Editor: Should citizens in developed countries
be more concerned for their safety today as they move about in public areas,
such as parks, gardens, or zoos than they might have been when you two and
Scott Newman wrote for MLO [July 2005 cover story, “The nature of
emerging zoonotic diseases: ecology, prediction, and prevention”]? What
major changes have come about in those three years that have either a)
improved conditions or b) made conditions worse? Do you have knowledge of
any global public-health organizations involved in any similar research
because of an “event”?
Lisa Schloegel: The current (2008) human
population was estimated at more than 6.5 billion. If current projections
hold, this number will increase to nearly 9 billion in less than 50 years.
As humans continue to populate the planet in ever-increasing numbers, we
will inevitably alter our interactions with animals and their pathogens,
leaving room for the opportunistic emergence and spread of zoonotic
diseases. There are numerous pathways whereby pathogens can spread from
animals to humans, or even from animal to animals. It is important that
people are aware of the hazards that may come about in everyday activity —
owning a pet, for instance. The 2003 outbreak of monkeypox originated from
imported Gambian rats for sale to the public in an Illinois pet shop. A
recently formed coalition — PetWatch — implemented by researchers at Brown
University, among others, helps guide the public in making smart pet choices
for healthy people and environments.
Jon Epstein: Three years is a very short time
frame, on a biological scale, so it is difficult to say whether the risk of
disease emergence in America — or globally, for that matter — has changed.
We can say that the rate of infectious-disease emergence, and particularly
zoonotic-disease emergence, has been significantly higher in the past 10 to
20 years, than in the previous 50 to 100 years, largely due to an increase
in the rate of anthropogenic (human-caused) activities that promote pathogen
emergence and movement. For example, global travel, which spread SARS
coronavirus from China to 25 other countries; trade, such as monkeypox virus
arriving in the United States through the importation of the African rats
Lisa mentioned; agricultural expansion where Nipah virus in Malaysia emerged
first in domestic pigs, then people, because of a combination of intensive
farming practices and fruit orchards planted next to the pig enclosures
which brought in fruit bats, the natural reservoir for the virus; and the
inappropriate use of antibiotics to combat diseases, which has resulted in
MRSA — methicillin-resistant Staphylococcus aureus in many parts of
the United States.
Should people be more concerned about their safety? I
think people certainly should be, by now, more aware about infectious
disease in general, especially with the media attention that SARS, avian
influenza, and West Nile virus have generated in recent times. On an
individual level, risk factors for becoming infected with a zoonotic
pathogen are highly variable, depending on where one lives and what one’s
lifestyle involves. I think most people do not realize that pathogens like
Bacillis anthracis,the bacteria that causes anthrax; Plasmodium
spp.,the protozoan that causes malaria; Yersinia pestis or
plague; and hantaviruses naturally occur here in the United States. Human
infections do occur but are relatively rare. What puts us at risk of
infection is partly the way we alter our environment such that there is more
contact between domestic or peri-domestic animals — like rodents — wildlife,
and people. Wildlife are not necessarily carrying more pathogens than
before; we are altering their habitats and encroaching on them, forcing them
to live with us, which also happens to put us at greater risk for disease to
spillover.
One of the most important activities, in terms of
research, that has increased in recent years is wildlife surveillance —
testing for diseases that may affect our health. Historically, we have not
had enough information about pathogens carried by different animals that may
affect human health nor an understanding of how they emerge. We have a long
way to go, but wildlife surveillance is an essential component in order to
being able to forecast zoonotic-disease outbreaks. Efforts are going on to
better understand the extent to which known pathogens, like H5N1 or Nipah
virus, occur in their natural reservoirs on a global scale. Many groups,
including the Consortium for Conservation Medicine (CCM) in partnership with
The Center for Infection and Immunity (CII) at Columbia University, and the
Global Viral Forecasting Initiative (GVFI) are working to discover new
pathogens in animals that may have the potential to infect people.
The bottom line is that we do not have to be
afraid
of wildlife, but we do need to recognize that animals — like people —
naturally carry a whole range of pathogens, some of which may cause disease
if they jump hosts, others of which may be completely benign. Understanding
what types of pathogens are out there in wildlife and the processes by which
those pathogens spill over from one group of animals to another or to people
are essential to reducing our risk of outbreaks in the future.
Editor: A New York Times article by
Elizabeth Svoboda about Nathan Wolfe, visiting professor of epidemiology at
Stanford, who, she wrote, is “slogging through the rain forests of Cameroon
and taking a lead role in an effort to identify illnesses involving virus
transmission between animals and humans … tto prevent disease outbreaks that
could become global.” Wolfe and colleagues started the GVFI with more than
100 scientists in nine countries. with field locations in China, Madagascar,
Malaysia, and Paraguay. How many groups like this are “out and about” in the
world doing similar research? You have said CCM is working with his group.
Were these field locations chosen because they contain environmental
“incubators.” Is the work being done in rain forests or jungles — or where
civilization already has encroached? Is there more possibility today that a
zoonotic disease could come from a domesticated animal or an animal that has
been bred or raised in captivity, such as in a zoo or a nature park?
Schloegel: When attempting to identify
pathogens that could become global epidemics, one component of human society
that cannot be overlooked is the live trade in animals for human consumption
on national and international scales. China is the world’s most populous
country, with a reputation for eating even the most obscure of wildlife —
whether it be mammalian, avian, reptilian, amphibian, or insect. The wet
markets in China are notorious for selling a wide range of live wild-caught
and captive-bred animals, bringing them into close proximity to each other
and humans. It is through this cultural legacy that SARS was believed to
have crossed the species barrier before making its worldwide debut. U.S.
Fish and Wildlife’s data on live wildlife imports into the United States
indicate that shipments have increased approximately 39% from 2000 to 2007.
