Infectious Agents and Diseases Flashcards
Define pathogen.
Anything that can produce disease. An infectious agent such as a virus, bacterium or other microorganism that causes disease in its host.
Define infection.
Invasion of a host’s bodily tissues by a disease-causing agent, the agent’s multiplication, and the reaction of host tissues to the agent and toxins it produces.
Define transmission.
Passing of a communicable disease from an infected host individual or group to another individual or group.
Define microorganism.
Microscopic organism, may be a single cell or a multicellular organism.
Define pathogenicity.
Potential capacity of a particular species of microorganism to cause disease.
Define zoonosis.
An infectious disease that is transmitted from animals to humans or from humans to animals (sometimes known as reverse zoonosis).
Define virulence.
Degree of pathogenicity within a group or species of agent of infectious disease, as indicated by host fatality rates and/or the ability of the agent to invade the tissues of the host.
Define vector.
Any living organism that carries and transmits an infectious agent into another living organism.
List the taxonomic categories of agents causing infectious diseases in animals and man.
Viruses
Bacteria
Fungi
Protozoa (parasite)
Helminths (parasite)
Arthropods (parasite)
Prions
Cancerous tissues
What is foot and mouth disease?
- Highly contagious
- Affects the upper alimentary tract and feet
- Affects cloven hoofed animals: all antelope, bovine, deer, goat, sheep and pig species
- Extremely resistant and can survive in environment for a very long time
- Severity depends on type/subtype and host animal species
What is rabies virus?
- Acute encephalitis in warm blooded animals
- Transmission mostly through bites from infected animals
- Long incubation period before clinical signs appear
- Fatal within 6-14 days of onset of clinical signs
- Globally important zoonosis
What is influenza virus?
- Waterborne, enteric infections in wildfowl
- Adapts readily to domestic fowl, domestic animals and humans
- Many different types varying in their pathogenicity for different species of animals and man, which can cause major pandemics
- Evolving constantly and new types appear every year through genetic mutation and recombination of genetic material
What is coronavirus?
- Well known as causing various diseases in poultry, pigs, cattle and cats for a long time
- Natural host are bats, very large number of coronaviruses
- Originally found in poultry in 1930s
What is E.coli?
- Gram-negative rods with flagella
- Can cause intestinal disease in many species of animals especially in young animals. Generally mild illness, lasts few days
- Some are zoonotic and can be acquired by humans
- Occasionally complications, especially in children and the elderly
What is brucella?
- Brucella abortus, Brucella melitensis, Brucella suis
- Intracellular, Gram-negative rods
- Abortion, infertility and chronic arthritis in ruminants, swine and humans
- Transmission via birth materials
- Important globally as a zoonosis
What is mycobacterium bovis?
- Bovine tuberculosis – still common in UK cattle
- Intracellular bacteria, causing chronic disease
- An important zoonosis
- Wildlife reservoirs complicate eradication from domesticated animals: badgers (in the UK and Spain) and possums (New Zealand)
What is bacillus anthracis?
- Anthrax
- Gram positive rod
- Causes rapid death in most animals
- Spores can stay alive for decades in the soil
- Disease reappears after floods, landslides etc.
- Affects farm animals, humans and also elephants, hippos
What is clostridium botulinum?
- Gram-positive rods
- Grows in anaerobic conditions
- Pathogenic effect through toxin production (‘Botox’)
- Toxins produce paralysis in extremely low quantities. Death through paralysis of respiratory muscles
- Typically seen in summer, such as waterfowl on stagnant water
- Toxic for humans: badly canned food
What are fungi?
- Large family of organisms, largely non-pathogenic
- Mushrooms may be edible, therapeutic, poisonous and hallucinogenic
- Yeasts: many are useful (baking, alcohol production) and some pathogenic (thrush, oral infections)
- Moulds: antimicrobial producing moulds (Penicillium), decaying of dead material and spores can be important causes of disease
What are some characteristics of pathogenic species of fungi?
- Causing dermal infections, ringworm
- Causing internal infections, coccidioidomycosis of lungs and aspergillosis
- Producing harmful mycotoxins: Aflatoxicosis (Aspergillus flavus) and Ergotism (Claviceps purpurea)
- In wildlife: white nose disease of bats in USA (Pseudogymnoascus destructans) and chytridiomycosis of amphibians (Batrachochytrium dendrobatidis)
What are protozoa?
- Single-celled eukaryotic microorganisms
- Many species are in water, in intestines (e.g. in rumen of ruminants) and in other body systems (mostly pathogens)
- Multiplication through binary fission, although sexual reproduction also occurs
What are coccidia?
- A broad group of protozoan parasites affecting all domestic animal species, most other mammals and many birds
- Many cause gastrointestinal diseases
- Others cause generalised systemic disease and reproductive losses
- Both direct and indirect life cycles
- Some infect a single host species and others have definitive and intermediate hosts
- Undergo sexual reproduction and have sexual life cycle stages
What is trypanosoma?
