prevalence of OPPVi
was determined in a 1991 study by Simard
and Morley (source 4) involving 14 047 sheep sourced from each of
provinces. National seroprevalence was
determined to be 18.8% +/- 0.3%, with a mean flock prevalence of 11.7%
(Graph 1). It
was also found that of 286 flocks, 62.9% of flocks had at least one
sheep. Of the sheep tested, 13 719 were female and had a seroprevalence
18.9%, while 14.5% of 560 male sheep were seropositive.
Other than in this study, there
has not been data reported in the literature to support a difference in
between the sexes. The percent seropositives, the percentage of flocks
least one seropositive sheep, and the mean flock prevalence all varied
considerably amongst provinces.
found by Simard and Morely, that
within OPPV infected flocks, the majority of infected sheep
seroresponse by 24-36 months of age. The
sheep with the highest prevalence at the time of collection were those
and 7 years (Graph 2).
authors explained the age specific
increase in prevalence up to six years of age as being related to the
length of exposure to horizontal transmission and the delay of
following infection. A decrease in
prevalence after eight years of age was noted and was suggested to be
result of culling and the death of sheep having clinical OPPV
infection earlier in life.
of the vertical transmission of OPPV, it was found that seropositive
a 39% probability of producing seropositive progeny compared to the 20%
probability for seronegative ewes.
During the first five months, virus can be isolated from ewes’
there is a 28% probability of the lambs becoming infected following
hours of contact time between the dam and the offspring. Droplets from
respiratory tract of infected animals can be a source of horizontal
transmission of infection for all ages of sheep.
to the literature, there may be a breed susceptibility component to
with OPPV. Higher prevalence of OPPV
has been found with one half Finnsheep crosses, while the Ile de France
certain strains of Icelandic sheep, and crosses between Icelandic and
Leicester breeds appear to be resistant.
In another study, Border Leicester sheep were significantly more
susceptible than Columbia
sheep. It has been suspected that
difference in the susceptibility of different breeds of sheep may be
with flock exposure and management practices (Simard). In
the study by Simard,
there appears to be a breed susceptibility, although the actual
of each breed would be a significant variable (Graph
Simard and Morely found that
seroprevalence of OPPV appeared to
increase with increasing flock size, which they suggested was a
larger flocks having been exposed to an extended period of horizontal
transmission due to their longer period of establishment.
J. R., Menzies P. I., Waltner-Toews D., et al collected sera from
sheep from 103 flocks in Ontario
in a 1994 study to determine OPPV prevalence in that province. The average number of ewes sampled per flock
was 37. The study found the
seroprevalence of OPPV within the province to be 20.9%, with 69.9% of
sampled having at least one seropositive sheep.
found by Campbell
positively associated with higher seroprevalence rates of OPPV
the number of years the owner had
practice of using foster ewes
the practice of allowing lambs older
day of age to have contact with other ewes that are lambing
transmission is highlighted by the positive association of OPPV
with average flock age. The greater the
number of years a farmer has had sheep means the flock has been in
for a longer period of time, increasing the potential for infection of
flock and the spread of OPPV within the flock.
Use of foster ewes supports the infection of lambs via ingestion
contaminated milk by allowing more lambs to come in contact with
seropositive ewes. It could be that flocks
that have a higher
prevalence of fostering do so because there is a higher incidence of
mastitis in the seropositive ewes.
A 2003 study of
culled ewes in Quebec
by Arsenault J., Girard C., Dubreuil P.,
et al found the seroprevalence of OPPV in Quebec
to be 44%, with older animals and ewes having higher odds of
the seropositive animals, 14 % and 40% had OPPV suggestive
lung and mammary gland lesions respectively.
It was found that overall 1% of seropositive sheep had both
microscopic lung lesions and that only three animals had arthritis on
examination. All three ewes with arthritis were > 3 years of
In terms of the
histological lung lesions, the prevalence in seropositive animals was
higher among sheep ≥4 years old or having a thin body condition score.
prevalence of moderate to marked, chronic, lymphocytic mastis, as
defined by the
authors, was not higher among ewes ≥4 years old, thin-ewes or ewes
histologic OPPV lung lesions. The author suggested that sheep sampled
study were generally too young when culled for there to be a detectable
increase in the prevalence or severity of lesions.
The level of mammary gland activity also could
not be associated with either the ewe serological status or with
induration in this study.
higher odds of having other lung diseases in the study.
It was speculated this may be the result of
higher animal density or poor ventilation in the flock of origin. Another speculation was that seropositivity
to OPPV may have increased the susceptibility to other lung
even before development of detectable OPPV lung lesions.
results suggest that there is no detrimental impact of OPPV
body score, which coincides with results of a previous study by Arsenault in 2001. Body condition score
was not associated with the presence of mammary gland or lung lesions.
examined, Arsenault also discovered
an absence of positive association between carcass condemnation and OPPV
seropositivity, which is consistent with the absence of detrimental
the disease on body condition score.
a 2006 study of OPPV prevalence in Alberta performed by Fournier
D., Campbell J. R., Middleton D. M., a
total of 353 sheep > 4 years of age from 180
flocks were tested. Determination of
histological lesions was used to estimate a 26.8% prevalence within the
The authors also determined seroprevalence and found that only
the animals tested seropositive using an agar gel immunodiffusion
but considered the histology to be the gold standard for determining
prevalence of OPPV.
association between lung and udder lesions in infected animals was also
examined, and there was found to be a lack of agreement between the
of histological lesions in the lung and histological lesions in the
udder. It was suggested by the authors
lack of agreement may be attributed to infection affecting these two
independently, or that there may be a difference in time course for the
development of lesions in the lung and udder.
The authors also suggested that it could be a combination of
should be kept in mind that sampling methods do affect the accuracy and
precision of prevalence estimates.
Variability of the prevalence within the population and the
tests utilized also play a role in determining the accuracy and
prevalence estimation (Campbell). Amongst the
above studies on OPPV
prevalence within Canada, the age of sheep sampled, the number of
sampled and the different methods employed all contribute to the
prevalence estimation. It can be taken
from these studies however, that the prevalence of OPPV in Canada is significant and widespread.