Emerging and Re-Emerging Infectious Diseases¶
Chapter 486 | Part 17: Global Medicine · Parts 17-18 – Global Medicine & Aging · Chapter 486
Key Clinical Points¶
- EIDs are infections recognized in humans for the first time (e.g., HIV, SARS, COVID-19).
- REIDs are previously known infections reappearing in new locations, as resistant forms, or after control.
- Established diseases have stable, predictable morbidity/mortality (e.g., TB, drug-susceptible malaria).
- Three primary mechanisms of emergence: dead-end infections, animal-to-human host switching, and mutations in human-adapted pathogens.
- Host-switching involves crossing a 'fitness valley' where viruses must adapt to new hosts.
- Determinants of emergence include agent (infectivity), host (behavior/demographics), and environment (climate/poverty).
- Clinical recognition requires: curiosity, sensing the unusual, epidemiological sense, and strong lab/public health ties.
- Influenza A is difficult to control due to genetic drift, shift, reassortment, and glycosylation.
- Dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) often occur with a second infection due to antibody-dependent enhancement.
- Historical pandemics include 1918 H1N1 (~50 million deaths), HIV/AIDS (>40 million), and COVID-19 (7+ million by 2024).
- Zika virus mortality is primarily fetal or related to severe congenital infections.
- Dengue can mimic highly contagious respiratory viruses in its explosive re-emergence.
DEFINITION & CLASSIFICATION¶
• Historical Context: Pre-neolithic era (before 10,000 BCE): Limited to skin and gastrointestinal organisms due to small clan sizes. Neolithic transition: Increased human crowding, animal-human contact, and waste accumulation led to a dramatic increase in infectious disease.
• Classification Categories: Table 486-1 provides the primary classifications:
Definition (Harrison's 22e): Emerging infectious diseases (EIDs) are those recognized in humans for the first time Examples: HIV/AIDS, Nipah virus, SARS, COVID-19.
Definition (Harrison's 22e): Re-emerging infectious diseases (REIDs) are those that have infected humans in the past and continue to reappear in new locations Subcategories: Accidental human release: Vaccine-derived polioviruses, 1979 Sverdlovsk anthrax leak. Human intent to harm (bioterrorism): 1997 Oregon salad bar poisonings, 2001 anthrax attacks.
Definition (Harrison's 22e): Established infectious diseases or endemic infectious diseases are those that have been prevalent for a sufficient period of time to allow for a relatively stable and predictable level of morbidity and mortality Examples: RSV, noroviruses, pneumococcal disease, drug-susceptible malaria, tuberculosis.
EPIDEMIOLOGY¶
• Global Burden: Historical Context: EIDs have been leading causes of death and social disruption for centuries. Major 20th/21st Century Pandemics: 1. 1918 H1N1 influenza (~50 million deaths). 2. HIV/AIDS (>40 million deaths). 3. COVID-19 (7+ million deaths by 2024).
• Historical Context of Emergence: Evidence: Large numbers of human pathogens today share nearly identical genetic/phenotypic forms, suggesting frequent emergence events since the Neolithic era.
ETIOLOGY & PATHOPHYSIOLOGY¶
• Mechanisms of Emergence: 1. Dead-end infections: Not transmitted onward; not likely to become epidemic. Examples: Coccidioidomycosis, histoplasmosis (excavation related), norovirus (cruise ships), food contamination. 2. Animal-to-human host switching: Accounts for almost all novel pandemics and many REIDs. Mechanism: Requires crossing a 'fitness valley' where the virus adapts to the new host. Figure 2 Context: Deep Fitness Valley: Requires significant mutation/adaptation (Progressive Adaptation). Shallow Fitness Valley: Easier transition; may result in 'Chance Transmission' where the virus is immediately fit for the new host. 3. Mutations in human-adapted pathogens: Example: Development of antibiotic resistance in previously susceptible bacterial species.
