Sussing out a fever of unknown origin


Just past dawn Saturday, a call from my answering service rouses me. Coal, the cat, is not moving. His owners are afraid he has been poisoned. I shake myself awake, collecting my thoughts.

Being finicky, cats are less prone to eating strange substances than dogs, so true cases of toxicity are infrequent in felines. There are always worrisome items like antifreeze or lilies, but, more often than not, what a cat owner thinks is poisoning is something more common, like a fever.

Sure enough, when Coal arrived at my office he had a temperature of 104.5, well over the normal 101-102.5. I examined him. No wounds. No abscesses. Heart and lungs sounded fine, though his breathing was rapid and shallow. His tail twitched when I squeezed his abdomen, but he didn’t seem like it hurt. His third eyelids were up. He was slightly dehydrated, but his gums were a healthy pink.

“It’s a UFO,” I said, my standard stupid joke. Actually, it’s an FUO — Fever of Unknown Origin. “We see a lot of these in cats. There are many possible causes.”

Fever in a previously healthy young cat is usually caused by a viral or bacterial infection. If viral, it could be one of the big, bad viruses, like Feline Leukemia or Feline Immunodeficiency virus. Or it could be a milder disease, some nonspecific virus we can’t test for but that will pass on its own with time and TLC. Bacterial infections can also range from mild to something potentially deadly, like tularemia.

“Does he hunt?” I asked. Knowing an outdoor cat’s preferred prey can help steer us in the right diagnostic direction. Cats who prefer bunnies to birds or mice have a greater risk of contracting tularemia. Coal did hunt occasionally, but they had not seen him with any rabbits.As we continued talking, I observed Coal, flat on his side on the table, eyes half closed. Not good.

Even with a fever, most cats stay at least marginally alert in strange surroundings like the vet’s office. Coal was really down and out. What’s going on? I wondered, patting him gently and gazing into his face. Tell me, little guy, what’s wrong? Coal stared back limply, and purred.

“I’m worried,” I said. “He’s so spiritless. We should run some tests.” His owners agreed.

We quickly ruled out Feline Leukemia and Immunodeficiency Viruses, then ran an in-house screen for tick-borne diseases. “This is designed specifically for dogs, ” I explained, “but I do occasionally use it for cats. It’s fast and may help us figure out what’s going on.”

Soon after, two bright blue spots appeared in the results window. Coal was positive for antibodies to the organism called Anaplasma.

Anaplasma phagocytophilum (previously known as Ehrlichia equi) is a bacterium transmitted by deer ticks. Depending on the strain, susceptible species include deer, coyotes, mountain lions, horses, llamas, people, dogs, and — rarely — cats. Clinical signs of illness usually occur one to two weeks after the tick bite. In dogs, typical symptoms are fever, lethargy, depression, anorexia, muscle pain, stiffness, lameness, and reluctance to move. Occasionally dogs may also exhibit vomiting, diarrhea, coughing, labored breathing, swollen lymph nodes, and/or central nervous system signs such as incoordination or seizures. There are few documented cases of anaplasmosis in cats but symptoms reported in kitties are similar — primarily fever, lethargy, depression, and anorexia.

Coal certainly fit that picture, but our positive in-hospital test only meant he had been exposed to the organism and made antibodies. It didn’t necessarily confirm a current infection. Definitive diagnosis of anaplasmosis can be made in two ways. The bacteria infect white blood cells, forming little microcolonies called morulae.

Seeing these morulae on a blood smear under the microscope is one way to confirm diagnosis. I made slides of Coal’s blood, stained them, and searched in vain for morulae, but the fact that I couldn’t find them didn’t rule out anaplasmosis. Even in confirmed cases, morulae are only rarely found in feline blood. The second way to confirm would be to send out blood for a moderately expensive test called polymerase chain reaction (PCR) assay. But Coal was too sick to wait for results. He needed treatment now.

The drug of choice for many tick-borne diseases, including anaplasmosis, is doxycycline. For cats, this presents a dilemma as they can have a quirky reaction to this antibiotic when it is given in tablet or capsule form — inflammation of the esophagus. This may lead to a permanent stricture that can interfere with normal passage of food from mouth to stomach. To avoid this problem, we ordered a liquid doxy preparation from a compounding pharmacy, and to cover our bases for other infections, such as tularemia, prescribed an additional antibiotic called enrofloxacin. I sent him home that first night, hoping I had correctly communed with Coal and interpreted the clues he was giving us.

Returning several days later for a recheck, his mom confided that she had phoned her “animal communicator” about Coal’s case. She knows I am a bit of a skeptic when it comes to things like over-the-phone divining of an animal’s medical condition, but, hey, who am I to say? People didn’t use to believe in bacteria or viruses.

“The animal communicator says Coal has a headache,” she said, then asked “Why would he have a headache?” Well, the CDC page on anaplasmosis in people lists headache as the second symptom, right after fever.

“Your animal communicator’s reading is consistent with my presumptive diagnosis,” I laughed.

Coal’s mom laughed too. “Know what she says cats call you veterinarians?” I couldn’t wait to hear. “The Smelly Humans.” I considered the appellation. I suppose it fit. We veterinarians spend our days among multiple animals, bodily fluids, disinfectants, and other odoriferous items.

Coal looked at me from where he sat on my exam table. He was clearly feeling better, no longer laid out flat as a pancake. “That’s Doctor Smelly Human to you,” I thought.

Coal just looked smug, and purred.