I was learning about
MRSA recently, a type of bacteria found in humans that's developed immunity to antibiotics. In the wiki article the following is said-
So in other words, despite doctors specifically trying to prevent this from occuring, penicillin immune S. Aureus bacteria went from under 1% of the population to over 90% worldwide and even as high as 98% in some regions
This is the development of natural immunity in a population. Imagine if the farming industry attempted something similar with livestock:
simply allow the survivors to reproduce
Nearly all S. Aureus bacteria is immune to penicillin now because the survivors reproduced
Natural immunity protects pathogens against the entire medical industry trying to kill them, yet this simple and obvious principle is ignored with livestock and the exact opposite is practiced with disastrous results
Natural immunity is an important angle to consider, but in the context of the current bird flu epidemic it’s important to consider how practical that is considering the nature of this virus itself and how it affects certain species.
Wild ducks are a main reservoir and have been, duck biology seems to grant them partial resistance, which is good for the ducks that manage to survive it, not so good for other species, especially raptors, vultures, pheasants, turkeys, chickens, etc which do not have that natural resistance.
For the virus itself it prefers a host species with that tolerance. It can continue to infect, replicate, and spread via the vector species, whereas with a species like chickens, it can infect an entire flock and within days that whole flock will be dead and if it hasn’t managed to spread to another host it will die with those chickens. Viruses are primarily spread via hosts that do not have severe immune responses and so tolerate the pathogen. Hosts that have severe immune responses usually die with the pathogen their immune system is trying to kill so they’re essentially dead ends for their species and the virus.
So that is the main issue, natural immunity exists for ducks, not so much for other species, they’re dead ends. A flock of chickens that gets hit with HPAI is pretty much guaranteed to die, there is no natural immunity that arises from it.
However there is LPAI. Low pathogenic avian influenza doesn’t hit the immune system of many species as hard as HPAI, because of this it has been known that birds infected with strains of LPAI have shown some immunity to HPAI. So HPAI will just eradicate a flock of chickens but if that same flock had contracted LPAI there’s a chance some may survive it.
LPAI is a riskier virus because of that however, it’s always one mutation away from leaving its benign habits and becoming a mass killer, and because it’s more widespread the greater the odds of doing so.
Another thing to consider is even with survivors natural immunity isn’t going to be hereditary. The next generation won’t inherit some encoded resistance to the virus if it returns to the flock.
A good comparison would be the Spanish flu vs Swine flu. When swine flu emerged in 2009 it was a mystery at first because it barely affected the elderly whereas it made younger people and especially pregnant women very sick. The reason was swine flu and Spanish flu are very similar, Spanish flu also affected pregnant women severely, however Spanish flu didn’t exactly spare the elderly in it’s first speed run around the globe whereas swine flu did spare the elderly. The reason why is because many people prior to the late 1950s had been exposed to the Spanish flu, their immune systems remembered it and recognized the key similarities in swine flu, thus they were spared, but they didn’t pass that immunity on. Their children and grandchildren had no immunity when swine flu appeared, it hit the young hard.
So in relation to bird flu, natural immunity/herd immunity isn’t really a thing unless the same virus or mostly the same is going around, if it’s a new strain that the immune system doesn’t recognize, there is no natural immunity.
The main reason the primary method of control was depopulation was it was effective. In previous outbreaks in Southeast Asia it did work. It stopped the spread of the virus. The difference between then and now is HPAI is not secluded in one small area, it’s endemic in nearly every environment now so depopulating a flock is no longer an effective means of control because it just keeps reappearing. Natural immunity is a limited means of combating the epidemic for the reasons I mentioned but there needs to be a different method than relying on the now redundant method of depopulation.
I can’t say what’s in the mind of viral researchers but my guess is that’s the conclusion they came to when exploration of hpai vaccines began in Europe ages ago and now here in the U.S with the zoetisvaccine that’s been under development the last few years. Bird flu is everywhere so vaccination now seems like it may be the primary method of limiting the loss of poultry. Proactive treatment rather than reactive depopulation. Vaccines through dead dna or rna also do not carry the risk of mutation like LPAI infection or another live vaccine.
