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Disease Transmission and Context

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Presentation on theme: "Disease Transmission and Context"— Presentation transcript:

1 Disease Transmission and Context
Nigel Paneth

2 Triumphs Of Epidemiology
Identification of water as a major reservoir and vehicle of communicable diseases such as cholera and typhoid fever ( ) Identification of arthropod vectors for many diseases - malaria, yellow fever, sleeping sickness, typhus ( ) Identification of the asymptomatic carrier as an important vector in typhoid, diptheria, polio ( )

3 More Triumphs Of Epidemiology
• Cigarette smoking found to be major cause of lung cancer, emphysema, and cardiovascular disease ( ). • Eradication of smallpox (1978). • Perinatal Hepatitis B infection necessary cause of hepatocellular carcinoma (commonest cancer in China, Southern Africa) ( s). • 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.

4 HOW DO EPIDEMIOLOGISTS STUDY OUTBREAKS OF DISEASE?

5 Concepts In Outbreak Investigation
QUANTIFYING THE EPIDEMIC (DESCRIPTIVE EPIDEMIOLOGY)  GETTING AT THE SOURCE (ANALYTIC EPIDEMIOLOGY)

6 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

7 Getting At The Source Mode of transmission Portal of entry Agent
Vector Vehicle Reservoir Portal of entry Agent

8 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.

9 VEHICLE – An inanimate object which serves to communicate disease
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.

10 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.

11 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.

12 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.

13 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) Five distinct modes of transmission were discovered in a period roughly contemporaneous with the period of microbial research I have just described. They were – transmission by hand – i.e. true physical contagion. READ SLIDE

14 MODES OF TRANSMISSION 1. DIRECT CONTACT
1848 – Semmelweis discovered that puerperal sepsis is transmitted manually from the autopsy room to the delivery room by doctors. READ. Semmelweis had no concept of microbes. All he knew was that if the doctors rinsed their hands in chloride of lime after doing autopsies but before delivering their patients and also between delivering patients , something on their hands would be removed and the hospital’s death rate from puerperal sepsis, which in bad years killed one in eight women admitted to the largest delivery service in Vienna, would drop down to very low levels.

15 MODES OF TRANSMISSION 2. 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. At about the same time, John Snow and William Budd, in England, showed that cholera and typhoid fever were transmitted by the fecal oral route, inadvertently, and in the water supply. Again, this work took place before there was firm knowledge of the germ theory of disease. By the 1860’s, still before any microbial agents of human disease had been identified in the laboratory, British and North American public health authorities made certain to keep raw sewage out of the water supply, and declines in typhoid fever and cholera followed. In Germany, by contrast, even though it was the country in which both the cholera bacillus and the typhoid fever agent was identified, the mode of transmission of cholera was not appreciated, sewage continued to flow into the water supply, and major waterborne epidemics of both cholera and typhoid fever took place in Germany 30 years after they had disappeared from the British and North American scene. German scientists were the leaders in clinical laboratory research, but they did not successfully engage in field research.

16 MODES OF TRANSMISSION: 4. THE ASYMPTOMATIC CARRIER
1893 Diphtheria Park and Beebe 1900 Typhoid Reed, Vaughan & Shakespeare 1905 Meningococcus Wechselbaum 1905 Polio Wickman American investigators showed that there was a carrier state in humans for both diphtheria and typhoid Park and Beebe in NYC, and Walter Reed, Victor Vaughan and Edwin Shakespeare working for the US Army. Wechselbaum, who discovered both the meningoccocus and the asymptomatic carrier state for this organism, is the only German scientist I have been able to identify who discovered a mode of transmission. But the most interesting discoveries in this area involved arthropod vectors.

17 1878- 1895: Filariasis, Texas Cattle Fever and Sleeping Sickness
Patrick Manson (UK) discovered that the larval stage of filaria, which causes filariasis, is found in mosquitoes.   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.

18 1897-1900: Malaria and Yellow Fever
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). 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).

19 1906-1909: Plague, Chagas Disease
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 ). 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).

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

21 The Seven Cholera Pandemics
# FIRST AWARENESS Restricted to Asia and Africa # CHOLERA IN EUROPE 1st European epidemic in John Snow, as young apprentice physician, sees cholera cases in Yorkshire.

22 The Seven Cholera Pandemics
# 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.

23 The Seven Cholera Pandemics
# SOME HAVE LEARNT Improved water supply in Great Britain and US considerably lowers mortality in the 1866 epidemic compared to earlier epidemics. # BUT OTHERS HAVEN’T 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.

24 The Seven Cholera Pandemics
# QUIET TIME IN US No epidemics in Western Hemisphere. # – present RETURN OF CHOLERA Less severe El Tor biotype predominates. 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. Major airplane importation into Los Angeles from Peru, but no secondary cases.


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