chapter 20

Anthrax

Cutaneous anthrax Localized skin infection 10-20% mortality if untreated Antibiotic therapy works well Gastrointestinal anthrax Eating animals infected with anthrax Bacilli/spores enter GI tract and infect bloodstream 50-60% mortality Inhalation anthrax Breathe in spores (high ID50) Bacteria go from lungs to bloodstream Mortality ~80% untreated, ~50% / treatment

Fig. 20.10 Recent disease emergences

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Table 20.1 Reportable Diseases

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High-speed transportation allows global dissemination of diseases

Fig. 20.5 Spread of the Black Death, 1345-1353. The rate of spread across Europe averaged about 5 km per day. Fig. 20.6 Spread of SARS. SARS is carried out of Guangdong Province on February 21, 2003. Within 23 days, more than 150 suspect and probable cases of SARS are reported from around the world. add pictures slide 10

Many human diseases are evolutionarily recent phenomena

For most of human evolution, disease patterns were very different than they have been for the last 5,000 to 8,000 years. Human population was not large enough to support disease that led to immunity. Small bands of hunters and gatherers had limited opportunities to infect other bands. Primitive humans had patterns of infectious disease characterized by zoonoses, chronic disease, and diseases with long latent periods.

Antigenic shift

If two different flu viruses infect the same cell, the progeny viruses package a random selection of the two parental types of chromosome, a process known as reassortment. Reassortment between avian and human strains can generate a novel human strain and cause a major pandemic. Perhaps the most lethal human disease epidemic ever was the 1918 flu pandemic Fig. 20.7 The reassortment process in flu viruses. add picture slide 14

Epidemic investigation follows standard procedures

In the United States, outbreak investigation is a state responsibility. For large, unusual, or particularly serious outbreaks, the Centers for Disease Control and Prevention (CDC) sends a investigation team. An investigation team formulates a case definition. a description of what symptoms are necessary to classify a particular case as part of the outbreak. The team determines the number of cases, the source, and how the outbreak is spreading.

Disease course

Incubation period A period between becoming infected and the onset of symptoms. The pathogen is multiplying and invading its target tissue but remains at low enough levels that symptoms are not yet apparent. Infectious period The period during which those infected can infect others. The pathogen levels can become high enough for transmission before they are high enough to cause symptoms. add picture slide 8

Influenza

Influenza (flu) commonly infects millions of Americans every year. Typically 30,000 to 40,000 Americans die every year. Periodically, a new variant of the flu virus appears and spreads. variant is a result of genetic recombination among different strains of flu virus Disease at expected frequency is endemic Endemic means "within a population." The term is also used to refer to the geographical distribution of disease. Malaria is endemic in the tropics worldwide, but is rare in the temperate zones. Fig. 20.1 Flu prevalence. add picture slide 3

The rate of transmission determines the course of epidemics

Rate of transmission—the average number of secondary cases that are caused by exposure to an infected individual. If the average rate of transmission is over 1.0, the outbreak will be expanding. When the rate drops below 1.0, the peak is past and the outbreak is dying out. The rate of transmission is the outcome of a complex mixture of factors that can include the properties of the pathogen, the host, the population, and the season.

Population structure is a major determinant of the rate of transmission

The two most important properties of a population are: Its density the proportion of members who are susceptible to the disease. Herd immunity—the ability of a population to resist serious epidemics because a significant proportion of its population is immune. Achieving herd immunity is an important goal of vaccination programs.

Some animal diseases are communicable to humans

These diseases are called zoonoses. The principal animal population that maintains the disease is the reservoir, or reservoir host. Plague, a disease of rodents, is a good example. Many zoonoses can be transferred from animals to humans, but cannot be transmitted from human to human; thus they are not contagious among humans.

Many of the diseases of urbanized humans came from domestic animals

When human populations became large, their herds of domestic animals became large as well. The large animal populations allowed disease to be maintained stably in their animal hosts. Transmission of rare animal pathogen variants that could infect humans became more likely.

Infectious disease is a major cause of death worldwide

AIDS, diarrheal diseases, tuberculosis, and malaria would be in the top 10 causes of death worldwide if they were listed separately, rather than combined with all other infectious diseases. add figures slide 21

Distinguishing between endemic and epidemic disease requires background information derived from surveillance

All countries conduct some degree of disease surveillance. International organizations conduct global surveillance of some diseases. Physicians are required by law to report every case of one of the reportable diseases that they see to state or national authorities. Data collected over a number of years reveal long-term trends and seasonal trends

New diseases are continuously emerging

Disease is the result of the interaction of pathogen and host. It is subject to natural selection on both short and long time scales. New diseases are termed emerging diseases. A major underlying reasons for emerging diseases is the environmental disruption that humans are causing. Humans have contact with animals with which they previously had little contact. increases the potential for transfer of pathogens from wild animal populations to humans.

Diseases that provoke immunity become childhood diseases

Diseases such as measles, mumps, and chicken pox These are diseases that people get only once and then become immune to re-infection for life. The only susceptible people would be children born since the last time the disease swept through. For such a disease to become endemic: the population needs an annual birth rate high enough to supply a sufficient number of susceptible children every year to sustain an unbroken chain of transmission

Species-specific strains of influenza virus cause avian and human flu

Influenza infects birds and humans, but it is not a true zoonosis for two reasons it is endemic in humans the viral strains that infect humans are distinct from those that infect birds You can't get the flu from a bird, but you can get it from a pig; a pig can get human flu and bird flu - Genetic rearrangement of flu genomes when a bird and human flu infect a single pig cell can create a flu strain with the potential to cause a pandemic (called antigenic shift )

An epidemic is the occurrence of disease at higher than expected frequency

Many diseases are present in populations at fairly constant levels. There may be frequently seasonal variations An epidemic infects far more people than normal. Epidemics do not have to involve large numbers of cases. When disease prevalence is normally very low, or even zero, a few cases can constitute an epidemic. Epidemiologists commonly use the term outbreak for localized epidemics. When an epidemic spreads to multiple continents, it is called a pandemic.