Back when feeding the animals was allowed at the San Diego Zoo, a group of visiting children fed part of their sack lunches to the 50 or so spider monkeys then on exhibit.
That night, the zoo veterinarian received an urgent call from a security guard saying that all the monkeys were having severe intestinal distress, bellowing loudly as they were soiling the cage. The animal doctor rushed down to the zoo and managed to stem the outbreak by pumping an orange-flavored medicine into the cage that, once lapped up by the monkeys, relieved their discomfort.
"That was a major incident, among many, that led to our policy of no feeding of animals (in the early 1960s)," said Werner Heuschele, then the veterinarian and today the zoo's head of infectious disease research. "The end to public feeding eliminated a lot of intestinal-type illnesses among our primates, as well as intestinal blockages in other animals, from simply eating too many peanuts."
Zoo officials worldwide worry as much about diseases transmitted from human to animal as those transmitted from animal to human, known as zoonoses (pronounced ZOO-NO-SIS). The diseases transmitted from human to animal can range from intestinal upset in monkeys to encephalitis in gibbons if the animals come in contact with the human herpes simplex (cold sore) virus.
Heuschele actually frets more about the human-to-animal potential because strict quarantine and other health procedures for zoo animals can detect diseases harmful to humans.
"All zoos are careful on these things," Heuschele said, citing the herpes virus carried by macaque monkeys which is capable of causing lethal encephalitis in humans. The deadly possibility was discovered the hard way, when researchers in the 1950s, raising monkeys to obtain polio vaccine, died after handling the animals' tissues.
"The monkeys' blood is routinely tested now during quarantine to see if there are antibodies to the virus," Heuschele said. "There have been no cases of (animal-to-keeper) transmission in zoos that I know of." During a one-month quarantine, all animals are also tested for tuberculosis and several other common diseases that could infect animals as well as people.
Heuschele helped formulate guidelines for zoo keepers that have been adopted by the American Assn. of Zoological Parks and Aquariums. The major effort concerns prevention, and includes a variety of periodic medical tests to make certain that workers have not contracted illnesses.
"Keepers should show good personal hygiene," Heuschele said. "And I've always felt that the animal care personnel should not wear the same shoes and uniforms home that they wear at work."
Because the transmission of zoonoses is a two-way street, Heuschele said that keepers, in the case of uniforms, could inadvertently bring diseases to the zoo from animals kept at home, particularly because most keepers are inclined to have pets.
"The best way to break the transmission either way is to wash and practice good habits," he said.
Keepers are told not to come to work when they have a cold or cold sores to eliminate possible harmful contact with their charges. Primates are especially vulnerable to catching such diseases, he said.
"We tell them, 'For God's sakes, stay away from primates in those cases!' " In particular, researchers have discovered that gibbons are unusually predisposed to suffer encephalitis when infected with the human herpes simplex virus. If a keeper or a zoo visitor has a cold sore blister, any spit or saliva transmitted to the gibbon could result in the animal's death. That is why the zoo stresses its "no feeding" policy so strongly with signs.
San Diego's mild year-round weather aids the zoo's prevention efforts, Heuschele said. Because the animals can be kept outside, illnesses such as flu do not spread as easily as they do in zoos where animals are kept indoors and breathe the same air as humans walking through to view them.
"For the same reason, we've had less tuberculosis (in our animals) than other zoos because of the climate," Heuschele said. Tuberculosis was a major scourge of zoos as late as the 1950s, he said, with "monkeys dying like flies" in many zoos in the eastern United States and Europe.
Traditionally, animals imported from foreign countries were quarantined to detect diseases that might affect domestic animals used for food production, such as cattle and swine. Only gradually did zoos adopt quarantines for protection of their own collections and for their workers.
"In the past, a zoo might have taken the health of their own animals lightly," Heuschele said, explaining why zoos allowed feeding by the public for so many years. "There was a feeling that, as far as the animal was concerned, we could always go back into the wild and get another . . . the (ability to go back to the wild) just isn't there anymore."
The zoo's feeding policy is now scientific. For example, members of the cat family--tigers, pumas and the like--are no longer fed an exclusive raw meat diet because it weakened their bone structure by upsetting the calcium balance. Older cats in certain zoos are often bow-legged because of that feeding practice.
Pet owners should treat their animals the same way, Heuschele said. "The quality of food to your animals should be the same as the quality you would want for your own," he said. "Otherwise, you are putting your animal in potential jeopardy (for diseases)."