1 Egtved Virus the Susceptibility of Brown Trout

8/13/2019 1 Egtved Virus the Susceptibility of Brown Trout

1/5

Egtved Virus The Susceptibility of Brown Troutand Rainbow Trout to Eight Virus Isolatesand the Significance of the Findingsfor the V S ControlP. E. v J ~ R G N S N I

On the basis of limited experimental evidence Rasmussen, 1965; J>rgensen, 1974;de Kinkelin et al., 1974) brown trout have generally been considered to be almostrefractory to natural infection with Egtved virus. Nevertheless VHS in a few rare caseshas been observed in brown trout under trout farm conditions. The author has isolatedEgtved virus twice from such cases, once 1969) from brown trout of Italian origin,once 1972) from brown trout in a Danish trout farm J>rgensen, unpublished results).Recently also de Kinkelin and Le Berre 1977) isolated Egtved virus from brown trout.The latter isolate, designated strain 23/75, was found to be pathogenic to brown troutas well as to rainbow trout in bath infection experiments.

The present experiments were carried out to make clear whether the pathogenicityof strain 23/75 to brown trout was a unique feature of that strain or whether otherEgtved virus isolates were also brown-trout-pathogenic when tested by means of bathinfection.

A total of eight virus isolates was examined, among others the reference strain ofeach of the three preliminarily identified serotypes, strain F 1 serotype 1), strain Heserotype 2; J>rgensen, 1972, 1974), and strain 23/75 serotype 3; Le Berre et ai.,1977).

aterial and MethodsGroups of 20 brown trout average body weight 6 g) and of 50 rainbow trout averagebody weight 7 g) were exposed to virus concentrations of approximately Ix1 S pfu perml ofwater for one hour. Negative controls received cell culture medium instead of virus.The aquaria were supplied with running tap water at a temperature of 9 _10C. Theobservation period was six weeks, during which all dead fish were examined for thepresence of VHS symptoms.

ResultsThe results of the experiments are shown in Table 1.

Among the eight virus isolates three were found to be pathogenic to brown troutas well as to rainbow trout. The remaining five were pathogenic only to rainbow trout.1 State Veterinary Serum Laboratory, Department for Jutland, a n g ~ v e j 2 8200 Arhus N, Denmark

W. Ahne (ed.),Fish Diseases

Springer-Verlag Berlin Heidelberg 1980


2/5

4 P.E.V. ].prgensenTable 1. Mortality in rainbow and brown trout after bath infection with 8 different strains ofEgtved virusVirus Mortality in Mortality in Sera- 50 pnt. Origin of Virus inoculumstrain rainbow trout brown trout type against virus strain produced in( ) ( ) anti-Fl aFl 10 0 1 1,500 Rainbow trout FHM cellsHe b 6 0 2 500 Rainbow trout -23/75 6 58 3 300 Brown troutJedsted 68 0 ? 1,200 Rainbow trout -Ans.p 74 79 ? 600 Brown troutN.543 96 0 ? 600 Rainbow trout -Bidstrup 62 0 ? 600 Rainbow trout -No S8 32 ? 300 Rainbow trout -Controls 0 0a SO plaque neutralization titerb Previously designated isolate 61 J.prgensen, 1972)

Strain n s ~ which caused the highest mortality in brown trout, was, like strain23/75, originally isolated from brown trout J ~ r g e n s e n , 1972, unpublished results),whereas strain No was isolated from rainbow trout.

The symptoms of the brown trout which died during the experiments Were clearlythose of VHS, and Egtved virus was readily isolated from affected individual fish. Serological identification of the reisolated virus strains was carried out by means of indirectimmunofluorescence utilizing a rabbit antiserum to strain F 1 J ~ r g e n s e n , 1974).

In 50 plaque neutralization tests all eight isolates were neutralized by a rabbitantiserum to strain F1 Table 1).

DiscussionThe results clearly show that brown trout are susceptible to some Egtved virus strainsunder the conditions of bath infection at a high virus concentration. Brown trout werenot found to be susceptible to strain He as reported by de Kinkelin and Le Berre1977). The reason for this discrepancy may be differences in the virus passage levels

or virus cultivation techniques.The above findings add an interesting detail to the knowledge of the epizootiology

of VHS. In spite of the pronounced difference between the virus titer in the water during the bath infection experiments 105 pfu per ml) and during a natural infection lessthan 1 TCIDs per ml) J ~ r g e n s e n , 1974) it must probably be expected that browntrout can become VHS-infected in streams receiving the outlet from VHS-infectedtrout farms at least under conditions of severe stress. t thus appears that brown troutmay make up a potential virus reservoir in the streams.When the severity of the above fmdings with regard to VHS control in trout farmsis estimated it has also to be considered that the wild fish population of many perhaps


3/5

Egtved Virus: The Susceptibility of Brown Trout and Rainbow Troutmost) European rivers includes a large number of rainbow trout, which have either es-caped from trout farms or been stocked into the rivers for fishery purposes.Since rainbow trout are more susceptible to VHS than brown trout it must probab-

5

ly be expected that the quantitatively most important virus reservoir exists in the wildrainbow trout.

n addition to rainbow and brown trout, pike may also playa role as carriers ofEgtved virus. At least this must be expected on the basis of a recent report by Meierand J ~ r g e n s e n (1979) of a severe spontaneous outbreak of virologically verified VHSin pike fry in Switzerland. The report in question represents the first demonstrationof VHS in nonsalmonid fish, and it may justify the feeling that more species of fishmay in time turn out to be susceptible to the disease.

