Epidemiology

The prevalence of OPPVi across Canada was determined in a 1991 study by Simard and Morley (source 4) involving 14 047 sheep sourced from each of the ten 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 seropositve sheep. Of the sheep tested, 13 719 were female and had a seroprevalence of 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 susceptibility between the sexes. The percent seropositives, the percentage of flocks with at least one seropositive sheep, and the mean flock prevalence all varied considerably amongst provinces.

 

It was found by Simard and Morely, that within OPPV infected flocks, the majority of infected sheep showed a seroresponse by 24-36 months of age.  The sheep with the highest prevalence at the time of collection were those age 6 and 7 years (Graph 2).  The authors explained the age specific increase in prevalence up to six years of age as being related to the longer length of exposure to horizontal transmission and the delay of seroconversion following infection.  A decrease in prevalence after eight years of age was noted and was suggested to be the result of culling and the death of sheep having clinical OPPV following an infection earlier in life. 

 

In terms of the vertical transmission of OPPV, it was found that seropositive dams had 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’ milk and there is a 28% probability of the lambs becoming infected following only ten hours of contact time between the dam and the offspring. Droplets from the respiratory tract of infected animals can be a source of horizontal transmission of infection for all ages of sheep.

 

According to the literature, there may be a breed susceptibility component to infection with OPPV.  Higher prevalence of OPPV has been found with one half Finnsheep crosses, while the Ile de France breed, certain strains of Icelandic sheep, and crosses between Icelandic and Border 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 associated with flock exposure and management practices (Simard). In the study by Simard, there appears to be a breed susceptibility, although the actual population size of each breed would be a significant variable (Graph 3).

 

Simard and Morely found that seroprevalence of OPPV appeared to increase with increasing flock size, which they suggested was a function of larger flocks having been exposed to an extended period of horizontal transmission due to their longer period of establishment.

 

Campbell J. R., Menzies P. I., Waltner-Toews D., et al collected sera from 3880 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 flocks sampled having at least one seropositive sheep.

 

Factors found by Campbell to be positively associated with higher seroprevalence rates of OPPV included:

 

 

The importance of horizontal transmission is highlighted by the positive association of OPPV prevalence with average flock age.  The greater the number of years a farmer has had sheep means the flock has been in existence for a longer period of time, increasing the potential for infection of the flock and the spread of OPPV within the flock.  Use of foster ewes supports the infection of lambs via ingestion of 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 indurative 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 seropositivity. Of the seropositive animals, 14 % and 40% had OPPV suggestive histological lung and mammary gland lesions respectively.  It was found that overall 1% of seropositive sheep had both gross and microscopic lung lesions and that only three animals had arthritis on gross examination. All three ewes with arthritis were > 3 years of age.

            In terms of the histological lung lesions, the prevalence in seropositive animals was not higher among sheep ≥4 years old or having a thin body condition score. The 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 having histologic OPPV lung lesions. The author suggested that sheep sampled in the 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 mammary gland induration in this study.

Seropositivity to OPPV was associated with 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 diseases even before development of detectable OPPV lung lesions.  

Arsenaul’s results suggest that there is no detrimental impact of OPPV infection on 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. When examined, Arsenault also discovered an absence of positive association between carcass condemnation and OPPV seropositivity, which is consistent with the absence of detrimental impact of the disease on body condition score.

            In 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 province (Graph 4).  The authors also determined seroprevalence and found that only 13% of the animals tested seropositive using an agar gel immunodiffusion (AGID) test, but considered the histology to be the gold standard for determining the prevalence of OPPV.  The association between lung and udder lesions in infected animals was also examined, and there was found to be a lack of agreement between the occurrence of histological lesions in the lung and histological lesions in the udder.  It was suggested by the authors that this lack of agreement may be attributed to infection affecting these two organs 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 these two scenarios.

            It 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 diagnostic tests utilized also play a role in determining the accuracy and precision of prevalence estimation (Campbell).  Amongst the above studies on OPPV prevalence within Canada, the age of sheep sampled, the number of sheep sampled and the different methods employed all contribute to the variability of prevalence estimation.  It can be taken from these studies however, that the prevalence of OPPV in Canada is significant and widespread.