Since parvo virus attacks rapidly dividing cells, the most commonly affected cells are those that line the intestines (hence the vomiting and diarrhea from cell loss in the intestines) and the bone marrow, making the white blood cell count of infected dogs dangerously low (another reason for no steroids). The even bigger complicating factor is that since the intestines are damaged by the virus, the normal bacteria in the intestines can escape the intestines, but these dogs can't fight the infection since their white cells are so low. This is why parvo patients are so critical, in addition to the large amount of fluid loss through vomiting and diarrhea.
The mainstay of treatment of parvo is hospitalization, aggressive intravenous fluids, and broad spectrum intravenous antibiotics. Many dogs are hospitalized for 3-5 days (though some longer). The prognosis for survival varies on how sick they are, and how well they respond to treatment, and if we are able to stave off infection until the GI cells and bone marrow recover. The cost will vary drastically from hospital to hospital and geographic location, so this is something I unfortunately can't help give information about. A word of caution though; this is a disease that I do not recommend treating at a hospital without 24 hour care. These patients can be so sick and complicated that they need someone working with them frequently.
It always makes things more difficult when financial constraints exist, however, if a puppy this young cannot keep food or water down, the chances of it becoming dehydrated quickly are very high, as are the chances of her blood sugar becoming dangerously low, since young puppies do not have large stores of sugar in their liver, so brief periods of not eating can lead to dangerously low blood sugar. Based on what you are describing, it really sounds as if your puppy needs further care (and testing for parvovirus if it hasn't yet been done).
One option would be to contact the humane society or SPCA in your area to see if they offer low cost hospital care. Another option is to apply for care credit (which can be done online at the link below). If you qualify, they can take care of all of the payments up front at a hospital that accepts care credit.
I hope this information is helpful and I will keep my fingers crossed for your little girl to be ok.
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