Disease Transmission and Context

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Disease Transmission and Context

• Nigel Paneth

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Triumphs Of Epidemiology

• Identification of water as a major reservoir and
  vehicle of communicable diseases such as cholera
  and typhoid fever (1849 - 1856)
• Identification of arthropod vectors for many
  diseases - malaria, yellow fever, sleeping
  sickness, typhus (1895-1909)
• Identification of the asymptomatic carrier as an
  important vector in typhoid, diptheria, polio
  (1893-1905)

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More Triumphs Of Epidemiology

• Cigarette smoking found to be major cause of
  lung cancer, emphysema, and cardiovascular
  disease (1951-1963).
• Eradication of smallpox (1978).
• Perinatal Hepatitis B infection necessary cause
  of hepatocellular carcinoma (commonest cancer in
  China, Southern Africa) (1970-80s).
• Identification of the AIDS syndrome, prediction
  that the cause was a sexually-transmitted virus
  (1981-3), and development of prevention measures
  BEFORE the virus was identified.

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HOW DO EPIDEMIOLOGISTS STUDY OUTBREAKS OF DISEASE?
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Concepts In Outbreak Investigation

• QUANTIFYING THE EPIDEMIC (DESCRIPTIVE
  EPIDEMIOLOGY)
•  GETTING AT THE SOURCE (ANALYTIC EPIDEMIOLOGY)

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Quantifying The Epidemic

• 1. Case definition
• 2. Epidemic curve
•  point source (common source, common
  vehicle)
• propagated
• 3. Attack Rate
• 4. Incubation period
• 5. Herd immunity

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Getting At The Source

• Mode of transmission
• Vector
• Vehicle
• Reservoir
•  
• Portal of entry
• Agent

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Sorting out the MODES OF COMMUNICATION of
disease, many of which involve vehicles, vectors
and reservoirs, is the province of
epidemiologists. Only work in the field can
uncover the way in which an agent links to a host
in the real world outside of the laboratory.
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VEHICLE An inanimate object which serves to
communicate disease. For example, a glass of
water containing microbes, or a dirty rag, etc.
VECTOR A live organism that serves to
communicate disease. For example, mosquitoes and
other arthropods.

• RESERVOIR A location that serves as a
  continuing source of disease for example, a
  water tower (common in legionella infections),
  the soil for tetanus, etc.

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• Snow discovered the waterborne route as a major
  mode of communication of disease, which turned
  out to apply not only to cholera, but also to
  typhoid fever and other infections.

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• In disease prevention, knowing the mode of
  communication is generally more important than
  identifying the specific agent. (Consider AIDS
  and SARS for example).
• Other routes of transmission were discovered
  after Snows work, especially arthropod vectors
  discovered between 1878 1911.

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• That period (1878 1911) can be viewed as the
  time of the GREAT VECTOR REVOLUTION.
• Occurring slightly later than the GREAT BACTERIAL
  REVOLUTION, it extended the findings of
  bacteriologists and provided information
  essential to disease control.

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DISCOVERY OF MODES OF TRANSMISSION

• DIRECT CONTACT
• FECAL-ORAL
• ASYMPTOMATIC CARRIER
• VEHICLE (water)
• VECTOR (arthropod)
• (two other modes of transmission were widely
  recognized by the 19th century sexual and
  airborne)

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MODES OF TRANSMISSION1. DIRECT CONTACT

• 1848 Semmelweis discovered that puerperal
  sepsis is transmitted manually from the autopsy
  room to the delivery room by doctors.

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MODES OF TRANSMISSION2. FECAL-ORAL ROUTE 3.
WATER AS VEHICLE

• In 1849 Snow published evidence that cholera is
  transmitted by the fecal oral route and by the
  water supply.
• In the 1850s, Budd showed that typhoid fever has
  an identical transmission pattern.

