Puppy Herpesvirus Infection Or Fading Puppy Syndrome: While there are many causes of "fading puppy syndrome", canine herpesvirus is perhaps the most important. Pups under four weeks "fade" or die suddenly. "Faders' stop feeding, cry, are lethargic, show discomfort when touched and may have a nasal discharge. Infections at five weeks of age or older cause only a mild nasal inflammation. At present there is little that can effectively be done either to treat young pups or to prevent this infection. Experimental vaccines have not been wholly effective. Herpesviruses (such as the human cold sore herpes virus) remain dormant in carriers and are activated by stress. Pregnancy is a stress.