School Papers

“…they both the microbes and humans and the

“…they rather went to the grave by thousands than into
the fields in mobs by thousands.” Daniel Defoe’s, ‘A Journal of the Plague Year’,
expresses the helpless state of men in the second visitation of plague, famously
called Black Death, that hit London from 1350 to 1490. Since time immemorial,
man has been plagued by such vicious widespread diseases. They’ve influenced
science, politics, religious thoughts, arts, literature, and in fact the very
course of human life on earth. It’s an ongoing struggle for survival for both
the microbes and humans and the former almost always seem to take us by
surprise. The post antibiotic era was one of 
over optimism, encapsulated  in
the words attributed to the then  US
Surgeon General William H. Stewart (1967),’ It’s time to close the books on
infectious diseases, declare the war against pestilence won, and shift national
resources to such chronic problems as cancer and heart disease.’ But the years
that followed witnessed several visitations by old as well as new foes and the
emerging failure of our key weapons against them.

                          An epidemic is a geographically localized occurrence
of disease at an expected frequency over a certain time period3. An
outbreak is restricted to a small geographical area or population while a
pandemic involves different countries and a large population3. There has been a
fourfold increase in outbreaks globally since 19801. The number of
diseases causing epidemics has also increased by 20 percent.1
The WHO
recognizes the following as diseases with the potential to cause epidemics:

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•        Chikungunya

•        Cholera

•        CCHF

•        Ebola virus disease

•        Hendra virus infection

•        Influenza

•        Lassa fever

•        Marburg virus disease

•        Meningitis

•           MERS-CoV

•           Monkeypox

•           Nipah virus infection

•           Plague

•           Rift Valley fever

•           SARS

•           Tularaemia

•           Yellow fever

•           Zika virus disease                     

 

                How do these diseases different from other
infectious diseases? The simple mathematical model of SIR- Susceptible,
Infected and Removed is a good tool to understand the dynamics of diseases
causing epidemic, though in reality disease dynamics is multi-dimensional and
variable.  An epidemic starts in a
susceptible population (S) as isolated cases. The infected (I) transmit the
disease to individuals who affect more individuals, setting up a chain
reaction. The rate of spread depends on the average number of secondary
infections from a primary case in a susceptible population which is called the
reproduction number (R0). For R0 to be high, the disease must be acute, have a
low latent period (the period between infection and disease manifestation) ,
considerable infectious period (when the patient is infectious), have an
effective mode of transmission and exposure between susceptible and infected. If so, there will be an
exponential increase in the number of people affected causing an explosive epidemic.
Another important characteristic is that such diseases cause debility and
mortality. This depends on the virulence of the organism. A widespread disease
which causes only mild self-limiting illness like self-limiting gastroenteritis
by Norovirus is not of much concern as there is not much harm to human life.  

            60% of diseases that cause
outbreaks are found to be zoonoses: transmitted to humans from animals1. And it
is highly probable that a new pandemic that is caused will also be zoonotic.
HIV spilled over from chimpanzees, Coronaviruses, Niaph and Ebola from wild
bats, Avain influenza from aquatic birds and chicken, Lassa fever from rats and
so on. Bats and rodents are the most common reservoirs. Phylogenetically
related organisms have a greater possibility of harboring organisms capable of
cross- transmission10. Animals capable of flight can cross borders and bring in
strange diseases to new places. This is one of the main concerns with avian
influenza where wild aquatic birds, especially migratory birds, can transmit a
new strain across the globe11. PREDICT, a program that is funded by the U.S.
Agency for International Development tries to predict future zoonoses by
discovering new viruses and collecting data 12. A similar initiative is the
Global Virome Project a venture to identify and sequence potential zoonotic
viruses. Vector-borne diseases pose a major threat in endemic areas and can
possibly spread worldwide. Zikka viruses, linked to congenital neurological
complications including microcephaly, has caused outbreaks in Africa, the
Americas, Asia and the Pacific.who Climate change expands the territories
inhabited by vectors and animals carrying diseases. Permissive climatic
conditions for Aedes mosquito in Brazil due to the effects of El Nino on the
climate was implicated in the explosive spread of Zikka13. Excessive
deforestation and loss of habitat is also associated with emerging and
re-emerging zoonosis. In short, it’s an interplay between the agent,
vector/reservoir, environment and man and we are more susceptible than ever.

                  The dawn of civilization
would probably have been a defining moment in the history of pandemics as there
was increased ease of spread and increased chance for getting infections from
animals. Today with the population growing at a global rate of 1.09% per year13,
with the grossly varying population density across the globe, we are more at
risk than ever. This coupled with increased international travel brings back
memories of rats with Yersinia pestis (causative agent of plague) aboard on
merchant ships, a seemingly simple prequel to the Black Death. Effective measures to check
travelers from places endemic for potentially dangerous diseases must be in
place. The Plague outbreak in Madagascar from the 1 August through 22 November
2017 caused a total of 2348 probable and suspected cases of plague, including
202 deaths (case fatality rate 8.6 %). But effective surveillance of travelers
and follow up of contacts helped check its spread beyond the country.   

                  An epidemic is usually self limiting.
This can again be describe using the SIR model. The affected people die or develop
protective immunity and go into the removed population (R). Hence the
susceptible population decreases over time, decreasing the rate of spread and
finally it ceases. If R0

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