This does not account for the numbers of livestock shipped across U.S.
borders each year, let alone animals sold on the black market. Oftentimes,
shipments are rarely identified to the species or even genus level; only a
small percentage of animals are actually monitored or quarantined for the
presence of diseases. With more and more animals being traded on an
international scale, it is imperative that we implement a system of checks
and balances to minimize the spread of potentially harmful pathogens.
Epstein: The CCM recently published a paper in
the journal Nature describing the major risk factors for the future
emergence of infectious diseases globally, based on looking at the past 60
years of disease-emergence events. In this analysis, areas that are
hypothesized to be more vulnerable to future zoonotic-disease emergence
include those parts of the world that 1) are in tropical latitudes, 2) are
in areas of high biodiversity, and 3) have high human-population density.
This intuitively makes sense because, with high animal diversity, there will
presumably be a commensurate level of pathogen diversity; and with high
human-population density, there will be a greater risk of resource overlap
between people and wildlife — which leads to increased connectivity and risk
of disease spillover. So, it is not simply that areas with vast rainforest
are more likely to have disease emergence, because while there may be high
animal biodiversity in remote tropical forests, if there are not people
around to get infected and spread these pathogens to other people, then
there is a relatively lower risk of zoonotic-pathogen emergence. In areas of
high interaction between wildlife and people, however, the risk of pathogen
spillover is increased.
Bushmeat hunting is an example of a human activity
that brings people into more frequent contact with a variety of animal
species that may live in remote areas of forest; but through hunting and
butchering practices, hunters may become exposed to animal pathogens. This
was most likely the case with HIV-1, a virus originally carried by
chimpanzees that may have been introduced to humans through hunting
activities. Thus, bushmeat hunters represent a group of people highly
exposed to a variety of wildlife species and, thus, a variety of potentially
zoonotic pathogens. Research being conducted by the GVFI with CCM is focused
on better understanding wildlife-hunting practices in various countries
around the world. The goal is to understand animal-pathogen diversity and
the frequency of hunter exposure. Then developing culturally relevant
educational programs that improve hunter safety and health may, one day,
reduce the risk of animal-pathogen emergence into human populations.
Editor: In your 2005 MLO article, you
pointed out that “predicting disease outbreaks is the ultimate goal” — which
is obviously the same reason Wolfe, et al, are doing all of this “slogging.”
Since “global warming” has become — pardon the pun — a “hot” topic, is there
a chance that the climate affects the rate of infectious-disease emergence?
Are your studies changing because you find indications of emerging pathogens
more often? Have fish or sea creatures ever become a threat in this way to
humans? As we learn more about “dead spaces” in our oceans, red tide along
our beaches, and the decimation of the aquatic food chain, do we know if
these disasters will have any effect(s) on the spread of zoonotic diseases?
While H5N1 is suspect in what might be the next pandemic flu, what other
diseases should we be watching for in the next five years?
Schloegel: Fish constitute more than 90% of
all live animal imports into the United States, a large percentage of which
are identified only as “tropical fish.” While this may be cause for alarm in
terms of pathogen spread, invasions by non-native imports and the spread of
diseases from fish to fish are of greatest concern. Any threats that do
exist between fish and humans are largely a result of parasitic infections
from eating raw or undercooked seafood. Some reptiles — iguanas, for
instance — are prone to salmonella, which is readily spread to humans if
proper caution is not taken. Frogs in the trade have been suggested as
possible reservoirs for West Nile virus; however, there is currently no data
to support that theory. The American bullfrog is farmed in the millions per
year in South America and Asia for frogs’ legs. In Brazil, these animals
were historically fed pellets containing meat products. After the most
recent outbreak of “mad-cow” disease, Brazil banned the use of meat in frog
feed to eliminate any possible spread to humans through consumption. The
feed is now supplemented with fish. Mammals, birds, and insects appear to be
the vectors of greatest concern when considering the risk of pathogen
emergence and spread in humans.
Epstein: I will comment on the final part of
your question. In trying to understand which pathogens have pandemic
potential, we look at pathogens that have caused human disease and have
shown signs of human-to-human transmission. One which has demonstrated this
ability is Nipah virus in Bangladesh where the virus has caused several
outbreaks since 2001, with case fatality rates averaging about 75% (and as
high as 92%). In Malaysia, there were no reported instances of
human-to-human transmission of Nipah virus — most human cases occurred from
exposure to infected pigs; but in several outbreaks in Bangladesh, there
have been clear chains of infection among individuals. Thus far, the chains
of transmission have been fairly limited, however, there is a concern that
with continued spillovers from bats, there will be opportunity for a viral
strain to evolve that is efficient at human-to-human transmission and that
may persist within a human population for long enough to spread beyond local
villages. Bangladesh is one of the most populous places on earth; and
fortunately, so far, the outbreaks have been localized and limited to fewer
than 300 cases in total. There is concern, however, that Nipah virus may one
day have the ability to cause more widespread morbidity and mortality. We
are currently studying the dynamics of Nipah virus in its fruit-bat
reservoir in collaboration with the CDC and scientists at the International
Center for Diarrheal Disease Research who are working with human and
domestic-animal populations, in an effort to reduce the risk of spillover
from bats to people, and, ultimately, prevent future outbreaks.
Jonathan H. Epstein, DVM, MPH, is a certified
international veterinary medicine senior research scientist at the
Consortium for Conservation Medicine, NY. Lisa Marie Schloegel, BA, is a PhD
candidate at Kingston University, England, and research consultant for the
Consortium for Conservation Medicine, NY. She is the current project manager
for the CCM’s Wildlife Trade Initiative. Find out more about Jon’s and
Lisa’s careers at
www.mlo-online.com.
