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Disease Epidemiology #2 > Host Community and multi-host pathogens > Flashcards

Host Community and multi-host pathogens Flashcards

1
Q

Reservoir competence

A
  • Ability of animal species to be a host for the pathogen
  • Refers to all the host traits that are important for the life cycle of the pathogen (susceptibility, pathogen abundance in tissues, clearance, transmission)
  • From the perspective of the host rather than the pathogen
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2
Q

Multi-host pathogens and reservoir competence

A

Hosts will differ in their reservoir competence

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3
Q

What are the key host features that determine the host’s contribution to the R0 of the pathogen?

A
  • Host reservoir competence
  • Host abundance/density
  • Therefore the composition of the host community will affect the prevalence and incidence of multi-host pathogens
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4
Q

West nile virus and reservoir hosts

A
  • Cycles between mosquitoes and birds and mammals (dead-end hosts)
  • Some birds develop high viremia which facilitates virus transmission to feeding mosquitoes
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5
Q

Viremia profile study for West Nile in different orders of birds

A
  • Birds were experimentally infected with West Nile
  • Song birds and shore birds had highest viremia and of longest duration
  • Parrots and fowl had lowest viremia and shortest duration
  • Birds with high viremias of long duration are most competent reservoir hosts
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6
Q

Calculation to determine reservoir competence

A

RC= susceptibility x infectiousness to mosquitoes x duration of viremia

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7
Q

Variation in West Nile virus competence among bird species

A

Most important reservoir hosts for West Nile Virus are blue jay, common grackle, house finch, American crow and house sparrow

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8
Q

If mosquitoes do not have a feeding preference, what would be expected to be seen?

A
  • They should feed on the birds with respect to their relative abundance
  • 80 of bird host A, then should be 80% of blood meals
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9
Q

Bird population sampling

A
  • 5 different sites in Maryland and Washington
  • Most common bird species from greatest to least: house sparrow, rock dove, European starling
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10
Q

Blood meal sampling of culex mosquitoes

A
  • Used PCR to determine what species the mosquitoes had fed on
  • Found that mosquitoes do not feed on birds in proportion of abundance
  • Mosquitoes fed more on robins than house sparrows
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11
Q

Infected mosquitoes produced by bird species

A
  • Robins were less than 4% of the birds but they produced ~60% of West nile virus infected mosquitoes
  • Sparrows were 56% of the birds, but produces ~24% of west nile virus infected mosquitoes
  • Showed that the mosquitoes feeding preference was important for which bird species contributed the most to the virus epidemiology
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12
Q

Host heterogeneity

A
  • Effects prevalence of multi-host pathogens

Factors include:
- Abundance: some host species more common than other s
- Reservoir competence: some host species have higher viremia and/or longer duration of infectious period
- Vector preferences: arthropod vectors prefer some hosts over others

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13
Q

Blacklegged ticks and Borrelia burgdorferi (Bbss)

A
  • Ticks are generalist vector, feeding on dozens of vertebrates
  • Bbss is a multi-host pathogen found in many vertebrate hosts and causes lymes disease
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14
Q

Nymphal infection prevalence and lyme disease risk

A

Risk of lyme disease depends on nymphal infection prevalence (NIP) which is the percent of nymphs infected with Bbss in the area
- Ex. 5/10= 50% vs. 8/10= 80%
- Assume that density of nymphs is same in both areas

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15
Q

Vertebrate species variation that effects R0 of Bbss

A
  • Body burden: 1 deer will feed more ticks than 1 mouse
  • Reservoir competence: Higher for mouse than deer
  • Density: density of deer is lower than mice
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16
Q

Different levels of biodiversity

A
  • High biodiversity: mice, possums, deer
  • Low biodiversity: only mice
  • Mice are good reservoir hosts, deer and possums are not. Nymphal infection prevalence (NIP) is low in high biodiversity areas compared to low biodiversity areas
  • Assume that density of infected nymphs is the same in both communities
17
Q

Nymph infection prevalence vs. diversity of host community

A

Study looked at calculating NIP for different host communities and density of mice. And then added other host species.
- Saw that as you add other hosts with lower reservoir competence, the NIP was reduced = Dilution effect

