Veterinary Epidemiology and Zoonosis

ENVIRONMENTAL DETERMINANTS

Location

• Local geological formations, vegetations and climate affect the spatial distribution of both animals and diseases.
  • Example: Jaw tumors in sheep are associated with the distribution of bracken fern.
• Noise is also associated with location.
• The temporal distribution - disease is also associated with location because of seasonal effect of climate.

Climate

• Two types
  • Macroclimate 
  •  Microclimate

• Macroclimate
  • Comprises of normal components of weather to which the animals are exposed: Rain fall, temperature, solar radiation, humidity and wind velocity.
  • Temperature: Newborn animals are more prone to hypothermia. 
  • Wind and rain increases heat loss from animals.
  • Cold stress predisposes animals to diseases.
  • Wind can also carry infectious agents (like FMD virus) and arthropod vectors (Culicoides spp. with bluetongue virus)
  • Solar radiation - primary determinant for the carcinogenic effect.Example: Cutaneous squamous cell carcinoma
  • Stratospheric ozone depletion - increases ultra violet radiation
  • Humidity: In low humidity, IBRT virus survives well and in high humidity, rhinovirus survives well.
  • Climatic impact can be measured in several ways. A common method is to calculate the Wind-chill index. This combines the effects of temperature and wind speed and especially important at temperatures below freezing.

• Microclimate
  • A microclimate is a climate that occurs in a small-defined space. This may be as small as within a few millimeters of a plant or animal’s surface (Terrestrial or Biological) or as large as a piggery or a calf house.
  • Terrestrial microclimate (over the surface of leaves) affects the development of arthropods and helminths.
  • Biological microclimate (over the surface of the host’s body) can change during the course of a disease, assisting its spread. Example: Sweating during parasitaemic phase in human malaria, increases the humidity of body surface and attracts mosquitoes.

Husbandry

• Housing: Well-designed, non-slippery floors and well-ventilated houses are important.
• Diet: Diet has obvious effects in diseases caused by energy, protein, vitamin and mineral deficiencies. Increased level of biotin reduces the incidence of foot lesions in sows.
• Management: Management determines the stocking density and production policy. Increased densities increase the challenge of microbial pathogens. Internal replacement policy i.e. maintaining a closed population is likely to induce diseases.
• Stress and stressors
  • Stress: Stress is the sum of the biological reactions to any adverse physical, mental and emotional stimulus that tends to disturb homeostasis.
  • Stressors: The factors that produce the stress are known as 'stressors'. Example: Climatic extremes and trauma.
• Selye's hypothesis:
  • Selye in 1936 described the effect of stress in the laboratory animal, which was, known as 'Selye's hypothesis’. Later on it is known as 'General adaptation syndrome'.
  • There are 3 phases in this syndrome,
    • General alarm reaction: occurs within 6-48 hours after exposure to a stressor. The size of various organs as spleen, lymph node, liver etc, will be decreased.
    • The phase of resistance: It begins after 48 hours wherein there will be release of corticosteroids and the thyrotrophic hormones.
    • The phase of reaction: Here it may end in death.