Unfortunately, the ocean harbors dangerous toxins and
disease-causing agents that can present serious threatsto human health. For example, some types of algae living
in the sea make toxic chemicals. These toxins, as
well as contaminants such as mercury, can move up
through the food web, transferring dangerous substances
to fish, birds, aquatic mammals, and, ultimately, humans.
The ocean is also home to several types of disease-causing viruses and bacteria that make people sick
when they eat tainted seafood.
Through the development of more effective threat detection
and monitoring systems and a deeper understanding
of the causes of ocean-related health threats, ocean
science can help prevent disease outbreaks and improve
public health.
The threat of harmful algal blooms
In this satellite image,
bloom of the toxic
alga
Karenia brevis is visible
along the west
coast of
Florida. (Image from
Jacques
Descloitres, NASA)Microscopic marine algae called phytoplankton serve a similar role in the sea as do plants on land. Through photosynthesis,
these tiny organisms create the foodstuffs that form the base of marine food webs. Some species, however, produce substances that are toxic to humans. Algal toxins can interfere with neural processes, causing paralysis, amnesia, nausea, diarrhea, and respiratory distress, depending on the specific toxin produced. Most of these effects are not permanent, but the effects of some toxins may persist for years, and severe cases may be deadly.
In addition to producing toxins, phytoplankton can multiply,
or "bloom," in such enormous numbers that when they die, the decay processes deplete the water of oxygen, causing fish and other sea creatures to flee or perish. Harmful algal blooms—also known as red tides—may present a serious threat to the health of humans
and marine ecosystems alike.
Algal toxins persist even when seafood is thoroughly cooked. According to the Centers for Disease Control and Prevention (CDC), marine toxins are tasteless and odorless, and, unlike pathogenic bacteria and viruses, they are usually not destroyed by cooking. To date, no antidotes to algal toxics are available.
Surprisingly, some toxic algae, such as
Karenia brevis, can sicken vacationers on the beach who never even enter
the water. Toxins from
K. brevis can become airborne within tiny droplets of seawater, which can cause asthma-like symptoms when inhaled. This presents a serious public health risk. Affected beaches often must be closed, and, as a result, local economies suffer. Winds may carry airborne toxins inland, possibly affecting the people living
in coastal communities. The amount of
K. brevis toxin necessary to cause severe coughing and sneezing is almost
undetectable using current chemical technologies.
The incidence and geographic spread of algal blooms is increasing.
Harmful algal bloom
(HAB) events have
spread
to all regions
of America's coastline.
(Image from
NOAA
COP/National HAB
Office-WHO1)Harmful algal blooms occur in all of the world's oceans and in all climatic regions. During the past three decades,
the recorded incidences of harmful algal blooms have increased significantly. Now, virtually all coastal regions of the United States are subject to this threat.
The frequency of harmful algal blooms is also increasing.
K. brevis, for instance, repeatedly causes red tides along the coast of Texas and the southwest coast of Florida,
delivering a blow to the Gulf Coast economy every year.
According to some estimates, this species alone causes economic losses of roughly $50 million per year between human illness and downturns in shellfish, finfish,
recreation, and tourism.
From Monsoons to Microbes identifies the need for closer
monitoring of algal blooms to help resolve why harmful algae are increasing in frequency and range. Closer monitoring will also allow earlier notification of public health authorities so that they can act to reduce exposure
of the public to algal toxins.
Determining the causes of harmful algal blooms should be a high research priority.
Although red tides have received much attention, the conditions that provoke toxic algal blooms remain largely
mysterious, and individual blooms may be triggered by different causes. The characterization of the life cycles of many harmful algal species is critical in allowing scientists to understand the root causes of blooms and to identify ecological controls on bloom dynamics.
From Monsoons to Microbes concludes that improved methods
are needed to identify the toxic algal species that make people sick. There is also a need to determine the physical, chemical, and biological factors that promote algal growth in order to improve the ability to predict,
manage, and potentially prevent harmful algal blooms.
Research Spotlight: A Positive Side of Toxic Algae?
Although harmful algal blooms are a significant threat to human health, their story has a surprising positive side as well. A team of researchers recently discovered that the toxic alga
K. brevis produces an anti-toxin chemical that can actually blocks the effects of the
K. brevis toxins on the human respiratory
system. Even more astounding, the researchers found that this chemical, called brevenal, helps to break up mucus in the lungs of cystic fibrosis patients. This promising new medical discovery is one positive outcome of research focused on toxic algae.
