serologic study of phocine distemper in a population of ... - BioOne

1 downloads 0 Views 886KB Size Report
Glasgow,. G61 1QH, Scotland. Scottish. Agricultural. Colleges. Veterinary. Investigation. Centre, Drummondhill,. Stratherick. Road,. Inverness, 1V2 4JZ, Scotland.
Journal

SEROLOGIC

STUDY

HARBOR

SEALS

OF PHOCINE

DISTEMPER

of

Wildlife

Diseases, © Wildlife

1992, pp. 21-27 1992

28(1), Disease

Association

IN A POPULATION

OF

IN SCOTLAND

P. M. Thompson, H. J. C. Comwell,2 H. M. Ross,3 Lighthouse Field Station, Department of Zoology, University Cromarty, IV1 1 8YJ, Scotland Department of Veterinary Pathology,

Veterinary

2

Bearsden, Glasgow, G61 1QH, Scotland Scottish Agricultural Colleges Veterinary Inverness, 1V2 4JZ, Scotland A serologic

Investigation

of

harbor

of the

University Centre,

(Phoca

seals

prevalence

of Glasgow,

Drummondhill,

Stratherick

vitulina)

from

of morbillivirus northeastern

Road,

was conducted in a where mortality was comparatively low during the 1988 phocine distemper virus outbreak. None of the 12 seals sampled before the epizootic were seropositive. Thirty-five (52%) of 68 seals sampled after the beginning of the epizootic were seropositive, although there were significant age-related differences in both the number of seropositive individuals and in antibody levels. Marking studies showed that most seropositive seals caught during the peak of the epizootic survived for several months. Thus, the low mortality observed in this population did not appear to result from a lack of contact with the ABSTRACT:

population

survey

School,

and D. Miller of Aberdeen,

antibodies Scotland,

virus.

Key

words:

serologic

Phoca

seal, study.

Harbor

field

survey,

vitulina,

Phocine

INTRODUCTION During among

1988,

a major

harbor

populations

seal in

North

to

20%

in

some

and

an

differed varying

considerably from only

northeastern Miller,

Scandinavian

on

Scotland 1991)

waters

to

in al.,

Although ondary

seals

generally

infections

pneumonia,

such

et

the

died as

primary

acute

of

of the

zootic was a previously undescribed billivirus, phocine distemper virus Kennedy

et

der, 1988). veloped for temper harbor pus)

1988; Osterhaus Virus-neutralization the closely related al.,

from

to PDV

the during

North 1988

Sea

had

(Osterhaus

been

morbillivirus taken from

serum

found different

antimor-

from

seals

samples

antibodies gray seals

in serum samples in the United King-

dom

between 1977 and 1987. If the epidemiology of the 1988 epizootic is to be understood in detail, data are required on the proportion of the population which came into contact with the virus and on the fate of those individuals

both gry-

exposed and

antibefore Nether-

in to a rehabilitation center be1984 and 1987. On the other hand, only samples from UK waters, Haret al. (1989) found no evidence of

epi-

and Vedtests decanine dis-

virus (CDV) confirmed that and gray seals (Halichoerus

morbillivirus

(1989)

Osterhaus et al. to an antigenically

wood

mor(PDV)

of

lands, bodies

sec-

bacterial

cause

prevalence

in

groenhispida)

(Dietz et al., 1989a). limited data are available populations In the

brought tween in the

1989b).

epizootic,

bodies in North Sea seal and during the epizootic.

billivirus

60%

(Dietz

the

morbillivirus,

and harp seals (Phoca ringed seals (Phoca

from Greenland However, only

L.)

killing

individuals (Dietz et al., et al., 1989). Harbor seals affected to some extent,

but mortality rates between populations, 10

vitulina Sea,

virus,

et al., 1989), landica) and

occurred

(Phoca

the

estimated 17,000 1989b; Harwood in all areas were

(Thompson

epizootic

distemper

Ved-

der, 1988; Harwood et al., 1989). In addition, morbillivirus antibodies were detected from other regions, in samples from Baikal seals (Phoca sibirica) (Likhoshway

exposed to assess mortality

to it. Such whether resulted

of contact

with

information can be used observed differences in from variations in rate the

virus,

in resistance to the virus. This paper presents data logic survey of a population 21

or of

variations

from a seroof harbor seals

22

JOURNAL

OF WILDLIFE

DISEASES,

VOL.

