gen microbio lecture 23: epidemiology

what is epidemiology?

it evaluates the occurence, determinants, distribution, and control of health and disease

john snow

• english physician

• father of epidemiology

• known for the cholera outbreak in london (1849)

cholera outbreak in london (1849)

• source was unknown

• mapped the location of diarrheal cases

• density of cholera cases and a single well on broad st. (epidemic ended)

• fecal material from cesspit leaked into drinking water

what was the hypothesis of the cholera outbreak in london?

if the cases are clustered geographically, it might determine the source

what does modern epidemiology include?

• infectious disease

• other health related issues (accidents, smoking, lead poisioning, aging)

what are the major organization for coordination of info in modern epidemiology?

• CDC (Centers for Disease Control and Prevention)

• WHO (World Health Organization)

sporadic

occasional and irregular intervals (isolated cases - no threat to the rest of the population)

endemic

steady, low level frequency at moderately regular interval

epidemic

suddenly higher than normal (influenza)

outbreak

localized epidemic

pandemic

increase in occurence in large population over very wide region

index case

patient zero

• 1st case

• track down and then contact trace (treat or seperate from population)

notifiable disease

required by law to report and every case goes to authorities

what does the national notifiable diseases surveillance system (NNDSS) do?

monitors and controls and prevents around 120 diseases

what are the two major types of epidemics?

1. common source

2. propagated epidemic

what is common source epidemic?

• single contaminated source (food or water)

• reaches peak level in a short period of time (1-2 weeks)

• moderately rapid decline in the number infected

what is an example of common source epidemic?

cholera

what is propagated epidemic?

• host to host epidemic

• introduction of a single infected individual into susceptible population

• initial infection propagated gradually (relatively slow and prolonged rise → infects multiple individuals and then there is a gradual decline in # infected)

highest point in susceptibles (1)

all individuals in population are susceptible to the pathogen

time = 0 (2)

• introduction of infected individual

• initiates the epidemic

• spreads and then reaches peak (day 15)

as individuals recover from disease (3)

• become immune and no longer transmit

• # of susceptible individuals decrease

threshold density (4)

• minimum # of individuals needed to propagate disease

• decline in susceptible to threshold density

as the number of cases decrease

the pathogen cannot propagate itself

herd immunity

the resistance of population to infection and pathogen spread

the larger the proportion of those immune,

the smaller the probability of contact between infected and susceptible individuals

why is the susceptible member of population enjoy the immunity from being the member of the group?

because the immunity is not self-made

why should you immunize large portions of susceptible population?

to maintain high levels of herd immunity

what are the restrictions of herd immunity? (aka when does it work and when does it not work?)

• it works only if spread is by person-person contact

• does not work if spread by contact between people

what is an example of herd immunity?

tetanus

• clostridium botulium

• soil

• transmission through the punctured skin

endemic disease can also be?

epidemic

what 2 things happen when there is an increase in # of susceptibles?

1. new individuals entering the population

2. antigenic shift of pathogen

what happens during antigenic shift of pathogen?

• major genetic change → changes antigenic character

• pathogen not recognized by immune system (ex. influenza virus)

components of infectious disease cycle that is responsible for particular epidemic

the control measures detected towards the most susceptible part of the cycle

types of control measures

1. reduces or eliminates source or reservoir of infection

2. isolation procedures

3. vaccination

reduces or eliminates source or reservoir of infection

• destruction of animal reservoir of infection

• destruction of vectors by spraying with insecticides

• chlorination or water supplies

• therapy to reduce infection

isolation procedures

• break connection between infected and susceptible

• private. closed room

• wash hands

• PPE

vaccination

• decrease number of susceptibles and increase levels of herd immunity

3 types of vaccines

1. killed organism

2. live, attenuated microbes

3. purifed components (subunits) from infectious agents

example of killed organism

hepatitis A vaccine, salk polio vaccine

live, attentuated (weakened) microbes example

sabin polio vaccine, BCG for tuberculosis

• this one is usually the best as weakened microbe replicates in normal body temperature and immune response develops

example of purified components for infectious agents

capsular antigens from streptococcus pneumoniae (all of them stimulate immunity)

polio infection pathway

1. poliovirus typically enters body through ingestion

2. replicates in the GI tract

3. moves to the regional lymph nodes and produces viremia (virus in the blood) - 75% of the cases, which is b

~1% of the cases of polio

the virus attacks the central nervous system (CNS) and causes paralysis in the body

what are the 2 vaccines?

1. salk vaccine

2. sabin vaccine

how does the salk vaccine work?

salk vaccine = IPV (inactivated polio vaccine)

• killed

• injected → antibody (Ab) response in the bloodstream

• Ab response prevents the viremia and paralysis by natural infection

• does not prevent GI replication with natural virus

how does the sabin vaccine work?

sabin vaccine = OPV (oral polio vaccine)

• replicates in the GI mucosa

• antibodies are secreted in the mucosa and prevents natural virus from replicating in the mucosa

can the virus in the vaccine revert to virulent form in the intestine

yes

ACIP (advisory committee on immunization practices)

• theres 2000 recommended that IPV can be used in the united states.

• polio is rate and fecal spread unlikely (eliminated VAPP from us)

what are the 4 ways to administer vaccines?

1. individually (influenza viral envelope)

2. polyvalent: control of multiple diseases (ex. MMR)

3. most during childhood (diseases can be devasting)

4. booster doses (2nd degree exposure to Ag provides more robust and long-lasting immunity)