L3 Host Shifting and Specificity

Host Range

The variety of host species a pathogen/parasite can invade, anagalous to ecological niches, determing its potential to shift hosts and influence disease emergence (e.g. rabies infects bats, dogs, foxes)

Host Shifting

The process where a parasite/pathogen transitions to a new host species promoting diversifitcation, triggering emerging infectious diseases and increasing selective pressures on hosts (e.g. plasmodium knowlesi from macaques to humans)

Ecological Traits

Characteristics of hosts or environments (e.g. habitat use, immune system) that influence parasite/pathogen infection and transmission, conserved across species, affecting host specificity (e.g. borellia optimised in white-footed mice).

Ecological Trait Analysis

The examination of species' ecological traits (e.g., diet, habitat use, social behavior, body size) to identify patterns that may influence their role as hosts or reservoirs of zoonotic pathogens. This approach helps predict which species are more likely to carry or transmit diseases to human

Phylogenetic Vs. function relatedness and Drivers

Phylogenetic relatedness

The evolutionary closeness between species. Species that are more closely related phylogenetically often share immune system traits, physiology, and susceptibility to similar pathogens—affecting how diseases may jump between them.

Drivers include:

• Shared ancestry

• Conserved immune responses

• Similar body physiology
  These increase the likelihood of pathogen sharing among closely related species (e.g., primates and humans).

Functional Relatedness

The similarity between species based on ecological roles or traits (e.g., diet, habitat, behavior), regardless of evolutionary history. Species with similar ecological functions may experience and spread pathogens in similar ways.

drivers include:

• Similar diets (e.g., frugivory)

• Shared habitats (e.g., forest canopy)

• Common behaviors (e.g., social group sizes)
  These traits can expose species to similar pathogen pools or vectors.

Comparative Apporaches

Methods that examine patterns across multiple species to identify traits or factors linked to zoonotic pathogen risk. By comparing species, researchers can test hypotheses about what makes some animals more likely to be disease reservoirs or bridge hosts.

Macroecological patterns

Large-scale ecological patterns observed across geographic regions or multiple species that influence disease dynamics, such as species richness, distribution, and biodiversity.

EXAMPLE:

Regions with high mammal biodiversity, like tropical rainforests, often harbor more zoonotic pathogens due to greater host variety, increasing spillover risk.

Ecological Niche

The range of environmental condtions and resources a species exploits, including host species, shaping its distribution and transmission

EXAMPLE

Lyme disease tied to tick vector niches

Fundamental Vs. Realised niche

Fundamental Niche: The full range of environmental conditions and resources a species could use without competition or predation.

Realised Niche: The actual conditions and resources a species uses when biotic interactions (competition, predation) are considered.

EXAMPLE

A rodent species might fundamentally be able to inhabit a wide range of habitats, but due to competition or predators, its realised niche may be limited to human settlements where it carries hantavirus.

Ecological fitting

The process whereby organisms colonise and persist in novel environements, use novel resources or form novel associations with other species as a result of the suites of traits that they carry at the time they encounter the novel condition

Example:
The ability of Borrelia burgdorferi (Lyme disease bacteria) to infect multiple mammal species, including humans, because these hosts share compatible immune system features, even without long-term evolutionary relationships.

Trade-off Hypothesis

Broader host specificity reduces optimal adaptation to a primary host, potentially lowering infection effeciency in specialist compared to generalists (Gaston et al. 2000)

Niche Breadth hypothesis

A broafer host range increases ecological opportunities, amplifying parasite prevelane by accessing more hosts (e.g. generalist parasiest like rabies has a higher spread potential)

Ecological opportunity

Availability of a new hosts or environments enabling parasite spreador host shifting, often outweighing evolutionary history in determining host range

Ecological Vs. Geographical barriers

Ecological Barriers: Differences in habitat, behavior, or ecological traits that prevent contact or transmission between hosts and pathogens.

EXAMPLE: Two bat species living in the same region but using different roosting sites (caves vs. trees) may rarely transmit viruses between them.

Geographical Barriers: Physical separations like mountains, rivers, or oceans that limit movement and gene flow between populations.

EXAMPLE: The Andes mountains limit the spread of certain rodent-borne hantaviruses by physically separating rodent populations.