28, NO.

1992

1, JANUARY

dom) and diazepam (Valium, Roche Products Ltd., Wellwyn Garden City, AL7 3AY, United Kingdom) (Baker et al., 1988) or tiletamine hydrochloride and zolazepam (Zoletil, Reading, Z.A.C., 17 rue des Marronniers, 94240 L’Hayles-roses, France) (Stirling and Sjare, 1988). Standard length and girth measurements were taken while seals were on the restraining board. All seals less than 1.1 m standard length were assumed to be juveniles in their first or second

LJ

year

(Harwood

et al.,

to determine seals

the than

longer

1989).

It was

precise age 1.1 m were

not

of older therefore

possible

animals; classed

as adult. Blood

Invenss

FIGURE

1.

Location

of

harbor

seal

haul

in

the

of the out

study

area

and

position

(#{149}).

sites

Firth,

Moray

Flrth

northeastern

(57#{176}35’N; 4#{176}O’W).Mortality

Scotland in

this

popu-

lation was low (10 to 20%), with most deaths occurring during August, September and October 1988 (Thompson and Miller, 1991). A capture and marking program was started early in 1988 and blood was collected Sera

routinely from analyzed to of morbillivirus

were

prevalence seals

caught

before,

epizootic. In made on seals to assess antibody

during,

addition, marked

the fate levels.

all captured seals. assess changes in antibodies in and

observations during the

of individuals

after

the

were epizootic

with

known

METHODS

Study area and capture The

Moray

least 1,100

Firth

harbor

techniques

contains

a population

seals which

haul-out

of at regularly

on intertidal part of the

sand and mudbanks in the inner firth (Fig. 1). Seals were captured by rushing at haul out groups with an inflatable boat or fourwheel drive vehicle and catching animals individually using hoop nets. They were then transfered to a restraining board and, where

lightly

with

necessary,

sedated

hydrochloride Lambert,

(Vetalar, Parke Pontypool NP4 8YA,

either

ketamine

Davis/Warner United King-

sampling

and marking

techniques

Blood samples were taken from the epidural vein of restrained seals using either plain or heparinized (heparin hydrochloride) blood vacuum tubes (Vacutainers, Becton Dickinson, UK Ltd., Between Towns Road, Cowley, Oxford, 0X4 3LY, United Kingdom). Serum or plasma was removed within 24 hr and frozen at -20 C for 12 mo. Between April 1988 and June 1989, blood samples were obtained from 79 individual harbor seals, with samples being taken before, during and after the period when most deaths occurred (Table 1). Repeat blood samples were also obtained from two individuals in different seasons. During August and September 1988, 28 harbor seals (14 adults, 14 juveniles) were caught at haul

out

sites

in the

Inverness

and

Dornoch

Firths. All seals were marked with a numbered tag in each hind flipper and twenty four individuals also were marked with a large colored number or letter (approximately 20 cm high) on their back (Thompson, 1989). Very high frequency (VHF) radio-transmitters were glued (Fedak et al., 1983) to the hair on the heads of six adult seals from the Inverness Firth. Throughout

1988

and

1989,

observations

were

made at known haul out sites at least twice each month. Haul out groups were counted using a 30 x 70 telescope and the presence of any colormarked individuals was noted. Flipper tags could not be read using a telescope but were used for the identification of carcasses or individuals that were recaptured after the color-marks had been lost in the moult. Radio-tagged individuals were located daily by triangulation during November 1988 and January 1989, and their presence and activity around haul out sites was monitored using automatic recording equipment (Thompson and Miller, 1990). Virus

neutralization

test

Neutralization tests were carried out in microtitration plates by a method based on that described by Appel and Robson (1973). Four-

THOMPSON

TABLE

after

1. Age a phocine

and sex distemper

of

harbor

seals

caught

and

AL.-PHOCINE

E

bled

in the

Moray

DISTEMPER

Firth

IN HARBOR

1988-1989,

before,

Pre-epizootic

April

Male

1988

epizootic

Aug-Sept

Post-epizootic

Feb-May

1989

Female

1

9

4

10

15

21

20

fold dilutions of test serum, from 1:16 to 1:16,384, were made up with graduated pipettes. The Onderstepoort-Bussell Strain (A. E. Churchhill,

