Canine Parvovirus disease is considered a disease of puppies and dogs under the age of 1 year old. It is a very serious disease with two clinical forms: myocarditis (cardiopulmonary failure, pulmonary edema, cyanosis and collapse) and gastroenteritis (anorexia, fever, vomiting, diarrhea and lethargy). Either way is a supra-acute disease and a dog who has this can die within hours. There is no specific treatment to eliminate the virus and the recovery depends in part on the time it took to identify the problem and the quality of supportive treatment given.
Because of these treatment problems and because of the speed the disease acts, it is the first disease against which a puppy is vaccinated. In order to achieve a good immunity against this virus at least two immunizations should take place in the early life of the dog; one usually at 6-8 weeks of age and second and third one at monthly intervals, after which yearly vaccinations are recommended. it is important that the dog is worm free at the moment of the immunization (fact true for all immunization shots) for the vaccine to fully engage the immune system and therefore for the dog to develop good response to the disease. If there are worm present in the digestive system their subproducts (proteins) will affect the response of the immune-system and the result of the vaccine will not be 100%. The efficiency of the result will decrease depending on the degree of worm infestation and on the types on gastrointestinal worms present.
Second aspect is the quality and type of the vaccine. There are many kinds of vaccines available (live modified, death, subunitar, and so on) and there are different manufacturers. It is very important that the vaccine is a high quality one with high density antigens. Also again dependent on the vaccine the storing conditions are extremely important, if the vaccine requires storing at fridge temperatures it is important that it is kept at those temperatures. If the storing conditions are not respected the vaccine will be unactivated and the result will be you giving a shot of sterilised water instead of antigen suspension.
The difference between the vaccines found at vet clinics and other suppliers is that a vet will always make sure he/she gives the best available product on the market as it is her/his responsibility if the animal treated will get the disease. Also vets do store medicines appropriately. Also a vet will be able to see through a consultation before actually giving the immunization shot if there is any underlying illness that can affect the outcome of the vaccination or if the vaccination has to be postponed.
Lately there were reports of Canine Parvovirus affecting older dog like yours,aged from 2 to 5 years old and it is still under study why.