Vasculitis associated with ovine progressive pneumonia virus infection in sheep

Vasculitis, involving small muscular arteries and arterioles, was found in 5 of 18 sheep naturally infected with ovine progressive pneumonia (OPP) virus and in 5 of 11 sheep experimentally infected with OPP virus. In order of frequency, arterial lesions were seen in carpal joint capsules, kidneys, meninges, brains, lungs, and tracheas. The lesions were intramural edema and hemorrhage, mononuclear cell infiltration, fibrinoid necrosis of media, and thrombosis. The vascular lesions were frequently associated with interstitial pneumonitis, arthritis, and encephalitis also induced by OPP virus.     

Mastitis associated with ovine progressive pneumonia virus infection in sheep

Eighteen ewes were experimentally infected with ovine progressive pneumonia virus and their mammary glands were examined for lesions and virus at 2.5 to 10 years postinoculation. Lesions were seen in 14 of 18 sheep; virus was isolated from 4 of 8 sheep. Lesion consisted of an interstitial accumulation of lymphocytes with periductal lymphoid nodules, and epithelial vacuolation and necrosis at the site of the lymphoid nodules.     

Intrauterine transmission of ovine progressive pneumonia virus

Ovine fetuses, newborn lambs, and ovine colostrum were examined for ovine progressive pneumonia virus. The lambs and colostrum were also examined for specific antibody. Virus was isolated from 1 fetus, from 2 newborn lambs, and from most samples of colostrum. The fetus was about 100 days old and was carried by a seronegative ewe in contact with seropositive sheep. Both newborn lambs were carried by seropositive ewes. One lamb was dead at birth; the other lamb was normal and had not nursed. Antibody specific for the virus was present in the colostrum of 12 of 14 seropositive ewes and in the serum of 8 of 11 lambs that had nursed seropositive ewes, but not in the serum of lambs that had not nursed.     

Effects on ovine fetuses of exposure to ovine progressive pneumonia virus

Fetuses of 20 pregnant ewes at 4 gestational periods (45, 55, 85, and 100 days) were inoculated with ovine progressive pneumonia virus. Fourteen of 16 fetuses exposed to virus before gestational day 80 were either resorbed or expelled, whereas 10 of 15 fetuses exposed to virus after day 80 were normal at birth. Three of the 9 expelled fetuses and 1 of 2 newborn lambs had accumulations of lymphoid cells in the lungs. Virus was readily isolated from the tissues of expelled fetuses and newborn lambs. Lambs did not have precipitating antibody to the virus at birth, but 3 to 5 lambs had specific antiviral antibody at 18 months of age.     

Ultrastructure and frequency of mastitis caused by ovine progressive pneumonia virus infection in sheep

Twenty-five sheep, experimentally (n = 15) or naturally (n = 6) infected with ovine progressive pneumonia virus and noninfected controls (n = 4), were evaluated for histological and ultrastructural lesions of mastitis. Histologically, nine of 15 experimentally infected sheep and all six naturally infected sheep had lympho-plasmacytic mastitis. Severity of the lesion increased with length of time after infection. Periductal lymphatic nodules were seen in five sheep experimentally infected for 2.8 years or longer and in five naturally infected sheep that were 3.7 years old or older. Ultrastructurally, responses to ovine progressive pneumonia virus were diffuse lympho-plasmacytic infiltrates in glandular interstitium, lymphocytic and occasional plasmacytic infiltrates in ductal walls and lumens, lymphoblasts surrounded by small lymphocytes in glandular interstitium, and degeneration of epithelium releasing cells and cellular debris into the lumen. Based on the prevalence of lesions, the mammary tissue was more susceptible to ovine progressive pneumonia virus than other target organs: lung, brain, and synovium. Lesions did not differ between breeds of sheep. Ovine progressive pneumonia virus was not seen in the mammary tissue but was isolated from 15 of 17 mammary glands.     

Elevated concentrations in serum immunoglobulins due to infection by ovine progressive pneumonia virus.

Sixty-seven serum samples were obtained from 2 sheep flocks. Agar gel immunodiffusion (AGID) was used to separate progressive pneumonia virus (PPV)-infected sheep from noninfected sheep by the presence of precipitating antibodies. Immunoglobulin (Ig), total protein, and albumin concentrations were then measured from all 67 sera to determine whether differences existed between PPV-infected and non-infected sheep. A significant difference (P less than 0.0005) was found in both total protein and Ig concentration between PPV-infected and noninfected sheep. This corresponding difference was absent in albumin measurements. The significant differences (P less than 0.0005) in Ig and total protein concentrations were then used to evaluate a field test for diagnosing progressive pneumonia. The possibility of using either total protein or Ig concentrations as a field test was found to be highly unlikely due to variation in individual values.     

