Although it is often believed that parasites are nuisances that cause illness and disease, such as malaria, it has been observed that they can also cause other changes within their hosts. Within recent studies, it has been noticed that the parasite, Ribeiroia ondatrae, has caused both morphological (physical) and behavioural changes in pacific tree frogs, Pseudacris regilla, which live in many standing bodies of water within the United States. Specifically, this parasite needs three hosts to complete its life cycle in which the frog is an ideal choice. The relationship between the frog and Ribeiroia ondatrae is parasitic, with the parasite benefiting at the expense of the frog.

        First, to begin its life cycle the parasite reproduces within freshwater snails which in turn release free swimming specialized worms which have suckers to penetrate into a host, such as frogs, and therefore transmit the parasite. It was shown in a study by Johnson and colleagues that frogs exposed to the parasite in early stages of life, such as tadpoles in their pre-limb and early-limb development stages, had a higher probability of being infected with the parasite. However, they found that there was a significant decrease in the chance of infection in later developmental stages. Often, limb malformations such as webbing, missing limbs and digits, extra limbs or feet, indicated that the frogs were infected with the parasite. Infection in early developmental stages often led to frogs with missing limbs, while infection in later developmental stages resulted in frogs with extra limbs or webbing.

       Due to the differences in the malformations which arise depending on when the frogs are infected with the parasite, it is said that there is a ‘critical window’ for infection. If the frogs are infected early in their development, the parasite has a higher chance of moving through its life cycle, and therefore it is the optimal, or critical, time for infection. However, if this critical window was missed, Johnson and his colleagues determined that the parasites had little effect on the frogs. Overall, it was concluded that host age and development stage influenced both the frogs’ susceptibility to the parasite as well as the diseases progression.              

Within a study done by Goodman and Johnson, it was found that over 50% of the frog population had been infected with the parasite within some lake systems, and often were more easily captured by predators. Frogs which suffered malformations were found unable to jump as far or fast as uninfected frogs, with their endurance reduced by 75%. Also, when timed, infected frogs had an average swimming speed 32% lower than that of uninfected frogs. Therefore, due to reduced performance in endurance, swimming speed, as well as reaction time and jump distance, infected frogs were more easily captured by predators. In general, their malformations reduced their performance and survival, causing infected frogs to have a 22% lower biweekly survival rate then uninfected frogs.

Through the parasites ability to reduce the frog’s chances of escape from predators, it therefore has an increased chance of moving through its life cycle. It was established in a study by Johnson and Goodman that birds are the optimal host for the parasites development, and that they rely on movement to spot their prey. They suggested that frogs with malformations have movement patterns which may allow birds to spot them more easily when hunting. Therefore, through the change in morphology which the parasite creates, there is also a change in behaviour, allowing them to be more easily predated and pass the parasite to its next host. Also, compared to uninfected frogs which are commonly found in bulrush and cattail, infected frogs were found to remain in more open areas such a bare earth habitats with small leaf litter, which provide less protection from predators. Thus, in addition to the infected frogs having decreased performance, as well a preference for open habitats allowing them to be more easily spotted by predators, they were therefore more likely to be predated upon by birds, enabling the parasite to move through its life cycle.

Overall, it was found that the frogs infected with Ribeiroia ondatrae faced both morphological and behavioural changes resulting in them being more easily predated upon, especially with regards to avian mammals such as birds. The parasite begins its life cycle within snails and is transmitted to frogs through the use of specialized worms. Through decreasing the frog’s performance as well as altering its habitat preference, the parasite makes frogs easy targets for predators, enabling it to complete its life cycle at the expense of the frog. Although one may think intuitively that a frog with more legs would find it easier to escape from predators, these studies have proven conclusively that more is not always better. 

Written by: Esme Batten, Beverly Allan, Shend Binxhiu, Olga Aksentyeva, and Daniela Azman