18
Q

Dilution effect hypothesis

A

Suggests that there is a negative relationship between disease risk and host diversity. So high diversity will dilute disease risk

19
Q

Forest fragmentation and Lyme disease risk

A
  • Biodiversity of vertebrate hosts is lower in smaller forest fragments. Results in mice doing very well in these areas and being in high abundance compared to larger forests
  • Highest nymph infection prevalence (NIP) in highly fragmented habitats with low biodiversity
20
Q

Assumptions of dilution hypothesis

A
  • Vector and pathogen are both generalists
  • Vertebrate hosts vary in their reservoir competence for the pathogen
  • Competent hosts achieves higher relative abundance in species-poor habitat
  • Increasing biodiversity favours noncompetent hosts
  • No increase in tick density with the addition of noncompetent hosts
21
Q

Conclusions on biodiversity and lyme disease risk

A
  • Vertebrate host species differ in reservoir competence for B. burgdorferi
  • White-footed mouse are highly competent reservoir hosts for B. burgdorferi
  • Habitats with low biodiversity have higher risk of lyme disease and vice versa
  • Highly fragmented forests have high density of mice and high nymph infection prevalence (NIP)
  • Habitat conservation and biodiversity preservation can reduce Lyme disease risk
  • Dilution hypothesis: preserving biodiversity will reduce zoonotic disease
22
Q

Amplification hypothesis

A
  • The opposite of the dilution hypothesis
  • Biodiversity increases disease risk
23
Q

Leishmaniasis

A
  • Vector borne disease
  • Protozoan parasite (Leishmania) transmitted by sand flies
  • Nicknamed chiclero’s ulcer
    o Cichleros- people who collect chicle, a latex produced by sapodilla tress
  • Infects humans who live and work in intact rainforests
24
Q

Amplification effect and leishmaniasis

A
  • Parasite has a positive association with biodiversity
25
Q

Bovine tuberculosis

A
  • Caused by Mycobacterium bovis
  • Multi-host pathogen infecting cattle and wildlife (badger)
  • Causes considerable economic losses to farmers
26
Q

Bovine TB in UK

A
  • Occurred from 1986-2010
  • Cattle routinely tested. Positive animals are slaughtered and if a herd tests positive, farmer loses official TB free status and is no longer able to transport and sale of cattle
  • Believed to be due to badgers
27
Q

Badger culling trials

A
  • Cull badgers in an attempt to control bovine TB
  • Previous studies showed opposite effects of culling badgers
  • New study at 10 locations. Some areas where badgers culled, others left alone for control
    »Active culling reduced numbers of TB in that area BUT areas surrounding cull saw an increase in TB in cattle
28
Q

Perturbation hypothesis

A
  • Culling badgers disrupted their social structure . Badgers moved in between culled and non-culled areas which led to increased exposure outside the culling zone
  • Effective TB control would require large scale culling of badgers
29
Q

Vaccine against TB

A
  • Bacillus Calmette-Guerin (BCG) vaccine (attenuated form) developed in 1921 to protect humans against TB.
  • Give as a single dose to children in endemic areas. Discontinued in Canada in 1970s. Only prevents 20% of children from getting it and prevents 50% that get infected from developing the disease
  • Found to protect mice, possums, badgers, deer, and cattle from TB
30
Q

TB vaccination of badgers in UK

A
  • They vaccinated badgers to reduce prevalence of TB
  • Vaccinated, and tested serum for antibodies
    »Seroconverted badgers were considered infected (non-vaccinated badgers seroconverted more quickly than vaccinated)
  • Vaccine efficacy increased over study
  • Post-mortem exams of badgers at end of trial saw 26% TB lesions in placebo group and 9% lesions in vaccinated group
31
Q

Does vaccinating badgers actually reduce TB in cattle?

A
  • Cannot say for certain whether it would or not
32
Q

Why do they not just vaccinate the cattle instead of the badgers?

A
  • Because if they did this then there wouldn’t be a way to tell whether the cattle population is TB free because vaccinated and infected would appear the same in testing

Disease Epidemiology #2 (11 decks)

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