ROTT N CHATTER, October 1992, Volume 1 Number 3

 

                            THE DOCTOR'S BAG

                            GAEA MITCHEL, DVM

 

I get lots of questions on Parvovirus, especially from my Rottweiler owner

clients.  There are lots of myths and misconceptions about the disease,

which increases the confusion.  I would like to take some time to outline

the disease itself, as well as immunization.

 

While Parvo is one of the most feared viral diseases of our dogs, the actual

severity of the symptoms varies widely.  When a susceptible dog is exposed

to a source of virulent Canine Parvo Virus (CPV), there is an incubation

period of 4-7 days.  Usually the first sign is vomiting, followed by

diarrhea within the next 24 hours.  In severe cases the dog will stop eating

and run a fever, develop a low white blood count (WBC), and bloody diarrhea.

Depression and death may follow.  (There are of course other causes with

many of these same symptoms, including distemper, hepatitis, coronavirus and

rotavirus.)  In young dogs, CPV can also cause cariomyopathy, a usually

fatal condition of the heart muscle.

 

However, in many cases CPV will produce a mild and self limiting diarrhea

lasting only 1-2 days, or may even cause no observable symptoms.  Studies

have shown that the severe forms of the disease usually involve concurrent

infections with other viruses of heavy parasite loads.  For example, a dog

exposed to CPV along with canine distemper or coronavirus will be much more

likely to suffer fatal symptoms.  Likewise, dogs with a heavy infestation of

Hookworm, Coccidia or Giardia have a greater risk of severe symptoms.

 

Following exposure to CPV, the virus will be shed in the stool beginning at

least one day prior to the onset of symptoms, and decreasing as an immune

response increases anitbody titers.  By the ninth to eleventh day,

postinfection shedding has usually stopped.  This virus-contaminated stool

is the source of infection for other dogs and the virus can survive up to

six months in the environment.

 

CPV is suspected initially based on symptoms and history.  A low WBC

supports the presumptive diagnosis, which can be confirmed by tests for the

virus in the stool, or by antibody titer testing.  Treatment consists mainly

of supportive care, with an emphasis on maintaining hydration with IV

fluids. Elimination of any parasite infections essential also.

 

It may surprise you to learn that the goal of immunization is not to prevent

infections, but to prevent disease.  The much-sought-after immune response

can not begin to occur until there is exposure to the virus, when it

hopefully destroys those viral particles before they replicate and produce

symptoms of the disease.  While this protects the immuned individual from

disease, it does not prevent the phase of infection that results in shedding

of the virus in the stool.

 

The greatest obstacle to successful immunization with CPV vaccine is,

ironically, the very thing that protects the new-born pup from disease. When

the pups receive colostrum, they get antibodies from the bitch at a level

directly related to the bitch's own titer (level of antibody).  When the pup

is vaccinated these maternal anitbodies tie up the viral particles before

they can stimulate the pup's immune system to develop a mechanism for

producing antibodies.  To add to the problems, maternal antibody

interference continues even at a level below that necessary to protect

against disease caused by CPV. The exact age and duration of this period of

susceptibility varies with the antibody of the bitch, the amount of

colostrum each pup received, and the rate at which the titer declines.  Add

to this the fact that the cell mediated immunity provided by T-cell

lymphocytes is evem more essential than circulating antibodies in the

survival of dogs exposed to CPV, and you can begin to appreciate how

difficult it is to give an accurate estimate of the exact age at which a pup

is successfully protected by vaccine-induced immunity.

 

While a test of antibody titer will show evidence of protective levels of

antibody, there is not test to indicate the existence of cell mediated

immunity.  The titer level that is considered effective varies depending on

the specific test and each lab will provide that information when the test

is run.  It is possible to have a dog with no circulating antibody, yet have

absolutely no symptoms with exposure to CPV.  The apparent failure of many

Rottweilers to develop an early protective titer following vaccination may

be due to a defect in the function of B-cell lymphocytes, yet these dogs may

still be protected by a T-cell mediated response.

 

I would like to thank Dr. Larry J. Swango, virologist at Auburn University

School of Veterinary Medicine, for his input into this article.