Thursday, 1 January 2015

Bats in a tree...

Meliandou and the burnt tree that
once housed a bat colony (from Fig 3, [1]).
While not snakes on a plane, I'm fairly sure the level of swearing has at times been at least as bad among those suffering from and dealing with the possible fall-out from these bats - if in fact they were the source for the biggest Ebola virus disease (EVD) epidemic on record.

A recent animal counting, trapping and testing study in Guinea included sampling in and around the village of Meliandou.[1] This village is, to the best of our knowledge, the site of the first animal-to-human, or zoonotic, transmission of the Ebola virus variant called Makona.[2]

The study team, made up of researchers affiliated with Germany, Sweden, Core d'Ivoire and Canada, did not find any decline in numbers of usually susceptible larger mammals around the index village; a sign during other outbreaks, of active local ebolavirus "activity". The team also found that primate hunting was not a big thing in this region, which is rather devoid of these and other Ebola virus mammalian host animals (including few of the Duiker, or forest antelope). Fruit bat hunting was common though.

The team captured 169 bats representing at least 13 different species and 6 families. But in the house of the 2-year old boy considered the epidemic's index case, fruits bats were not eaten and no bat hunters resided there. No Ebola virus RNA was detected in any bats and antibody screening results from bat blood were inconclusive. 

These findings led the authors to study Meliandou, resulting in an hypothesis that a nearby hollow tree that once housed a large colony of free-tailed bats [locally described as lolibelo - small and smelly bats - otherwise known to belong to the species of insectivorous bat, Mops condylurus of the family Molossidea; [3], may have been the source of  infection. Why only one child was infected this way when the tree was a site of frequent play by many children is not known. The tree was burned out in March 2014 which caused many bat deaths, some of which were collected for consumption. Sequencing of a PCR-amplified mitochondrial DNA segment found that in 5 of 11 ash and soil samples from around the tree, contained traces of Mops condylurus genetic material. So that species was at least there.

So, this is all quite far from a conclusive link between the 2-year old boy and these bats. But it does read as though every avenue has been tested in this village, perhaps apart from better animal antibody testing (serology), and some serology on the blood of those villagers who remain alive in Meliandou. 

Serology testing is going to be very important for answering many questions around EVD and this outbreak and epidemic. 

Of course this will raise the usual question of whether we cull all bats to prevent this from ever happening again. Don't be ignorant! Bats have very important roles in pollinating and thus in keeping our ecosystem going. Should we kill all bees because they sting us? I'm pretty sure I've been stung by a bee more times than I've had Ebola/Hendra/SARS/Nipah/MERS/Lyssavirus or any other bat-hosted virus infection. Killing off everything to prevent a very rare zoonotic event when better knowledge can resolve the problem is just a typically short-sighted and knee-jerk human reaction (not a fan-can you guess?).

One question that does still remain, and one that is of extreme interest to me, is how often mild disease results from an Ebola virus infection? Good, robust serology methods to the rescue.


References...

  1. Investigating the zoonotic origin of the West African Ebola epidemic. EMBO Molecular Medicine(2014). http://embomolmed.embopress.org/content/embomm/early/2014/12/29/emmm.201404792.full.pdf
  2. Nomenclature- and Database-Compatible Names for the Two Ebola Virus Variants that Emerged in Guinea and the Democratic Republic of the Congo in 2014. Viruses 2014, 6(11), 4760-4799.
    http://www.mdpi.com/1999-4915/6/11/4760
  3. Mops condylurus via the IUCN Red List of threatened species (listed as of least concern)
    http://www.iucnredlist.org/details/full/13838/0