Unit 1 -- Introduction

Overview of Animal Health Management

Importance of Animals to Humankind

1. spiritual, emotional (earliest-dove or pigeon), companionship -- dog, cat
   
   
2. service -- horse, oxen, elephant, camel
   
   
3. later domesticated for food, fiber -- importance of ruminants in converting unusable products into digestible food for humans
   
   
4. sharing of disease -- more than 80% of human infections shared with another species -- human health is inexorably tied to animal health

a. Rabies
b. Tuberculosis, Malta fever (Brucellosis), Plague
c. recent times -- Salmonella, E. coli 0-157, H7
d.  West Nile virus
e.  Hantavirus Pulmonary Syndrome

 

Complex Relationship between Health and Disease

Three factors interact and determine whether an animal will be healthy or sick. These three factors are:

1. AGENT - infectious organism
   
   
2. HOST - animal of interest
   
   
3. ENVIRONMENT of the host

Overview of the Role of the Agent in Disease (Infectious disease)

A. Agent-Host Interplay

1. parasitic relationship - one organism (agent) benefits at the expense of the other -- what we classically consider disease agent
   
   
2. symbiotic relationship - both organisms benefit from the relationship (example ruminant and the microflora of rumen); in this relationship there is no disease.
   
   
3. commensal relationship - only one organism derives benefit from close living arrangements, but the other is not harmed (example of normal skin and gut microflora). If equilibrium is upset, a commensal may become a parasite and actually cause harm to the host (disease).

B. Characteristics of an Infectious Agent (parasitic relationship)

1. Ability to multiply - generation time of the agent
   
   
2. Ability to spread from one host to another
   
   a. horizontal spread
   b. vertical spread
   
   
3. Rapid rate of evolution (mutation) - gives rise to different strains with varying virulence, species selectivity, etc. The dose needed to cause disease is related to virulence of organism
   
   
4. Pathogenicity - invasiveness (ability to become established in cells, tissues and cause damage)
   
   a. production of enzymes to digest/destroy protective covering
   b. pili/fimbriae - mechanism to attach
   c. production of toxins
   
   
5. Physical nature of the agent can affect it's location and survival
   
   a. size (very small agents can be inhaled deeply into lungs)
   b. +/- cell wall, spore former, etc.
   c. intracellular existence/survival

Overview of the Role of the Host Animal in Disease

A. Intrinsic Disease (strictly host determined)

1. Hereditary -- entropion, spider lamb
   
   
2. Congenital -- hydrocephalus, spina bifida, cleft palate

B. Characteristics of Host species that influence outcome of infectious disease

1. anatomic characteristics -- sites of entry for organisms
   
   a. skin - scaly, dry, low temperature, but secretes sweat, sebum; can be penetrated by bites, abrasions, etc.
   b. hair - protects and entraps agents, but also holds them close to body
   c. mucous membranes - site of attachment / defense of mucus layer, cilia
   
   
2. sex - some diseases are sex specific, such as mastitis in female, orchidis in male
   
   
3. age - young are often susceptible to organisms that live as commensal in adult
   
   
4. breed differences -- squameous cell carcinoma in white faced breeds of cattle
   
   
5. immune competence of host
   
   
6. level of nutrition and stress of the host

Overview of the Role of the Environment in Disease

A. Diseases caused by environmental factors

1. Nutrition -- deficiencies, excesses
   
   
2. Mechanical/thermal injury
   
   
3. Toxins -- chemical, plant

B. Environmental factors that affect infectious disease

1. Temperature
   
   
2. Humidity
   
   
3. Degree of wetness or dryness of the environment (leptospirosis)
   
   
4. Grazing characteristics -- overgrazing can lead to greater exposure to anthrax spores in soil, grazing in heavy fog, dewy conditions can lead to lungworms (fog fever)
   
   
5. Econiche of organisms -- may depend on certain type of intermediate host (i.e. snail as intermediate host of liver fluke, specific species of mosquito or tick for blood borne organisms
   
   
6. Prevailing winds -- foot & mouth disease spread to Denmark from Germany, Pseudorabies (PRV) spread between hog farms
   
   
7. Social distance/population density of the animals within the environment

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