Better tracking can help prevent human exposure to algal toxins.
Scientists are now developing exciting new technologies
for identifying algal blooms, including ways to spot them
from space.
Better tracking helps health officials
protect the public from
exposure to
toxic algae. One method of preventing
exposure is to
temporarily close beaches
or fishing areas. (Image from Woods
Hole
Oceanographic Institution)Scientists have also been designing a number
of promising tools to detect the presence of algal
toxins rapidly and to track their route of transfer throughout
the environment. Being able to predict when a harmful
algal bloom will become dangerous for humans
would make health officials better prepared to make management decisions that protect the public from exposure,
such as temporary beach and fishing closures.
Epidemiology—the study of the occurrences of diseases in populations—can be used to identify disease "hot spots." By investigating what sick people have in common,
scientists can often trace the cause of the problem, such as an algal bloom, and warn the public accordingly.
Without such warnings, people can become ill and not know why. In many cases, illnesses related to marine toxins go unreported.
Epidemiological studies also alert the medical community
to the presence of harmful algal blooms or other events so they recognize the symptoms in their patients.
From Monsoons to Microbes concludes that there is a need both to document the incidence of toxin-related illness in coastal areas and among travelers who visit high-risk areas and to train public health authorities in
coastal states to recognize and respond to toxin-related
illnesses. Tracking has been most useful in cases
where the acute effects from toxic algae resulted
in a cluster of illnesses. It is also important
to consider sublethal or subsymptomatic
effects that can result from
low-level exposure to toxic
algae, which are only
now being studied
and understood.
The Role of the Ocean in Spreading Disease
The
ocean
is
home
to
several
types
of
disease-causing
viruses
and
bacteria
(pathogens).
These
waterborne
diseases
can
either
originate
in
the
ocean
or
originate
on
land
but
be
transmitted
through
seawater.
Like
the
public
health
threats
posed
by
harmful
algal
blooms,
the
threats
of
waterborne
diseases
can
be
reduced
through
improved tracking, environmental monitoring, epidemiology, and basic research.
Most waterborne disease agents enter the human body when people eat tainted seafood or swallow contaminated
seawater. Some agents, however, can enter through broken skin or mucous membranes; the free-swimming larvae of avian schistosomes can even penetrate unbroken human skin and cause a problem called swimmer's itch.
Microbes in the vibrio genus are an example
of bacterial pathogens that originate in the ocean. These bacteria
multiply in marine waters and include
Vibrio cholerae (which causes cholera) and other life-threatening pathogens.
Although cholera occurs mostly in developing countries,
vibrio-related
diseases
threaten
human
health
everywhere—including the United States.
Vibrio vulnificus is one of the bacteria in
the Vibrio
genus that live in seawater and
cause cholera and other
diseases. (Image
from CDC/ Colorized by James Gathany)A major source of waterborne illness is seafood consumption.
In the United States, the most common vibrio infections are caused by
Vibrio vulnificus and
Vibrio parahaemolyticus, and these infections occur most frequently
in the Gulf Coast region. According to the CDC, about 59 percent of all vibrio infections in this country are food borne. Of these, 4 percent are directly linked to the consumption of oysters.
The CDC estimates that about 8,000 cases of mostly mild vibrio infections occur each year, but many people do not see a doctor, so their infections go unreported. The Gulf Coast reports the highest number of vibrio infections, with about 100-200 seriously ill individuals per year. These infections can be deadly in individuals who have a weakened immune system. Among those seriously ill who seek medical attention,
V. vulnificus hospitalizes 93 percent, and 38 percent of these patients die.
The CDC suggests that people should avoid harvesting and consuming shellfish during warmer weather, when disease-causing bacteria are more likely to be present. It also suggests that people should thoroughly cook oysters
and
use
technologies
such
as
irradiation
and
pasteurization
to
eliminate
vibrio
in
shellfish.
Viruses, too, can be lethal, and even a single ingested virus can cause an infection. Rotaviruses, for example, are unusually robust and can be found in seawater that has been contaminated by sewage. Rotaviruses cause severe diarrhea in young children, killing an estimated 870,000 children in the world each year, mostly through their exposure to human sewage.
