As a general rule, there are two broad categories of avian influenza; LPAI (Low Pathogenic Avian Influenza) and HPAI (Highly Pathogenic Avian Influenza).
- LPAI viruses are quite common in wild birds, cause little illness, and only rarely death. They are not considered to be a serious threat to human health, although H5 & H7 strains have the potential to mutate into HPAI strains.
- HPAI viruses are more dangerous, can produce high morbidity and mortality in wild birds and poultry, and can sometimes infect humans with serious result.
From the mid-1990s to 2012, the HPAI virus of greatest concern was H5N1, which had caused the deaths of millions of heads of poultry, and claimed the lives of hundreds of people.
While high on the list of viruses with pandemic potential, H5N1 continues to have two saving graces:
- While it can jump to humans, it isn't easily spread from human-to-human
- And with the exception of a few species of waterfowl, it produces visible (often fatal) illness in most bird species.
This meant that poultry farmers could quickly see that something was amiss when it infected their (unvaccinated) flocks, sick or dying wild birds could help us track its progress as it spread, and this likely helped to slow its spread.
And while there are plenty of other avian viruses (mostly LPAI, including H5N2, H7N7, H7N3, H9N2) circulating in wild birds, and occasionally showing up in poultry flocks, only H5N1 was considered a serious threat to human health.
At least until the spring of 2013, when the `rules' abruptly changed with the emergence of LPAI H7N9 in eastern China. While `low path' in birds - including poultry - this new virus proved to be very pathogenic in humans.
In only three years, H7N9 has nearly caught up with H5N1 in terms of documented human infections, even though H5N1 had nearly a 15 year head start.
Since H7N9 infected birds appear healthy, it is often only after someone in contact with those birds falls ill that we realized a flock - or a live market stall - was infected. Making, in essence, humans the sentinel species for H7N9.
As H7N9 is only currently reported in China, and given the difficulty of studying an avian virus that travels in stealth mode', we don't know a whole lot about how the virus is transmitted in wild and migratory birds.
The assumption, however, is that H7N9 - like H5N1 and H5N8 before it - won't stay put forever. It will eventually expand via wild and migratory birds beyond China's borders.
In an attempt to assess the ability of peridomestic birds (those that live in or near human habitats) to carry and spread H7N9, researchers have experimentally infected European Starlings.
This from the Journal of Wildlife Diseases.
Experimental Challenge of a Peridomestic Avian Species, European Starlings ( Sturnus vulgaris ), with Novel Influenza A H7N9 Virus from China.
AbstractIn 2013 a novel avian influenza H7N9 virus was isolated from several critically ill patients in China, and infection with this virus has since caused more than 200 human deaths. Live poultry markets are the likely locations of virus exposure to humans.
Peridomestic avian species also may play important roles in the transmission and maintenance of H7N9 at live poultry markets. We experimentally challenged wild European Starlings ( Sturnus vulgaris ) with the novel H7N9 virus and measured virus excretion, clinical signs, and infectious dose.
We found that European Starlings can be infected with this virus when inoculated with relatively high doses, and we predict that infected birds excrete sufficient amounts of virus to transmit to other birds, including domestic chickens. Infected European Starlings showed no clinical signs or mortality after infection with H7N9.
This abundant peridomestic bird may be a source of the novel H7N9 virus in live poultry markets and may have roles in virus transmission to poultry and humans.
Some related research shows that not all avian species are equal when it comes to shedding (and spreading) H7N9, as a study that appeared in the Journal Virology two weeks ago fingered quail as being vastly superior at shedding the H7N9 virus than chickens.
Between its stealthy attributes, and its propensity to reassort with other avian subtypes (see EID Journal: H7N9’s Evolution During China’s Third Wave – Guangdong Province), H7N9 holds a position very near the top of our list of viruses with pandemic potential.
Making it imperative we learn as much as we can about how it behaves and propagates in the wild before it shows up in Europe, Africa, Asia or the Americas.