Chances are, when you bring in your dog for their annual check-up, you’ve heard us say we are going to draw a few drops of blood for their yearly heartworm test. On the invoice when you check out, it is listed as “Heartworm 4Dx Test”. So what is it exactly and what do we mean?
Just as Rx is short hand for prescription in medical jargon, Dx is short for diagnosis. A 4Dx is therefore a four way diagnosis done on a blood sample for 1. Heartworm, 2. Lyme, 3. Anaplasma, and 4. Ehrlichia. These parasites and diseases are blood based and not to be confused with common intestinal parasites that are detected in a fecal sample. This test can be sent to the lab or done in-house depending on the needs of the patient.
The first disease, heartworm, is a blood borne parasite transmitted by mosquitoes. It really is a worm that matures to live in the heart and associated great vessels of the dog (and other species too but for our purposes we are just talking about dogs). It is a potentially fatal disease that can cause heart failure, lung damage, and damage to other organs. For the heartworm portion of the 4DX, it is testing for antigen which is the actual worm itself that will cause an immune reaction. So if your dog tests positive, that means the test is picking up actual worm.
The other three diseases in the 4Dx are bacterial tick-borne diseases (meaning they are spread by ticks to dogs). Lyme disease in dogs is primarily transmitted via the deer tick (Ixodes) and can cause fever, swollen painful joints, enlarged lymph nodes, and other organ failure. Ehrlichiosis is primarily transmitted via the brown dog tick and Anaplasmosis is, again, transmitted primarily via the deer tick. Both diseases create a similar clinical syndrome of fever, malaise, poor appetite, occasionally enlarged lymph nodes, and bruising due to low platelet numbers. The 4Dx differs for these three diseases from the heartworm portion in that a positive test is detecting antibodies to each bacterium in the bloodstream. Antibodies are proteins made by the immune system in response to a disease or foreign invader. So if your dog tests positive for one of these three, it is only indicating exposure to the bacteria.
What if my dog is positive? Good question but the answer to this depends on which one is positive. Remember, the heartworm portion is detecting actual worm. A positive heartworm test is worth repeating and performing other tests such as testing for microfilaria (microscopic baby worms in the blood) to ensure it is not a false positive. It is rare but can happen. If those tests confirm the positive, then your dog really does have heartworm and we will initiate staging and treatment.
If any of the other 3 is positive, we are only picking up antibodies and the only thing we can say definitively is that your dog has been exposed to the disease. The good news is that the vast majority of dogs exposed to one of these three tick borne diseases will clear it on their own and never get sick! If a dog is sick and showing clinical signs consistent with one of the diseases, and the 4Dx Snap test is positive, it is probably best to start treatment.
It is a little trickier if a dog is healthy. For Lyme positive dogs, we typically do further testing including a C6 test to see if the antibody level is significant and requires treatment and a urinalysis to check for protein that would indicate secondary renal issues caused by Lyme (Lyme nephritis). For Anaplasma or Ehrlichia positive dogs, we recommend a CBC (complete blood count) to check for low numbers of white blood cells or platelets. If these are normal, then no treatment is required. As a follow-up, after treating heartworm, a dog will eventually go back to being negative. With Lyme, Anaplasma, or Ehrlichia, the test will stay positive for years as the antibodies will always stay with the dog.
I hope this clears up any confusion about what we mean by heartworm test and 4Dx. This is a valuable test to help keep your dog happy and healthy. Most importantly, all four diseases are preventable with heartworm prevention and good quality tick control. Ask us for more info on prevention at you next annual wellness exam! We look forward to helping.
article written by:
Jessica Smith, DVM
Companion Animal Hospital Mount Prospect
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