LESSON 1: ECOLOGICAL ISSUES IN EPI

• Classical epidemiologists have long regarded their field as “human ecology,” “medical ecology,” or “geographic medicine,” because an important characteristic of epidemiology is its ecological perspective.
• One of the most important insights of ecological thinking is that as people change one part of a system, they inevitably change other parts.
• An epidemiologist is constantly alert for possible negative side effects that a medical or health inter- vention might produce.
• UNINTENDED CONSEQUENCES FROM SOLUTION

  
• STEPS IN SOLUTION OF PUBLIC HEALTH PROBLEMS AND UNINTENDED CREATION OF NEW PROBLEMS:

  1. Vaccination and patterns of immunity
  2. Effects of sanitation
  3. vector control and land use patterns
  4. river dam construction and patterns of diseases
• Understanding herd immunity is essential to any discussion of current ecological problems in immunization.
• A vaccine provides herd immunity
• The degree of immunity necessary to eliminate a disease from a population varies depending on the type of infectious organism, the time of year, and the density and social patterns of the population.
• The examples of diphtheria, smallpox, and poliomyelitis are used here to illustrate issues concerning vaccination programs and population immunity, and syphilis is used to illustrate natural herd immunity to infection.
• Vaccine-produced immunity in humans tends to decrease over time.
• people who had been vaccinated against diphtheria were exposed more frequently to the causative agent, and this exposure could result in a mild reinfection. The reinfection would produce a natural booster effect and maintain a high level of immunity.
• Recommendation for diphtheria: Additional single vaccination for adults to provide booster.
• Diphtheria is caused by bacteria called: Corynebacterium diptheriae
• Early attempts at preventing smallpox included actions reportedly by a Buddhist nun who would grind scabs from patients with the mild form and blow into the nose of nonimmune individuals; this was called variolation.
• The term vaccination comes from vaca, or “cow”; epidemiologists noted that milkmaids developed the less severe form of smallpox.
• Smallpox was caused by the virus: variola
• dominant form of smallpox is: variola minor (alastrim). Mild from of smallpox, often disfiguring but have a low mortality rate.
• However, alastrim provided individual and herd immunity against the much more disfiguring and often fatal variola major form of the disease (classical smallpox).
• Fortunately, the smallpox vaccine was effective against both forms of smallpox, and the immuniza- tion program was successful in eradicating both variola minor and variola major.
• The need for herd immunity was also shown by poliomyelitis.
• Poliomyelitis was caused by the: Poliovirus
• live, attenuated Sabin oral polio vaccine (OPV) was approved in the early 1960s. produced cell-mediated immunity, preventing the poliovirus from replicating in the intestine, and it also provided herd immunity.
• inactivated or killed polio vaccine (IPV), which became available in 1955, provided protection to the immunized individual, but did not produce much herd immunity. Although it stimulated the production of blood antibodies against the three types of poliovirus, it did not produce cell-mediated immunity in the intestine, where the polioviruses multiplied.
• Syphilis is caused by infection with bacteria known as spirochetes and progresses in several stages.
• In the primary stage, syphilis produces a highly infectious skin lesion known as a chancre, which is filled with spirochete organisms.
• In the secondary stage, a rash or other lesions may appear; these also subside spontane- ously. A latent period follows, after which a tertiary stage may occur. Untreated infection typically results in immunity to future infection by the disease agent, but this immunity is not absolute.
• Ironically, when penicillin came into general use, syphilis infections were killed so quickly that chancre immu- nity did not develop, and high-risk individuals continued to repeatedly reacquire and spread the disease.
• diarrheal diseases were the primary killer of children, and tuberculosis was the leading cause of adult mortality.
• Sanitary revolution- most important factor in reducing infant mortality. However, the reduction of infant mortality contributed in a major way to increasing the effective birth rate and the overall rate of population growth.
• sanitary revolution was therefore one of the causes of today’s worldwide population problem.
• reduction in infant mortality temporarily helps to produce a significant difference between the birth and death rates in a population, resulting in rapid population growth, the demographic gap.
• improvements in sanitation were a fundamental cause of the appearance of epidemic paralytic poliomyelitis late in the 19th century.
• Control of the vector, tse tse fly, enabled herders to keep larger numbers of cattle, and this led to overgrazing. Overgrazed areas were subject to frequent droughts, and some became dust bowls with little vegetation.The results were often famine and starvation for cattle and humans.
• after the dam was erected, the incidence of schistosomiasis increased in the areas supplied by the dam, just as epidemi- ologists predicted.
• The relationship between malnutrition and infection is similarly complex. Not only does malnutrition make infec- tions worse, but infections make malnutrition worse as well.
• Ecological and genetic factors can also interact to produce new strains of influenza virus.
• Antigenic shift- and the genetic material of the two influenza strains may mix in the pigs, producing a new variant of influenza.
• and the new virus from antigenic shift may produce a pandemic, or widespread, outbreak of influenza that could involve multiple continents.
• If the genetic changes in the influenza virus are minor, the phenomenon is called an antigenic drift, but this still can produce major regional outbreaks of influenza.