Visiting Veterinarian : Bugs in the system
By Michelle Gerhard Jasny V.M.D.
Published: May 22, 2008
If you're a long-time Island resident, you undoubtedly know people who have had Lyme disease. You may have had it yourself. More than once. You've probably heard of someone who had babesiosis, tularemia, even Rocky Mountain Spotted Fever. Ehrlichia infection is the new kid on the block, but lately more and more clients nod their heads in recognition when I talk about this tick-borne disease.
Before we begin our discussion, there is a small matter of nomenclature to review, so bear with me. In the beginning was the genus Ehrlichia. Remember high school biology? Closely related organisms are grouped together by genus. For example, your Airedale Annie is in the genus Canis, along with coyotes, wolves, and jackals. The genus Ehrlichia initially lumped together a bunch of bacteria that cause a bunch of diseases. Potomac Horse Fever. Salmon Poisoning Disease (which doesn't make fish sick, but infects dogs via ingestion of raw salmon). Tick fever of small ruminants. Human granulocytic ehrlichiosis. But as genetic evaluation became more sophisticated, scientists reclassified these organisms into four separate groups.
Today we will focus on the bug now called Anaplasma phagocytophilum, formerly know as Ehrlichia equi. Why? Because it can cause a disease called canine anaplasmosis which may be confused clinically with Lyme or other tick-borne infections.
Imagine Annie the Airedale gets bitten by a tick carrying Anaplasma - most likely a deer tick. That tick probably needs to be attached for 24 hours to transmit the disease, but we all know even the most vigilant owners can overlook a teensy tick on a big, fuzzy pooch. And even the best flea and tick products aren't 100 percent effective. So Annie gets infected. Now what? Acute signs vary. She may have fever, loss of appetite, swollen glands, and joint pain resulting in lethargy and reluctance to move. This may last a few days or more. Less commonly reported signs include coughing, labored breathing, vomiting, diarrhea, even seizures. Annie looks miserable. She hasn't eaten in days and refuses to chase the tennis ball, an unheard of turn of events. You bring her in to see me. Her temperature is 104 F. I give her a good look-see and say, "I can't find any source for the fever. Let's run some tests."
We start with the 4DX Snap test right at the office. It looks for antibodies to Lyme disease, heartworm, Ehrlichia canis (a disease we don't often see here on the Vineyard), and our new friend, Anaplasma phagocytophilum. It takes less than ten minutes, so we sit and chat while waiting to see what develops. (Don't get me started on the Democratic primary). Oh good! Annie is negative for heartworm, Lyme, and E.canis. But wait! A blue spot is forming at the top quadrant of the test window. Aha! Positive for Anaplasma! So now we know Annie has anaplasmosis, right? Wrong. A positive test only tells us that Annie's body has produced antibodies to that organism. It doesn't necessarily indicate current infection. Dogs who have been exposed may have antibodies for years. In acute illness, we must correlate 4DX test results with symptoms. Sure, Annie could have anaplasmosis, but it could also be Rocky Mountain Spotted Fever, or a number of other diseases.
At this point in the process, you and your veterinarian should talk through all the possible scenarios. There is a confirmatory test called an Ehrlichia PCR that is very accurate in identifying true infections. It's a bit pricey and may take weeks to get results, but it is the gold standard. Faster tests such as a complete blood count (CBC) can give your veterinarian additional information about Annie's condition, and in a few cases, can provide definitive diagnosis if Anaplasma organisms are actually seen inside the blood cells.
Another important issue to consider is that cross-infection with more than one tick-borne disease is an increasingly common phenomenon. Those nasty ticks can pack a lot of pathology in their tiny bloodsucking bodies. Annie may have more than one of these diseases simultaneously. A complete "tick panel" can help pin down the specifics.
"I think we can make a presumptive diagnosis of tick-borne disease," I may say, reaching for a vial of antibiotics. Treatment is fairly standard: 5 to 10 mg/kg of doxycycline twice daily for a month. The bigger Annie is, the more she needs. For example, I give a 44-pound border collie 100 mg twice daily. An 88-pound Chesapeake Bay retriever gets twice that. Some veterinarians dose even higher.
Interestingly, many human physicians seem to use a one-size-fits-all doxycycline dose with their adult patients. A petite 100-pound dancer may receive the same dose as a hefty 250-pound wrestler. Hence I feel more confident when I occasionally scale back the dose a bit on those huge mastiff-style dogs, in the interest of minimizing side effects.
Doxycycline can cause an upset stomach in some individuals, so it is best to give it with food. Calcium and iron bind doxycycline, preventing proper absorption, so the medication should not be given simultaneously with antacids, like Tums, or with iron-containing vitamins. What about dairy? Some studies have shown that taking doxycycline with milk products does not significantly alter absorption, but there is still debate on the issue. It's simple enough to give Annie foods other than dairy with her pills.
This all sounds straightforward, but sometimes the initial signs of Anaplasma infection are so mild you don't notice. What then? Annie may self-cure, like when we get over a cold or the flu without treatment. But there is also evidence that chronic, subclinical infections may occur and eventually lead to serious illness.
What do you do when a hale and hearty Annie has her annual physical and tests positive for Anaplasma on a routine 4DX screening? There is controversy over the clinical significance of positive tests in asymptomatic dogs. As I often tell owners, only half-joking, we don't really know what we're talking about when it comes to tick-borne diseases. My philosophy is based on the following facts: 1.) We live in an area with a much higher-than-average incidence of these diseases. 2.) To date, it appears that the vast majority of dogs without clinical symptoms who test positive for Anaplasma antibodies are not truly infected. They just have persistent antibodies. 3.) I have seen more than one case where an asymptomatic dog turned out to have a real Anaplasma infection that warranted treatment. If your budget allows, run as many tests as your veterinarian advises. We are doing the most thorough job we can with the current state of knowledge. Here in tick-laden paradise, it may be better to be safe than sorry.