CLINICAL FEATURES¶
• Clinical Recognition: Mechanism: Clinicians identify new diseases when signs/symptoms are not typical of known conditions. Historical Examples: 1. 1918 Influenza: Novel pneumonia-associated case-fatality (viral + bacterial). 2. Dengue Hemorrhagic Fever: Novel shock and death complications in the 1950s. 3. AHC (1969) & Rift Valley Fever (1977): Identified by specialists (ophthalmologists) noticing unexpected clinical presentations. 4. AIDS (1981): Identified by pathologists, epidemiologists, and clinicians. 5. Enterovirus D68 (2014): Identified by pediatric neurologists.
DIFFERENTIAL DIAGNOSIS¶
• Distinguishing EIDs vs. REIDs: Significance: Clinicians are often the first to see EIDs but have more expertise in recognizing REIDs.
• Specific Clinical Distinctions: 1. Influenza vs. Dengue: Observation: Dengue shows greater case-clustering than influenza in shared environments. 2. Dengue Shock Syndrome (DSS): Pattern: Bizarre age-specific mortality (infants <1 and older children; spares middle ages). 3. Legionellosis vs. DSS: Distinction: Legionellosis was often 'unknown' because pathogens could not be isolated/identified by labs. 4. SARS: Clinical Profile: Respiratory/constitutional symptoms with ~10% case-fatality rate.
DIAGNOSTIC APPROACH¶
- Clinical Recognition: Based on ecologic perspectives of infectious disease occurrence.
- Laboratory Testing: Viral Culture: For identification of viral agents. Serology: To detect antibodies/antigens. PCR: Used for SARS-CoV-2, HIV, etc. Bacterial Culture: Specifically for enteric pathogens.
- Epidemiologic Characteristics: Application: Identifying norovirus outbreaks or identifying bacterial pathogens in community settings.
MANAGEMENT & TREATMENT¶
- HIV/AIDS: Treatment: Combination antiretroviral therapies.
- Dengue: Treatment: Oral rehydration solutions to reverse shock syndrome.
- Influenza: Prevention: Vaccines (noted as incompletely effective).
- Ebola: Management: Standard public health measures alone (no vaccine or proven treatment currently available).
PROGNOSIS & COMPLICATIONS¶
• Mortality Statistics: 1. 1918 H1N1 influenza: ~50 million deaths. 2. HIV/AIDS: >40 million deaths. 3. COVID-19: >7 million deaths (by 2024). 4. Ebola: >15,000 deaths. 5. Zika: ~1000 deaths (mostly fetal or congenital complications).
SPECIAL POPULATIONS¶
• Pregnancy: Zika Virus: Mortality not fully established; most deaths are fetal/congenital. • Pediatrics: Dengue Shock Syndrome (DSS): High risk in infants <1 and older toddlers/school-aged children; spares middle age group. • Immunocompromised: HIV/AIDS: Primary focus for this population.
KEY PEARLS & HIGH-YIELD POINTS¶
• Clinician Skills for EID Recognition: 1. Abiding curiosity. 2. Sensing the 'out of ordinary' and pursuing it via history, exam, and labs. 3. Epidemiologic sense (understanding how a disease was acquired: environmental, respiratory, etc.). 4. Identifying clusters (common-source vs. zoonotic vs. person-to-person). 5. Determining incubation periods and duration of infectivity. 6. Close relationship with diagnostic laboratories. 7. Collaboration with specialists and researchers. 8. Relationship with public health workers for reporting and outbreak awareness. 9. Regular reading of medical literature.
• Clinical Traps: 1. Dengue Mimicry: Dengue can be so explosive it mimics highly contagious respiratory viruses. 2. Vaccine Fear: Concern that vaccines might potentiate severe disease (e.g., in dengue). 3. Overconfidence: The 1960s belief that infectious diseases were 'conquered' led to a lack of preparedness for the AIDS pandemic. 4. Diagnostic Gap: Clinicians may be unable to identify specific pathogens (like Legionella) if labs cannot isolate them.
