Disease Transmission and Context

Nigel Paneth

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)

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.

HOW DO EPIDEMIOLOGISTS STUDY OUTBREAKS OF

DISEASE?

Concepts In Outbreak Investigation

• QUANTIFYING THE EPIDEMIC (DESCRIPTIVE EPIDEMIOLOGY)

•  GETTING AT THE SOURCE (ANALYTIC EPIDEMIOLOGY)

Quantifying The Epidemic

1. Case definition2. Epidemic curve

· point source (common source, common vehicle)· propagated

3. Attack Rate4. Incubation period5. Herd immunity

Getting At The Source • Mode of transmission

– Vector

– Vehicle

– Reservoir 

• Portal of entry• Agent

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.

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.

• 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.

• 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 Snow’s work, especially arthropod vectors discovered between 1878 – 1911.

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.

DISCOVERY OF MODES OF TRANSMISSION

1. DIRECT CONTACT2. FECAL-ORAL 3. ASYMPTOMATIC CARRIER4. VEHICLE (water)5. VECTOR (arthropod)

(two other modes of transmission were widely recognized by the 19th century – sexual and airborne)

MODES OF TRANSMISSION1. DIRECT CONTACT

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

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 1850’s, Budd showed that typhoid fever has an identical transmission pattern.

MODES OF TRANSMISSION:4. THE ASYMPTOMATIC CARRIER

• 1893 Diphtheria Park and Beebe

• 1900 Typhoid Reed, Vaughan & Shakespeare

• 1905 Meningococcus Wechselbaum

• 1905 Polio Wickman

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.

1878- 1895:Filariasis, Texas Cattle Fever

and Sleeping Sickness

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 30’s, by Rivers).

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).

1911 - Typhus

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

The Seven Cholera Pandemics

#1. 1817-1823 FIRST AWARENESS

Restricted to Asia and Africa

#2. 1826-1837CHOLERA IN EUROPE

1st European epidemic in 1831-2. John Snow, as young apprentice physician, sees cholera cases in Yorkshire.

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.

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 HAVEN’TThough 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.

The Seven Cholera Pandemics#6. 1902-1923 QUIET TIME IN USNo 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.