Note: I was called away unexpectedly in the middle of editing this post and should have clarified that the exact subtype has not been firmly established. The assumption of H5N1 is probable based on history, but with the recent arrival of H5N8 in Europe, that isn’t necessary a given. - MPC
Unless you’ve been following avian influenza for a very long time, Nigeria probably seems an unlikely place to find the H5N1 virus. But during the great H5N1 diaspora of 2005-2006 - when the virus moved from being essentially a problem for a a handful of Southeast Asian nations to a Eurasia-wide threat (see H5N8: A Case Of Deja Flu?) – Nigeria was an important hub of activity.
When you look at a map of the migratory bird flyways (see below), you see that Nigeria sits at the southern intersection of no fewer than three migratory flyways – routes that begin in the northern climes of Russia, Mongolia, and China where H5N1 is known to circulate in wild birds – and that cross both Europe and the Middle East.
One of the reasons we don’t hear much regarding H5N1 out of Nigeria anymore is that there is precious little disease surveillance and reporting available outside of the larger cities that are controlled by the central government, and many of the northern rural regions are under the control of the militant Islamist movement Boko Harum.
Given the civil and religious strife – particularly in the north of Nigeria (there was another huge massacre reported yesterday) – it is somewhat remarkable that we have even this lone report.
As to how prevalent H5N1 really is in Nigerian poultry – or in any of the their neighboring countries – that is something that we simply don’t have the surveillance tools in place to calculate.
We do have this study, published in 2014, that found among a small group of Nigerian poultry workers surveyed, that a small percentage have antibodies to the H5N1 virus. Suggesting, at least, previous exposure and (likely) subclinical infection with the virus.
In recent years Nigeria has experienced sporadic incursions of highly pathogenic H5N1 avian influenza among poultry. In 2008, 316 poultry-exposed agricultural workers, and 54 age-group matched non-poultry exposed adults living in the Enugu or Ebonyi States of Nigeria were enrolled and then contacted monthly for 24 months to identify acute influenza-like-illnesses.
Annual follow-up sera and questionnaire data were collected at 12 and 24 months. Participants reporting influenza-like illness completed additional questionnaires, and provided nasal and pharyngeal swabs and acute and convalescent sera. Swab and sera specimens were studied for evidence of influenza A virus infection. Sera were examined for elevated antibodies against 12 avian influenza viruses by microneutralization and 3 human viruses by hemagglutination inhibition.
Four (3.2%) of the 124 acute influenza-like-illness investigations yielded molecular evidence of influenza, but virus could not be cultured. Serial serum samples from five poultry-exposed subjects had a ≥4-fold change in microneutralization titers against A/CK/Nigeria/07/1132123(H5N1), with three of those having titers ≥1:80 (maximum 1:1,280). Three of the five subjects (60%) reported a preceding influenza-like illness. Hemagglutination inhibition titers were ≥4-fold increases against one of the human viruses in 260 participants.
While cross-reactivity from antibodies against other influenza viruses cannot be ruled out as a partial confounder, over the course of the 2-year follow-up, at least 3 of 316 (0.9%) poultry-exposed subjects had evidence for subclinical HPAI H5N1 infections. If these data represent true infections, it seems imperative to increase monitoring for avian influenza among Nigeria's poultry and poultry workers.