Emerging Infections 3

Rickettsioses present some of the oldest recognized infectious diseases. Ricketts proved that the wood tick Dermacentor andersoni, was involved in the transmission of the Rocky Mountain spotted fever (RMSF). In 1910, the first cases of Mediterranean spotted fever (MSF) were reported in Tunis, Tunisia. The role of Rhipicephalus sanguineus was established in 1930. Prior to 1984, only eight rickettsioses were recognized, and in the subsequent 13 years, seven new rickettsial diseases were described. The advent of novel diagnostic tools has dramatically improved the efficiency of diagnosis of rickettsioses and the recognition of new rickettsial species. Cell culture is the most widely used system for primary isolation. The oriental rat flea, Xenopsylla cheopis, is the main flea vector of flea-borne bacterial diseases, including rickettsioses. These fleas are associated with Old World commensal rats, and consequently, the disease associated with these fleas is prevalent in harbors and large cities, The cat flea, Ctenocephalides felis, is the vector for cat-scratch disease and cat-flea typhus. Eleven new rickettsioses have been described since 1974. An entirely new clinical entity, such as the one caused by R. slovaca, which results in disease without the typical rash associated with other spotted fevers was described. The future of such new diseases is unpredictable because rickettsioses have not been systematically studied in such cases. This is an area for future research.

The aquaculture industry, which is increasingly being developed, has not yet been recognized to result in significant human disease. Aquaculture in North America involves diverse farming systems in diverse areas. The criticisms concern contamination of the environment by aquaculture systems through unwanted obstructions to coastal navigation, unsightly cages or pens, aquaculture effluents such as excess food and chemotherapeutics, and the use of nonnative species or native species that are either domesticated or genetically different from wild stocks. The level of contamination of aquaculture products with pathogenic bacteria depends on the environment and the bacteriological quality of the water where the fish are cultured. It should be noted that nonindigenous bacteria of fecal origin could be introduced into aquaculture ponds via contamination by birds and wild animals associated with farm waters. Streptococcus iniae has also been reported to be the causative agent of ongoing infection and excess mortality of tilapia in Texas aquaculture farms. Overcrowding in farms and during transport may have contributed to the increasing importance of streptococcal infections in fish. Finally, although S. iniae commonly colonized the surfaces of tilapia and other species of fish, isolates are genetically diverse. Although S. iniae is capable of causing invasive disease in humans, serious disease appears to be rare, and if people take the proper precautionary measures when handling whole, uncooked fish, infections caused by S. iniae can be prevented.

Microsporidia have risen from obscure and anecdotal causes of keratitis and systemic disease in humans to become one of the most medically significant groups of emerging infectious agents. This chapter focuses on what is known regarding the epidemiology of human microsporidiosis and what we may be poised to better understand in the very near future. Topics covered include prevalence and geographic distribution, case demographics and populations at risk, and potential modes of transmission. The most common clinical presentation of Encephalitozoon intestinalis infection, similar to that of Encephalitozoon bieneusi infection, is severe chronic diarrhea, which often progresses to malabsorption and wasting syndrome. Surveys for antibodies to microsporidia in human sera have focused exclusively on human exposure to Encephalitozoon species. Although most recognized cases of human microsporidiosis are associated with some form of immunosuppression, reports describing microsporidial infections in HIV-negative, immunocompetent patients are also increasing. Microsporidia have been isolated from ditch water in a mosquito larval habitat in Florida. The only two factors which were associated with microsporidian infection were swimming in a pool and male homosexuality. One of the most exciting observations in the prevention of microsporidiosis in persons with AIDS has been the role of highly active antiretroviral therapy (HAART) in raising CD4 lymphocyte counts and heightening immunity. A high standard of personal hygiene seems mandatory for patients infected with microsporidia. Cohabitating sexual partners of infected patients should be offered screening for microsporidiosis regardless of their HIV status.

