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Disease Epidemiology > Disease Transmission > Flashcards

Disease Transmission Flashcards

1
Q

Taxonomic overview of pathogens and parasites

A

Prokaryotes- viruses and bacteria

Eukaryotes- protozoans, fungi, helminths (nematoda, platyhelminthes- cestoda, trematoda, arthropods- ticks, lice, mites, flies, mosquitoes)

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2
Q

Macroparasites (helminths, arthropods)

A
  • Multicellular, and grow and develop inside host
  • Large and few within a host (think 2 to thousands)
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3
Q

Microparasites (viruses, bacteria, protozoans) presence in host

A
  • Small and replicate rapidly inside host
  • Small and numerous within a host (think millions)
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4
Q

microparasite models

A
  • SIR models used to model microparasites
    S= susceptible hosts
    I= infected hosts (shedding)
    R= Recovered hosts
  • all infected individuals are the same
  • probability of transmission or recovery does not depend on number of microparasites in host
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5
Q

Macroparasite models

A
  • helminth worms have different stages in their lifecycles (eggs, larvae, adult). Models of these parasites must track the developmental stage
  • helminth worms are aggregated in hosts. There is a small percentage of host population carrying most of the worms
  • parasite burden will influence transmission. More worms present = more worms shedding= more spread
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6
Q

Effect on host fitness for microparasites vs. macroparasites

A

Microparasties= mortality. Microparasites have the ability to overload the host very quickly

macroparasites= morbidity

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7
Q

macro vs. microparasites: multiplication inside a host and growth and development

A

Multiplication inside host= microparasites; rarely macroparasites

Growth and development= macroparasites

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8
Q

macro vs. microparasites: transmission

A

Macroparasites: direct transmission and complex life cycles

Microparasites: direct transmission and vector-borne

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9
Q

Steps in the parasite lifecycle

A
  1. Finding a host
  2. Infection through the outer barrier and establishment of the host (through skin and mucosal membranes
  3. Growth or multiplication of the parasite inside the host
  4. Reproduction (Eg. By exchange of genetic material between co-infecting strains)
  5. Development of transmission stages, and transmission to the next host
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10
Q

Lifecycle of Rabies

A
  1. Dog bite breaks skin and dog saliva containing the virus enters the tissues
  2. Virus uses PNS as transport system to enter the CNS including the brain (CNS are preferred tissues for rabies virus)
  3. Infection of CNS causes changes in behaviour (increasing aggression and biting) that enhance the virus transmission
  4. Rabies virus contaminates salivary glands so that infected host will transmit virus to a naïve host following a bite
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11
Q

Pathology and its link with mode of transmission

A

The pathogens lifecycle determines mode of transmission, tissue tropism, pathology, and disease symptoms

Ex. respiratory pathogens: Colonize the mucous membranes of the resp system and cause symptoms like mucous production and coughing that facilitate pathogen transmission

Ex. Vector borne pathogens: Colonize the circulatory system to facilitate acquisition by blood-feeding arthropod vectors

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12
Q

Disease transmission

A

The process where a pathogen/parasite transits from an infected host to an uninfected host.

Important for determining biology of infectious disease and determining control for infectious diseases.

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13
Q

Types of mode of transmission

A
  1. Horizontal (indirect and direct)
  2. Vertical
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14
Q

Horizontal transmission

A

Transmission between unrelated individuals (same or different generations)

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15
Q

Types of horizontal transmission

A
  1. Indirect
  2. Direct
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16
Q

Indirect Horizontal

A

Requires multiple species involvement for the life cycle to be complete (arthropod vector and intermediate host)

Either vector-borne lifecycles or complex life cycles

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17
Q

Direct horizontal

A
  1. Close contact (skin-skin, sexual, aerosol, secretions, carcasses)
  2. Contaminative (air-borne, water-borne, soil-borne, fomites)
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18
Q

Direct transmission- skin-skin (cattle ringworm)

A

Cattle ringworm is a fungal skin disease in cattle

Causative agent: Trichophyton verrucosom

Direct close contact transmission from skin to skin contact between healthy and unhealthy animal

