Flu the Farmer
In our H7N9 post we described the possibility reducing finishing time may select for greater virulence in influenzas. That is, reducing the age at which poultry are sacrificed may select for increasing the damage influenza incurs.
There may be immunological fallout as well,
By increasing the throughput speed, and reducing the age of food animals at slaughter, the livestock industry may also be selecting for strains able to transmit in the face of younger, more robust immune systems, including, should spillover occur, in humans.
A friend and colleague points out that younger immune systems in poultry may not be the more robust. Indeed, it takes up to six weeks for a bird’s full immunity to come online. As JJ Dibner and colleagues describe it,
The seeding of the bursa by lymphocytes occurs between embryonic Days 10 and 15. These cells are committed B cells but are capable of only IgM expression at hatch. The secondary immune organs, such as the spleen, cecal tonsils, Meckel’s diverticulum, Harderian gland, and the diffuse lymphoid tissue of the gut and respiratory systems are incomplete at hatch. There are B cells in the cecal tonsils, but these only express IgM. Similarly, there are T cells in the lamina propria and epithelium of the gut and in other secondary immune organs, but these do not develop helper or cytotoxic capability until some period after hatch. The ability to mount a secondary response, as indicated by the presence of germinal centers or circulating IgG and IgA, begins to appear between 1 and 4 wk of post-hatch life in the broiler chick.
Dibner’s team recommend reversing what was at publication the industrial practice of waiting to feed and water hatchlings. The earlier new poults are fed and watered–and, interestingly, by feeding, the earlier they are exposed to antigens–the faster the immune system develops.
The notion of immune development–and the health costs of too much biosecurity–introduces an interesting convergence here.
The industry makes an economic distinction between food animals of different ages. If neonatals get sick and die, no problem. Little investment–feed per unit gain–is lost. But if poultry or livestock approaching sacrifice get sick, that’s bad business. The company has invested considerable feed by this point. There is, then, a premium placed on a well-oiled immune response in poultry approaching sacrifice.
Some preliminary modeling a group I’m a part of is now conducting suggests influenza may be on a similar schedule.
We found the individual host’s recovery time post-infection and the expected fitness of a flu strain are apparently independent of the duration of a poultry cohort, even under industrial conditions. Influenza’s infectious period (no more than a week for any individual host) will likely never approach the finishing time, even under the most rapid throughput speed (presently 40 days).
However, selection occurring at another level of organization—namely across the cohort—may better bear down on the relationship between finishing time and virulence:
Even as individual hosts die off, the outbreak chain continues to propagate on the farm over several generations until a strain can infect enough birds to threaten violating biocontainment and spreading to the next barn or farm. If so, the time it takes to successfully propagate to such a threshold of infected birds may approach the finishing time. From this perspective, it’s the final number of birds infected that is the controlling variable, not the individual infectious period.
If so, and we’re still modeling this mind you, reducing finishing time could indeed affect the evolution of virulence. A shortened finishing time may select for strains able to reach the propagation threshold faster (before the cohort is slaughtered).
By that backdoor–by which both flu and farmer put a premium on a near-slaughter bird–evolving virulence in the face of an increasingly active, if not robust, immune system may indeed be selected for.
Incredibly, it would also mean flu’s now a farmer too, husbanding cohorts of infecteds not for market but the next available barn of fresh susceptibles.