One very big mistake i wish i knew early on was the effects that a live vaccine can have on your flock. Im not talking about Mareks vaccine. Although Mareks is a live vaccine, it is NOT the actual virus. The other vaccines wich are live ARE the actual virus. Theory being that they give the flock a small amount of virus, and immunity kicks in, fights the virus, and your flock is immune. What i didnt know is what happens if your flock CANT fight off the virus! I learned a very sad and hard lesson that year and lost my ENTIRE flock. Needless to say now, the ONLY vaccine i give is the Mareks vaccine!!! Such a hard and awful lesson. Such a tragic ending!!! I learned the ONLY vaccine they really need is Mareks. No problems with that and it protects the flock! Not potentially kills them!!
Unfortunately, not completely accurate so let me explain.
Vaccines against viral diseases are three basic types: "killed", "modified live", or "recombinant". I'll explain what that means, but there aren't commonly used vaccines anymore that are just "live"--meaning that small amounts of disease-causing virus are injected into the individual.
(1) Killed vaccines are the whole virus, killed in some fashion. It is possible that some virus escapes being killed, but extremely rare. Unfortunately, this is the weakest vaccine and usually requires an immune-stimulating booster (an "adjuvant") to create protective immunity. One example of an adjuvant used in the past is aluminum.
(2) Modified live is the live virus, but disabled in such a way that it can't cause disease. This type of vaccine is very effective at stimulating the immune system because it uses the whole virus, but there is a rare chance that a random mutation will give a virus particle back its virulence.
(3) Recombinant vaccines are made by taking a small bit of the virus that is capable of stimulating the immune system to produce antibodies against it, then inserting the gene for producing that bit into cells/bacteria that are capable of mass-producing it. This is the safest, most targeted version as the whole virus is never used and therefore can never cause the disease.
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Okay, that covered the types of vaccines, but now let's talk about the Marek's vaccine. Except...there isn't THE Marek's vaccine...there are many versions!
Three potential things can be included in a Marek's vaccine. One is the similar but harmless-to-chickens Herpesvirus of Turkeys (HVT). This is similar enough to the Marek's Disease Virus (MDV) that it can stimulate a mild immune response when used in a vaccine given to chickens. So, this was the first vaccine used against MD. Unfortunately, over time it became less and less effective as the virus developed resistance, so then a non-pathogenic (can't cause disease) strain of MDV was added to the vaccine. And then in some even more resistant areas, a modified strain of pathogenic MDV had to be added. So, you have vaccines that can have HVT, MDV (non-pathogenic), or MDV (pathogenic) strain in them...or some vaccines actually contain more than one strain.
So unless you know which particular Marek's vaccine was given, you won't know what type of vaccine (killed, modified live, recombinant) it was or what strains it contained. Although I don't think there are any recombinant MD vaccines available...too expensive.
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Okay, this all pertained to viral vaccines. Bacterial vaccines (such as tetanus, Lyme's) are handled differently and vary widely. And I can't bring myself to think of the "vaccine" against coccidiosis as a vaccine...it really is inducing a low-level version of the disease instead of just simulating it.