The Emergence of Zoonotic Diseases Understanding the Impact on Animal and Human Health: Workshop Summary (2002) / Chapter Skim
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3 Factors of Emergence
3 Factors of Emergence
Pages 26-63
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From page 26... ...
While we currently know the complete genome sequences of many influenza viruses, we do not understand the molecular basis of pathogenesis and are unable to predict which combinations of viral genes have pandemic potential.
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Thus, pig tracheal cells can be infected not only by human influenza viruses but also by avian viruses. However, the direct chicken-to-human transmission of HSN1 vi .
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From page 28... ...
Examples of Recent Outbreaks in Birds The avian HSN1 influenza virus that was transmitted to poultry and humans in 1997 in Hong Kong caused high mortality in both species, killing more than 70 percent of chickens and six of the 18 infected humans. (Hong Kong's location at the crossroads of many trade routes makes it particularly susceptible to the outbreak of new diseases.)
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From page 29... ...
Further characterization of the viruses revealed that the human isolate from Hong Kong and the quail isolates shared similar genetic traits with the HSNl-like viruses from chickens and humans in Hong Kong in 1997. Thus, while avian influenza viruses can transmit directly to humans and cause disease, additional mutations and/or reassortant events are probably required to permit efficient human-to-human spread.
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ASSESSING THE THREAT AND THE OPPORTUNITIES ACROSS THE SPECTRUM OF ZOONOTIC DISEASES Paid W Ewald, Ph.D.
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In contrast to the oft-mentioned examples of newly emerging acute infectious diseases, the most damaging chronic diseases are already globally distributed and prevalent. Some of these diseases are now causing damage in human populations that is comparable to the damage that is merely feared for emerging acute infectious diseases.
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From page 32... ...
Neuronal damage sufficient to reduce a person's IQ by 5 percent, for example, would not be acceptable for humans but probably would be acceptable for cats. If the reservoir host is not a valued animal, then discovery of zoonotic causes of already common chronic diseases may provide other less technologically sophisticated options.
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From page 33... ...
The potential applicability to other highly damaging chronic diseases is apparent from the current evidence on infectious causation of diseases for which causation is still controversial. Atherosclerosis, for example, is associated with infections by Chlamy~lia pneumoniae, Porphyromonas gingivalis, Actinobacillus actinomycetocomitans, Bacillus forsythus, and cytomegalovirus, with
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From page 34... ...
The proposed importance of infectious causation of chronic diseases emphasizes an irony in the attention devoted to emerging infectious diseases over the past two decades. This attention was triggered largely by the AIDS experience, in which a lethal disease arose from an exotic source and spread pandemically.
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In both poor and rich countries, grave dangers are posed by the long-standing chronic diseases that are or may soon be recognized as caused by infection. Leaders of the effort to awaken concern over infectious diseases have emphasized the danger from resurgence of known acute infectious diseases, and to some extent the growing recognition of infectious causation of chronic diseases, but most of the media attention has focused on the exotic acute diseases.
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From page 36... ...
Another IOM report in 1998, on the benefits and risks of using drugs in food animals, reached a stronger conclusion: "There is a link between the use of antibiotics in food animals, the development of resistant microbes, and the zoonotic spread of pathogens to humans." . Because of the mounting evidence of risk to humans, FDA/CVM believes that there are issues we must address.
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From page 37... ...
Simply put, these are the drugs of last resort with a life-threatening disease. We would put drugs in Category 1 particularly if they were important for the treatment of foodborne disease, because many zoonotic diseases are foodborne in nature.
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We felt that the only way to really get on top of this problem was to put a system in place to start tracking resistance. So, in 1996, we started the National Antimicrobial Resistance Monitoring System (NARMS)
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There was some concern that antimicrobial resistance is a politically hot issue, and that FDA therefore may fee! pressure to do things that are not well based in science.
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Within this framework, consider the following three emerging zoonotic diseases, which illustrate the complex interaction of various ecologic factors on disease emergence and how little we really understand about the basic ecology of most zoonotic disease agents. Venezuelan Hemorrhagic Fever In 1989, physicians in central Venezuela began to report cases of a
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From page 41... ...
The majority of cases have occurred during the dry season, when there is considerable agricultural activity in the endemic region. VHF has many clinical and epidemiological similarities with the other arenaviral hemorrhagic fevers (Lassa fever, Argentine hemorrhagic fever, and Bolivian hemorrhagic fever)
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From page 42... ...
is a severe viral hemorrhagic fever that is transmitted by the bite of infected mosquitoes. The current geographic distribution of the disease includes tropical regions of sub-Saharan Africa and South America.
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From page 43... ...
This undoubtedly is an underestimate, since milder cases of the disease often are not recognized and the more severe forms can be confused with other endemic diseases, such as hepatitis B and D, leptospirosis, and dengue hemorrhagic fever. Current knowledge about the ecology of sylvan YF in South America is still incomplete.
