AVBS3001- AGENTS OF DISEASE

Goals of this Unit- Name infectious diseases, What are their features, Recognise pathogenic traits

Define Disease

A change in the state of **health** of a host resulting in the **inability** of the host or part thereof, to **carry out normal function due to the actions of an agents.**

Define Clinical disease

Change that is obvious with readily detectable signs of dysfunction or illness

* You can visible see symptoms

Define Subclinical disease

Not readily detectable signs of dysfunction or illness, rather subtle changes in productivity, growth and fitness which may only be detectable using laboratory tests

Can a disease be subclinical to clincal

An animal can easily change from subclinical to clinical depending on the environment of the animal is situated on

What agents cause disease

* physical agents
* genetic or inherited
* Chemical agents
* Infectious agents
* Most common
* Can invade and multiply in/on a host
* Produce chemicals in the host or its environment (toxins)
* Multi-cellular, single cell or non-cellular
* Transmissible (direct or in-direct)
* Infectious agents are the most significant

How can we protect animals from infectious

* Understanding infectious agents that affect animals
* How they interact with the host and the environment
* Creating strategies to favour the HOST
* Breed and age
* Vax and worming
* Feed and shelter
* Biosecurity and control of animal/human movement

Factors that influence the Host pathogen environment interaction (HPEI)

* The pathogen itself; agent of disease
* The effect on the host; pathology
* Host response; immunity, inflammation and repair
* Environment; the external world relates to the host and the pathogen

Disease will result when the interactions favour the pathogen and a pathological host response

Disease manifestation can give us a clue about the identity of the infectious causative agent

Moraxella catarrhalis

* It contains a special feature inn attaching to the cornea
* The pillai of this pathogen gives it the ability to infect and attach to the cow’s eye causing the cloudiness -→ pilla are more invasive
* The pink causes cornea cloudus

Understanding HPEI

1. Diagnose the cause of an infectious disease
* Through sample collection
* Or culture and detection methods

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2. Provide effective treatment of animals
* Antibiotics
3. Prevention and control
* By understanding the environment!!
* Survival in the environment
* Husbandry and stock density
* Housing and feed
* Climate
* Vaccination

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Understanding HPEI -→ Better infectious disease management

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Define Infection

Infectious agent is present within or on the animal. The agent may multiple, grow or invade the host.

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Infection =/ Disease

HPEI determines the disease/ health status resulting from an infection

Define Carrier

Can be an animal, often resistant to disease development, but able to spread an infectious agent to others.

Define Parasitism

When a symbiotic relationship between an organism and the host causes harm in the host (DISEASE).

* Any organism that cause harm to the host it inhabits can be classified as a parasite
* Infectious agents can be said to have parasitic properties that enable them to be pathogen (i.e) cause disease

Host-infectious agent relos

* parasitism
* Mutualism
* Commensalism

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Define Mutualism

When the symbiotic relationship between an organism and the host results in benefits to both.

ie. the rumen microflora → ferments plant CHO -→ Energy for rumination

The bacteria needs the host in order for survival and likewise

Define Commensalism

When the symbiotic relationship exists between an organism and the host results in no obvious harm to the host

* No obvious harm
* Shows some benefit for the host e.g bacterial found normally on your skin such as Staphylococcus aureus

Mutualistic and commensally can turn towards parasitism

Changes in host and environmental factors

* Lactic acidosis in cattle occurs with sudden introduction of high grain diets and overgrowth of Streptococcus bovis
* Cut and abrasions on the skin resulting in Staphylococcus aureus dermatitis

What are the consequences of animal infectious diseases

* Animal productivity and diversity
* Supply and demand
* Farming industries
* Food safety and security
* Ecological balance
* Impact upon public health
* Indirectly and directly
* Financial
* Nutritional
* Psychological

Zoonosis

Diseases caused by unfectious agents that can be transmitted between animal and human

* 75% of emerging human infectious disease have an animal origin
* Swine flu, Avian influenza and COVID

Causation

Infectious diseases are caused by the action of an infectious agents in the host.

Factors that influence disease risk

* **Host Factors**
* Animal species
* Genotype
* age
* Nutritional status
* reproductive status
* Past exposure/immunity -→ often protects the host, but may sensitive the host (allergic reaction)
* Concurrent disease/injuries
* Immune competence → Lack of passive transfer of antibodies increases risk of infectious disease in neonates
* Behaviours
* **Parasitic factors**
* Strains and virulence - virulence is a measure of pathogenicity → more virulent, less bias needed to cause disease and associated disease severity
* Higher does → greater chance of disease
* Methods and duration of exposure
* route and length of expsoure can influence host response
* Properties of virulence factors
* adapt to the host
* ability to invade and evade host host responses
* trigger pathology
* Often associated with virulence
* **Environmental factors**
* Climate- feed availbility, drought, rain, humidity and insects
* Topography
* Population density - spread of disease
* Food, water, soil and air - source of infectious agent

How do we establish causation

Establishing evidence of a disease process associated with a pathogen

* **Substantial**
* The mere present of an agent is usually not enough


* Significant concentration and specific responses relating to virulence traits
* Need to make biological sense
* SLIDE 37 lec 1

How do we establish casaution