1. Definition of infectious diseases, sources of infection, factors influencing infectious diseases (pathogen-host relationship, protection of the host, environmental factors) Flashcards
What is infectious disease:
Agent - lower ranked (taxonomically)
Host - higher ranked
Exposition: agent infects host
Disposition: host has to be able to be infected
Phases of infection: entry, colonization, replication, damage of the host, transmission
What is infectious disease:
Agent - lower ranked (taxonomically)
Host - higher ranked
Exposition: agent infects host
Disposition: host has to be able to be infected
Phases of infection: entry, colonization, replication, damage of the host, transmission
Importance of infectious diseases
All areas of vet med, economic impact, trade of animals and animal products, zoonoses, food hygiene and safety (25% of human death is due to infective agents)
Sources of infection
Generally from infected animals/humans
Direct: tissue of infected animal in contact with tissue of susceptible host, e.g. rabies bite
Indirect:
- excretions, secretions
- products of animal origin
- drinking water, feed, soil, environment
- aerogenic
- arthropods: true vectors, mechanical vectors
- nosocomial infections
- iatrogenic infections
Transmission of diseases: 2 types
Vertical and horizontal
Vertical transmission
Parent to offspring
Germinative
Intrauterine
Galactogen
Horizontal transmission
Spread in the same group, infecting each other
Types of zoonoses:
- Orthozoonoses
- Cyclozoonoses
- Metazoonoses
- Sapronoses
Orthozoonoses, explanation and example
Animal to host (direct and indirect)
e.g. rabies
Cyclozoonoses, explanation and example
Spread by arthropods, circulated infection between arthropod and maintaining host - humans can accidentally enter this cycle
- Requires more than one vertebrate
e.g. Lyme disease, tick borne incephalitis, taenia sp.
Metazoonoses, explanation and example
Requires both a vertebrate and invertebrate host to complete life cycle
All arboviruses + Rickettsia, Leishmania
Black plague –> fleas
Sapronoses
From a source like soil, water, decaying plants etc. Replicate in non-living matter
Like some protozoa etc
Way of infection
Place of entry
Natural orifices
Percutaneous
Forms of infection
Simple infection - ONE AGENT
Repeated infection - ALSO ONE AGENT
- Reinfection: after the infection has cleared up
- Exacerbation: the agent is isolated in nodule/abscess then break out causing secondary infection
- Superinfection: infected animal is infected again with the same infection e.g. TB
Infection of multiple agents:
Secondary infection (complication)
Mixed infection - e.g. kennel cough
Outcome of the infection depends on:
Agent - host - environment
- Agent subcategories
- 1 Host-parasite relationship
- 2 Pathogenicity
- 3 Virulence
- 4 Invasiveness
- 5 Amount of agent
- 6 Way of infection
- Agent subcategories
- 1 Host-parasite relationship
- 2 Pathogenicity
- 3 Virulence
- 4 Invasiveness
- 5 Amount of agent
- 6 Way of infection
1.1 Host-parasite relationship
Symbiosis, mutualism, commensalism, parasitism
“normal” microflora
1.2 Pathogenicity
Euryxen: wide host range (rabies, anthrax)
Stenoxen: narrow host range (ASF, EIA)
Obligate pathogen: no predisp needed
Facultative pathogen: predisp needed
1.3 Virulence
Virulence factors, MLD, LD, change of virulence
How able is the agent to cause disease in amount
Avirulent and low virulent strains can be used for vaccination
1.4 Invasiveness
Intracellular - protected from body
Extracellular
1.5 Amount of agent
A minimum amount is required. If it is more virulent, less pathogens are needed
1.6 Way of infection
Has to be optimal
Henle Koch postulates: detect agent, isolate in pure culture, should provoke the same clinical signs/PM lesions:
The microbe is present in all cases of the disease
The microbe does not occur in other diseases
The agent can be isolated in pure culture
The same disease can be caused with the isolated agent and it can be re-isolated
- Host
- 1 Species, age
- 2 Race, individual resistance
- 3 Production
- 4 Non-specific resistance
- 5 Specific resistance
- 6 Effect of immune suppression
- 7 Fetal immune reaction
- 8 Immune reaction of newborns
- 9 Cellular immune reaction
2.1 Species, age
Has to have the appropriate receptors to the agent - stenoxen/euryxen
2.2 Race, individual resistance
Lines, e.g. certain poultry hybrids susceptible to Mareks
2.3 Production
Feeding, laying - intensively fattening animals predisposed to certain diseases
2.4 Non-specific resistance
Skin, mm, cilia, excretions
MPS
Complement, properdin, opsonin systems
IL, IF, TNF
2.5 Specific resistance
Targeting certain agents
Passive immunity: natural (maternal), induced (hyperimmune serum)
Active immunity: natural, induced (vaccine)
Humoral resistance: IgA - local protection, IgG - circulation etc.
Cellular resistance: intracellular bacteria, viruses
2.6 Effect of immune suppression
Decreases resistance of animal
Toxicosis, medicine, mycotoxins, parturition etc
2.7 Fetal immune reaction
In ovo vaccination - immune after hatching
2.8 Immune reaction of newborns
More susceptible
Calf: Ig 4-32 days - immune reaction improves with time
Calf, piglet: local immune reaction occurs immediately after birth, disappears
2.9 Cellular immune reaction
0-2 weeks long, limited at beginnning then faster
- Environment
- Nutrition, management
- 2 Survival of the agent in the environment
- 3 Environmental effects on animals
- 4 Predisposing factors to facultative pathogens
3.1. Nutrition, management
Poor nutrition (deficiencies, mycotoxins) and management (overcrowding, ventilation, humidity)
Technology: weaning, grouping, treatment, castration, sheering, transport
Physiological effects: laying, pregnant, fattening, use of animal
3.3 Environmental effects on animals
Weather, temp, humidity
