Cleaning up a Messy Diagnosis
Disease treatments usually work best when veterinarians know what disease they are treating; specific treatments require a specific diagnosis. At the San Diego Zoo Global Wildlife Disease Laboratories, we provide diagnostic support to our veterinarians. Using special diagnostic tools and expertise, we can help deliver a specific diagnosis in difficult cases.
But rendering a specific diagnosis can be a challenge when a disease only causes vague, non-specific signs. It’s even worse when the patients can’t talk and don’t complain. I love to complain when I’m sick, but for wild animals, complaining only draws the attention of predators. Even if you are a cheetah and a predator yourself, you don’t want to draw the attention of larger predators. As a result, wild animals will generally mask all signs of illness as long as they can, which makes it more difficult to identify and characterize the signs of disease.
One of the problematic diseases of cheetahs is caused by a feline herpesvirus. It usually starts with mild, non-specific upper respiratory tract signs, such as sneezing or a runny nose, but a number of relatively innocuous things can also cause these types of clinical signs. We can use specific diagnostic tests to distinguish cheetah herpesvirus infections from the more innocuous things, but they are not widely used because they are expensive and require capturing and restraining an animal in order to get the necessary samples.
Capture and restraint is always a risky business, both for the animals and for the veterinarians. Since most mild upper respiratory tract diseases will resolve spontaneously, diagnostic testing is seldom pursued—it is just too risky. However, cheetah herpesvirus infections may not resolve spontaneously. For reasons that are not entirely clear, some individuals will progress to severe disease, such as pneumonia or eye infections that can damage the cornea. In addition, infected mothers can spread the disease to their cubs through licking, and the disease is often more severe in cubs.
So, there is our challenge – how can we help clinical veterinarians reach a more rapid and specific diagnosis of cheetah herpesvirus infections so they can be treated, without forcing them to capture every animal with mild upper respiratory tract signs for sample collection? The answer is to call in your epidemiologist.
Epidemiologists specialize in identifying population-level patterns of disease, which is just what we need here. One of the clinical veterinarians at the Safari Park, Dr. Nadine Lamberski, recruited our epidemiologist in the Wildlife Disease Laboratories, Carmel Witte, to tackle this problem. Carmel then enlisted the help of a half-dozen other zoos with large cheetah populations to generate the large data sets she would need for her analysis. The idea was to see whether the nature, pattern, and duration of clinical signs in cheetahs with confirmed herpesvirus infections could lead to a diagnostic map that would allow a clinical veterinarian to make a diagnosis without having to capture animals for diagnostic sample collection.
All of the participating zoos sent us their medical records for cheetahs and Carmel began the arduous task of reviewing and compiling the medical data for 549 animals. Placed in a single pile, the stack of records would have been nearly 12 feet tall! The goal was to extract and summarize any information that could be related to a herpesvirus infection, including what clinical signs were present, how long they lasted, what medicine was used, and whether any diagnostic tests were performed that confirmed the presence of the herpesvirus.
Next, we wanted to determine whether particular clinical signs grouped together in cheetahs with confirmed herpesvirus infections, to see whether they resulted in an identifiable clinical syndrome distinguishable from the more innocuous diseases. Using an analytical technique called “multiple correspondence analysis” we were able to characterize a clinical syndrome in confirmed animals, and correspondingly identify 58 additional cheetahs that had the same syndrome, but lacked the confirmatory diagnostic tests.
These epidemiologic methods allowed us to use animal health data that spanned 19 years to generate a more specific definition of the clinical syndrome associated with herpesvirus infections in cheetahs. This will now allow veterinarians to more reliably diagnose this disease without animal capture and restraint, simply by carefully documenting the constellation of clinical signs presented by a cheetah.
This work has recently been published in the Journal of the Zoo and Wildlife Medicine, so now veterinarians in zoos and conservation programs around the world can also use this information to more quickly diagnose and treat cheetah herpesvirus infections. New diagnostic tools such as this help us maintain the health and well-being of our animals by allowing more rapid treatment targeted to the individuals that need it most. Even if our animals are trying their best to hide the signs of their illness, we now have the tools we need to outsmart them.
Bruce A. Rideout, D.V.M., Ph.D., D.A.C.V.P.
Director, San Diego Zoo Global Wildlife Disease Laboratories