Prevalence is the proportion of existing cases of disease present in a designated population at a given point in time. Usually, prevalence refers to a “point prevalence” (ie, the prevalence at one point in time).
Prevalence is calculated as the number of cases at a designated time divided by the number of individuals at risk at the designated time; it is usually expressed as a percentage.
Prevalence = (No. of cases / PAR) x 100%
At a single point in time (eg, based on the results of a serosurvey of dogs in the practice area), 237 dogs of 6,821 dogs with active records in a practice had coccidioidomycosis. In this scenario, the prevalence of coccidioidomycosis at the time of the serosurvey would be 3.5%.
Prevalence = (No. of cases / PAR) × 100% = (237 / 6,281) × 100% = 0.035 × 100% = 3.5%
In a class of 102 veterinary students, 7 were married at the start of year 1. In this scenario, the prevalence of married veterinary students in a class at the start of year 1 would be 6.9%.
Prevalence = (No. of cases / PAR) × 100% = (7 / 102) × 100% = 0.069 × 100% = 6.9%
Period prevalence differs from point prevalence in that it includes the number of existing cases at the start plus new cases that occurred in a designated population during a time interval of interest.
Period prevalence = (No. of cases [preexisting + new] / PAR) × 100%
In addition to 237 dogs with known coccidioidomycosis, the practice diagnosed 542 clinical cases in a particular year among their patient population of 6,821 dogs. In this scenario, the period prevalence of coccidioidomycosis for the year would be 11.4%.
Period prevalence = (No. of cases [preexisting + new] / PAR) × 100% = (237 + 542) / 6,821 × 100% = 0.114 × 100% = 11.4%
Incidence is the number of new cases of a disease in a population that is at risk (and disease-free) within a specified period of time.
Incidence is calculated as the number of newly diagnosed cases divided by the total population at risk over a specified period of time. Incidence can be calculated as incidence risk (cumulative incidence) or a true rate (incidence density).
Cumulative incidence (incidence risk)
Cumulative incidence (also called incidence risk or incidence proportion) quantifies the risk of new disease occurrence (ie, the probability of an animal developing a disease in a defined time period). Case-fatality rate is a cumulative incidence for death due to a given cause.
Cumulative incidence is calculated as the proportion of animals developing a disease in a defined time period; it is usually expressed as a percentage.
Cumulative incidence = (No. of new cases in time period / PAR) × 100%
On a small dairy farm with 102 cows, 13 developed ketosis over the course of a year. In this scenario, the cumulative incidence would be 12.7% per year. In other words, the estimated probability (incidence risk) that a lactating dairy cow develops ketosis over the next year is 12.7%.
Cumulative incidence = (No. of new cases in time period / PAR) × 100% = (13 / 102) × 100% = 0.127 × 100% = 12.7% per year
Note that it is essential to quote the relevant time period as part of the cumulative incidence (ie, the risk of developing disease over the next year or the next week are very different).
In outbreak investigations, attack rate is often used as a measure of disease frequency. Note that attack rate is not a true rate but actually a proportion.
Attack rate is calculated as the cumulative incidence of disease in outbreak situations.
Attack rate = (No. of new cases since onset of outbreak / PAR at onset of outbreak) × 100%
In an outbreak of upper respiratory disease, 53 out of 77 cats in a municipal shelter became infected. In this scenario, the attack rate is 69% during the outbreak.
Attack rate = (No. of new cases since onset of outbreak / PAR at onset of outbreak) × 100% = (53 / 77) × 100% = 0.69 × 100% = 69% during outbreak
Incidence density (incidence rate)
Incidence rate quantifies the rate at which new events occur in a population.
Incidence rate is calculated as the number of new cases in a population divided by animal-time at risk during a given time period. There are several methods of calculating animal-time at risk (eg, the sum of each animal’s time at risk or average population during the time interval).
Incidence rate = No. of new cases / Total animal-time of observation
Of 50 laboratory gerbils (Meriones unguiculatus) kept in a biomedical research facility, 25 developed aural cholesteatomas by 2 years of age. In this scenario, the incidence rate is 2.5 cases / 10 gerbil-years.
Incidence rate = (No. of new cases / Total animal-time of observation) = 25 cases / (50 gerbils x 2 years each) = 0.25 cases / gerbil-year = 2.5 cases / 10 gerbil-years
Note that incidence rate is typically adjusted to have at least one digit to the left of the decimal place by applying a multiplier to both the numeric quantity and the units of animal-time.
Simple count of the number of new cases of disease in a population.
Without information about the population at risk, an incidence count can be misleading as a measure of disease frequency.
PAR = Population at risk. The population at risk is defined by susceptibility to the condition of interest, time period of interest, geography, species, breed, age, sex, etc.