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MODES OF TRANSMISSION4. THE ASYMPTOMATIC CARRIER

• 1893 Diphtheria Park and Beebe
• 1900 Typhoid Reed, Vaughan
• Shakespeare
• 1905 Meningococcus Wechselbaum
• 1905 Polio Wickman

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1878- 1895Filariasis, Texas Cattle Fever and
Sleeping Sickness

• 1878 - Patrick Manson (UK) discovered that the
  larval stage of filaria, which causes filariasis,
  is found in mosquitoes.
•   1892 - Smith and Kilbourne (US) discovered
  that Texas Cattle Fever is transmitted
  perinatally by ticks (They also identified the
  causative babesia organism).
• 1895 Bruce (UK) discovered that African
  trypanosomias or sleeping sickness is transmitted
  by the bite of the Tse-tse fly.

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1897-1900 Malaria and Yellow Fever

• 1897 - Ronald Ross discovered that malaria is
  transmitted by mosquitoes. Some credit the
  discovery to the Italian scientist, Grassi.(The
  French physician Laveran had disovered the agent,
  plasmodium, in 1880).
• 1900 - Walter Reed discovered that yellow fever
  is transmitted by mosquitoes. William Gorgas
  uses this information to rid Havana of Yellow
  Fever, and later to do the same in the canal
  zone, permitting construction of the Panama
  canal.(The virus was discovered in the 30s, by
  Rivers).

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1906-1909 Plague, Chagas Disease

• 1906 - The Indian Plague Commission proved that
  fleas carried by rats transmit Plague, though
  some credit the French investigator, Simond, in
  1898, with this discovery. (The plague bacillus
  had been discovered by Yersin or Ogata (disputed)
  in Hong-Kong 1894-1896).
•  
• 1909 - Chagas found that the trypanosome that
  causes Chagas disease or American trypanosomiasis
  is transmitted by blood-sucking cone-nosed or
  kissing bugs (reduviidae). (Chagas and Cruz
  discovered the specific trypanosome at the same
  time).

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1911 - Typhus

• Charles Nicolle demonstrated that typhus is
  transmitted by lice. (Rickettsia were discovered
  by Ricketts at about the same time).

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The Seven Cholera Pandemics

• 1. 1817-1823 FIRST AWARENESS
• Restricted to Asia and Africa
• 2. 1826-1837 CHOLERA IN EUROPE
• 1st European epidemic in 1831-2. John Snow, as
  young apprentice physician, sees cholera cases
  in Yorkshire.

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The Seven Cholera Pandemics

• 3. 1846-1862. SNOW FIGURES IT OUT
• London and New York hit hard in 1848/9 and
  1853/4 (more than 10,000 deaths in each city in
  each epidemic).
• Golden Square epidemic of 1854 leaves 500 dead
  within a 250 yard radius of a single water pump.
• Official Board of Health investigation denies
  waterborne transmission, attributes London
  epidemic to miasmas arising from the Thames.

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The Seven Cholera Pandemics

• 4. 1864-1875 SOME HAVE LEARNT
• Improved water supply in Great Britain and US
  considerably lowers mortality in the 1866
  epidemic compared to earlier epidemics.
• 5. 1881-1896 BUT OTHERS HAVENT
• Though Koch had identified vibrio comma in 1883,
  Hamburg, under influence of Von Pettenkoffer, who
  did not believe in direct waterborne
  transmission, experiences 10,000 cholera deaths
  in 1893, from a clearly waterborne source.

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The Seven Cholera Pandemics

• 6. 1902-1923 QUIET TIME IN US
• No epidemics in Western Hemisphere.
• 7. 1961 present RETURN OF CHOLERA
• Less severe El Tor biotype predominates.
• 1978 - Cholera returns to North America in with
  sporadic shellfish-associated cases in Louisiana
  and Texas.
• 1991 -First epidemic in South America this
  century begins in Peru in January 1991, with
  360,000 cases in 13 countries so far, mostly
  waterborne.
• 1992 - Major airplane importation into Los
  Angeles from Peru, but no secondary cases.