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Category: Clinical Microbiology
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While it is widely recognized that diseases in humans are emerging at an unprecedented rate, an often overlooked phenomenon is that the same issue is present in animal populations, where new diseases are occurring at an even faster rate than in humans.
This important new volume provides an in-depth review of various emerging diseases in animals and highlights the importance of veterinary medicine in the control of emerging diseases. It discusses the need for an awareness of emerging diseases in animals in preparation for emerging diseases in humans and offers an understanding of the serious negative consequences of animal diseases on human populations, including zoonotic potential, ecological impact, and compromising of the food chain.
Electronic only, 310 pages, illustrations, index.
This chapter focuses on constructing a framework for exploring the underlying reasons for the emergence of new animal diseases. Foot-and-mouth disease (FMD), the most contagious disease known to exist for either animals or humans, causes a transient but severe drop in production. Susceptible animals, which include all cloven-hoofed species, develop big painful blisters in the oral cavity and on the feet. Habitat alterations can lead to new diseases by a variety of means. First, destruction of existing habitat will cause a change in an animal's or a population's behavior. Second, environmental disruption can cause a change in vector populations and patterns. The plethora of diseases that have emerged from fruit bats underscores not only the ease with which diseases can move from one area to another but also the tremendous repository of viruses contained within sylvatic species of animals. The recent emergence of West Nile encephalitis virus emphasizes the importance of thorough pathologic investigations of wild animals that died in captivity and the necessity of an integrated network for reporting animal and human diseases. Artificially feeding wild animals, while seeming to be an innocuous and perhaps altruistic practice, can have dramatic effects on animal and even human health interests. Any decisions made concerning unusual animal movements, environmental disruption, or altered husbandry should be thoroughly reviewed prior to implementation to determine potential impacts on animal and human health.
This chapter introduces a proposed scheme for use as a reference in placing animal pathogens into biosafety classifications. The importance of an accepted animal disease classification is apparent; however, the universally accepted guidelines for placing pathogens that only infect animals into biosafety risk categories are poorly defined compared with the guidelines for human pathogens. A biosafety classification scheme for animal pathogens is not intended to place undue burden on researchers but to enhance the quality of research and efforts to improve animal health. The natural starting point for proper biosecurity classification of strict animal pathogens is risk assessment. A consensus on animal pathogen risk groups was reached at the 5th International Veterinary Biosafety Workshop in 1996. The emphasis of biosafety risk classification was on pathogens that infect only animals (not affecting human health). Laboratory work allows the containment and control of infectious materials during procedures by utilizing standard laboratory practices and techniques and also specialized primary barrier equipment. Animals (livestock and poultry species) infected with disease agents present unique challenges for biosecurity control.
This chapter deals with the history of agroterrorism, biological crimes, and biological warfare directed toward animal agriculture, specifically horses, cattle (both beef and dairy), swine, sheep, goats, and poultry. A recent investigative report concludes that it was an ambitious and well-planned program, conducted on three continents, but that the success of the attacks was questionable. In a very recent excellent investigative report on the introduction of rabbit calicivirus disease into New Zealand, Steve Goldstein offers a serious, thorough review of agroterrorism and biological crimes. The animal pathogens and the species affected that are most often mentioned as being the most important as potential agroterrorism attack agents are, for cattle, foot-and-mouth disease virus and rinderpest virus; for swine, foot-and-mouth disease virus, classical swine fever virus, and African swine fever virus; and for poultry, avian influenza virus and Newcastle disease virus. With U.S. government funding and attention focused on countering biological weapons targeted at humans, agencies and groups are just now becoming aware of the threat posed by agroterrorism and biological warfare directed against the nation's animals and crops. The current laws that authorize investigations of potential terrorism rest on assumptions that may be misaligned for preventing and punishing agroterrorism. The biocrimal acts and the consequent economic impact clearly demonstrate that agriculture can be the target of an economically devastating terrorist or criminal attack.
