influenza virus
- Nipah virus
- Marburg virus
- SARS virus
- West Nile virus
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.
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. 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.