Factors Affecting Disease Patterns and Measures

Factors Affecting Disease Patterns

• Pathogenicity: Ability of a disease-producing agent (DPA) to cause disease or morbidity.
• Virulence: Severity of a specific DPA strain.
• Pathogenicity and Virulence Measurement: Ratio of clinical and subclinical cases, Case mortality.
• Duration of Infectious State (Period of Communicability): Time during which an infected host can transmit the DPA.
• Ease of Communicability: Droplets from an infected host within a confined population can cause secondary infections, known as the Secondary Attack Rate (SAR).
• SAR Calculation: (No. of cases in outbreak - 1ry case(s)) / (Total No. of susceptible animals - 1ry case(s)).

Measures of Disease Occurrence

• Prevalence "P": Amount of infected animals in a population at a specific time, regardless of when the infection occurred.
• Prevalence Calculation: (No. of individuals infected at a particular point in time) / (No. of all individuals in the population at risk).
• Incidence "I": Number of new cases infected in a population over a specific time period.
• Cumulative Incidence (CI): Proportion of new cases infected during a specific period compared to the number of healthy individuals at the beginning of that period.
• CI Calculation: (No. of individuals that become diseased during a particular period of time) / (No. of healthy individuals in that population at the beginning of that period).
• CI Range: Values between 0 and 1.

Relationship Between Prevalence and Incidence

• Prevalence depends on the duration of the disease "D" and the disease incidence "I".
• Prevalence Equation: P = I x D
• Incidence Equation: I = P/D.

Other Measurement Factors

• Mortality Rate: (No. of dead cases at a given time) / (total No. of animals at risk).
• Case Fatality Rate: (No. of dead animals at a given time) / (No. of sick animals at the same period).

Disease Determinants (Causation)

• Infection: Invasion of a living organism by a DPA.
• Infectivity: Ability of a DPA to establish itself in a host, measured by ID50 (the amount required to infect 50% of susceptible hosts under experimental conditions).
• Pathogenicity: Ability of a DPA to produce disease in a susceptible host.
• Virulence: Severity of a disease in the host, measured by LD50 (the amount required to kill 50% of exposed hosts under experimental conditions).
• Carrier State: Infected host that transmits a DPA without showing clinical signs.
• Incubatory Carrier: Infected host that transmits a DPA during the incubation period.
• Antigenic Variation: DPA can evade host defenses by changing its antigenic characteristics.
• Incubation Period: Time between infection and the appearance of clinical signs.
• Prepatent Period: Time between infection and the first detection of the DPA.
• Period of Communicability: Time during which an infected host can transmit the DPA.

Etiology of Diseases "Determinants"

• Diseases can be caused by single or multiple factors.

Classification of Disease Determinants

• Primary Causes: Essential for the disease to occur.
  • Intrinsic Factors: Causal agent is part of the host.
    • Hereditary Diseases: Genetically transmitted conditions.
    • Metabolic and Hormonal Diseases: Disruptions in metabolic or hormonal processes.
    • Behavioral Disorders: Abnormal behaviors
  • Extrinsic Factors: Causal agent is not part of the host.
    • Non-living Agents: Physical or chemical agents causing harm.
    • Living Agents: Biological organisms causing disease.
• Secondary Causes: Non-specific factors that contribute to disease occurrence.
  • Intrinsic Factors: Inherited or acquired host characteristics.
    • Age, Sex, Species, Breed, and Strain: Influence susceptibility to disease.
    • Body Confirmation, Metabolism, Hormonal Balance, State of Nutrition, Degree of Function and Fatigue, Physiological State: Can increase or decrease disease risk.
  • Extrinsic Factors (from 2ry): Environmental factors.
    • Climate and Weather, Animal Management (feeding, stable sanitation, handling, overcrowding, etc.): Impact disease development.

Host-Parasite Relationship

• Outcome of an invasion depends on host defenses and DPA pathogenicity/virulence.

Microbial Factors

• Pathogenicity: DPA's ability to cause disease.
• Virulence: DPA's quantitative ability to cause disease, measured by LD50 (the amount required to kill 50% of animals under experimental conditions).

Virulence Factors

• Adherence Factors: Structures like pili help DPAs adhere to host cells, initiating infection.
• Invasiveness: DPA's ability to invade tissues, multiply, and spread. Capsulation, which protects against phagocytosis, is an example.
• Toxigenicity: DPAs produce toxins that harm host tissues.
  • Exotoxins: Secreted outside the cell and diffuse into the surrounding medium.
  • Endotoxins: Released when bacterial cells die.
• Production of Certain Enzymes: Enzymes secreted by DPAs can contribute to disease progression.

Resistance Factors

• Host protective mechanisms, divided into two categories:

• Non-specific Protective Mechanisms (1ry defenses) = innate immunity: Operate against most invaders.

  • Mechanical Barriers: Physical barriers preventing entry.
  • Skin: Intact skin prevents most DPAs from entering.
  • Mucous Membranes: Mucous secretions trap and remove invaders.
  • Gastrointestinal Tract (GIT): Secretions and enzymes inactivate pathogens.
  • Vagina: Acidic environment created by lactobacilli inhibits infection.

Explore the critical factors influencing disease patterns, including pathogenicity, virulence, and the ease of communicability. This quiz also covers measures of disease occurrence such as prevalence and the calculation methods used in epidemiology. Test your understanding of these key concepts in disease dynamics.