K. Molnar - Veterinary Medical Research Institute, Hungarian Academy of Sciences (Budapest, Hungary)
Reports on fish diseases transmitted by birds were received from a total of 11 countries: United Kingdom, Ireland, France, Federal Republic of Germany, the Netherlands, Belgium, Denmark, Finland, Poland, Romania and Hungary, but other reports contained some data giving a total of 17 which contained data that could be evaluated.
Pond Fish Culture
All reports emphasize that birds cause significant damage by spreading diseases in pond farms. There are well substantiated data on the role of birds in transmitting parasitic fish diseases in pond farms. Observations confirm their role in the transmission of: Viral Haemorhagic Septicaemis (VHS), Infectious Pancreatic Necrosis (IPN), Spring Viremia of Carp (SVC), and bacterial diseases from one place to another. Of the parasitic diseases, the trematode Diplostomum spathaceum and the cestode Ligula intestinalis are the most important. The adults of Diplostomum live in the intestines of gulls, whereas their metacercariae occupy the eye of fishes. This parasite is the causative agent of lens cataracts of fish in numerous European countries. The larvae of Ligula intestinalis, a helminth colonizing the intestine of aquatic birds, cause significant damage to pond farm fishes in certain countries. Besides these two parasites which are spread all over Europe, other trematode metacercariae, e.g., the larvae of the black spot disease agent Posthodiplostomum cuticola, might give rise to problems in fish ponds. This disease is transmitted by herons. Infections caused by Diphyllobothrium dendriticum and D. norvegicum were reported from Finland and the Netherlands and Valipora sp. (Cestoda) spread by herons causes infection among pond farm fishes in Poland.
The stress caused by aquatic birds can be considerable. In ponds overrun by birds fish must hide, do not feed regularly and consequently suffer retardation in growth. In winter ponds devoid of ice cover, grebes, diving ducks and goosanders cause significant damage and disturb the resting period for fish. Gulls might also do damage in these ponds by inflicting wounds with their beak on fish coming near to the surface or to the inflow because of other diseases or problems, such as, lack of oxygen. These wounds aggravate the primary disorder.
The most significant damage is done by gulls and terns during fishing out of the ponds. At such times these birds lift out numerous fish from the overcrowded fish population; since they are unable to consume the relatively large fish, they drop them back into the water repeatedly. The beak-inflicted skin injuries can give rise to fungal and bacterial complications and in Hungary and the Netherlands up to 100% of carp bear evidence of such damage.
Trout Culture
In trout culture bird-transmitted fish diseases have lesser importance. Lens cataract develop as a result of diplostomosis in most cases. In the affected farms, the fish population is protected by scaring away the birds or by covering the ponds with nets. In these farms, birds can cause damage primarily by introducing viral diseases.
Open Waters
In open waters, the major damage by birds is due to their role in spreading diplostomosis and ligulosis. Diplostomosis is spread by gulls (Laridae), whereas ligulosis is spread primarily by herons (Ardeidae). Black spot disease caused by Posthodiplostomum cuticola (Metacercariae) is spread by herons, the eye disease caused by Tylodelphis clacata is spread by grebes (Podicipinae), and Apophallus muehlingi and Metagonimus yokogawai metacercariases transmitted by gulls and terns are widespread in natural waters. Trout and coregonids in northern countries may develop severe infections by larvae of the cestodes Diphyllobothrium dendriticum and D. norvegium. Wounds inflicted by birds and the subsequently developing mycotic complications are frequently observed also on fish living in natural waters.
Additional information on this subject is given in Part C: Fish Diseases section.
