Infectious diseases

0.5-1.5 million immigrants and refugees were admitted to the USA each year. In contrast to the previous waves of immigration many of these individuals came from the parts of the world where certain infections such as tuberculosis are common. This is an important factor in the resurgence of tuberculosis in the USA as the percentage of patients who were foreign-born increased from 22 % in 1986 to 37 % in 1996.

A variety of human behaviors also influence the emergence of the infectious diseases. The impact of the sexual revolution on the frequency of gonorrhoea, syphilis, and HIV is evident. Perhaps less evident is the impact of other changes such as changes in eating habits. There are changes in the types of food that people eat, how this food is prepared, and where the food is prepared. This can result in the new exposures to unfamiliar food or the dependency upon others to handle and prepare food safely. All these factors have contributed to the emergence of some of the newer food borne diseases. Another important factor influencing the emergence of resistance has been the unnecessary use of antimicrobial agents. In 1992 over 110 million courses of antimicrobial drugs were prescribed to outpatients in the USA. Since three-quarters of these drugs are prescribed for upper respiratory infections that are often caused by viruses, over half of these 110 million courses may be unnecessary.

Technology and industry. The impact of technology and industry falls into three general areas. These include new technologies and products, changes in food production processing and preservation, and changes in industrial demographics. New technologies and products may have unexpected disease implications such as the association of air conditioning and whirlpool spase with Legionnaires disease, or new tampons with toxic shock syndrome, or the fast-food hamburger with E. coli 0157:H7.

The second area, the changes in how food is produced, processed, and preserved has also been important. For example, in the last 50 years many of the new-agricultural production strategies involve intensive rearing of young animals under environmental conditions that are conducive to the transmission of the infectious diseases. These production strategies often depend upon increased antimicrobial use. Thus, only substituting young animals for children, the situation is similar to the day care setting and has resulted in an increase in antimicrobial resistance for organisms that are transmitted through the food chain from animals to humans. Between 1979 and 1989,the frequency of drug resistance in human Salmonella isolates almost doubled from 17 % to 31 %. Today the resistance in humans is the result of antimicrobial use in animals. The many changes in food processing and preservation are also influencing disease emergence. The recent emphasis on "natural" foods has led to use of fewer preservatives or secondary barriers to prevent spoilage. Thus, some foods are protected only by refrigeration. This has resulted in increasing problems with organisms that grow in the cold, such as Listeria or Yersinia. The lack of secondary barriers also

Introduction of infectious diseases 11

increases the risk of food handling errors leading to diseases such as botulism. In the last 10 years, several outbreaks of botulism have occurred when "keep refrigerated" foods were not kept refrigerated.

The third change, that of industrial demographics, is characterized by consolidations of industry, larger market size, and wider geographic distribution for a variety of food products. Although these changes have the potential for greater quality control and better safety,when something goes wrong,it can really go wrong. Thus, in 1994, an ice cream product produced by a single company in Minnesota led to thousands of cases of salmonellosis in over 41 states.

Economic development and land use. Changes in economic development and land use are often cited in discussions of emerging viral diseases. Encroachment on rain forests, for instance, may lead to exposure to new agents such as Ebola or Marburg viruses. However, such changes are also influencing the emergence of other infectious diseases. For example, population growth and spread lead to environmental change and pollution. The inadequacies of hygiene and sanitation that exist in many of the "mega cities" in the developing world are potential ticking time bombs for the emergence of infectious diseases. Other types of development and land use practices are contributing to specific problems. Conservation activities, such as those directed toward deer populations, have contributed to the emergence of Lyme disease. Coastal agriculture expansion is leading to blooms of toxic microorganisms, while coastal population growth is leading to human faecal contamination of shellfish beds and transmission of a variety of viral and bacterial pathogens.

International travel and commerce. Advances in technology have had a rapid impact on international travel and commerce. A person or a food can be almost anywhere in the world in 24-48 hours. This facility in travel and commerce has increased the potential for the introduction of emerging pathogens to new geographic areas by infected travellers, by contaminated food, or even by transporting vehicles.

Microbial adaptation and change. As humankind is instituting a number of changes, the microbes themselves are changing. This is leading to the evolution of new pathogens, the development of new virulence factors, the development of antimicrobial resistance, and tolerance to adverse environmental conditions. A good example of this microbial change has been the emergence of ?. coli 0157:H7, which probably evolved from an entero-pathogenic E. coli that acquired Shigella genes. As a food borne pathogen, it combines the worst of Shigella and Salmonella. Like Shigella, this organism has a low infectious dose, requiring only a few organisms to cause disease. This leads to subsequent person-to-person transmission once the organism is introduced into a community and also poses a high risk for cross-contamination in the kitchen. This organism is more similar to Salmonella in its tolerance to adverse environmental conditions. Thus, it has been associated with outbreaks that were caused by foods with pH 4.0, conditions* that are usually inhibitory to most bacterial pathogens.

Date: 2014-12-21; view: 1041