Emerging and re-emerging diseases
What is an Emerging Infectious Disease?
1. Established infectious disease currently undergoing increase incidence (e.g. TB, cholera, dengue fever)
2. Newly identified agent but circulating for some time (e.g. West Nile virus, Lyme disease)
3. Newly evolving/occurring infections (e.g. Ebola, HIV, SARS)
Favouring factors for EID in non-humans
Increased density of monotypic domestic animal populations
Increased transport of animals/animal products
Climate change + Land-use changes + Biodiversity decline = wildlife crowding + altered animal migrations
Closer contact with humans (edge-effect)
Intentional movement: Horticulture, food/pet trade, agriculture, forestry, ecological restoration, aquaculture.
Unintentional movement: Ignorant possessions, ship ballasting, horticulture, natural dispersal, food/pet trade.
Rabbits and myxomatosis
Background: European rabbit moved to Australia and the Americas. Picked up Myxoma virus from the Eastern
cottontail (north America) and the Jungle rabbit (south America). This virus is not severe to native American rabbits
but fatal to European rabbits. First recorded outbreak was in laboratory rabbits at the Institute of Hygiene in
Montevideo, Uruguay in 1896.
Experiments (1936-38): Tested in Australia, Sweden, Skokholm island (UK) and Vejrø island (Denmark). They realised
that the virus was highly species-specific with fatality rates of >99%. However, there were not enough mosquitos to
act as vectors. Thus, MYXV unlikely to be successful in controlling rabbits – need more vectors and no foxes. (Kerr 2012)
Experiments (1950s): Further experiments in Australia – virus spread locally and then died out due to lack of
mosquitos. However, there was a flood which resulted in a spread of 1600km from south to north and 1800km from
east to west in the first 3 months. MYXV spread for the next 3 years with case fatality of 40-99.8% and massive
reduction in populations. Shows the importance of ecology/environment in establish disease transmission.
Social & environmental factors (McMichael 2005)
Demographic characteristics and processes – human mobility (e.g. Schistosomiasis came from Africa).
Land use, climate and other environmental changes, encroachment on new habitats
Consumptions behaviours (eating, drinking, changes in culinary culture)
Other behaviours (sexual contacts, IV drug use, medical technology etc)
Host condition (malnutrition, diabetes, immune status etc)
Few diseases affect one group exclusively – relations between host
populations impact disease emergence.
Zoonotic link: Species jumpers – no longer dependent on animals.
Often ecological cause like habitat changes. 75% of human EID are zoonoses –
from bushmeat, roadkill cuisine, sick animals easier to capture. Risk for
researchers.
Spill-over to domestic animals or humans from wildlife leads to substantial amplification of the disease because we
live in dense populations – early detection and effective response system needed.
Ebola virus: Came from bats. The specific cultural and lifestyle of susceptible individuals (social
transmission) makes it difficult to eliminate ebola e.g. traditional funeral practises.
The many Ebola variants also make it difficult to control. 3 main types = Zaire, Sudan and Bundibugyo.
Geographical isolation contributes to the differing phenotypes of these 3 strains.
Coronavirus: SARS 2002. MERS 2012. COVID-19 2019. Understanding ecology of the all the coronaviruses
(including ones that do not spread to humans) would help us to understand the more severe ones.
(Cui et al. 2019) https://cmr.asm.org/content/33/4/e00028-20/figures-only
Summary
, The complex nature of EID – including the role of invasive species in introducing them into non-humans and how we
try to use EID in control of the invasive species (myxomatosis).
Historically, currently and in the future, EID will blight human and animal populations.
The greatest challenge at the moment is to have early warning tools which will allow to prepare for the outbreak and
work on novel approaches.
Key questions
1. what is an EID?
an Emerging Infectious Disease – can be newly identified, newly evolving or an established disease
that is experiencing a resurgence.
2. What influences the development of EID in human population and the importance of zoonotic link.
75% of human EID are zoonoses – they come from animals. Zoonotic link.
Increased human movement, transport of animals, climate change factors and other social and
environmental factors all affect the spread of an infectious disease.
3. why are some problematic and others not?
Some have a high fatality rate and may be made up of many different strains with varying severity.
