FREYA NOTES Veterinary Epidemiology

• Epidemiology: Study of disease outbreaks.

Objectives of Veterinary Epidemiology

• Determine the origin of a disease when the cause is known.
• Investigate and control diseases with unknown or poorly understood causes.
• Gather info on the ecology and natural history of diseases.
• Assess the economic, social, and welfare effects of diseases, including assessing the costs and benefits of control programs.
• Develop, monitor, and evaluate disease control programs.

Determinants of Disease

• Agent, host, and environmental factors determine disease patterns.

Agent Factors

• Infectivity is the amount of organism required to start an infection.
• Pathogenicity is the ability of an agent to cause clinical signs in infected hosts.
• Stability is the length of time an organism remains infective outside a host.
• Virulence refers to the agent's ability to cause severe disease or death in hosts.
• Immunogenicity is the ability of an agent to illicit inflammatory response in a host.
• Antigenic variation refers to an agent's ability to undergo genetic changes and alter its characteristics.

Host Factors

• Animals that cannot transmit an agent to another animal are considered dead-ends.
• Reservoirs are where agents normally live and replicate.
• Susceptibility refers to a host's vulnerability to infection.
• Infectiousness is the duration of the period when an animal can transmit an agent and the amount of agent transmitted.

Environmental Factors

• Disease patterns are influenced by climate, water systems and quality, food availability, and geology.

Disease Spectrum

• Latent period: Time from infection to becoming infectious.
• Incubation period: Time from infection to the presentation of clinical signs.
• Infectious period: Time when an infectious agent can be transmitted from an infected individual to susceptible individuals.

Methods of Disease Transmission

• Vertical transmission occurs from parents to offspring.
  • Hereditary transmission involves the genome of parents
  • Congenital transmission occurs at birth.
• Horizontal transmission is indirect.
  • Mechanical vectors are physical carriers of agents, such as fleas.
  • Biological vectors involve the agent undergoing part of its lifecycle within the vector.
  • Fomites are inanimate carriers like feed and trucks.

Survival Challenges for Agents

• Host challenges include immune responses.
• Capsules, intracellular life, and fecundity can help agents respond to host challenges.
• Environmental challenges include temperature, desiccation, UV light, and pH levels.
• Agents may use avoidance strategies, resistance forms, rapid infection cycles, persistence within a host, or extension of host range.

Causation, Association, and Confounding

• Causation is the relationship between cause and effect.
• Association is the degree of dependence between variables.
• Confounding occurs when factors appear connected but are not.

Role of Epidemiology in Non-Infectious Diseases

• Epidemiology is used to identify risk factors, track disease trends, evaluate interventions, guide health policy, and measure disease burden.

Reed-Frost Model

• The Reed-Frost model represents a propagating epidemic by dividing a population into susceptible, infected, and immune individuals.
• The model has limitations, including its inability to account for non-infectious diseases, diseases that do not confer immunity, or open populations.

Quantifying Disease

• Incidence is the number of new cases in a population over a specific period.
• Prevalence measures the total number of cases in a population at a given time.
• Mortality rate counts deaths in a population over a specific period.
• Attack rate is the proportion of an exposed population that develops a disease.
• Case-fatality rate is the proportion of diseased individuals who die.
• Crude rates are general rates that do not adjust for population differences.
• Incidence density measures new cases per unit of person-time at risk.
• The susceptible fraction is the proportion of a population at risk of developing a disease.
• Neonatal mortality rate is the number of newborn deaths per 1,000 live births.
• Crude birth rate is the number of live births per 1,000 individuals per year.
• Postnatal mortality rate is the number of deaths of infants aged 28 days to 1 year per 1,000 live births.
• Cumulative incidence is proportion that develops a disease over a specific time period

Prevalence Measurements

• Counts lack context.
• Percentages allow better comparison between groups.
• Proportions are fractions where the numerator is part of the denominator.
• Ratios are fractions where the numerator is not a subset of the denominator.
• Rates incorporate change over time and are useful for comparisons.

Incidence Rate

• Incidence rate is the number of new cases between T=0 and T=1 divided by the total animal-time-risk in the population at risk.

Incidence Risk

• Incidence risk refers to the number of new cases divided by the population at risk at the start of the period, also known as Cumulative Incidence.

Attack Rate

• Attack rate is the number of animals affected divided by the number of animals exposed.

Disease Frequency Measurements

• Disease frequency can be measured by prevalence, incidence rate, and incidence risk.

Calculations

• Incidence risk or rate may be better suited when a disease has a short duration.
• Prevalence is useful when calculating overall economic burden.

Confounding Factors

• Standardization is a method used to manage confounding factors, involving the comparison of populations with a hypothetical standard population.

Categorical Variables

• Represent qualitative characteristics.
• Nominal variables are subjective and have no inherent order.
• Ordinal variables have inherent order and are represented numerically

Numerical Variables

• Represent quantities, including discrete (counts) and continuous (measurements).

Epidemic Patterns

• A point-source epidemic occurs when many animals are exposed to a disease source over a short time.
• A propagating epidemic occurs when transmission is gradual among individuals.
• Sporadic diseases occur irregularly.
• Epidemics are sudden increases in disease cases beyond normal expectations.
• Endemic diseases are constantly present in a specific area.
• Pandemics are global epidemics that affect multiple countries or continents.

Hill's Causal Criteria

• Hill's causal criteria assess causality in epidemiology and determine the likelihood of a causal association between exposure and disease.
• The criteria are more applicable to modern epidemiology and are flexible and multifactorial.
• The subjectivity in interpretation and inability to always conduct RCTs are weaknesses of Hill's causal criteria.

Koch's Postulates

• Koch's postulates are designed to establish microbial causation for infectious diseases.
• Koch's postulates are useful for identifying bacterial and viral pathogens but have limitations regarding chronic diseases and ethical concerns.

Component Cause

• Component cause factors contribute to disease but are not enough to cause it alone.

Necessary Cause

• Necessary cause factors must be present for a disease to occur.

Sufficient Cause

• Sufficient cause factors can cause disease on their own.

Epidemiological Model

• A “causal pie” should be complete; each slice representing a component cause.

Disease Outbreak Investigation Steps

1. Confirm existence of an outbreak
2. Establish a provisional diagnosis
3. Determine magnitude of the outbreak
4. Characterize outbreak
5. Develop a working hypothesis
6. Evaluate working hypothesis
7. Data collection and analysis
8. Implement control and preventatives measures
9. Report and communicate findings

Diagnosing Outbreaks

• Diagnosis can be done via farm records, vet records, interviews, and review of literature.

Criteria for Judging Causation

• Koch's postulates, Evan's unified concept of causation, and Hill's criteria are all used.

Evans Unified Concept of Causation:

1. Individuals with the disease should be significantly higher in those exposed to the supposed cause than in those who are not.
2. Exposure to the supposed cause should be more commonly present in those with than those without the disease, when all other risk factors are held constant.
3. The number of new cases of disease should be significantly higher in those exposed to the supposed cause than in those not so exposed, as shown in prospective studies

Hills Guidelines for Causation (1965):

• Temporality, strength of association, dose-response, consistence, and compatibility.

Component Causes

• Component causes includes the individual causes.

Determining Disease

• Disease can be determined via the time, where, and which animals are infected.