Diplostomum spathaceum Effects on Fish Behavior: Adaptive or Pathogenic?

            Many parasites adapt the ability to manipulate the behavior of their host in order to increase transmission rates. However, these behavior manipulations may be mistaken as pathogenic consequences. This phenomenon can be observed in Diplostomum spathaceum, a digenean trematode that infects the lens of freshwater fish, the intermediate host, or host in which the parasite does not sexually reproduce, resulting in blindness. The parasite is transmitted when infected fish are consumed by birds, the definitive host of D. spathaceum, the host in which the parasite sexually reproduces. Research has suggested that due to D. spathaceum, infected fish have difficulty feeding, reduced shoaling, the grouping together of fish for protection from predators, impaired crypsis, the ability to blend in with the environment to avoid predators, and limited escape behavior. These behavior alterations may increase the predation of infected fish. Further experiments have indicated that predation to the intermediate host may be increased to only birds, therefore increasing transmission. On the other hand, ocular immune privilege, the weakened immune response that resides in the eye, allows D. spathaceum to survive in their intermediate hosts for longer periods of time. Because of this, it is difficult to determine whether the resulting vision impairment in fish is a pathogenic consequence and increased transmission is a beneficial coincidence for D. spathaceum, or if it is an adaption of the parasite.