- African trypanosomiasis (sleeping sickness): anaemia, weight loss and death in domestic animals and cerebral symptoms and death in humans
- Extracellular blood parasite
- Transmitted by tsetse fly
What are macroparasites?
Multicellular organisms from various phyla:
- Helminths (worms) – roundworms, flatworms (flukes), tapeworms
- Insects – biting flies that may be disease vectors, blow flies, nuisance flies
- Acari (8 legged) – ticks and mites
What are helminths?
- From microscopic to metres long
- Various locations: alimentary canal often as adults, liver e.g. liver fluke, both as larvae and adults, and other organ systems, often as larval stages
- Often complicated life cycles, involving various hosts for different stages of development Three major groups: nematodes (roundworms), trematodes (flatworms/flukes) and cestodes (tapeworms)
- Many different species in each group
What are nematodes?
- Roundworms
- Mostly live in the gastrointestinal tract
- Also found in lungs, heart, blood, liver, kidneys, skin, eye
- Found in almost all domestic and wild animals
- Many are harmless
- Some cause a variety of diseases and sometimes death
What are trematodes?
- Flatworms
- Parasitic species referred to as ‘flukes’
- Generally have snail intermediate hosts
- Liver fluke of cattle and sheep is the classic example
- Very common cause of disease and losses in farm animals
- Often zoonotic
What are cestodes?
- Tapeworm
- Many species cause serious zoonoses of public health concern in the UK and globally
- Intermediate hosts may be animals or insects, depending on cestode species
- Larval stages may cause pathogenic cysts in intermediate hosts and sometimes in humans
What are arthropods?
- Insects: 6 legs, with a head, thorax and abdomen
- Acari: 8 legs, with cephalothorax and abdomen
- Many different species in each group
- Some important as disease vectors
- Others are pathogens in their own right
What are the arthropod insects?
- Important as disease vectors: mosquitoes, midges, tsetse flies, sandflies, fleas
- Pathogens in tehri own right: fleas, horseflies, horn flies, blowflies, midges
- Fly strike: larvae penetrate skin and cause severe lesions
What are acari?
- Ticks: many species, feed on blood of host, cause blood loss, irritation and may transmit diseases
- Mites: microscopic organisms/sometimes just visible, live on or burrow in skin, causing irritation and lesions, can cause death in serious situations
What are prions?
- Abnormal brain protein – misfolded proteins that cause normal proteins to misfold
- Accumulation of prion protein causes brain cell death in many animal species and man
- Always fatal
- Highly resistant to cleaning and disinfection
- Bovine spongiform encephalopathy (BSE) is a famous example
- Scrapie in sheep
What is a canine transmissible venereal tumour?
A transmissible cancerous tissue.
- Transmitted through sexual contact
- First emerged in a dog that lived about 11,000 years ago
- All CTVT tumours carry the DNA belonging to this “founder dog”
- Generally self-limiting, regression after 3 to 6 months
What is facial tumour disease in Tasmanian devils?
Transmissible cancerous tissues
- Transmitted through fighting
- Tumours interfere with feeding, death through starvation
- Serious concerns for the conservation of the species
How can pathogens be transmitted via ingestion?
After ingestion, pathogens may affect:
- The intestinal tract: E.coli, salmonella, many viruses, some coronaviruses via the faeco-oral route. Gastrointestinal nematodes, transmitted via pasture or soil
- Other organ systems: liver fluke, larval tapeworm cysts in peritoneal cavity, lungs, liver, brain, brucella abortus in reproductive tract and joints, bovine TB anywhere
How can pathogens be transmitted by inhalation?
- Pathogen enters lungs
- Depending on pathogen type, lungs mainly affected or other organ systems involved
- Foot and mouth disease – wind borne spread is possible, hence initial infection may be by inhalation. Clinical signs are nevertheless in virus predilection sites in upper alimentary tract and coronary band of hooves.
- TB is acquired by inhalation initially develops in lungs and eventual spread to affect other parts of the body.
How can pathogens be transmitted by inoculation and some examples?
Pathogen is injected into the body through vectors such as ticks or being insects. May cause local disease, but mostly cause systematic disease. Examples:
- Virus – West Nile virus and Bluetongue
- Bacteria – tick-borne fever and tick pyaemia
- Protozoa – plasmodium and trypanosoma
What is deposition?
Pathogen is deposited one the skin and spreads across or penetrates the skin: blow fly larvae, mites and ringworm. Biting insects ticks actively seek out their hosts: questing by ticks and attraction of flying biting insects to odours.
What is vertical transmission?
- Transmission of infection from 1 generation to the next
- Disease agent crosses the placenta or perinatal transmission
- Important in many viruses, bacteria and parasites
What are 4 infectious agents that occur via vertical transmission?
- Bovine viral diarrhoea virus
- Mycobacterium avium paratuberculosis – a bacterial disease of cattle
- Neospara caninum – a protozoan affecting cattle (Johne’s)
- Toxocara canis – a common roundworm of dogs