Appearance of ovine progressive pneumonia virus outside the United States

A recently isolated Israeli retrovirus from a sheep with maedi-visna was compared with other retroviruses, using cDNA-RNA hybridization in solution. The Israeli isolate was shown to have close, if not identical, genetic homologic features with the ovine progressive pneumonia virus reported in the United States, rather than with those of the maedivisna viruses of European origin.     

Prevalence of Anaplasma phagocytophilum infection and effect on lamb growth

Background

A major challenge in sheep farming during the grazing season along the coast of south-western Norway is tick-borne fever (TBF) caused by the bacteria Anaplasma phagocytophilum that is transmitted by the tick Ixodes ricinus.

Methods

A study was carried out in 2007 and 2008 to examine the prevalence of A. phagocytophilum infection and effect on weaning weight in lambs. The study included 1208 lambs from farms in Sunndal Ram Circle in Møre and Romsdal County in Mid-Norway, where ticks are frequently observed. All lambs were blood sampled and serum was analyzed by an indirect fluorescent antibody assay (IFA) to determine an antibody status (positive or negative) to A. phagocytophilum infection. Weight and weight gain and possible effect of infection were analyzed using ANOVA and the MIXED procedure in SAS.

Results

The overall prevalence of infection with A. phagocytophilum was 55%. A lower weaning weight of 3% (1.34 kg, p < 0.01) was estimated in lambs seropositive to an A. phagocytophilum infection compared to seronegative lambs at an average age of 137 days.

Conclusions

The results show that A. phagocytophilum infection has an effect on lamb weight gain. The study also support previous findings that A. phagocytophilum infection is widespread in areas where ticks are prevalent, even in flocks treated prophylactic with acaricides.     

Failure of experimental vaccines to protect against infection with ovine progressive pneumonia (maedi-visna) virus

Cell culture medium was harvested from cells infected with ovine progressive pneumonia (OPP) virus and used to prepare killed virus vaccines. Virus was inactivated by either heat, formalin, or ethyleneimine and used either without adjuvant, with Freund incomplete adjuvant, or with aluminum hydroxide adjuvant to vaccinate sheep. The sheep produced precipitating antibody against the virus but were not protected against infection when challenged with live OPP virus.     

Usefulness of subjective ovine milk scores: I. Associations with range ewe characteristics and lamb production

Range ewes are commonly evaluated for milking ability by producers to determine the ewe's ability to rear lamb(s). The U.S. Sheep Experiment Station has subjectively scored (low, average, high) a ewe's milking ability within 24 h of lambing for many years. The relationship of subjective milk scores with lamb production was investigated using lambing records of Columbia (n = 1,731), Polypay (n = 1,129), Rambouillet (n = 1,704), and Targhee (n = 1,638) ewes. The incidence of high milk scores increased from less than 10% at first parity to 29 to 40% at second and greater parities. At maturity, Columbia ewes (38%) had the highest percentage of high milk scores. A positive association existed between ewe BW and her milk score at third and later parities. Ewes with high milk scores gave birth to heavier lambs (P < 0.05), whereas ewes with low milk scores were associated with lighter (P < 0.05) lambs at birth. Ewes with low milk scores weaned less (P < 0.05) total weight than ewes with better milk scores across all age groups for all breeds. Lighter weaned litter weights from ewes with low milk scores were linked to lighter birth weights and fewer weaned lambs. Differences for litter weight weaned between ewes with average and high milk scores were generally observed at 2 and 3 yr of age, when litter weights were heavier among ewes with high milk scores (P < 0.05) for all breeds. Between the ages of 1 and 3 yr, Columbia, Polypay, Rambouillet, and Targhee ewes with an average milk score weaned heavier (P < 0.05) litters (average differences of 10, 9, 13, and 12%, respectively) than ewes with low milk scores. For all breeds at all ages, individual lamb weaning weights were heavier (P < 0.05) when they were reared by ewes with high milk scores compared to lambs reared by ewes with low milk scores. Results suggest that milk score is an economically important trait in these four breeds and should be considered in management and breeding objectives; at a minimum, the incidence of low milk scores should be kept as small as possible.     

