Why are some pathogens species-specific, and how can others cross species barriers?

Sourav PanNovember 9, 2024

Answered step-by-step

Some pathogens are species-specific due to a combination of genetic, molecular, and ecological factors, while others can cross species barriers through various mechanisms. Here’s an overview of why these differences exist:

Reasons for Species-Specific Pathogens

1. Host-Specific Receptors:
   • Pathogens often rely on specific interactions with host cell receptors to gain entry and establish infection. For example, certain bacterial surface proteins (adhesins) are tailored to bind to unique receptors present on the target host cells. In Listeria monocytogenes, the proteins InlA and InlB specifically recognize human E-cadherin and other human receptors, which restricts their ability to infect non-human hosts. This specificity limits the pathogen’s ability to infect a broad range of species.
2. Evolutionary Adaptation:
   • Over time, pathogens may evolve to adapt to their specific host environments, leading to specialization. This adaptation can involve changes in virulence factors that are optimized for a particular host’s immune system or physiology. For instance, many human pathogens have evolved mechanisms to evade the human immune response, making them less effective in other species.
3. Genetic Barriers:
   • Genetic differences between species can create barriers to infection. For example, even closely related species may have variations in their immune responses or cellular structures that prevent successful pathogen entry or replication. The genetic incompatibility can also result in differing responses to the same pathogen, further reinforcing species-specificity.
4. Co-evolution:
   • Pathogens and their hosts often co-evolve, leading to a dynamic relationship where both parties influence each other’s evolutionary trajectories. This co-evolution can result in highly specialized interactions that are not easily transferable to other species .

Mechanisms for Crossing Species Barriers

1. Mutation and Adaptation:
   • Some pathogens, particularly viruses, have high mutation rates that allow them to rapidly adapt to new hosts. For example, avian influenza viruses can mutate and reassort their genetic material when infecting different bird species or mammals, enabling them to cross species barriers and potentially infect humans. The rapid evolution of these pathogens increases their chances of overcoming host-specific barriers.
2. Environmental Factors:
   • Changes in environmental conditions can facilitate cross-species transmission. For instance, habitat destruction or climate change may bring humans into closer contact with wildlife, increasing opportunities for zoonotic diseases (diseases that jump from animals to humans). Areas with high biodiversity often serve as hotspots for such transmissions.
3. Close Genetic Relationships:
   • Pathogens are more likely to successfully infect new hosts that are closely related genetically. For example, HIV crossed from primates to humans due to the close evolutionary relationship between these groups . Similar immunological responses and life-history traits among closely related species can facilitate successful infections.
4. Intermediate Hosts:
   • Some pathogens utilize intermediate hosts as bridges between species. For example, certain viruses may infect an animal that is not a primary host but can still support viral replication before spilling over into a new host species (e.g., bats as reservoirs for coronaviruses).
5. Human Activities:
   • Human activities such as agriculture, trade, and travel can also promote the spread of pathogens across species boundaries. The movement of livestock and wildlife increases the likelihood of pathogen transmission between different animal populations and ultimately to humans