Parasite induced behavioural changes in fish

Numerous studies have been conducted on the hypothesis of parasites influencing the behaviour of their hosts. Many authors have been bias in trying to prove that this is a form of manipulation and that parasites increase their transmission success by changing host behaviour. Not much research has been conducted in Africa regarding parasite induced behaviour. During the present study fish were collected from the Okavango and Orange-Vaal River Systems, respectively situated in Botswana and South Africa. Within these ecologically diverse systems a wide variety of fish were found to be infected with diplostomatid metacercariae occurring inside their eyes and / or brains. These digenetic trematodes are known to cause blindness and other pathological effects, known as diplostomiasis, especially in fish reared in aquaculture. Diplostomatids have a three-host life cycle which involves different species of snails, fishes and piscivorous birds. Fish act as the second intermediate hosts and need to be eaten by a piscivorous bird in order for the diplostomatids to be trophically transmitted and the life cycle to be completed. Seven different metacercarial types and cysts were identified and some were collected from both the eyes and brains of infected fish species. The results signified a moderate prevalence of these parasites amongst the different fish species, whilst the intensity of infection proved to be low. No marked altered pathology was observed in infected fish brains but noticeable histopathological changes were noted within eyes infected with diplostomatid cysts. To determine the effect these infections have on the behaviour of natural populations of fish, two behavioural experiments were conducted. The results indicate that infected fish show similar behaviour than uninfected fish and therefore the null hypothesis (Ho) is accepted.