This chapter reviews the microbiological risks of xenotransplantation as they currently exist, with the caveat that the technical tools of molecular biology are rapidly adding to our knowledge in leaps and bounds. The ethical issue for xenotransplantation from the microbiological aspect is the risk of introducing an unknown infectious agent into a naive human population versus the individual benefit of a life-giving organ transplantation and improved quality of life. There is a large database of information on pigs, from basic anatomy and physiology to specifics of their biochemistry and genome. Production of pigs in barrier facilities operated under very high standards of animal husbandry can consistently yield qualified-pathogen-free animals. The gibbon ape leukemia virus (GALV) behaves as an exogenous retrovirus, but DNA evidence points to the endogenous retrovirus of Mus caroli as the origin. Certainly, the expanding interest in pigs as organ donors has placed the subject of endogenous retroviruses, in particular the porcine endogenous retrovirus (PERV), front and center in the microbiological risk analysis of xenotransplantation. PERV mRNA has been identified at variable levels in all tissues examined, indicating that viral expression is not restricted. Transmission electron microscopy of tissues was negative, although virus-like particles (VLP) were detected in serum. Although the VLP did not bind antibodies against recombinant gag or whole virus, product-enhanced reverse transcriptase analysis produced positive results.
This chapter describes a remarkable series of new zoonotic viruses, including Hendra virus (HeV) and Nipah virus (NV), that have emerged from fruit bats in Australia and Malaysia. An apparently new paramyxovirus was isolated as the causative agent and was called Menangle virus (MenV). An initial diagnosis of Japanese encephalitis was changed when a paramyxovirus was isolated from the brain of a person and quickly identified as a virus resembling HeV. It was subsequently confirmed in the farmers’ pigs and also in dogs, cats, and horses. The major pathological changes were bronchopneumonia, with syncytium formation in respiratory alveoli in some pigs, and some animals showed meningitis. In humans, the disease was largely confined to pig farmers and to some abattoir workers, possibly from respiratory excretions from the pigs. There is evidence of greater contact between humans and their domestic animals and the bats as we encroach on the rainforest. MenV, once it was introduced into pigs, affected large numbers of sows and their offspring as well as two humans. Four new viruses have been isolated that have fruit bats in southeast Asia and Australia as natural hosts. These were HeV, Australian bat lyssavirus (ABL), MenV, and NV. The first, HeV, was probably only isolated because it was from an outbreak of disease with numbers of animals and humans far greater than what would be expected from the virus's level of contagiousness.
In general, influenza viruses replicate in epithelial cells of the upper respiratory tract or gastrointestinal tract, and the outcomes of such infections include no clinical signs, upper respiratory disease, pneumonia, and occasionally death. In birds, most infections are subclinical. However, in poultry, influenza infections have caused clinical respiratory disease or drops in egg production, and a few specific avian influenza virus strains have caused severe disseminated infections with systemic disease and high death losses. Interspecies transmission between different hosts within separate classes is even less frequent, as has occurred rarely with chicken-to-human or free-flying duck-to-pig transmission. One exception to the above rule has been the ease and frequency of transfer of swine H1N1 viruses to turkey breeder hens, but these are sporadic and isolated occurrences. The ability of influenza viruses to spread and produce infectious progeny and lesions is related to the cleavage of the hemagglutinin in various cell types. In mammals, influenza viruses primarily replicate in the respiratory tract and cause associated clinical signs and lesions. Virus replication occurs predominantly in the epithelium of the nasal cavity and results in clinical rhinitis, the most frequent clinical presentation in human patients, horses, and pigs. In Minnesota, traditional and molecular epidemiology has demonstrated the direct transfer of mildly pathogenic avian influenza viruses from free-flying birds (primarily mallard ducks) to range turkeys reared outdoors during annual wild-bird migrations.
There are specific transmissible spongiform encephalopathies (TSEs) which affect humans and others which affect animals. Evidence suggests that bovine spongiform encephalopathy (BSE) has crossed the species barrier to cause variant Creutzfeldt-Jakob disease (vCJD) in humans. The primary focus of this chapter is on BSE and its assumed relationship with vCJD. The chapter also provides more detailed summaries on scrapie and chronic wasting disease (CWD). The clinical, pathological, and molecular genetic features of the transmissible spongiform encephalopathies have led to speculation on the nature of the etiologic agent and the pathogenic mechanisms of the disease. A transmissible spongiform encephalopathy has been diagnosed in eight species of captive wild ruminants as well as exotic (cheetahs, pumas, a tiger, and an ocelot) and domestic cats. Isolations from the distal ileum were made in experimentally infected calves that were 4 months old, at 10,14, and 18 months after dosing. This study has also identified infectivity in bone marrow, trigeminal ganglion, dorsal root ganglion, brain, and spinal cord. The diagnosis of BSE is based on the occurrence of clinical signs of the disease and currently is confirmed by postmortem histopathological examination of brain tissue. Due to the many unknowns surrounding CWD, much research is under way to better characterize the disease, determine host range, develop and validate diagnostic tests, and understand the epidemiology. The results of this work will assist in developing science-based methods for prevention and control.