n rivers with several VHS-infected trout farms situated next to each other alongthe stream and with free movement of wild fish up and down the stream it is likely,considering the susceptibility of several species of fish to the virus, that a smaller orlarger percentage of the wild fish are, at least periodically, harboring the virus. Againstthat background it might appear necessary to remove the wild fish population beforeprevention of VHS in the trout farms is attempted, since persistence of the infection inthe wild fish might lead to reinfection of sanitized trout farms.

n Denmark control of VHS in trout farms has been practiced for more than 15years without measures being taken to remove the wild fish populations J ~ r g e n s e n ,1974). The basis for this policy was the recognition that it would be very difficult orimpossible to obtain permission for such steps from government authorities and private owners and that it would be extremely difficult to remove all wild fish from astream, even if permission was obtained.The principle of the VHS control program is that all VHS-infected farms along astream, starting at the top of the stream, are emptied, disinfected, kept dry for one

A B C D E F GA.

1968 0 + + + + +B 1969 0 E+ E+ + + + +1970 0 E E + + + +C 1971 0 0 0 E E E E1972 0 0 0 0 0 0 01973 0 0 0 0 0 0 0D 1974 0 0 0 0 0 0 01975 0 0 0 0 0 0 0E. 1976 0 0 0 0 0 0 01977 0 0 0 0 0 0 0F 1978 0 0 0 0 0 0 01979 0 0 0 0 0 0 0G

+ VHS infection; stamping out procedure; 0 indicates freedom from VHS;wo signs indicate two events in one year

Fig. 1 Development of the VHS control program in 7 trout farms A-G) at the river Lille A(The figure is based on information obtained from Dr N P Kehlet, Veterinary Services, Copen-hagen)


4/5

6 P.E.V. ~ r g e n s e n

A B C D E F G H1966 + + + + + + + + +1967 E + E + + + + + +1968 0 E 0 E + + + + +1969 0 0 0 0 + + + + +1970 + 0 0 + + + + + +1971 E 0 0 E + + + + +1972 0 + +E + + + + + +1973 0 E 0 E E E E E E1974 0 0 0 0 0 0 0 0 01975 0 0 0 0 0 0 0 0 01976 0 0 0 0 0 0 0 0 01977 0 0 0 0 0 0 0 0 01978 0 0 0 0 0 0 0 0 01979 0 0 0 0 0 0 0 0 0+ indicates VHS infection; indicates stamping outprocedure; 0 indicates freedom from VHS; wo signs in-dicate two events in one year

Fig. 2. Development of the VHS control program in 9 trout farms (A-I ) at the river SimestedA (The figure s based on information obtained from Dr. N. P. Kehlet, Veterinary Services, Co-penhagen)

month, and then repopulated with fish from VHS-free farms. As shown in Figs. 1and there are cases in which this policy has been very successful, since it has led sofar to freedom from VHS in all the farms involved for periods of 8 and 6 years respectively.

I t is not known whether in those cases the wild fish in the streams were actuallycarriers of Egtved virus or not. f they were it might be suspected that the duration ofthe virus carrier state was short, since the VHS eradication program was successful.Probably the removal of the common sources of virus, i.e., the densely populatedVHS-infected trout farms, caused a drop in the virus concentration in the streams as aside effect of the sanitation. Perhaps the virus concentration reached a level at whichnew infection of wild fish did not occur and at which already infected fish were ableto clear themselves of the infection.

Since there is no experimental evidence available which supports the above hypothesis it is put forward only as a working theory. In other streams the VHS control program has been less successful, since reinfection has sometimes occurred shortly afterthe repopulation of the farms. It is not clear at present to what extent this has beendue to persistence of the infection in the wild fish and to what extent to other sourcesof virus such as closely situated VHS-infected farms from which virus may be transferred by animals, personnel, trucks, and so on.

It would be very valuable to have experiments initiated which could help inestimating the relative significance of each of those two fundamentally differentcauses of VHS reinfection in sanitized trout farms.


5/5

Egtved Virus: The Susceptibility of Brown Trout and Rainbow Trout

eferences

Berre M Le, Kinkelin P de, Metzger A (1977) Identification serologique des Rhabdovirus des sal-monides. Proc 2nd Copraq Meet (FAO/EIFAC - OlE), Brest 1977, pp 390 393

7

Jrgensen PEV (1972) Egtved virus: Antigenic variation in 76 virus isolates examined in neutralization tests and by means of the fluorescent antibody technique. In: Mawdsley-Thomas (ed)Symp Zool Soc London No 30, Diseases of fish. Academic Press, London New York,pp 333 340

jrgensen PEV (1974) A study of viral diseases n Danish rainbow trout, their diagnosis and control Thesis, pp 101, commissioned by A/S C.F. Mortensen, Biilowsvej 5c, 1870 Copenhagen VKinkelin P de, Berre M Le (1977) Isolement d un Rhabdovirus pathogene de la truite Fario (Saimotrutta). C R Acad Sci Ser D 284: 101 104

Kinkelin P de, Berre M Le, Meurillon A (1974) Septicemie hemorrhagique virale: Demonstration del etat refractaire du saumon (Oncorhynchus kisutch) et de la truite Fario (Salmo trutta). BullFr Piscic 253: 166 176

Meier W Jrgensen PEV (1979) Isolation of Egtved virus from pike fry (Esoc lucius) with hemorrhagic symptoms. Proc 3rd Copraq Meet (FAOjEIFAC - OlE), Munich 1979 in pressRasmussen CJ (1965) A biological study of the Egtved disease (INUL). Ann N Y Acad Sci 126:

427 460

Documents

1 Egtved Virus the Susceptibility of Brown Trout