Symptoms: patches of hair loss, desquamation (skin peeling), formation of thick crusts

Economics: spoils milk, meat, and leather quality

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19
Q

Direct transmission- sexual (brucellosis)

A

Brucellosis affects many animals including dogs, cats, swine and goats

  • Causative agent: Brucella canis
  • Infected dogs have bacteria in their genital secretions. Oral transmission (licking genitals) or sexual transmission
  • Causes reproductive problems (infertility, abortions) but few other symptoms
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20
Q

Direct transmission- biting (FIV in cats)

A

Feline immunodeficiency virus affects cats worldwide (2.5-4.4%)

  • Causes AID like symptoms in cats, but it is not typically fatal; compromises the immune system by infecting the white blood cells
  • Transmitted via deep bite wounds. Low risk of transmission via sharing water bowls, food, or litter box
  • Most commonly occurs in males due to higher levels of fighting, dominance, biting
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21
Q

Direct contaminative- air-borne (Foot and Mouth Disease)

A
  • Causative agent: FMD virus (picornavirus)
  • Highly infectious disease. Virus has very high survival rate (relative humidity greater than 55%
  • Symptoms: blisters inside mouth, foamy saliva and drooling, blisters on feet that can rupture
  • Affects ungulates (cattle, buffalo, sheep, goats, pigs, antelope, deer, bison); rarely affects humans
    –> Pigs: more resistant to virus, but when infected they shed high number of virus.
    –> Cattle have low infectivity threshold and get virus easily
  • Transmitted by close animal-animal contact, fomites, food, motor vehicles, long-distance aerosol
  • Aerosol: allows for high virus emission. Weather will favour low virus aerosol emission: gentle winds and stable atmosphere. Large numbers of livestock can be exposed to virus plume for hours… Ex. 1981 transmission from France to UK (250km)… usually only travels less than 10km on land.
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22
Q

Direct contaminative- waterborne (giardia)

A
  • Beaver fever
  • Hosts: humans, beavers, cows, rodents, sheep
  • Infection occurs by ingestion of contaminated food or water or animal-animal contact
  • Symptoms: diarrhea (which further facilitates transmission
  • Cysts can survive for 3 months in water
  • In 2013, ~280 million people had symptomatic giardiasis
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23
Q

Direct contaminative- soil-borne (anthrax)

A
  • Gram positive bacteria (Bacillus anthracis)
  • Found in soil and affects domestic and wild animals world wide, and can infect humans
  • Endemic in latin America, sub-saharan Africa, asia; rare in N. America
  1. spores in the soil (can last for decades)
  2. Grass-eating animals will encounter spores, ingest or breath them in
  3. Spores become active growing cells. Bacteria multiply inside host, and move into circulatory system, producing endotoxins that cause severe illness and death of host. Carcass contains millions of spores that contaminate the soil for its next victim
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24
Q

Anthrax bioweapon

A

English bioweapon in WWII. In 1942, exploded bombs on Gruinard Island. Places 80 sheep on island, all sheep became infected and died within days.

In 1986, decontamination efforts began. Removed top soil and sprayed over 280,000 kg of formaldehyde on island. Placed test flock back on island, and they remained healthy. In 1990, 48 yrs later, the island was considered safe for human habitation.

25
Q

Vertical transmission

A

Transmission between mother and offspring

26
Q

Vertebrate pathogens and vertical transmission

A

Vertebrate pathogens with vertical transmission will almost always have horizontal transmission as well

Ex. Salmonella in chickens

27
Q

Vertical and horizontal transmission of Salmonella in Chickens

A

Salmonella causes GI disease in many vertebrates species. Persists in the environment for long periods of time.

Horizontal transfer: fecal-oral route

Vertical transfer: from mother to egg. Salmonella colonizes the ovaries and oviduct. Infected hens lay infected eggs. Can then be passed on to humans.

