Outside conditions are crappy again this week. HOT and HUMID! No sneezing last week during a good cool snap. Now 2 of my youngest are sneezing again. They have been integrated with the flock for almost a month now and none of the other birds are affected. What can be wrong. Even the third of the 3 new birds is fine as can be and she came from the same breeder. Not even a runny nose. I'm clueless.
Hmmmm...without doing some deeper research the only thing that comes to my mind with what you descirbe, and mind you I'm not an expert, is residual CRD. Otherwise, I'm thinking what else changes with the weather...and why sneezing. Only other thought would it be has to be in the environment and what grows in hot humid weather but slackens in cool moist? Mold? (Although in Oregon our mold grows with the constant cool moisture.) Chickens sometimes sneeze with dust, and will simply just sneeze infrequently, but repetitive sneezing indicates a cause that should be addressed.
In my mind a viral infection would have spread to the rest of the flock almost immediately (been there, done that). Bacterial infections can spread more slowly, and sporadically, but having only the same 2 birds repeatedly being the only affected and brought on by weather change leads me to think CRD in those 2 birds....which means there is some risk of transmission to the others, how much, many think is debatable and dependent upon the immune system of the individual bird. (Many feel ACV and garlic help to boost the immune system and continuously feed with that as well as making sure worms infestation is kept at a minimum.)
CRD is prevalent in the environment. Most commonly CRD is transmitted from infected momma hen to egg to hatched baby chick, although it can be passed from mature bird to mature bird, but this is not as common. The initial MG infection would have been mild and is now producing a latent CRD (chronic respiratory disease) with weather changes...sort of like chicken asthma. CRD can be chronic. No fatalities normally occur with "true" CRD. It has been the bane of the commercial industry for years as it can reduce egg production, and if the MG strain occurs with a secondary infection of e-coli, fatalities do as well....but the symptoms of that involve the air sacs and are more dubious.
Symptoms for CRD can be very mild, and sneezing is a symptom...one of the few diseases, other than IB, that is listed as producing sneezing from the materials I rely on (linked below). The Rooster Booster Triple Action Multi-Wormer should help put that to right (make sure you have the product with Bacitracin for the CRD) as that's what the commercial growers use to keep mild CRD at bay through continuous feed. Stronger antibiotics are necessary for a more acute CRD/MG infection.
There is a government certification system in place for breeders and commercial growers to help iradicate the disease by culling all involved so that it is no longer passed to the next generation through hatching eggs or bird to bird transmission as a lot of industry dollars go to antibiotics to treat it. Some prefer to breed for natural resistance to diseases believing a hardier chicken needs less intervention because vaccines and antibiotics can breed more virulent bugs (Marek's is experiencing this with the current vaccination).
For further research, below are links to respiratory infections in chickens that I have found helpful.
Hope the information is useful to you.
Lady of McCamley
http://www.barnyardhealth.com/resinindompo.html
http://www.localharvest.org/blog/26992/entry/respiratory_disease_in_chickens
http://msucares.com/poultry/diseases/diseases.html
