Multiple legs on frogs, reckless behaviour in fish, black ants turning red. These are just a few of the effects of the microscopic organisms that are present in almost every ecosystem on earth. Parasites are organisms that take advantage of a host organism in order to carry out its life cycle. Parasites have complex life cycles, usually involving more than one host. These parasites can be harmful or even lethal to their hosts, as the intermediate hosts must be eaten for the parasite to be passed on to the definitive (final) host. In the case of some frogs, the parasite Ribeiroia ondatrae can cause mutations that make the frog more susceptible to predation. Frogs infected with this parasite suffered from extra, missing or malformed hind legs. These malformations caused a reduction in the distance the frogs were able to jump and also reduced their endurance and ability to swim quickly. It is therefore more difficult for these frogs to escape from predators. As well, a study done by Goodman and Johnson in 2011 showed that frogs infected with the R. ondatrae parasite are more likely to spend time in open field environments, leaving themselves open to predators such as garter snakes and birds. As a result, frogs infected with these parasites rarely lived to adulthood.
Now don’t go thinking that these frogs are completely defenseless against these parasites, evolution has equipped all organisms with the ability to fight them. There are two main “battlefronts” when it comes to resisting a disease. Resistance is the ability of the host to avoid being infected by the parasite. If the host is unlucky enough to become infected, there are multiple ways that the host can prevent the parasite from hurting it. These include immune responses, in which white blood cells and antigens storm to the site of the infection to destroy it, but the organism can fight the infection in other ways, such as increasing its body temperature in order to kill off the parasite.
Like humans with the common cold, the ability of a frog to resist parasites changes throughout its lifetime. Frogs exposed to the R. ondatrae as eggs or as adults do not experience the serious limb malformations seen in frogs infected as tadpoles. A recent study found that frogs are most susceptible to the parasites in the pre-limb or early limb growth phase. This study determined that the frog’s resistance remained fairly constant through its entire life, and that the changes in frog’s susceptibility to the parasites were a result of changes in the frog’s tolerance. Like us humans, frogs need time for their immune systems to develop in order to properly fight the parasite. As well, parasites have a more serious effect on smaller frogs, so frogs that have been growing for longer have a better chance of surviving the infection.
Understanding these sorts of interactions is more important than ever with the threat of climate change looming. Warming of the environment could cause these parasites to attack the frogs sooner, when they are more susceptible to infection, causing severe malformations that leave the frogs open to predation. This would result in mass mortality in the frog population. Most parasites are harmful to their hosts. With the current changes to the climate, malformations and behavioural changes could become commonplace. It’s a fact that parasites exist in almost every ecosystem on earth. Although you can’t see them, parasites are everywhere, and they can have a disastrous effect on the world around them.
