1. Definition of infectious diseases, sources of infection, factors influencing infectious diseases (pathogen-host relationship, protection of the host, environmental factors).

Question 1

Epidemiology definition?

Answer 1

Epidemiology: the study of diseases in populations and of factors that determine their occurrence

Question 2

Importance of infectious diseases?

Answer 2

Importance of infectious diseases

• All areas of veterinary medicine
• Great economic impact ➝ GB foot and mouth diseases in 2001 30 billion GBP
• Trade of animals and animal products is limited
• Zoonoses
• Food hygiene and food safety
• 25% of human death cases are caused by infective agents (without COVID-19)
• COVID-19

Question 3

Infectious disease?

Answer 3

Infectious disease:

Infectious diseases

• Agent ➝ lower ranked (can cause a disease in a taxonomically higher ranked creature)
• Host ➝ higher ranked (host has to be exposed and ready to receive disease)
• Exposition - disposition
• Phases of infection

1. Entry
2. Colonisation
3. Replication
4. Damage of the host
5. Transmission to other susceptible animals

Question 4

History of Infectious diseases?

Answer 4

History of infectious diseases

• Galenos (2nd Century BC): miasmatic and infectious diseases
• Varro (2nd Cent BC): living agents
• Francastoro (16th Cent): ‘contagium animatum’
• Leeuvenhoek (17-18th C): microscope
• Jenner: Pox vaccination
• Pollender: bacteria in the blood of animals with anthrax
• Pasteur: culture and identification of bacteria, rabies, vaccines
• Koch: identification of different bacteria, tuberculosis
• Mechni: phagocytosis
• Behring: antibodies in the blood
• Ivanovsky: tobacco mosaic virus
• Ehrlich: antibacterial treatment, Salvarsan
• Domagk: sulphonamides
• Fleming: penicillin
• Watson and Crick: DNA structure
• Mullis 1983: polymerase chain reaction (PCR)
• Eradication of infectious diseases
• Smallpox 1979
• Rinderpest 2011 (animal disease)

Question 5

Sources of infection?

Answer 5

Sources of infection

• Source: generally infected animals/humans
• Direct: when the tissues of the infected animals have direct contact with the tissues of susceptible host
• Indirect
• Drinking water, feed
• Soil, environment
• Aerosol ➝ respiratory diseases leading to coughing or exhalation of air (air contains agents)
• Arthropods (true vectors and mechanical transmission)
• Rodents
• Iatrogenic infections
• Nosocomial infections (iatrogenic) ➝ in hospitals, source can be other patients or the environment
• Excretions, secretions (blood, faeces, urine, saliva, milk, semen)
• Products of animal origin (meat, milk, skin, wool hair)

Question 6

Transmission of diseases?

Answer 6

Transmission of diseases

• Vertical ➝ transmission from one generation to the next generation (parent to offspring)

I. Germinative ➝ germ cells infected, infected birds lay infected eggs

II. Intrauterine ➝ generalised disease spreads in host and infects uterus, including foetus leading to consequences

III. Galactogen ➝ generalised diseases shed through discharges including milk

• Horizontal ➝ shedding of virus which is inhaled, spread in the same population

Question 7

Zoonoses(animals ‘humans)?

Answer 7

Zoonoses (animals ↔ humans)

• Diseases and infections that are naturally transmitted between vertebrate animals and man (WHO, 1951)
• 60% of infectious diseases are zoonoses
• 75% of new and emerging diseases are zoonoses
• One Health
• Orthozoonoses ➝ direct and indirect transmission
• Cyclozoonoses ➝ maintained by more hosts
• Metazoonoses ➝ maintained by one vertebrate and one invertebrate species
• Sapronoses ➝ there is a maintaining host but the infection happens from the environment (soil, food)

Question 8

Ways of infection?

Answer 8

Ways of infection

Infection

• Ways of infection (place of entry)

• Natural orifices

‣ Per os

‣ Intranasal

‣ Genital tract

‣ Udder

‣ Conjunctiva

• Per cutaneous (wound, arthropods)
• Optimal entrance ➝ most effective infection
• Place of entry can influence the outcome of the infection

Question 9

Forms of infection?

Answer 9

Forms of infection

• Forms of infection

• Single infection
• Repeated infection

‣ Reinfection

‣ Exacerbation

‣ Superinfection

• Infection by multiple agents

‣ Secondary infection (complication) ➝ quite common

‣ Mixed infection

Question 10

Agent, Host parasite relationship pathogenecity?

Answer 10

Outcome of the infection (agent-host-environment)

• Agent

• Relationship of the agent and the host (pathogenicity, virulence)

‣ Host-parasite relationsip

๏ Symbiosis, mutualism, commensalism, parasitism

๏ ‘normal’ microflora

‣ Pathogenicity (ability of agent to cause disease)

๏ Host spectrum: euryxen (broad range of hosts) - stenoxen (narrow range of hosts)

๏ Pathogenicity: obligate pathogen (can be present without clinical signs) - facultative pathogen (needs some

predisposing or environmental factors)

๏ All viruses are cell parasites

- Amount of the agent

- Way of the infection (optimal)

Question 11

Virulence, Invasiveness amount and way of infection?

Answer 11

• Virulence (grade of disease, can be low or high virulence)

‣ Virulence variants

‣ Virulence factors (help agents cause disease), MLD (minimal lethal dose), LD50 (lethal dose 50, lowest amount of

agent which will kiss 50% of animals)

‣ Change of virulence ➝ spontaneous, induced

‣ Virulence factors can be lost as a result of mutations

‣ Virulence can increase if we have infections with serial passage of the agent (shedding)

‣ Artificial virulence ➝ vaccines

• Invasiveness: intracellular, extracellular

‣ If agent within cell ➝ cells can protect the agents, so treatment is more difficult

• Amount of the agent
• Way of infection (optimal is important)
• Henle-Koch postulates

‣ The microbe is present in all cases of the disease

‣ The microbe does not occur in other disease

‣ The agent can be isolated in pure culture

‣ The same disease can be caused with the isolated agent, and it can reisolated

Question 12

Host?

Answer 12

Host

• Species, age (species specific resistance): receptor
• Race, individual resistance (illness)
• Production: feeding, laying
• Species related resistance
• Non-specific resistance

‣ Skin, mucous membranes (pH, fatty acids, bile, lysosyme, cilia)

‣ Macrophage-phagocyte system (MPS)

‣ Complement, properdin, opsonin

‣ IL, IF, TNF

• Specific resistance

‣ Passive immunity: natural, induced

‣ Active immunity: natural, induced

‣ Humoral resistance: IgA, IgG

‣ Cellular resistance: bacteria, viruses

‣ Effect of immune suppression: toxicoses, medicine, mycotoxins, some agents, parturition

๏ Diseases caused by facultative pathogenic agents

๏ Efficacy of vaccinations

‣ Foetal immune reaction: hierarchy, age ➝ foetuses have an active immune response

‣ Immune reaction of newborns (age related)

๏ Calf: Ig 4-32 days

๏ Calf, piglet: local immune reaction immediately (later disappears, no immune memory)

๏ Cellular immune reaction: 0-2 weeks long, limited

Question 13

Enviroment?

Answer 13

Environment:

• Nutrition
• Management
• Survival of the agent in the environment
• Environmental effects on animals: weather, temperature, humidity
• Predispose to facultative pathogens
• Nutrition: deficiencies, mycotoxins
• Management: overcrowding, ventilation, humidity
• Technology: weaning, grouping, treatment, castration, sheering, transport
• Physiological effects and use of animals