You may have heard about a disease called Ebola on the news, in the last few years. According to the United States Centers for Disease Control, nearly 30,000 cases occurred worldwide between 2014 and 2016. Approximately, 12,000 people died. Most of the people affected were in three West African countries: Guinea, Liberia, and Sierra Leone. Chimpanzees are vulnerable to Ebola too. Just like in humans, the disease spreads quickly among chimps after a breakout. Even natural barriers like rivers can't seem to stop it, and it's also extremely deadly. More than 75 percent of all chimpanzees infected will die from the disease. It's not surprising then that Ebola has killed many chimps in several large outbreak events over the last 30 or so years. In fact, scientists from the Jane Goodall Institute estimate that, since the 1990s, about one-third of all chimpanzees and gorillas have been killed by Ebola. Luckily, there has never been an Ebola outbreak among the Gombe chimpanzees. In fact, the CDC has no record of Ebola cases in Tanzania in chimps or in people. That doesn't mean that the Gombe chimps are safe from all disease risks. Here's a graph of the known causes of death in Gombe. On the x-axis is the cause of death for those deaths that are known: disease, aggression, injury, orphaning, maternal disability, and poaching. On the y-axis is the percent of known deaths with this cause. If all chimpanzees died of a particular cause, that bar would reach 100 percent and the other bars will all remain at zero. Here we see that disease is the greatest cause of death at Gombe, and in fact, accounts for more than half of all known deaths. Other causes account for much fewer deaths. Here we've lumped disease into a single category, but we've observed a variety of different illnesses at Gombe. Polio or poliomyelitis is a disease caused by the polio virus. It's typically only found in humans, but because chimpanzees and humans are so closely related, polio can pass between us. In some cases, people infected with polio have no symptoms at all or only very minor ones. But if the virus is able to infect the central nervous system, the spinal cord, and brain, it can cause paralysis. This is what happened in the Gombe chimpanzees in 1966. Six individuals died or disappeared during the epidemic. Six more survived the disease but were permanently disabled. Faben, this young male lost use of his arm. All the survivors learned to compensate for the paralysis and were able to get around on their remaining functional limbs. One of the more shocking disease outbreaks in Gombe came in 1997, when 19 individuals develop Sarcoptic mange. This skin disease is caused by parasitic mites that burrow into the skin, causing hair loss and an itchy scabby allergic reaction. Mange is highly contagious since the tiny parasites can be spread through physical contact, like social grooming in chimpanzees. When mange struck the Gombe community, it was a ghostly site. Here's a picture of Fifi during the outbreak sitting next to an unaffected individual. With her bald, flaky skin, she's hardly recognizable. But mange harmed more than just appearances. When Fifi made a full recovery, her infant died during the outbreak along with two others. The polio and mange epidemics were devastating and very hard for researchers to watch. But the most deadly diseases in Gombe are less dramatic, respiratory infections. These are just like the colds and flus we humans experience, with symptoms like chest congestion and runny noses, like Gremlin when this picture was taken. In fact, chimpanzees are highly susceptible to respiratory infections carried by humans, and the Gombe chimpanzees are exposed to researchers and tourists nearly every day. Of course, we take precautions to protect them from our pathogens. Researchers observe a period of quarantine upon arriving in the park staying out of the forest for a week in order to make sure that they're healthy before spending time in proximity to the chimps. Everyone who observes the chimps, including tourists, must maintain a distance of seven meters from all individuals. In spite of these measures, over 30 chimps have died due to respiratory illness since the onset of the research project. Most of these deaths happened during four respiratory epidemics. Samples taken during an epidemic in 2000 showed that the disease was caused by bacteria and viruses that probably came from humans. Some of the victims didn't die directly from an infection, but after being orphaned or neglected by a sick mother. For example, one of Gremlin's infants died while she was very ill during the 1987 epidemic. Most recently, in the spring of 2015, two vibrant promising young adult males, Sinbad shown here and Titan, died from a respiratory illness. It was a tremendous loss for the Kasakela community. The diseases we've discussed so far can be described as acute. Though they can be deadly, they last only a short time. But there's another disease afflicting the Gombe chimpanzees, this one chronic or long-lasting. It's called Simian Immunodeficiency Virus in chimpanzees or SIVcpz. Simian means monkey. Immunodeficiency means that the virus attacks and weakens the immune system, which is supposed to protect your body from sickness and help it to heal. That name may sound familiar to you because of the closely related human immunodeficiency virus or HIV. Just like all living things, viruses evolve. Mutations arise and the genetic code changes over time. We call the new versions that evolved strains and we can make a family tree of all the different strains. This tree shows the relationships between different forms of HIV and SIV. It turns out that HIV is a cousin of SIV. Most humans infected with HIV are infected with the HIV-1 strain, which is shown at the top in purple. You can see that this virus is nested within the SIV chimpanzee strains in pink. On the other end of the diagram are HIV-2, a more rare form of the human disease, an SIV AGM and SIVMM, two strains found in a variety of African monkey species. Scientists now believe that the virus originally evolved in monkeys and later it spread to chimpanzees, probably when chimps hunted and ate infected monkeys. Research from our colleague Dr. Beatrice Hahn, showed that SIV then jumped from chimpanzees into humans becoming HIV. This probably occurred through bushmeat or human hunting and eating of chimpanzees as meat. SIV is present in many monkey species and seems not to cause too much harm to its hosts. This is not the case in humans where HIV can develop over time into the deadly AIDS, Acquired Immune Deficiency Syndrome. For many years, researchers believed that chimpanzees were more like monkeys than humans when it came to SIVcpz. They didn't think it had many real health effects and the SIV positive chimps would live just as long as those without the virus. But many years of data from Gombe revealed a different story. It's true that some SIV-positive chimps live long healthy lives, showing few or no symptoms of the virus. But overall infected individuals are more likely to die younger than uninfected individuals. The disease reduces the fertility of infected females. They were less likely to give birth and their infants were less likely to survive. In a few cases, we have been able to examine the bodies of individuals after they die. Those that were positive for SIV showed signs of organ failure and other symptoms similar to those seen in human AIDS patients. This leads us to believe that chimpanzees do suffer severe health consequences from SIVcpz. This juvenile was SIV-positive and went into a dramatic decline. Here he is just a few days before his death. Notice how skinny and unkempt he was. He was so weak that he couldn't groom himself nor keep up with the others to be socially groomed. As a result, his fur and mouth became encrusted with fruit. In Gombe, we do everything we can to keep the chimpanzees healthy. We take precautions to avoid passing our own germs to them and we even occasionally provide medicine to sick chimps, especially when it seems that humans are responsible for their health problems. We hope to have a healthy, thriving population of chimps for many years to come.
