Abstract
The epidemiology of bovine herpesvirus 1 (BHV1) infection in Norfolk
was studied by scrutinizing records of submissions to Norwich
Veterinary Investigation Centre and undertaking a cross-sectional
analysis of serological findings from breeding herds in a disease
monitoring scheme. Longitudinal studies examined the feasibility of
eliminating BHV1 infection from individual herds by a test-and-cull
programme and of maintaining seronegative status by employing
security measures including serological screening of replacements.
Alternative strategies were adopted in two heavily infected herds. A
review of the appropriate literature and a description of the Norfolk
cattle industry are also presented.
Thirteen incidents of BHV1 fetopathy and 83 outbreaks of systemic
infectious bovine rhinotracheitis (IBR), including 13 clinically mild
low morbidity recrudescences in dairy herds, were confirmed between
1980 and 1989. These affected about 11 per cent of dairy herds and
five per cent of suckler herds. Outbreaks occurred mainly during the
winter months in rearing/fattening herds but throughout the year
amongst adult breeding cattle. BHV1 antibodies were present in 510
(26.7 per cent) of 1908 sera and 122 (44.5 per cent) of 274 herds
tested. There were no significant differences in antibody prevalence
between herd types or between systemic illness and fetopathy
submission categories but fewer sera from apparently healthy cattle
were seropositive (Pc.001). Antibody prevalence increased
significantly with age after three years.
BHV1 antibodies were detected in the sera of 639 (15.1 per cent)
cattle from 4219 aged at least two years in 56 scheme herds with no
history of clinical IBR or IBR vaccination; almost 50 per cent of
cattle aged 10 years or more were seropositive. Antibody prevalence
amongst purchased cattle was much greater than in homebred cattle
(Pc.001 in dairy herds, P<.05 in suckler herds). Reactors were
present in 40 of the 56 herds: in seven herds more than 40 per cent
of adult cattle were seropositive. In these high prevalence (HP)
herds most cows seroconverted to BHV1 within two years of first
calving whereas there was virtually no evidence of active infection
in the 33 low prevalence (LP) herds. Maternally derived antibodies
waned by about six months and young stock in infected herds
subsequently remained seronegative provided they were kept apart from
the cows and external sources of infection.
BHV1 infection was readily eliminated from LP herds; serological
freedom was retained during surveillance periods of up to four years
by strict adherence to disease security rules. The repeated use of
inactivated BHV1 vaccine to reduce virus shedding appeared to prevent
further spread within an HP suckler herd; combined with partial
segregation it was also used in the successful phased removal of
reactors from an HP dairy herd within 30 months.
