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Saturday, May 25, 2013

By Michelle Gerhard Jasny V.M.D.
Published: June 18, 2009

Keaton, a four-year-old golden retriever, arrived at my office one spring day for a sudden onset of lethargy and lack of appetite. The only abnormality on physical examination was a whopping fever of 104. Twenty-five years ago, these cases were easy. "Your dog has Rocky Mountain Spotted Fever," I would pronounce, then equivocate slightly: "Either that, or Lyme disease."

The explanations were not too time-consuming, treatment not too expensive. We had no in-house tests for tick-borne diseases and virtually everyone declined send-out tests to veterinary reference labs. I would give a few injections, then send owners home with a vial of pills, assuring them their pups would be right as rain in a few days - and most were.

Nowadays, we know so much more about tick-borne diseases and have a wider array of diagnostic options, which is great except all these medical advances are making my job way complicated.

"Your dog probably has one of the tick-borne diseases," I told Keaton's owner. "Could be Lyme, Anaplasmosis, or Rocky Mountain Spotted Fever." I flipped back through Keaton's record.

Bear with me. This is where it gets confusing. For eight years now, we have had the 3DX test we can run quickly right in the office. It screens for antibodies to Borrelia burgdorferi, the organism that causes Lyme, to Ehrlichia canis, which we rarely see on the Vineyard except in dogs that contracted it elsewhere, and to heartworm. A positive test does not necessarily mean a dog is currently infected. It means he has been exposed to the organism and his body has responded by making antibodies. The test is not affected by vaccination nor does it indicate immunity against reinfection. Keaton first tested 3DX Lyme positive three years ago, at which time we treated with antibiotics.

Not unusually, Keaton continued to test positive in subsequent years. His owner followed up with the Lyme Quantitative C6 titer, a send-out test that became available in 2005, which helps differentiate between exposure and infection. Keaton's QC6 supported the conclusion that he had persistent antibodies, not active Lyme infection.

But wait. There's more. In 2007, the 4DX test became available. It screens for antibodies to one additional tick-borne disease caused by Anaplasma phagocytophilum, an organism that used to be called Ehrlichia equi, until scientists reclassified and renamed it a few years back. Keaton first tested Anaplasma positive during a routine screen at his 2008 annual physical. His diligent owner once again approved further tests, this time one called an Ehrlichia PCR, which confirmed that Keaton, like many other Vineyard dogs, had persistent antibodies, but was not currently infected - which brings us to his visit in May 2009, when he is one sick puppy.

"It's probably Rocky Mountain Spotted Fever," I began, "or it could be Lyme, or Anaplasmosis. We'll do the 4DX test right here, but since he's been testing positive for years, that may not help much."

I mixed three drops of blood with the proper reagent, poured it into the little plastic apparatus, then snapped it shut to activate. In a few minutes, we would look for blue dots in the result window. The 4DX works much like reading a home pregnancy test. Theoretically it is strictly qualitative, not quantitative. In other words, any color change at all is considered positive, whether it is a faint pale dot or a dark, intense color. Now a human pregnancy test is also qualitative. Either you are pregnant or you are not. But the higher the levels of pregnancy hormones in an expectant mother's urine, the darker the result may appear in the window. I find the depth of the blue color on the 4DX sometimes correlates with antibody levels, and that truly infected dogs often have darker spots. Although not scientifically documented, I still use this observation as a small piece of the diagnostic puzzle. When Keaton's 4DX showed the exact same faint blue dots he had years past, I found it hard to believe these indicated current, active infections.

"Positive for Lyme and Anaplasma antibodies," I announced. "But that's no big surprise. I still think it's Rocky Mountain Spotted Fever. The treatments are all similar but if we want definitive diagnosis, we need more tests."

Keaton's dad readily agreed, but there was one more thing to clarify. Different diseases have different amounts of time between infection, onset of clinical signs, and antibody production. For example, with Lyme, an animal almost invariably produces antibodies by the time symptoms occur. If your dog is sick and the 4DX is negative, it ain't Lyme. Not so with Rocky Mountain. A dog can get sick so fast after infection that he arrives at the veterinarian's before his immune system is making antibodies.

"Sometimes the initial Rocky Mountain test is negative," I explained. "To know for sure, we might need a convalescent titer in two to three weeks. The good news is that, because antibodies to Rocky Mountain Spotted Fever don't persist for very long, a high titer in a sick dog is pretty definitive for infection."

I gave Keaton an injection to bring his fever down, another of intravenous antibiotics, and sent him home with a vial of pills. Over time, lab results trickled in. Lyme Quantitative C6: very low. I called the owner. It wasn't Lyme disease. PCR tests: negative. I called the owner. It wasn't Anaplasmosis. First Rocky Mountain titer: negative.

Sigh. I called the owner.

Three weeks later, this dedicated dad brought Keaton back for a convalescent titer. A few days later we had our answer. The convalescent titer was sky high. It was Rocky Mountain Spotted Fever. I was happy to have a definitive diagnosis, and that Keaton was already feeling right as rain - but 25 years ago, these cases sure seemed a lot easier.