28
Q

Vertical and horizontal transmission of Pestiviruses in livestock

A

Bovine viral diarrhea in cattle, classical swine fever in pigs, border disease in sheep

Horizontal transmission: shed in fecal material, urine, and nasal secretions

Vertical transmission: virus crosses the placenta and infects fetus. Leads to abortion of persistently infected individuals (super spreaders)

29
Q

Persistently infected individuals of Pestiviruses

A

Persistently infected (PI) individuals that are super spreaders

Infection of dam must be during gestation (days 30-120) to cause immunotolerance and birth of a PI calf. This is because the calf does not have a competent immune system at the time of the infection, so the virus enters the cells and the body accepts it as self.

30
Q

Vector-borne life cycles

A

requires an arthropod vector and a vertebrate host

31
Q

Complex lifecycles

A

Helminth parasites require an intermediate host and final host

32
Q

Arthropod vectors

A

mosquitoes, ticks, sandflies, assassin bugs

33
Q

How do arthropod vectors transmit pathogens?

A

Hematophagous (blood-feeding) or blood meal

34
Q

Vector-borne pathogens

A

viruses, bacteria, protozoan parasites, nematode worms

35
Q

Two critical steps of a lifecycle of a vector borne pathogen

A
  1. Uninfected vectors must acquire VBP from an infected vertebrate host
  2. Infected vectors must transmit the VBP to other uninfected hosts
36
Q

How does vector become infected and able to further pass on a pathogen?

A

Vector does not become infected immediately. The VBP is in the midgut which is not an ideal place for further transmission. The pathogen will travel through the hemocoel to the salivary glands (migration may take days to weeks). Once inside the salivary gland, the vector is considered infectious

Time this process takes is very dependent on temperature

37
Q

EIP

A

Time for a vector to become infectious after feeding on infected host

Dependent on temp. Higher temperatures increase pathogen replication and migration, and therefore a shorter EIP

38
Q

Condition of vector-borne transmission

A

Vector must take at least 2 blood meals on different vertebrate hosts

Ex. Dengue virus: Uninfected mosquito bites infected human, contracts the disease, and bites uninfected human, human then becomes infected

39
Q

Pros of a vector borne lifestyle

A
  • Easier to spread over long distance as vector is able to move
  • Single infected individual can propagate infection in multiple individuals
  • Motivated vector- arthropods will actively seek out host for a pathogen
40
Q

Cons of a vector-borne lifestyle

A
  • Need to evolve ability to live in both arthropod vector and vertebrate host. This can lead to competition and more complicated lifestyle
  • Infected vector may feed on wrong vertebrate host
41
Q

Vector borne lifestyle- West Nile Fever

A
  • A mosquito-borne disease (Mosquitoes- genus Culex). Mosquitos act as bridge vector, able to bite birds, horses, and humans.
  • Birds are reservoirs, mammals are dead-end or incompetent hosts.
  • Mosquitos bite birds 80% of time and then bite mammals 20% of time, allowing cycle to complete.
42
Q

Bridge Vector

A

Vectors that are specialized to bite both the reservoir host and the dead-end host

43
Q

Reservoir hosts or competent hosts

A

Hosts with high viral titres so they are able to transmit infection on to vector and therefore the next host.

44
Q

Dead-end hosts or incompetent hosts

A

Hosts with low viral titres which means it is too low to pass infection on

45
Q

Viremia

A

Level of virus in the blood

Low viremia- less likely to allow for virus to be passed on (likely in dead end hosts)

High viremia- more likely to allow for transmission of virus (likely in reservoir or competent hosts)

46
Q

Effects of infection on both reservoir or dead-end hosts

A

Infection in either type of host can result in death or individual becoming asymptomatic. So mortality and morbidity are not good predictors of host competence

Ex. West Nile- high mortality rate in birds (outbreaks often noticed by dead crows), 40% mortality in horses, 80% humans are asymptomatic

47
Q

Vector Competence

A

The ability of a vector to transmit a disease.

