One of the fun things about working in a zoo is that each day brings something new and different; we never know what we will be working on from one day to the next. Most of our work in the Wildlife Disease Laboratories, San Diego Zoo Institute for Conservation Research, involves examining animal tissues and fluids for disease surveillance, diagnostics, and research designed to improve the health and well-being of the animals in our care. But every once in a while we have to solve a mystery about strange things found in our animal exhibits. Animal keepers are very vigilant in monitoring their animals and exhibit environments. If some unidentified thing found in an exhibit might be an indication of a disease problem in an animal, or a health threat to an animal, it is our job to identify the object and determine its significance.
Some time ago we had a five-year-old, pregnant lion-tailed macaque at the Zoo that had been a good mother to her previous two offspring; this time she delivered an infant that was found cold and weak shortly after birth. The veterinarians were concerned that this might be evidence of an infection in the baby, and possibly the mother as well. One of the best ways to diagnose a neonatal infection is to examine the placenta, so the keeper wisely set about scouring the enclosure and eventually found something that was the right color and consistency to be a piece of placenta. It was submitted to us for an urgent examination. We quickly processed it for microscopic examination and began our investigation.
Although the sample looked like a piece of placenta to the naked eye, it didn’t look anything like a placenta under the microscope (see photo at right). If it wasn’t a placenta, what was it?
The first thing to notice is the presence of thin pink outlines surrounding clear spaces. Within these clear spaces are translucent objects with dark centers.
When we see strange translucent structures like this, one quick evaluation tool we often turn to is examination under polarized light. Some objects transmit polarized light while others block it, which gives us information about the structural nature of the material.
This photo at left shows the translucent structures illuminated with polarized light. You can see they glow pink and blue. This helps me confirm my suspicion: this is not a piece of placenta but a piece of sweet potato! The thin, pink outlines are the plant cell walls and the small translucent objects are starch granules (the things that make sweet potatoes rich in carbohydrates). Sweet potatoes are a normal part of the diet of these animals, so this sample represented a small piece of food that had been lost in the exhibit and had become desiccated just enough to make it difficult to identify.
In the end, no placenta was ever found, but the story had a happy ending, as the infant quickly recovered and was successfully reared by his mother. Although our investigation in this case took an unexpected turn and ended up not being as useful as we had hoped in guiding the immediate treatment of the infant, in other cases it does. Stay tuned for our next installment, when we reveal how you can tell whether something that looks like vomit really is. Until then, I can’t wait to see what diagnostic challenges we’ll face tomorrow.
Bruce Rideout is the director of the Wildlife Disease Laboratories, San Diego Zoo Institute for Conservation Research.



































