CIDD researchers are exploring what underlies the emergence and persistence of emerging and re-emerging diseases — from the cellular mechanisms involved in invasion of the host; to host immune responses; to the evolution of viruses by mutation, recombination and reassortment; to the spatial and social arrangement of host populations that predispose them to successful invasion.
Zoonoses and more
Sometimes, a parasite spills over from its usual host species into a novel host. When it is transmitted from individual to individual within the new host population, it has the potential to become endemic or epidemic.
Many well-known emerging diseases are zoonoses — where humans are infected by an animal disease agent that does not normally depend on human hosts to complete its life cycle. West Nile Virus, rabies and Lyme Disease are all examples. CIDD researchers are investigating some zoonotic diseases (e.g. influenza, tick-borne encephalitis), as well as emerging diseases resulting from other routes of transmission:
- From humans to animals (e.g. tuberculosis into mongeese)
- From wild to domestic or game animals (e.g. Louping ill from hares into grouse and sheep) and vice versa (e.g. canine distemper into wolves and hunting dogs)
- From one wild animal species to another (e.g. simian foamy viruses, phocine distemper virus)
From spillover to persistence
In most cases, a parasite challenging a host from a different species will fail to infect it. Even where a parasite does manage to infect an individual of a new species, the spillover is often a dead end for the parasite — it does not transmit adequately to new hosts. However, in some cases, the parasite transmits and persists.
CIDD researchers are investigating what makes species jumps more likely, and what enables parasites to transmit and persist in novel host populations. We are exploring variables that increase the likelihood of a spillover event resulting in persistence, including:
- Exposure risk: the probability of a new host species being exposed to a parasite. Environmental and anthropogenic factors may increase this risk by increasing the probability of contact between infected hosts or vectors and the new host species.
- The parasite's ability to invade host tissues, establish there, and transmit before the original infected individual dies. Selection may favor phenotypic or genetic change in the disease agent — it may adapt to its new host environment.
- Heterogeneities in factors affecting disease dynamics — such as duration of the infectious period, frequency of contacts between infectious and susceptible hosts, and probability of transmission per contact. In many cases, emerging disease outbreaks are short-lived because on average each infected host transmits to so few other individuals that the disease dies out. In other cases, the disease dies out because infection rates are so high that the disease passes rapidly through a local population, using up the supply of susceptible individuals faster than they can be replenished by birth or immigration. Persistence is most likely when each infected individual infects one or more susceptible individuals, and the supply of susceptibles is greater than or equal to the rate at which susceptibles are removed by infection, death or vaccination. Both the infectious period and the supply rate of susceptibles can be affected by the parasite's virulence.
Controlling emerging diseases
CIDD research has implications for control of emerging diseases, particularly for strategies that attempt to:
- Prevent spillover, e.g. by reducing contact rates between infective individuals of reservoir species and new hosts.
- Reduce transmission in the novel host population — for instance by quarantining or culling infected individuals, or by vaccinating susceptible hosts