Veterinary Epidemiology and Zoonosis

HOST DETERMINANTS

• Genotype
• Age
• Sex
• Species and Breed
• Coat colour
• Physiological status
• Size and confrontation

Genotype

• Genotype is the genetic constitution of a host.
• Gene alterations are considered to be the major cause for genetic diseases.
• This altered gene structure may be inherited by succeeding generations.
• Example: Haemophilia A and B in dogs.
  • Genetic diseases belong to one of the following three categories.
    • Chromosomal disorders
    • Mendalian disorders
    • Multifactorial disorders
  • Example: Hip displasia in dogs due to genetic weakness of muscle supporting the hip joint and condition in early life favours separation of joint.
  • Multifactorial disorders occur when factors pass beyond a threshold level or value beyond which disease occurs.
  • Although an individual is inherited with right number and combination of genes, when exposed to environmental determinants to pass beyond a threshold level beyond which the disease occurs.
  • Example: Congenital heart defects -Patent ductus arteriosus.

Age

• In general young animals are more likely to be affected by many bacterial and viral diseases.
• This is because of an absence of acquired immunity (maternally derived antibodies) (Example: Parvoviral infection in young pups)
  • Low non-immunological host resistance.
  • In contrast many protozoan and rickettsial infections induce milder response in young than old.

Sex

• It includes
  • Hormonal determinants
  • Occupational determinants
  • Social and ethological determinants
  • Genetic determinants
• Hormonal determinants
  • Bitches are more likely to develop signs of diabetes mellitus after oestrus, possibly more than male dogs, due to increased insulin requirement during oestrus.
  • Less incidence of mammary carcinoma in neutralized bitches.
• Occupational determinants
  • Increased risk of heartworm infection in male dogs, resulting from increased occupational exposure of male dogs during hunting (mosquitoes are vectors).
• Social and ethological determinants (Behavioral patterns)
  • Example: Competition between male dogs for breeding with females resulting in transmission of rabies due to bite wounds.
• Genetic determinants
  • Inherited either by sex linked, sex limited or sex influenced.
  • Example: Haemophilia A and B, which associated with ’X’ chromosomes and are inherited recessively, the defects being predominant in males.

Species and breed

• They vary in their susceptibility and responses to different infectious agents.
• Example: In Rottweilers and Doberman dog breeds, parvoviral enteritis is more common and severe. In Boxers, mycotic diseases (Coccidiodomycosis) are more common.

Coat colour

• White cat prone for cutaneous squamous cell carcinoma, which is related to lack of pigmentation.

Physiological status

• Metabolic diseases immediately after calving due to physiological stress.
  • Example: Downer cow syndrome and milk fever.

Size and confrontation

• Cows with small pelvic outlet relative to their size - predisposed to dystocia.
  • Example: Hip dysplasia and osteosarcoma are common in larger breeds than smaller breeds.