HC25: Epidemiology

Definition of epidemiology

Epidemiology is the study of the occurrence and determinants of illnesses and their spread in the population. It is about the effects between:

• Environment
• Host
• Microorganism

Incidence, prevalence and attack rate

The 3 most important terms in epidemiology are:

• Incidence: the part of the population which develops the disease in a certain time period
  • Number of new cases (n)/(population number (N) x time period)
• Prevalence: the part of the population which has the disease at a certain time point
  • Number of existent cases (n)/population number (N)
• Attack rate: the percentage of "attacked" people
  • Number of people affected/population number (N)

Case:

The following data is available:

• 10 beds, 10 days
• 5 surgical site infections
• 24 patients
• 95 patient days

The following calculations can be done:

• The prevalence on day 3 is: 1/9 = 11%
• The prevalence on day 10 is: 2/8 = 25%
• The incidence is 5/95 patient days
  • 100 patient days is 1 patient staying in the hospital for 100 days, or 4 patients staying for 25 days
• The attack rate is 5/24 = 21%

Endemic, epidemic and pandemic

3 important terms that describe to what extent a disease has spread are:

• Endemic: the disease occurs continuously in a certain part of the population, but doesn't spread any further
  • For example malaria in sub-Saharan Africa
• Epidemic: the disease occurs more frequently than normal and there are more patients than expected
  • For example Ebola in 2015/2016
• Pandemic: an epidemic on worldwide level
  • Can be caused by a DNA-shift → a part of the genome of the pathogen changes

Filoviruses

Currently the Ebola virus is causing an epidemic. In Africa, confirmed cases of Ebola HF have been reported in many countries.

Structure:

Marburg and Ebola are filoviruses, which have a distinguishing structure:

• Negative-stranded RNA virus
• Envelope
• Threadlike structure
• Very broad cell tropism → can infect nearly every cell in the body

Transmission:

Fruitbats form the normal reservoir of Ebola. The hosts are reindeers and monkeys → these animals are infected, but the virus normally stays inside the rainforest. People living around the rainforest can get infest by eating meat of these animals. In conclusion, filoviruses can be transmitted in multiple ways:

• Primary transmission: contact with fruitbats/infected mammals
  • Bushmeat
• Interhuman transmission: contact between body secretions
  • Sweat, mucosae, bloodstream, non-intact skin, aerosols (limited)
• Nosocomial transmission: inadequate sterilization of materials
  • One of the biggest outbreaks of Ebola was caused by hospital needles not being properly sterilized

Incubation period:

The incubation period is the moment of infection up to the moment of the first symptoms. It is important to know the incubation period to determine how long a patient has to be isolated. In this period, the patient has a clinical disease.

Both Ebola and Marburg take a while to incubate:

• Ebola
  • Se: 6-8 days
  • Ob: 3-12 days
• Marburg
  • Se: 10-13 days
  • Ob: 5-9 days

After exposure, there also is a period the microorganism isn't transmittable to other people → the latent period. Afterwards there is a period where this is possible → the infectious period. Usually the latent period is shorter than the incubation time.

A serial interval is the time between the start of the disease between person 1 and person 2.

Symptoms and course:

Marburg and Ebola have very similar symptoms:

• Fever
  • Sometimes with cold chills
  • Laboratory: leukopenia
    • Mainly lymphocytopenia
  • Mortality mainly in the 2nd week
• Headache
  • Laboratory: trombopenia
  • >50% mortality
• Myalgia
  • Laboratory: elevated liver enzymes
  • Convalesce takes weeks/months
• GE: diarrhea, vomiting
  • Laboratory: prolonged PT, APTT and elevated D-dimer
• Minor bleeding
  • Conjuntivae
  • Petechiae
  • Prolonged bleeding time after blood draw
• Rash
  • In 25%-50% of cases
• Major bleeding

Filoviruses can easily be diagnosed in the laboratory. Eventually, more than 50% of patients will die.

Reproductive rate

A primary case is the first patient with a disease in a certain population. Secondary cases will follow. The clue for this is the basic reproductive rate (R0) → the average number of secondary cases per patient with a specific infection in a nonimmune (susceptible) population.

The reproductive rate (R) can be calculated as follows:

• R = R0x proportion nonimmune population
  • If R = 1 → stable endemic disease
  • If R < 1 → decreasing incidence
  • If R > 1 → epidemic

If the entire population is susceptible, the basic reproductive rate is equal to the reproductive rate → R = R0.

An example is an infection with R0= 5. If 50% of the population isn't immune, then:

• R = 5 x 0,5 = 2,5 → epidemic

It is desired that R < 1, so that R0x nonimmune population < 1:

• 5 x nonimmune population < 1
• Nonimmune population < 0,2
• Needed vaccination grade > 80%

Measles:

Often, in the Bible-belt children aren't vaccinated due to religious beliefs. Because the non-immune population is very high, many people get sick when there's an outbreak of for example the measles.

Infection chain

The infection chain consists of all places where multiplication of the microorganism takes place. A reservoir can infect a source, which in turn can infect a host. The host can also infect the source. It is called transmission when the source infects the host.

Influenza A virus:

An example is the influenza A virus:

• Reservoir: humans, swines, avians, equines, marine mammals
  • An ongoing source
• Source: humans
• Transmission
  • Direct contact: touching, kissing, sexual contact
  • Indirect contact: blood, food, objects, hands
  • Air: spores, droplet nuclei, droplets, skin flakes, dust particles
  • Vectors: insects or arachnids that transmit an infectious disease

Zoonosis

Zoonosis is when an infectious disease is transmissible under natural conditions from vertebrate animals to humans.

Echinococcus granulosis:

An example of zoonosis is an echinococcus granulosis infection:

• Source: larvae in dog feces
• Transmission: oral ingestion of worm eggs
• Host: sheep

If the parasite mistakes humans for sheep, the parasite will infect the human. It will form large cysts in the bowel → the "waterlily sign" is distinguishable for echinococcus granulosis.

Multiple measures can be taken to prevent and control echinococcus granulosis:

• De-worming of infected dogs
• Hand washing
• Burn or burying infected carcasses

Echinococcus granulosis isn't prevalent in the Netherlands and other European countries anymore.

Antimicrobial treatment

A major epidemic that is nearing is antimicrobial resistance (AMR). There is a certain algorithm that is applied to determine which antimicrobiotic to prescribe:

1. What is the a priori chance of the presence of a certain infectious disease, and which factors influence this?
2. What are the estimated consequences of the antibiotics for this patient?
3. How do the disadvantages of the treatment relate to the advantages?