Occurs because vector-borne pathogens have evolved to become adapted to certain species of arthropod vectors

Ex. Only 2 out of 90 tick species can transmit lyme disease

48
Q

TYPICAL ONE HOST LIFECYCLE: Parasitic roundworms and direct lifecycles

A
  1. L3 larvae eaten by vertebrate host; develop into adult worms
  2. Adult worms produce fertilized eggs, expelled with fecal material
  3. Eggs hatch (L1, L2, L3, in dung)- L1 and L2 feed on bacteria in the dung, L3 larvae move onto pasture waiting for host to ingest.
49
Q

Requirements for microparasite complex or indirect lifecycles

A
  1. One or more intermediate hosts and one final vertebrate host
  2. Sexual reproduction often takes place in the final vertebrate host and results in the shedding of an infectious stage in the environment
  3. Often food chains facilitate trophic transmission of parasites from intermediate hosts to final host
50
Q

Host types in complex or indirect lifecycle

A
  1. Final or definitive host- an organism in which the parasite reaches maturity and reproduces sexually
  2. Intermediate host- an organism that harbours the immature stages of the parasite and is required by the parasite to undergo development and complete its life cycle
51
Q

Parasitic helminth examples

A
  • Roundworms
  • Tapeworms
  • Flukes
  • Thorny-headed worms
52
Q

COMPLEX LIFECYCLE- FASCIOLOSIS IN LIVESTOCK

A

A disease of cattle and sheep; common liver fluke (Fasciola hepatica). Found in 70 countries

Steps:
1. Adult worms live in sheep liver and produce eggs
2. Egg hatches into miracidium
3. Miracidium enter the snail, develop into free-swimming cercariae
4. Cercariae leave snail and encyst on water plants
5. Cysts eaten by vertebrate hosts

**Remember that the lifecycle must be respected. Miracidium cannot infect cattle, must go into snail first.

53
Q

COMPLEX LIFECYCLE- TAPEWORM, ECHINOCOCCUS MULTILOCULARIS

A
  1. Adult tapeworm living in definitive (final) host = canids, producing eggs that are released in feces
  2. Rodents (intermediate hosts) ingest the eggs which hatch into oncospheres that target the liver and develop into cysts
  3. Cysts develop into protoscolices on intestines
  4. Canids eat the intermediate hosts, and protoscolices develop into adult worms
54
Q

Alveolar echinococcosis

A

If a dog or human eats the eggs instead of the protoscolices. The tapeworm treats them as the intermediate host, and the oncospheres target the liver and cause decreased organ function.

For lifecycle to continue, something would need to eat the dog or human. Therefore skipping steps does not really work, lifecycle would likely end.

55
Q

Cons of complex lifecycle

A
  • Need to evolve ability to make a living in many different hosts
  • Expect probability of completing a life cycle to decrease with number of hosts
    EX. IF PROB OF ENCOUNTERING A HOST IS 20%, COST OF ENCOUNTERING 3 DIFFERENT HOSTS IS 0.2 ^3 = 0.8%
56
Q

Pros of a complex lifecycle

A

Can use both predators and prey (trophic level interactions) to increase chances of infection

  • Smaller hosts are more abundant
  • Larger hosts are less abundant
  • Large hosts encounter lots of smaller host, therefore spread can occur to the larger animals
  • Predators are best for adult parasites- larger body so more space, and longer lifecycles
57
Q

Pros to using a predator in a lifecycle

A
  • Predator is bigger host, more resources
  • Predator is longer-lived, more time to reproduce
  • Parasite can grow larger, produce more offspring
  • Adding the predator offsets losses due to predation
58
Q

Pros to using prey in a lifecycle

A
  • Predators unlikely to ingest immature stages (eggs) in the environment
  • Prey is more common, easier to encounter for parasites
  • Smaller prey items more likely to feed on immature stages in environment
  • High predation rates concentrate parasite into original predator host
59
Q

Trophic Transmission

A
  • A highly efficient form of transmission because food webs are everywhere in nature
  • Many predator species are solitary and encounters between adults is uncommon. However, predators encounter prey often. So infecting prey items is a great way for parasites to meet predators.
  • Also delaying sexual maturation until you reach final host means create space for adult parasite and a much longer lifespan

**Complex lifecycles are adaptive because they allow helminth populations to use abundant but small intermediate hosts to encounter the rare, large predator hosts that provide great environments for adult parasites

Disease Epidemiology (9 decks)

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