of

UK

Intervet,

Ltd.,

Science

Park,

Cambridge CB4 4FP, United Kingdom) of canine distemper virus (CDV) was diluted to 80320 tissue culture infectious doses 50% (TCID5O) per ml of growth medium with 5% fetal bovine serum (FBS) and 1 ml of this added to an equal volume of each serum dilution. The serum-virus mixtures were left in the dark at room temperature (18-20 C) for 1 hr and at 4 C for a further 1.25 hr to allow neutralization to proceed. Each mixture (0.2 ml/well) was then inoculated into four wells of Vero cell monolayers in a 96 well microtiter plate (Nunclon, Gibco Ltd., P.O. Box 35, Trident House, Paisley, PA3 4EF, Scotland). These cultures had been prepared the previous day by seeding the wells with a suspension containing 200,000 cells per ml, 0.2 ml per well. Growth medium consisted of medium 199 with 25 mM Hepes buffer and L-Glutamine and 10%

FBS. incubated atmosphere containing 3% CO2. Each well was then examined microscopically and the proportion of wells showing the characteristic cytopathogenic effect was recorded for each serum dilution. Titers were expressed as the reciprocal of the serum dilution which reduced the proportion of wells infected from 100% to 50%. All samples with titers 90 were assumed to be seropositive. Serum and virus controls were included on each occasion that the test was carried out. Following

at

inoculation,

37 C for

plates

were

in a humidified

3 days

Male

Unknown

2 1988

Total

formerly

during

and

Adult

Juvenile

During

23

epizootic.

period

Sampling

SEALS

5

-

1

1

Thirty-five

(52%)

10

23

16

of

= 5.25, 1988 and Furthermore,

seropositive

higher

(

adults

than

adult

titer

2.69,

=

those

33

significant

samples

spring

1989

taken

2.41,

=

were

log10

in autumn P

samtiters

juveniles titer

There in

0.05)

significantly

juvenile

(t-test,

af-

P < 1989 log10

seropositive

differences

tween

1 df, spring the

P

Survival of marked seals with known antibody titers Eleven

color-marked

negative.

Three

seals

of these

were

sero-

individuals

were

found dead 4 to 6 wk after capture and another three were seen alive and apparently healthy after 21 to 30 wk (Table 2).

Log

Antibody

Titer

3.

2

12

2

21

2

RESULTS 12

Changes

in the prevalence

41

and

3

1

2

level of antibodies Antibody

all low, ples first

titers

ranging

taken deaths

quently body

after due had

(Fig.

before from 8

much 2).

August to PDV higher

the

epizootic to 32. Blood 1988, were levels

when seen, of

MAMJJ

were samthe freanti-

A

SOND

JFMA

1988

FIGURE

free-living

2.

temper

epizootic

(August

to September

(February

1989

Serum

harbor

to May

M

morbillivirus seals

(April

from

before

1988), 1988),

1989).

antibody the

during and

after

titers

phocine the

epizootic

the

epizootic

in

dis-

24

JOURNAL

OF

DISEASES,

WiLDLIFE

VOL. 28, NO. 1, JANUARY

% Positive Ad

Ad

71%

1992

One

juvenile

after signs

3 wk. It displayed of PDV infection

63%

1989b)

36%

L

tive

been died after was

0 Outbreak

3.

seropositive epizootic.

Poet-Outbreak

88

Aug-Sept

FIGURE

Age

Feb-May

differences

seals caught sizes are

Sample

89

in the proportion and

during presented

after

of

the

in Table

its titer

was none (see

remained

found of the Dietz at

Two of thirteen color-marked seals were recovered dead,

after

During

(32038)

8.

sick classic et al., After

treatment with antibiotics it recovered and was released to the wild. The remaining seals were either not subsequently seen or were seen alive only soon after marking.

Juv

50

and

seal

1988 1.

marking.

An adult

seroposi1 and 3 wk

female

which

had

radio-tagged (32041) probably also because her radio-signal disappeared less than 1 wk. This disappearance preceded by several days of unusual

behavior when she spent almost all her time either on the shore or at the surface. A further six individuals were observed alive between 13 and 44 wk, well beyond the period when deaths due to PDV were

TABLE 2. 1988 when

Fate most

of seals with and without morbillivirus seal deaths were seen in the Moray

antibodies

Antibody

Seal Seals

number

with

no

antibodies

against

M

32

8.29.88

32032

A

M

32

9.2.88

32035

J

32036

16