Understanding the epidemiology and causes of outbreaks
of waterborne pathogens will allow better control of these outbreaks and will protect public health.
Case Study: Public Health Effort in Bangladesh
In tracking
V. cholerae around the world since the 1970s, U.S. marine microbiologist Dr. Rita Colwell noticed that four to six weeks after the seasonal peaks in sea-surface temperatures in the Bay of Bengal, cholera outbreaks in Bangladesh increased dramatically.
Once she understood the effects of changes in sea-surface temperature on cholera incidence, Dr. Colwell was able to warn residents in a particular rural village of an impending outbreak by monitoring satellite data. She met with the women, teaching
them how to filter pathogens from the family drinking water by placing four to eight layers of clean sari cloth over the mouth of the water pitcher.
Tests show that Dr. Colwell's filtration method removes about 99 percent of
V. cholerae. A three-year study demonstrated a 50 percent reduction in cholera infections in villages where the women filter water through sari cloth.
The ocean can influence human health even hundreds of miles inland.
Many diseases are not directly present in seawater but are carried by other animals, called vectors, which can be influenced by changes in weather mediated by the ocean. Worldwide, changing weather and climate patterns
affect the incidence of such diseases as malaria, yellow fever, hantavirus, and others that are transmitted
by insects, mice, or other animals.
Ocean and climatic conditions often influence outbreaks of waterborne diseases.
Environmental changes can affect
the dynamics of waterborne diseases.
When sea-surface temperatures increase,
pathogens can become more
concentrated in seawater, threatening to
contaminate seafood and drinking water
supplies in coastal communities. When sea
levels rise, low-lying areas can become inundated
with contaminated water. Recognizing
environmental clues such as higher sea-surface
temperature or rises in the sea level allows public
health officials to take action to help prevent people from being exposed to waterborne diseases.
Effective public education not only alerts health care providers but also allows the individuals in a community to reduce their risk of exposure to waterborne pathogens.
Case Study: El Niño Linked to Hantavirus Outbreak in the Southwest United States.
El Niño is a shift in weather patterns that is caused by natural fluctuations in ocean dynamics. It brings unusually warm and wet weather to certain parts of the world, including the west coasts of North and South America.
Though it is an ocean-based phenomenon, the effects of El Niño can be felt far inland. In 1993, for example, an outbreak of hantavirus, which causes acute respiratory distress, occurred in the "four corners" area of New Mexico, Arizona, Colorado, and Utah. In retrospect, scientists observed that the unusually heavy rainfalls from the 1992-1993 El Niño led to an abundant food supply for deer mice—the most common reservoir of hantavirus in the southwestern United States. The abundance of food caused a population explosion among deer mice, which commonly live in and around houses and stables. The urine of these mice, which was infected with hantavirus, mingled with the dust in the air after drying. People who inhaled this infected dust got sick, and more than half of those infected died.
There is a growing need for rapid, inexpensive tests for detecting waterborne pathogens.
The CDC, the Environmental Protection Agency (EPA), and the Council of State and Territorial Epidemiologists maintain a collaborative surveillance system of waterborne
disease outbreaks. This monitoring is instrumental in determining national and regional trends of various infections, which then helps to guide prevention programs
throughout the country.
In 2000, the U.S. Congress passed the Beaches Environmental
Assessment and Coastal Health Act to better protect
seashore bathers from harmful pathogens. Of the 4,025 beaches monitored by the EPA in 2005, 28 percent
were affected by advisories or closings.
In the United States, testing for coliform bacteria—the bacteria found in human or animal wastes—constitutes the cornerstone of all monitoring and regulatory programs.
This testing has been effective in reducing exposure
to waterborne disease outbreaks that arise from fecal
contamination.
In addition to the bacterial contamination revealed in coliform tests, viruses can be transmitted by seawater. However, viral monitoring has not yet been included in the routine water quality tests.
The EPA is working to design a water quality test that will identify both bacterial
and viral threats and provide results within two hours so that beaches and oyster and clam-producing areas can be temporarily closed in a timely manner whenever necessary.
From Monsoons to Microbes emphasizes the need to apply
newly developed tests to environmental monitoring and quality assessment, but it also notes that they must be field-tested for applicability in different geographic locations. If the field tests are successful, the new methods
should be adopted by enforcement agencies as standard
methods.
Table of Contents