Minimum intravenous infectious dose of ovine progressive pneumonia virus (OPPV)

The minimum intravenous infectious dose for ovine progressive pneumonia virus (OPPV) WLC1 was determined using twenty-four 6 month-old lambs. Twelve groups of two 6 month-old lambs were inoculated intravenously (i.v.) with tissue culture fluid containing ovine progressive pneumonia virus (OPPV) WLC1 titers ranging from 10^7.6 TCID₅₀/lamb down to 10^−3.4 TCID₅₀/lamb and were monitored for seroconversion using the OPPV agar gel immunodiffusion assay (AGID). Fifteen of the 16 lambs given equal or greater than 10^0.6 TCID₅₀ seroconverted, and virus could be isolated from peripheral blood leukocytes in 13 out of the 15 of these lambs. None of the eight lambs receiving less than 10^0.6 TCID₅₀ seroconverted during the 12 months. The results of this study indicated that 10^0.6 or 4 TCID₅₀/lamb given i.v. was capable of establishing infection.     

Breed susceptibility to ovine progressive pneumonia (maedi/visna) virus

In this retrospective study of breed differences in susceptibility to disease caused by ovine progressive pneumonia (OPP) virus, 29 Border Leicester sheep were compared with 46 Columbia sheep. As judged by frequency and severity of clinical signs and lesions attributable to the infection, Border Leicester sheep were markedly more susceptible than Columbia sheep and experimentally infected sheep were slightly more susceptible than naturally infected sheep. Differences in susceptibility to infection by the virus were not determined.     

Diagnostic features of ovine progressive pneumonia.

Ovine progressive pneumonia, a chronic, insidious disease of adult sheep, has a relentless course leading to dyspnea, emaciation, and death. Clinical observations and serologic tests are adequate for making a tentative diagnosis. The agar gel immunodiffusion test seems to be the best serologic procedure for indicating infection with the virus but cannot be used to predict morphologic changes or clinical disease, inasmuch as many clinically unaffected animals carry the virus. A definitive diagnosis is based on finding lesions and isolating virus. Affected lungs are large and heavy as a result of interstitial accumulation of lymphoid cells and fibromuscular tissue. Frequently, interstitial lesions are accompanied by bronchopneumonia from secondary bacterial infection. The causal virus can be isolated from infected lungs by cocultivation with primary ovine or bovine cells.     

Eradication of ovine progressive pneumonia from sheep flocks

Two management methods for controlling ovine progressive pneumonia in sheep were evaluated. By a test and cull method, wherein seropositive sheep were culled from the flock, the causative virus was eradicated from a closed flock and controlled in an open flock. By an isolate-and-test method, wherein lambs were removed from infected ewes before nursing and reared in isolation, the virus was eradicated in 1 of 2 attempts to establish virus-free flocks. It was concluded that either method should be effective in controlling ovine progressive pneumonia. Results indicated that annual serologic monitoring of a virus-free flock would be necessary to ensure that the virus-free status is maintained.     

An investigation of the possible effects of subclinical iodine deficiency on ewe fertility and perinatal lamb mortality

A controlled supplementation trial was performed in an assumed iodine-deficient Romney ewe flock in the Manawatu. Significantly more twin litters were born to iodine-supplemented ewes than to control ewes. Furthermore, the perinatal mortality rate in lambs born to iodine-supplemented dams was significantly lower than in lambs born to control dams. Ewe age and the use of different lambing paddocks for iodine-supplemented and control ewes may have been confounding factors. In both iodine-supplemented and control ewes, serum T₃ and T₄ concentration values remained above the currently used “normal” range throughout the trial period.     

An investigation of the possible effects of subclinical iodine deficiency on ewe fertility and perinatal lamb mortality

A controlled supplementation trial was performed in an assumed iodine-deficient Romney ewe flock in the Manawatu. Significantly more twin litters were born to iodine-supplemented ewes than to control ewes. Furthermore, the perinatal mortality rate in lambs born to iodine-supplemented dams was significantly lower than in lambs born to control dams. Ewe age and the use of different lambing paddocks for iodine-supplemented and control ewes may have been confounding factors. In both iodine-supplemented and control ewes, serum T3, and T4, concentration values remained above the currently used normal range throughout the trial period.     

Experimental infection of sheep by caprine arthritis-encephalitis virus and goats by progressive pneumonia virus

The lentiviruses, caprine arthritis-encephalitis virus (CAEV) and progressive pneumonia virus (PPV) of sheep, cause major diseases in their respective hosts; however, the infectivity of these viruses for closely related species has not been determined. Experiments were conducted to determine whether CAEV would infect sheep and whether PPV would infect goats. Upon inoculation with CAEV, lambs developed a nonsuppurative arthritis and antibody to CAEV, and the virus was isolated up to 4 months later. Exposure of 3 lambs to CAEV-infected adult goats did not lead to demonstrable infection after 18 months. Young goats inoculated with PPV replicated the virus and developed arthritis and antiviral antibody. These results demonstrate that these distinctly different lentiviruses may infect and cause diseases in species other than their accustomed host. Presently used techniques may not be effective in differentiating which lentivirus is responsible for infection of sheep and goats. Our results also indicate that mixing sheep and goats may adversely influence attempts to eradicate lentiviruses from these species.     