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Many unemployed miners and their families migrated into the area around Santa Cruz because of the economic prosperity and the availability of agricultural land and jobs. Most of the migrants, as well as a majority of the residents of Santa Cruz, lacked immunity to yellow fever virus.
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The parasite is introduced into the domestic cycle when infected wild animals visit houses to scavenge for food. During such visits, so-called "peridomestic" sand flies feed on the infected wild animals, pick up the parasite, and subsequently transmit it to dogs, which then act as domestic reservoirs.
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From page 46... ...
These three examples are meant to illustrate the importance of ecologic factors in the maintenance and emergence of zoonotic diseases. The current paradigm of biomedical research, which focuses on mechanistic studies at the cellular, molecular, and genetic levels, frequently overlooks the importance of ecological factors in the development of human disease.
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From page 47... ...
Chief, Arbovirus Diseases Branch Division of Vector-Borne Infectious Diseases National Center for Infectious Diseases Centers for Disease Control and Prevention A number of vectorborne zoonotic diseases, both bacterial and viral, have emerged in recent years or are now increasing in many regions. The agents that cause some of these diseases, such as plague and tularemia, also are of concern because of their potential use by bioterrorists.
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The case of West Nile virus may illustrate how the federal government responds to an emerging infectious disease, as well as some of the policy issues and problems that can arise. The virus was discovered in Uganda in 1937, and major outbreaks of the disease periodically occurred worldwide.
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Federal, state, and local public health officials are now using the guidelines in an effort to minimize the public and veterinary health impact of West Nile virus. To aid in this control effort, Congress has given CDC $2.7 million to enhance surveillance efforts in states along the Eastern seaboard, because of concern that migrating birds will carry the virus southward from New York.
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During the past decade, our laboratory in the Center for Medical, Agricultural, and Veterinary Entomology has developed risk assessment models for a number of diseases, including malaria, Lyme disease, and dengue hemorrhagic tever. The laboratory also has developed models for predicting the population dynamics of some of the mosquito species that serve as disease vectors, such as the mosquitoes that transmit Venezuelan equine encephalitis.
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This type of information linking the extent of potential control measures to expected disease outcomes has been lacking in most control programs. The new method of targeted source reduction is being considered by the World Health Organization and the Pan American Health Organization and is in operation in countries in Southeast Asia and South America.
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With further advancement, it also may be possible to derive models of vectorborne diseases, including West Nile virus and St. Louis encephalitis.
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These locations are of special concern because of their increasing role as wildlife habitat, the greater interface between humans and wildlife that takes place within those environments, the paucity of knowledge about disease in those wildlife populations, and the general lack of orderly management for wildlife within those environments. In the wild, several trends are contributing to the growing importance of zoonotic diseases.
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Environmental factors are the driving force for many emerging diseases of wildlife. In general, wildlife disease prevention and control will be most effective when environmental conditions are understood and the anthropogenic actions causing those conditions are addressed.
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A reasonable question is: Why is more not being done to address disease within wildlife populations? One contributing factor is that while most people believe it is possible to deal with disease threats involving humans and domestic animals, similar confidence is lacking regarding our ability to
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From page 56... ...
As a result, disease outbreaks in wildlife do not have a mandated responsibility to be investigated or dealt with. Because of the differences in agency responsibilities, agriculture agencies do not become involved unless the outbreak is known to be, or has a high probability of being, a disease of major concern for domestic animals.
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The CDC has proposed developing a national electronic disease surveillance network for state and federal public health information on emerging infectious diseases, and this network should be expanded to include wildlife surveillance information on emerging .
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It is not sufficient to rely on the diverse scientific meetings that currently incorporate wildlife diseases as agenda topics. The North American Wildlife Conference can provide an appropriate forum for bringing wildlife disease issues before those individuals who manage public lands, and organizers of the conference should be encouraged to develop a regular forum devoted to emerging diseases.
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From page 59... ...
These agents cause fatal human and animal neurological diseases, which are characterized by a long symptom-free incubation period followed by a short acute phase. Many such diseases have been described over the years, but attention has been focused on them most recently by the massive outbreak of BSE in cows that started in the United Kingdom in 1986.
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the disease was caused by an "old" agent, (3) the disease had a very long incubation period, (4)
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SIV infection has not been shown to cause disease in its natural hosts. This issue is obviously difficult to address in the wild but has been addressed by the epidemiological studies of sooty mangabey and African green monkey populations bred in primate centers.
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From page 62... ...
This issue may be more easily addressed in simian models, by comparing SIV infection in species, such as the sooty mangabey and the rhesus macaque, that are resistant and susceptible to disease. Sooty mangabeys range from Sierra Leone and Liberia to the western half of Ivory Coast.
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From page 63... ...
Epidemiologists and historians have documented multiple instances of reuse of nonsterile needles or even of direct arm-arm vaccination in Africa since the beginning of the 20th century. The main reason why serial intravenous passages can promote SIV adaptation is that they provide the setting for successive viral jumps from primary infection to primary infection.
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