There are currently six named species of Brucella plus the newly discovered group of brucellae infecting marine mammals. Brucella melitensis is primarily a disease of goats and sheep that is present in most areas of the world where goats are raised. In humans, B. melitensis is considered the principal cause of brucellosis and is clinically the most severe. In the United States, disease caused by B. melitensis in animals is considered exotic, and nearly all cases of human infection are due to the consumption of imported unpasteurized goat cheese. Bison migrations outside Yellowstone National Park (YNP) boundaries have occurred with increased frequency since 1980, usually during the winter. When the bison migrate onto private land, they pose a potential threat of spreading brucellosis to domestic cattle, thereby putting the state's class free status at risk. The development of effective strategies to control and eradicate brucellosis from bison and elk in the greater Yellowstone area (GYA), from feral swine across the southern and central states, from reindeer and caribou in the arctic, and perhaps finally from marine mammals presents an unprecedented challenge. With the continuing emergence of this disease and these disease issues in free-ranging populations of wildlife, solutions can only be found based on good science, public education, and innovative, collaborative work between research scientists, regulatory and wildlife veterinarians, and wildlife managers.
In livestock, the main sources of economic loss caused by leptospirosis include abortions, stillbirths, birth of weak neonates, animal deaths, loss of milk production, and costs of veterinary care, treatment, and vaccines. Animals that have been exposed to or infected with Leptospira are not eligible for import or export because of the risk of transmission of the disease. Perhaps the most confusing facet of leptospirosis is the classification and nomenclature of the causative agent. Bacteriologists recognized early on that there were clinically and epidemiologically distinct types of leptospirosis. Clinicians recognized that there were different types of leptospirosis and that it was important to identify the type of leptospire, and therefore the likely animal reservoir, to control outbreaks of leptospirosis in their patients. In addition, investigators often study one particular serovar in one particular host (often not the natural host), and it is difficult to compare results between studies. However, several pathogenic mechanisms have been proposed to be important in the development of leptospirosis, including bacterial motility and adhesion, bacterial toxins, persistence of infection, and immune-mediated tissue damage. LPS is postulated to be important in the development of the endothelial damage and platelet aggregation that lead to thrombocytopenia in acute leptospirosis. The difficulties in recognition and diagnosis of this infectious disease are legendary, and the availability of quality diagnostic support is crucial. Physicians, veterinarians, and public health authorities must work closely together to recognize, document, and control this important zoonotic disease.
Enterohemorrhagic Escherichia coli, or pathogenic Shiga toxin-producing E. coli (STEC), is distinguished from other pathogenic E. coli by its ability to produce Shiga toxins designated Stxl, Stx2, Stx2c, and Stx2e. E. coli O157:H7 is highly pathogenic, possesses numerous virulence factors, and is the most common cause of STEC disease in humans in many parts of the world. The accumulation of virulence factors into pathogenicity islands may have resulted from continued acquisition of genes from bacteriophages or plasmids that integrate at common integration sites such as the bacterial tRNA genes. The prevalence of E. coli 0157:H7 was higher in herds that weaned calves abruptly than in herds that used a variety of methods to gradually wean calves. Some of the most numerous bacterial species in the rumen and colon, such as Bacteroides spp. and Prevotella spp., can hydrolyze esculin to the aglycone esculetin, thus producing inhibitory aglycones in the gastrointestinal tract. The major inhibitory factor under aerobic conditions is pyocyanin and under anerobic conditions appears to be fluorescein. The possibility of adding specific feed substances, feed additives, or competitive exclusion bacteria to the diets of cattle to reduce fecal shedding and rumen proliferation of STEC is promising but will require further study. Ruminants used as food are an important source of food-borne STEC infections in humans. Shedding of STEC is widespread in healthy cattle throughout the world.
