By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
A puppy with a bloody diarrhea could have a parasite problem, a virus other than parvovirus, stress colitis, an intestinal foreign body, or may simply have eaten something that disagreed with him. It is important to confirm the diagnosis of parvovirus before embarking on what could be the wrong treatment.
The Fecal Parvo ELISA Test
The ELISA test has become the most common test for parvovirus in puppies. ELISA stands for Enzyme Linked ImmunoSorbent Assay. This sounds complicated and high tech but is actually the same type of technology used in home pregnancy test kits. The parvo ELISA test is available as a kit and is performed in the vet’s office in about 15 minutes or less. There are many different brands and testing is sensitive in its ability to detect virus in stool.
The test has some limitations that are important to realize. Recent vaccination with a live vaccine (the type of vaccine that is most effective) may interfere with the test results. This means that the test may detect the live virus from the vaccine and show a positive reading when, in fact, the puppy does not have a parvo infection. Classically, this interference occurs 5 to 12 days after vaccination so if a positive fecal ELISA test is obtained within this period after vaccination, additional tests may be recommended.
Further, the puppy could be infected but no longer shedding virus in its stool. Or the virus particles may be so thoroughly coated with antibodies that they cannot react with the chemicals of the test. In almost all cases, one can trust a negative ELISA reading but it is important to realize that no test is perfect.
The Drop in White Cell Count
Recall that one of the first actions of the canine parvovirus is to inhibit white blood cell division in the bone marrow. The virus essentially turns the immune system off before making its deadly way to the GI tract. This is a feature of parvovirus infection in all species, which means that a characteristic drop in white blood cell count is seen on a blood panel when the infection is active and real. If there is any question about the veracity of an ELISA test result, the presence or absence of a white cell count drop can be used to clarify the situation but it should be mentioned that the positive ELISA test often precedes the white cell drop by a couple of days. White blood cell count is commonly monitored through the treatment of infection. The white blood cell numbers bottom out during the brunt of infection and recovery of the white blood cell numbers heralds recovery.
Polymerase chain reaction (PCR) testing is probably the newest method of testing for parvo. A fecal sample must be submitted to a reference laboratory for PCR testing; this is not the sort of test that can be done while you wait. The DNA of the parvovirus is detected and amplified so as to be able to detect even small amounts of it. PCR testing is generally too sensitive to be clinically helpful yet canine parvovirus is commonly included on PCR panels that test for multiple organisms. The problem with PCR testing is that, because it amplifies DNA, very small amounts of virus can be detected which means that positive results will turn up on dogs that are vaccinated or who are passing insignificant amounts of virus. PCR testing is very helpful for many infections but often clouds the issue for parvovirus.
Photo courtesy of CDC Public Health Image Library.
There are two types of antibody titers that can be run: IgG and IgM. With the advent of ELISA testing, titers are not frequently used in making this diagnosis any more. The IgG titer is a more long-lasting antibody level. A high IgG titer would probably indicate active infection in a puppy that is old enough to generate antibodies and who has not yet received any vaccinations. Most of the time, the IgG titer simply reflects antibodies generated by vaccination. The IgM titer reflects recent antibody production so if a vaccinated puppy had not been vaccinated recently, a high IgM titer might indicate active infection.
Because parvovirus infected puppies have frequently received vaccinations in their recent past and are frequently too young to generate their own antibodies (which is how vaccinated puppies get infected in the first place), these test results are difficult to interpret. It is easy to see why the ELISA test that directly detects the virus has become so popular.
Titering is mostly used nowadays to determine if a dog has adequate protection against parvo, either through vaccination or prior exposure. In a shelter setting, a dog of unknown vaccination status can be tested to determine if he is considered protected from infection or not. In the pet setting, a dog can be tested to determine if a vaccine is needed or not. Certain antibody levels are associated with protection; however, because of other contributing branches of the immune system, an inadequate titer does not necessarily mean a dog is vulnerable.
Parvovirus lesions in the GI tract are of a classical appearance. There is no mistaking them under the microscope. Unfortunately, tissue samples of the GI tract are not readily available and most infected puppies are not good surgical candidates. Still, if a puppy has died and the cause is unclear, submitting samples of the GI tract can generally confirm or rule out a parvovirus diagnosis provided the tissue has not degenerated.
It is also important to realize that puppies with parvovirus infection may have concurrent intestinal parasites or may have swallowed foreign bodies.
There will be complicating conditions that must be monitored. This means that other tests will be required during the management of the parvo patient. The above tests are simply those that can be used to confirm the parvovirus diagnosis.