Introduction
Disease associated with the canine parvovirus
started appearing around 1978. It is postulated that this virus is a
mutation of the feline distemper virus. Since so few dogs had
developed natural antibodies in the late 1970's, a large number of
dogs died from this disease. In some cases, the virus affected the
heart, and caused death within a few hours. We can still remember
people coming into our clinic in droves to get their dogs vaccinated.
There was no parvo vaccine for dogs then, since we did not even know
what parvo was. The vaccine we gave initially was the cat feline
distemper vaccine since there was no dog parvo vaccine available at
the time. Our universities and drug companies immediately responded
to the challenge, and identified the virus and manufactured a highly
effective vaccine.
20 years later most dogs have encountered the parvovirus, either
naturally (maternally) or through vaccines. These natural antibodies pass on
to puppies (called maternal antibodies) when they nurse in the first few days
of life. Maternal antibodies initially give a pup protection from parvovirus,
along with many other viruses and bacteria. They last for a variable period
of time, and start diminishing by around 2 months of age, yet can stay around
until 5 months. It is at this time that a pup starts becoming vulnerable to
the parvovirus, so we begin vaccines then. We never know exactly when these
maternal antibodies diminish, which is why we give the parvo vaccine in a series.
If we start the series much before 2 months of age we are wasting it because
the maternal antibodies will negate the effects of the vaccine. In essence,
we are trying to give the vaccine just as the maternal antibodies are diminished
and just before the pup becomes susceptible to an exposure to the parvovirus.
There are reports of people and cats getting this
disease, but in our locale this is almost non-existent. Pigs can get
their own version of parvovirus, but it does not affect dogs or act
in a similar manner.
This disease is still prevalent, as evidenced by
the number of e-mails we receive regarding this topic. Many of these
dogs would not suffer this problem if they were properly vaccinated.
Preventing this disease is dramatically more effective (not to
mention less expensive) than treating it.
Cause
The disease is caused by a highly contagious virus that is
transmitted mostly by dogs orally contacting infected feces. Being a virus,
they contain only DNA
or RNA, and are
not capable of reproducing unless they invade a cell. Once inside the cell they
take over and force the cell to produce so many new virus particles that the
cell eventually bursts, releasing these new virus particles into the bloodstream
and tissues so they can invade other cells. The only thing that can stop this
is the immune system.
Viruses are the smallest of know living organisms, and can
only be seen with special microscopes called scanning electron microscopes,
that cost millions of dollars (ours is on order). The parvovirus is extremely
small (the Latin word for small is parvo)- just 1 thimble full of stool can
contain millions of virus particles. It is easy to see why contamination occurs
so readily. Incubation period varies from 5-10 days.As
in many viral diseases of the intestinal tract, some dogs can pick up the disease
and shed the virus without significant symptoms in themselves.
This
is an electron microscope view of a cluster of parvo
virus particles. It is courtesy of Dr. Cornelia
Büchen-Osmond Biosphere 2 Center. "Universal
Virus database of the International Committee on
Taxonomy of Viruses, ICTVdB.
The scale at the bottom shows the length of 100 nanometers
(nm). A
nanometer is 1 billionth of a meter, so it is obvious these particles
are real small
The virus has 3 basic strains:
1. CVP2 which was the first one found in
1978
2. CVP2a came on the scene in the early
80's
3. The strain most prevalent today is
CVP2b
Eventually a new strain will appear because the
virus will adapt to the immune system of dogs.
The parvovirus can linger in the environment for
many months, allowing it to affect other dogs. It can withstand
common household disinfectants (except bleach) and can withstand
freezing winter temperatures. It can be spread on the hair and feet
of dogs, in addition to shoes, clothes, and eating
utensils.
We tend to see parvo in dogs that have other
diseases, especially intestinal
parasites (worms). This might be because these dogs
have immune systems that are weakened, making them more susceptible to a viral
infection. Also,
the parvo virus capitalizes on the fact that the lining of the intestines of these
dogs with intestinal parasites are damaged and susceptible to the parvovirus.
Pathophysiology
Parvovirus has a predilection for rapidly dividing
cells (similar to cancer). The rapidly dividing cells in a dog are
the intestines, bone marrow, and the immune system. When the virus
infects these areas the lining of the intestine literally dies, the
bone marrow cannot make red or white blood cells in adequate
quantity, and the immune system can become impaired.
Initially the virus replicates in the tonsils
(which are lymph nodes) after oral ingestion. The virus rapidly
multiplies and enters the bloodstream within a few days. From here it
spreads to those parts of the body that contain rapidly dividing
cells as mentioned above.
The main cause of death in parvo infected dogs is
septicemia. Poisons from bacteria that are attacking the susceptible
lining of the intestines release toxins into the bloodstream. These
toxins add to the complications of a puppy that is dehydrated from
vomiting, diarrhea and not eating, is hypoglycemic from not eating,
and has electrolyte imbalances from vomiting and not eating. The
bodies immune system becomes overwhelmed and death ensues if
treatment is not instituted early and aggressive enough. Even if
toxins are not released by bacteria, the dehydration and electrolyte
imbalance that occurs can lead to shock and eventual
death.
In actuality, whether a pup recovers from the
virus depends on a race between parvovirus particles causing
septicemia and dehydration, and the immune system's ability to
neutralize the virus. Fortunately, for most pups that get medical
care, the good guys win the race.
