Visiting Veterinarian

Tickle-ish problem

By Michele Gerhard Jasny V.M.D. - August 17, 2006

What is the only thing two veterinarians can agree on? This question was posed by Dr. Adam Birkenheuer at a recent conference in Hyannis. Dr. Birkenheuer is an internal medicine specialist on the faculty of the College of Veterinary Medicine of North Carolina State University. I was attending his lectures on infectious diseases in my ongoing quest to look like I know what I'm talking about when faced with Tickle, the bluetick hound, with tick-borne disease. Twenty years ago, things seemed less complicated. There was only one veterinary clinic on the Vineyard and, despite Dr. Birkenheuer's query, all three docs working there agreed. Dogs with high fevers and sudden onset of lethargy, anorexia, and hind-end stiffness probably had Rocky Mountain Spotted Fever. Dogs with low-grade fevers and a single hot swollen joint probably had Lyme. Ehrlichia? Babesia? Bartonella? We had heard of them, but did not consider them to be common risks to Island pets. Testing for these diseases, even for Lyme, could be difficult and expensive, and many owners opted just to have us treat their pets empirically. Since we rarely confirmed our diagnoses, we went blithely along assuming we were correct. We all agreed, but in many cases we were probably all wrong. Today, these other tick-borne diseases are taking on new notoriety among Islanders. Used to be everyone we knew had Lyme, right? Now it is becoming more and more common to hear of friends with ehrlichiosis or babesiosis. The same phenomenon is occurring in the animal population. Are these diseases becoming more prevalent? Or are we just diagnosing them more frequently? Should you have Tickle tested? If so, what tests? And then, what do those tests mean?

Erlichiosis 101

Let's talk about ehrlichiosis. There are a number of related bacteria that have different names but cause similar clinical symptoms including Ehrlichia, Anaplasma, and Neorickettsia. (Scientists seem to be discovering new ones or renaming old ones almost weekly.) These bugs infect many different animals including people, horses, dogs, and cats. The one that most commonly causes disease in Vineyard dogs used to be called Ehrlichia equi but scientists recently changed the name to Anaplasma phagocytophilum. Officially that changes the name of the disease from ehrlichiosis to anaplasmosis...unless Tickle got infected down south with Ehrlichia canis. Then it's still ehrlichiosis. (Luckily for horse vets, it's just called Potomac Horse Fever, regardless of changes in nomenclature of the organism). Just remember that if your vet says Tickle has anaplasmosis or ehrlichiosis, it's the same thing for all practical purposes. Except one. Although symptoms and treatment may be identical, diagnostic tests are different.

For years now, veterinarians have used a test called the 3DX that screens for Ehrlichia canis (as well as Lyme and heartworm). That's great for vets in areas where E. canis is common. Not so for Islanders. We need a test for Anaplasma phagocytophilum (the bacteria formerly known as E. equi.). Well, we're in luck. A new test has just come out called the 4DX which screens for both. Great! Except for one problem. I call it the Blue Dot Dilemma. Stay with me now. If you've never seen one of these gadgets, the test is a little white rectangular box with a window. You mix a few drops of Tickle's blood with a special reagent, pour it into the box, activate the test by "snapping" it, wait eight minutes, and examine the result window. If one blue dot appears, the positive control, that shows the test worked correctly. If you see a second blue dot at the ehrlichia, anaplasma, or Lyme location in the window, you then know...hmmmm...you then know what?

What you actually know is that Tickle has antibodies in his blood to that organism. This means he has been exposed to the bug. It does not necessarily mean he is currently infected. Now if Tickle comes into my office showing symptoms consistent with these diseases - including lethargy, fever, lameness, poor appetite, bleeding (especially bloody noses), weight loss, vomiting, and/or diarrhea - I'm gonna take a positive result very seriously. But what if Tickle looks completely healthy and I am just running the test as a routine screening? This is where the Blue Dot Dilemma arises. Does Tickle have anaplasmosis (or ehrlichiosis, or Lyme) or does he just have antibodies to the organism? Do we treat the blue dot? Or do we treat only dogs who show obvious disease? We have been struggling with the Lyme Blue Dot Dilemma for years. Part of the difficulty is that, unlike more clear-cut diseases like Rocky Mountain Spotted Fever where Tickle is either better or dead in a month, Lyme, ehrlichiosis, and anaplasmosis may cause sub-clinical and chronic infections. In other words, Tickle may be truly infected and not show any symptoms for a while... a long while... and then suddenly get very sick. Other tests such as a complete blood count may reveal hidden signs, such as a low platelet count, but since the symptoms in these diseases tend to wax and wane, ancillary tests may be normal even in a truly infected dog.

No clear answer

The most definitive test currently available is the polymerase chain reaction test (PCR) which looks for an actual piece of the disease-causing organism (called an antigen) in Tickle's blood. It's a pretty sure thing that if you've got anaplasma antigen in your blood, you've got anaplasmosis. The test isn't perfect. You can still miss an occasional case. It's pricey and it takes a long time to get results. The biggest glitch is this. Say Tickle comes into my office with some very vague signs. We get a blue dot on our anaplasma test. We're not sure this is the problem, but we decide to put him on antibiotics and see if he gets better. After a week, he's not better and we decide we need to know for sure if he has anaplasmosis or if we should be looking for other explanations. We run a PCR, right? Wrong. It's too late. The antibiotics we gave may have cleared the organism from Tickle's blood, resulting in a negative PCR, even if he originally did have an anaplasma infection. But then wouldn't Tickle be all better? Not necessarily. The response to these infections is highly variable from individual to individual. Symptoms are caused directly by the organism, but also by Tickle's own immune system's response. That's why clearing the organism doesn't always clear the illness.

So what is the only thing two veterinarians can agree on? That the third veterinarian is wrong. Okay, it's a joke.... But to paraphrase Dr. Birkenheuer, when it comes to the Blue Dot Dilemma and the diagnosis and treatment of many tick-borne diseases, we're still in the realm of "we have no earthly idea...." Because we are in a geographic area with a proven high incidence of anaplasmosis and Lyme disease, I prefer to err on the side of over-treating, and I'm starting to save blood samples drawn prior to giving Tickle antibiotics in case we want to do PCR testing later. Would those two other hypothetical veterinarians agree? You'd have to ask them.