This chapter reviews the epidemiological features of the global dissemination of multiresistant Salmonella enterica serovar typhimurium DT104, discusses the possible origins of this clone, hypothesizes about the possible mechanisms by which rapid global dissemination and displacement of other clones may have occurred, and discusses possible means of control. The clonal nature of mr-DT104 is well established by the genetics of antimicrobial resistance and by restriction fragment length polymorphism analyses. Nosocomial salmonellosis associated with nontyphoidal salmonellae occurs in some developing countries. Four hypothetical models are discussed with respect to their ability to explain the ecological characteristics typical of the agent. These models include: model 1: the enlarged niche hypothesis, model 2: the dissemination fitness hypothesis, model 3: the chance dissemination event hypothesis and model 4: the selection focus hypothesis. To date, the discussion of control efforts for mr-DT104 and similar clones has been limited primarily to debates on the use of antimicrobials in livestock, notably subtherapeutic antimicrobials. The epidemiology of multiresistant serovar Typhimurium is not confined by national borders but is global in nature, as well illustrated by mr-DT104. The available evidence indicates that these efforts should center on selection foci, such as calf-raising operations, which serve as reservoirs of serovar Typhimurium and as sites for clonal competition among epidemic multiresistant strains.
Carrión's disease, the first identified bartonellosis, is a hemolytic, vasculoproliferative disorder of people in the Andes Mountains of Peru, Colombia, and Ecuador. The disease, caused by Bartonella (Rochalimaea) quintana, went largely ignored until late in the 20th century, when it was found as a complicating infection of immune-compromised people causing a condition termed bacillary angiomatosis (BA). Two new pathogenic species, B. henselae and B. elizabethae, were also isolated from BA patients. Erythrocyte invasion is a strategy utilized by Bartonella spp. Blood or tissue culture is one of the most clinically useful tools to document active infection. B. henselae bacteremia can persist in clinically healthy cats for months to sometimes years. The presence of bacteremia can be intermittent or recurrent in some cats, making multiple sampling necessary. Blood for culture should be taken in an aseptic manner and placed into sterile EDTA tubes or directly into centrifugation-lysis tubes. Blood samples can be frozen and thawed prior to culture to produce erythrocyte lysis, which improves the sensitivity of isolation and rapidity of colony formation. Vaccination of cats would seem desirable to decrease potential human exposure to Bartonella. Cats that are treated and recover from Bartonella bacteremia following experimental intradermal infections with cultured organisms appear to be resistant to homologous strains on rechallenge.
One of the most notable effects of the plague was its influence on the development of modern medicine. The life cycle of plague is maintained in complex enzootic and epizootic episodes that involve numerous rodent reservoirs and flea vectors throughout the world. During the course of human history, pandemics of plague have occurred under two geographic settings, classified as either urban or sylvatic. The increasing popularity of the domestic ferret (Mustela putorius furo) as a pet might also raise concern about the rise of another potentially susceptible reservoir that has a close association with human households. This concern is tempered by the results of one study that involved eight domestic ferrets experimentally infected with Yersinia pestis. This study showed that domestic ferrets developed high serum antibody titers and showed no clinical signs of disease; Y. pestis could not be detected in tissues 21 days postinfection. In humans, plague is generally classified as bubonic, septicemic, or pneumonic. The pathogenesis in naturally infected cats is an acute disease depicted by fever, lethargy, buboes, formation of abscesses, and occasionally pneumonia. The standard method of serodiagnosis is the passive hemagglutination assay or enzyme-linked immunosorbent assay with F1 antigen. In the event of a bioterrorism incident involving Y. pestis, it will be critical that human health care first responders, veterinarians, and public health officials be familiar with the normal geographic distribution, clinical presentation, and diagnosis of the disease in both humans and animals.
Mycobacterium tuberculosis and Mycobacterium bovis are members of a closely related group of organisms referred to as the M. tuberculosis complex, which also includes Mycobacterium africanum and Mycobacterium microti. M. tuberculosis is the primary cause of tuberculosis in humans and, world wide, is the leading cause of death from a single infectious agent. Animals with potential exposure to M. bovis through contact with infected cattle are skin tested and, in some cases, slaughtered to prevent the spread of tuberculosis to new herds. Tuberculosis caused by M. bovis and M. tuberculosis has been reported in zoos, game parks, and other exotic animal collections in the United States and throughout the world. The presence of a wildlife reservoir of tuberculosis in Michigan may prevent the United States from achieving the goal of eradication of tuberculosis in animals. To comprehend the pathogenesis of a disease, it is important to understand the routes of infection and mode of transmission between hosts as well as the host response and characteristics intrinsic to the pathogen. Tuberculosis in animals is chiefly acquired by inhalation or ingestion. Follow-up surveys of the wildlife in the area revealed no further evidence of tuberculosis in any of the wild animals that were examined. The best long-term approach to control of tuberculosis in the United Kingdom appears to be the development of a vaccine for cattle and improved diagnostic tests to discriminate infected from vaccinated cattle.
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