Reference Tables¶
TABLE 486-1 Emerging and Re-Emerging Infectious Diseases: Definitions, Categories, and Examples Emerging infectious…¶
Harrison's 22e, p.3853
- Emerging infectious diseases (EIDs) are those recognized in humans for the first
time, e.g., HIV/AIDS, Nipah virus infection, or severe acute respiratory syndrome
(SARS) and COVID-19.
Re-emerging infectious diseases (REIDs) are those that have infected
humans in the past and continue to reappear in new locations (e.g., West
Nile virus in the United States and Russia in 1999), reappear in resistant or
otherwise phenotypically different forms (e.g., influenza, methicillin-resistant
Staphylococcus aureus, drug-resistant falciparum malaria), or reappear after
apparent control or elimination (e.g., poliomyelitis in parts of Africa, cholera in
Haiti in 2010 and elsewhere in association with natural disasters, wars, and mass
migrations) or under unusual circumstances (e.g., deliberately released agents,
including the 2001 anthrax bioterrorism attacks). Important subcategories of
REIDs include the following:
REIDs related to accidental human release—e.g., vaccine-derived polioviruses,
epizootic vaccinia virus, and the 1979 Sverdlovsk laboratory explosion releasing
anthrax spores
REIDs caused by human intent to harm (bioterrorism)—e.g., the 1997 Oregon
salad bar poisonings and the 2001 anthrax spore attacks in the United States
Established infectious diseases or endemic infectious diseases are those that
have been prevalent for a sufficient period of time to allow for a relatively stable
and predictable level of morbidity and mortality (e.g., many viral and bacterial
respiratory and diarrheal diseases, including respiratory syncytial virus, endemic
coronaviruses, noroviruses, pneumococcal disease, drug-susceptible malaria
and tuberculosis, and many other tropical diseases such as helminthic and other
parasitic diseases, many nosocomial infections).
TABLE 486-2 Selected Emerging Infectious Diseases of Note, 430 BCE to 2024 AD YEAR 430 BCE 541 1340s 1494 c. 1500 1520…¶
Harrison's 22e, p.3854
| YEAR | NAME | DEATHS | COMMENTS | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 430 BCE | “Plague of Athens” | ~100,000 | First identified transregional pandemic | |||||||||||||||
| Justinian plague (Yersinia pestis) | 30–50 million | |||||||||||||||||
| 1340s | “Black Death” (Yersinia pestis) | ~50 million | Pandemic; killed at least one-quarter of the known world population | |||||||||||||||
| Syphilis (Treponema pallidum) | >50,000 | |||||||||||||||||
| c. 1500 | Tuberculosis | High millions | Ancient disease; became pandemic in Middle Ages | |||||||||||||||
| Hueyzahuatl (Variola major) | 3.5 million | |||||||||||||||||
| 1793–1798 | “The American plague” | ~25,000 | Yellow fever terrorized colonial America | |||||||||||||||
| Second cholera pandemic (Paris) | 18,402 | |||||||||||||||||
| 1918 | “Spanish” influenza | ~50 million | Led to additional pandemics in 1957, 1968, 2009 | |||||||||||||||
| Ebola | More than 15,000 deaths | |||||||||||||||||
| 1981 | Acute hemorrhagic conjunctivitis | Rare deaths | First recognized in 1969; pandemic in 1981 | |||||||||||||||
| HIV/AIDS | > 40 million | |||||||||||||||||
| 2002 | SARS | 774 | Near-pandemic | |||||||||||||||
| H1N1 “swine flu” | 284,000 | |||||||||||||||||
| 2014 | Chikungunya | Uncommon but high morbidity | Pandemic, mosquito-borne | |||||||||||||||
| Zika | ~1000?* | |||||||||||||||||
| Hepatitis C vCJD yme isease |
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| L f |
assa ever |
HI | V |