When the symptoms of two infections mirror one another, misdiagnosis may occur.
A 16-week-old kitten shelter kitten develops symptoms consistent FIP, but no evidence of coronavirus infection is found. Instead, her cause of death was determined to be a parasitic infection which caused inflammation and necrosis in her brain.
This case study comes down to a game of cat and mouse. It starts when a seven-week-old kitten was surrendered to a shelter along with her mother and her littermates. She was underweight and infested with fleas and ticks. She was dewormed and vaccinated and sent into foster care. At nine-weeks-old she was returned to the shelter for spay and was adopted along with one of her siblings. After about one week in her new home, her adoptive family reported that she was less active than her littermate, cried often, wanted to be held and was weaker than her sibling. She was also inappropriately urinating and defecating around the home. On exam she was found to have lost a significant amount of weight (half a pound) since adoption. She had a low-grade fever and ulcers in her mouth. She was presumptively diagnosed with an upper respiratory infection. She received fluid support and was monitored in the shelter until she recovered. At this time, she was sent out for foster and continued monitoring. Her foster parent brought her to the shelter every two weeks for check-ups, she was gaining weight as expected, and no problems were reported in her new home.
A little more than a month into her stay in foster care, she was returned to the shelter where it became clear she was unwell. She had a fever of 103.8F and was minimally responsive to interaction. Laboratory testing revealed anemia, elevated liver enzymes, and high circulating globulin with a corresponding decreased in albumin. These signs are characteristics of dry FIP, particularly when they occur in a young cat with a history of weight loss and fever. Given the severity of her disease the shelter made the decision to have her humanely euthanized.
Her tissues were tested for feline coronavirus but curiously, no coronavirus proteins or RNA could be detected. The cause of her mysterious illness was ultimately discovered in her brain where she was infected with the parasite Toxoplasma gondii. Toxoplasmosis is a single celled, microscopic, spore forming parasite that completes its life cycle in the intestines of the cat. Cats are the only host that shed toxoplasmosis in their feces. All other warm-blooded animals can develop tissue stages of the parasite. Cats are infected by eating the tissue of infected animals – often mice. The kitten in this case study may have been infected by eating prey supplied by her mother or through transplacental transmission.
In humans, transplacental infection has been documented to cause abortion, stillbirth, brain damage, eye damage, and blindness. Even with treatment, the disease can be fatal if the tissue damage is severe. Thankfully, most infected individuals (cats and humans) show no symptoms. This is true for transplacental infections as well – most children are asymptomatic and, in this case, none of the kitten’s littermates were reported to be affected.
Despite good clinical insight, appropriate diagnostic tests are often part of the decision-making process for veterinarians – and this case highlights the importance of ruling out other conditions, such as toxoplasmosis, when making the diagnosis of FIP. Antibody titers are often used to diagnose toxoplasmosis and treatment involves a 4–6-week course of anti-parasitic medication. Unfortunately, not all cats will respond to treatment, particularly when inflammation is severe enough to cause neurologic signs, such as the mental dullness reported in this case. Good, readily accessible diagnostic tests will always help with tough clinical decision-making.