Parvo can last in the environment for typically one year, but up to two years. After that, it's dead. If the dog got over parvo, it will have some immunity still. Hopefully, however, the owners would have kept up with yearly boosters.
It isn't critters that bring it, so much as other dogs and people (by bringing it in on shoes, or from touching an infected dog) as the route is fecal-oral. So, basically, an infected dog has to have defecated in the yard, or a person has to have tracked feces from an infected dog into the yard. Panleukopenia can very VERY rarely cross over, and that's a concern with stray and feral cats, but it's not something worth nuking your yard over. Sufficient bleach will kill all the plants.
Yearly vs. every 3-year vaccines depends on the amount of risk, to some degree. One hospital that I worked at was in a high risk area for parvo (sometimes multiple cases every day, during summer) and we vaccinated all dogs for it yearly. Another hospital saw maybe one or two cases a year, and thus after the first yearly booster would vaccinate every 3 years.
Because I am a vet tech and thus can easily bring stuff home on accident, I vaccinate my dog yearly.
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