The effect of ewe maternal behaviour score on lamb and litter survival

The study was carried out on a commercial New Zealand sheep farm with high ewe reproductive rates and lamb survival produced through intensive selection in its Coopworth flock for maternal ability.Heritability and repeatability estimates were derived for ewe maternal behaviour score (MBS) and litter survival (LIS). Heritability estimates were derived for lamb survival as a trait of the lamb (LAS) for all lambs, for twin (LAS2) and for triplet (LAS3) lambs.MBS and LIS were measured on 1954 dams, for a maximum of four parities: 1997, 1998, 1999 and 2000. MBS was measured at tagging on a 5-point scale (1=poor, 5=excellent) when the dam's lambs were between 12 and 36 h old. The mean MBS in this study was 3.3 and increased with litter size. LIS was measured from birth to weaning. Mean litter survival was 83%. LIS increased significantly as MBS increased (P<0.01). LIS decreased as the size of the litter increased (P<0.01). Age of dam was a nonsignificant effect on LIS (P>0.05).LAS was measured from birth to weaning on 4171 Coopworth lambs. Mean LAS was higher for lambs born as twins compared to lambs born as singles and lowest for lambs born as triplets (P<0.01). LAS was lower for lambs born to dams aged 2 years. This effect was significant for all lambs, regardless of litter size at birth and for the triplet lamb data set (P<0.01). The effects of age of dam and sex of lamb on twin lamb survival were not significant (P>0.05). Ewe lamb survival rate was higher when compared to ram lambs in the full data set, however the relationship was reversed for the triplet lamb data set where ram lamb survival was greatest (P<0.01). LAS decreased as the MBS of its dam increased (P<0.01). The relationship was significant for lambs in the full data set and the twin data set (P<0.05).MBS and LIS were under minimal genetic control. The heritability and repeatability for MBS were both 0.09. The heritability and repeatability for dam LIS were 0.0 and 0.11. Heritability for LAS over all lambs attributed to direct effects was 0.14, while the heritability attributed to maternal effects was 0.11. The heritability for twin (LAS2) and triplet (LAS3) lamb survival differed. Heritability attributed to direct and maternal effects were 0.0 and 0.21, respectively, for twin lambs and 0.08 and 0.16, respectively, for triplets.The genetic correlation between maternal and direct effect for LAS was −0.74. It is possible that the genes that regulate physiological and biochemical processes for survival are incompatible with the genes that enhance ewe-lamb bonding. For example, the genes that regulate the physiological factors to reduce gregariousness at parturition may in fact be the same genes that encourage isolation in the neonate from its littermates and dam.There is minimal genetic variation in this flock for lamb survival and maternal traits. Low genetic variation suggests that selection will be ineffective, and that farmers must consider environment and management techniques for improving lamb survival.     

Microimmunodiffusion test for diagnosis of ovine progressive pneumonia.

Two microimmunodiffusion tests (MIDT) for detection and measurement of ovine progressive pneumonia antibody are described. Substrates of various salt concentrations and pH were used to determine the optimal conditions for the tests. In comparisons between two MIDT and one macroimmunodiffusion test, sera from cull ewes were used. The MIDT require less reagents and were more responsive than the macroimmunodiffusion test. After extended incubation of the test materials, results in all three tests were comparable.     

Prevalence of ovine progressive pneumonia in a sampling of cull sheep from western and midwestern United States.

Ovine progressive pneumonia was found to be prevalent in many major sheep-producing areas in the United States. The incidence was determined in a sampling of cull sheep (5 to 10 years of age) at slaughter. Diagnostic criteria were based on finding viral-specific immunoglobulins, virus, or lesions. Each method indicated a high incidence in samplings from midwestern and northwestern states and a relatively low incidence in the sampling from southwestern states. Of those sheep examined, precipitating immuno-globulin to the causal virus was seen in 1.0% to 67.5%, virus was isolated from the lungs of 0% to 46.2%, and lymphoid hyperplasia was seen in the lungs of 0% to 38.5%. The most common lesions in the lungs (from which virus was isolated) were multiple lymphoid nodules and increased fibromuscular tissue.