14 - epidemiology

human health

complete physical, mental, and social well-being

• not just absence of disease

public health cycle

surveillance → studies → evaluate interventions

epidemiology

basic science of public health

• studies factors which determine presence + absence of diseases + disorders

Ascaris model

• direct

• no IH 

• no amplification

Ascaris step 1

adults in SI

Ascaris step 2

unfertilized + fertilized eggs in feces

Ascaris step 3

embryonated eggs with L3 larvae

Ascaris step 4

ingestion of embryonated eggs

Ascaris step 5 + 6

hatched larvar enter circulation + migrate to lungs

Ascaris step 7

larvae coughed up + swallowed

• re-enters gastrointestinal tract

• matures in SI

label:

A, E, r, rA and β

A = adults

E = eggs

r = egg prod/ adult

rA = net reproductive rate

β = probability of egg making it to next host

time 0:

A = 10 adults

r = 20 eggs/ adult

rA = ?

β = 0.5

rA = 20 × 10 = 200 eggs

time 1:

rA x β = ?

Total # A in population at end of time 1 = ?

new rA = ?

β = 0.5

rA x β = 200 × 0.5 = 100 adults

Total # A in population at end of time 1 = 10 + 100 = 110

new rA = 20 × 110 = 2200 eggs

time 2:

rA x β = ?

Total # A in population at end of time 2 = ?

rA x β = 2200 × 0.5 = 1100 adults

Total # A in population at end of time 2 = 110 + 1100 = 1210 adults

μ1

probability of adult worm death

μ2

probability of egg death rate

Aμ1

number of adult worms removed from population

Aμ2

number of eggs removed from pop.

model components - parasite reproduction + transmission

rA and β

model components - parasite elimination

Aμ1 and Eμ2

Ascaris control

• increase parasitic elimination

• decrease parasite transmission

• chemo + education

how to increase parasitic elimination

Aμ1: increase μ2 by treating host

Eμ2: increase μ2 by killing eggs in ext env

how to decrease parasite transmission

• lower:

  • contacts

  • # eggs

  • # hosts

  • rA

Ro

basic reproductive ratio

• avg # of 2° cases a typical single infected case will cause

• pop w/ NO immunty

• NO interventions

Ascaris Ro

number of eggs produces by adult that surviveto produce reproductive adults

• threshold level of survival is Ro = 1

• eradication Ro < 1

Ro of:

• smallpox

• ascaris

• measles

• malaria

• smallpox = 2

• ascaris = 5

• measles = 8

• malaria = 50

eradication

no longer exists anywhere in world

• not rlly realistic

elimination

localized eradication

• doesn’t exist in specific location

smallpox eradication

natural exposure → long- lasting immunity

smallpox eradication - vaccine

• long-lasting protective immunity

• 1 dose

• easy to administer

• efective delivery + education campaign

• fear of disease → public acceptance

why are parasites difficult to eradicate?

no vaccine available for any

which parasite is close to being eradicated

dracunculus

• not bc of vax, bc of control + other measures

how would parasites become eradicated?

education + control, NOT VACCINE

Enterobius vermicularis types of infection

retroinfection and autoinfection

retroinfection

eggs hatch in perianal area

• L1 migrate back to rectum + colon

Enterobius vermicularis step 1

eggs on perianal fold

Enterobius vermicularis step 2

1. larva in eggs mature w/in 4-6 hrs

2. embryonated eggs ingested by human

Enterobius vermicularis step 3

larvae hatch in SI

Enterobius vermicularis step 4

adults in lumen of cecum

Enterobius vermicularis step 5

gravid female migrates to perianal region at night to lay eggs

Enterobius vermicularis eggs

flattened side

Enterobius vermicularis adults

• alae

• female = pinworm

• male = curved posterior

Enterobius vermicularis pathology

• most asymptomatic

• damage w/in intestine + around anus

  • inflammation + bacterial invasion

• sleepless + irritability

Enterobius vermicularis diagnosis

scotch tape test

Enterobius vermicularis treatment

pyrantel, albendazole, mebendazole

how do young children contract Enterobius vermicularis?

• sleeping → itch → fingers in mouth

• airborne