In the United States it is important to keep in mind that imported cases among tourists returning from countries where acute Chagas' disease is endemic have not been reported and that autochthonous Trypanosoma cruzi infections are extremely rare. Even though T. cruzi-infected triatomine insects are present in many parts of the western and southern United States, only four autochthonous cases of acute Chagas' disease have been reported. Importantly, none of the imported cases occurred in tourists returning to the United States, but two such instances have been reported in Europe. Although the number of imported and autochthonous cases of acute Chagas’ disease that go unrecognized may be several times the number described, the fact is that acute T. cruzi infection is rare in the United States, and there is no reason to expect that this situation will change. The courses of acute Chagas’ disease in the transfusion recipients were particularly severe because of immunosuppressive treatments that they were receiving, and no doubt this contributed to the specific diagnosis. Finally, an antifungal agent, bis-triazole (00807), was recently reported to cure acute T. cruzi infections in mice, but it is still in the early stages of development. Management of the occasional severely ill patient with chagasic meningoencephalitis or myocarditis is for the most part supportive. Cardiac transplantation is an option for patients with end-stage chagasic heart disease.

In 1997, a huge outbreak of foot-and-mouth disease (FMD) emerged in Taiwan. Although the source of the outbreak was never confirmed, there were strong suspicions that it resulted from illegal importation of infected animals. All of these large-scale outbreaks of diseases emergent in countries that have been free of them usually result in massive depopulation of herds or flocks. This depopulation in and of itself can have significant impacts on humans. First, for each food-producing animal that is killed, there is considerable resource wastage; that is, all of the materials required to grow the animal are never recovered in the form of consumable protein. Second, there can be major environmental problems associated with the disposal of carcasses. In general, the animals and animal products that cross borders are subject to stringent regulatory controls related to those diseases with which the scientific agricultural community is already familiar. In animals there are numerous examples of alterations in ecology that have resulted in new diseases. As animals are moved around and grouped in new ways, as their environments are modified, and as technological advances to increase production are promoted, there is tremendous potential for endogenous flora to wedge into new gaps. Husbandry changes, the correlate of which in human medicine would be lifestyle changes, are responsible for a number of emerging diseases in animals. There are a variety of emerging diseases of animals, and they are caused by many of the same factors that are at play in human diseases.

Due to increased awareness of infectious diseases, there has been renewed interest and revitalization in infectious disease pathology. Recent experience at the Centers for Disease Control and Prevention (CDC) and other institutions has shown that use of a combination of traditional morphology with immunologic and molecular pathology techniques is an extremely useful approach for the confirmation of diagnoses for patients with otherwise unexplained illnesses. Visualization of microbial antigens or nucleic acids in the context of pathology allows the pathologist to assess the clinical significance of serologic test results or microbial isolation. By comparison, the use of molecular pathology techniques offers several distinct advantages over traditional microbiologic methods, including speed, sensitivity, reduced risk of exposure of laboratory personnel to the agent, and tissue localization of pathogens. Epidemiologic, clinical, and histopathologic findings enable CDC pathologists to use the most appropriate initial immunologic and molecular pathology tests. The chapter highlights some of the contributions of CDC pathologists in addressing some of the challenges posed by new, emerging, and reemerging infectious diseases. Historically, pathologists and the discipline of pathology have played pivotal roles in the discovery and characterization of the rickettsioses and, more recently, the ehrlichioses. Disease recognition by infectious disease pathologists can play a key role in the study of emerging infectious diseases. The chapter provides several examples of the frontline role of pathology in guiding the early phases of epidemiologic investigations of infectious diseases.

The recognition of and response to actual terrorist use of an infectious agent are complicated. In contrast to chemical agents or explosive devices, whose destructive impacts are immediately recognizable, covert delivery of biological agents in the absence of notification of intent would cause disease only after an incubation period during which exposed persons could be widely dispersed. Immunohistochemical staining of autopsy tissues confirmed the existence of a previously unrecognized hantavirus approximately 1 month after the initial recognition of cases of the syndrome, but several months elapsed before the virus was finally isolated in cell culture. This experience highlights the critical importance of highly trained field and laboratory personnel, appropriate biocontainment facilities, and the laboratory reagents and sophisticated molecular techniques, all of which must be available for the detection of candidate agents that terrorists might use. The potential for bioterrorism emphasizes the urgency of strengthening the public health capacity to detect and respond to emerging infectious diseases, particularly the need to forge and strengthen partnerships between the public health and emergency response sectors. Centers for Disease Control and Prevention (CDC) maintains expertise in the detection of virtually all infectious agents pathogenic for humans and is able to use modem molecular techniques, traditional procedures for organism cultivation. Whether epidemics are the result of terrorism or natural factors, CDC and the public health community remain at the forefront of the global epidemic response.