Symptoms
The majority of dogs presented with parvovirus
show signs of fever, lethargy, vomiting, diarrhea, and lack of
appetite. In severe cases the diarrhea is very watery and frequently
bloody, with a telltale odor. They are very ill, with significant
abdominal pain. The virus is so strong that it literally causes the
lining of the intestines to slough. It is painful to eat, and with
the severe diarrhea and vomiting that is present, they rapidly become
dehydrated. The also have a disruption in their electrolytes (sodium,
potassium, chlorine) that adds to the weakness.
There is a complication that can occur from all
the intestinal activity regarding vomiting and diarrhea. It is called
an intussusception, which is literally a telescoping of the intestine
into itself. This will cause the intestine to die, resulting in death
of the pup. Treatment is surgical, unfortunately, these pups are in
no shape for surgery. Luckily we do not encounter this very often, if
at all.
In the peracute form of this disease the virus
attacks the heart and causes rapid death. Fortunately, it is rare to
encounter this nowadays.
Diagnosis
The diagnosis
of canine parvovirus is frequently made by age of pet (usually under 6 months
of age), symptoms exhibited, and physical exam. Other diseases can mimic the
signs of parvovirus, so x-rays and routine blood
samples are sometimes run to help eliminate them
as a cause. A
CBC (complete blood count) might show a reduced white blood cell level, an indication
that a virus is present in the body. A
blood sample can be run looking for Parvo antibodies, but the results are open
to interpretation as to whether the dog has an active infection or not.
An accurate,
simple to perform, and inexpensive test has been developed to test the feces
for the virus. It is called a CITE test, and can be performed by our in hospital
lab within 15 minutes. Recent vaccinations can interfere with the interpretation
of this test. No test is perfect though, and like many diagnostic tests for
diseases, there can be false positives and false negatives. Lets look at how
we perform and interpret this test:
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A sample of feces is obtained from the rectum and
placed in the first test well.
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A special probe is placed sequentially in the 4
test wells over a 5 minute period of time.
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The results are obtained by analyzing
the blue dots on the bottom of the probe.
The test on the left is negative, the
one on the right is positive.
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Here are all the possible
interpretations
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Treatment
Dogs with parvovirus need immediate veterinary
care because they are usually very sick. They should not be
treated at home if they are significantly ill. This care
involves large amounts of intravenous
fluids, added electrolytes, antibiotics,
and special medications to minimize vomiting. It is common for them
to be hospitalized for 5 days. We will monitor red blood cell counts
and protein levels to identify those dogs that are not responding to
routine treatment. If the protein level becomes low we institute
therapy with additional fluids (called colloids) to combat the
problem. Dogs that continue to decline in spite of therapy may also
need a blood transfusion because they can become anemic and deplete
their protein. We will also treat the internal parasites that can be
an integral part of this disease.
We do not feed them until they have gone 24 hours
without vomiting. We will send your dog home if it is eating and not
vomiting for 24 hours. We expect it to have a persistent soft stool
or diarrhea for several days after returning home.
Special precautions
are taken when we treat parvo dogs. They are put in a special
area so that we can monitor them closely. They are also
isolated so that other dogs in the hospital are not exposed to
the virus. Staff members that treat them wear disposable gloves
and gowns, and clean their shoes in a special chlorine foot
bath to prevent hospital contamination of the
virus.
Fortunately, most dogs recover with our intensive
therapy, although there still is an occasional dog that does not.
This may be due to a weak immune system that can not produce adequate
antibodies, or a particularly strong (virulent) strain of the virus
that we encounter occasionally. Dobermans and Rottweilers seem to be
especially sensitive to this virus, and have the most difficult time
recovering from an infection.
Once your dog is well on its way to recovery we
will send it home with antibiotics to be given orally along with a
bland food. Even though the pup might be ravenous, feed the food in
small amounts frequently. It should be rechecked within a week to
make sure it is putting on weight and thriving. Only then will we
continue (or in some cases begin) its routine vaccination series.
Since the pup is potentially contagious to other dogs, it is well
advised to keep it away from other animals for at least 30 days since
it can still spread the virus.
Pups that have recovered from parvo do not get the
disease later in life. They can completely recover and lead a normal
life.
Prevention
As with all infectious
diseases, minimizing exposure from infected animals is the most effective means
of prevention. Since infected dogs shed large amounts of virus in their stool,
contamination is always a possibility. The virus is quite resistant in the environment,
especially in public areas that are not disinfected. This is a good reason to
keep your pup away from these areas until it is older, worm free, and had its
full series of dog vaccines.
Since this disease occurs mostly in puppies,
worms
(internal parasites) and poor nutrition add susceptibility. Puppies
should be wormed frequently until they are 3 months old.
Any dog you already
have in the household before you exposed it to a parvo dog you recently brought
in (whether it died or recovered from the parvo) should be current on its vaccines
and should have minimal exposure, if possible, to the contaminated areas. It
is rare for an adult dog that is current on its parvo vaccine (yearly boosters)
to get parvo.
If you had a dog die of parvo we recommend
thorough cleaning with diluted bleach (1:30 with water, or 4 ounces
of Clorox in a gallon of water) and waiting 1-2 months before
introducing a new dog to the area. Spray the yard as best as possible
with a hose and keep new dogs away from the area for 1-2 months.
Never put bleach on your dog.
Vaccines are highly effective. Ideally, we should
vaccinate pups every 2 weeks starting at 6 weeks of age and lasting
until 5 months of age. This is not realistic for most people though.
Fortunately, parvo vaccines given at 8, 12, and 16 weeks of age are
highly effective. Puppies should not be exposed to other dogs or the
feces of other dogs until the vaccine series is complete.
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