# 2103
Most of us tend to take a rather simplistic view about how epidemics and pandemics arise. We assume a single virulent pathogen gets introduced into a susceptible population and then spreads rapidly.
But the question arises, why today and not yesterday? Or why tomorrow, and not today?
There are multiple (and quite nasty) pathogens circulating in this world of ours, and yet they are held at bay - at least most of the time.
Are there other factors at work here?
Scientists at scientists at the University of California, Davis, the University of Illinois and the University of Minnesota believe they may have found at least one contributing factor.
In their study:
. . . the authors propose climate change, or extremes in weather, as a contributing factor in the generation of some disease outbreaks.
Here is the abstract from the PloS One article (reformatted for readability). It is a fascinating study.
Extreme climatic conditions may alter historic host-pathogen relationships and synchronize the temporal and spatial convergence of multiple infectious agents, triggering epidemics with far greater mortality than those due to single pathogens.
Here we present the first data to clearly illustrate how climate extremes can promote a complex interplay between epidemic and endemic pathogens that are normally tolerated in isolation, but with co-infection, result in catastrophic mortality.
A 1994 canine distemper virus (CDV) epidemic in Serengeti lions (Panthera leo) coincided with the death of a third of the population, and a second high-mortality CDV epidemic struck the nearby Ngorongoro Crater lion population in 2001. The extent of adult mortalities was unusual for CDV and prompted an investigation into contributing factors.
Serological analyses indicated that at least five “silent” CDV epidemics swept through the same two lion populations between 1976 and 2006 without clinical signs or measurable mortality, indicating that CDV was not necessarily fatal.
Clinical and pathology findings suggested that hemoparsitism was a major contributing factor during fatal epidemics. Using quantitative real-time PCR, we measured the magnitude of hemoparasite infections in these populations over 22 years and demonstrated significantly higher levels of Babesia during the 1994 and 2001 epidemics.
Babesia levels correlated with mortalities and extent of CDV exposure within prides. The common event preceding the two high mortality CDV outbreaks was extreme drought conditions with wide-spread herbivore die-offs, most notably of Cape buffalo (Syncerus caffer).
As a consequence of high tick numbers after the resumption of rains and heavy tick infestations of starving buffalo, the lions were infected by unusually high numbers of Babesia, infections that were magnified by the immunosuppressive effects of coincident CDV, leading to unprecedented mortality.
Such mass mortality events may become increasingly common if climate extremes disrupt historic stable relationships between co-existing pathogens and their susceptible hosts.
The authors have shown what they believe to be a direct link between climatic conditions, co-infections, and a highly pathogenic outbreak of a disease not usually associated with a high mortality rate.
Canine Distemper Virus (CDV) is rarely fatal in lions, yet two documented outbreaks in recent years have yielded up to a 40% mortality rate.
Both were preceded by a severe drought. Water Buffaloes were left debilitated by these climatic conditions, and in their weakened state became easier prey for lions, and at the same time suffered heightened (Babesia ladened) tick infestations.
Lions, who often have low levels of Babesia Leo, and tolerate it without ill effects, suddenly were exposed to Babesia sp., and began to suffer from its effects.
Concurrently, CVD which is normally mild in African Lions, spread through the population - weakening the lions and allowing the Babesiosis infection to bloom - resulting in a high mortality rate.
A complex and circuitous chain of events leading to an epidemic.
As the authors point out, this may have ramifications that go well beyond the lions of the Serengeti.
If extreme weather events become increasingly frequent owing to global climate change, the consequent synchronization of proliferating pathogens or their vectors may cause disease to become a major threat to historically stable populations that had previously coexisted with multiple viral and parasitic pathogens.
It may be that a warmer climate will help introduce vectors, such as ticks and mosquitoes, to areas that currently don't suffer from them. Tropical diseases such as Malaria, or Dengue may one day affect population centers where they are currently unknown.
Shifting patterns of rainfall or drought, along with temperature changes, could also affect the geographic spread and prevalence of some diseases. And as we've seen in this study, co-infections of one pathogen could exacerbate the symptoms of another.
Obviously, the interaction between climate, extremes in weather, and disease is a complex field where we've still got a lot to learning to do.
