Understanding and Managing Small Ruminant Lentivirus in Your Sheep and Goats

Virus Indicators

General information is lacking on the seroprevalence, risk factors, and subsequent economic effects of raising orphaned lambs, reduced lamb weaning weights, and early culling of ewes infected with Ovine Progressive Pneumonia (OPP), or Caprine Arthritis Encephalitis (CAE) in goats. OPP and CAE are both the same Small Ruminant Lentivirus (SRLV). Biosecurity measures for SRLV control on Pennsylvania operations are not widely implemented.

"OPP is a slow and silent productivity monster. Why?

Ewe lambs born to your best ewes can get this disease from others in the flock. These ewe lambs may have the genetic potential to give you twins or even triplets every year. But year after year she seems to lose her ability to raise them, even though their udder seems to be full of milk. Lambs fail to thrive. They either starve, get grafted, or orphaned. By 4 to 5 years old these ewes fail to regain condition after weaning their lambs and tend to hang at the back of the flock when moving them. These ewes tend to be pointlessly treated multiple times with dewormers or antibiotics and, eventually, tend to be culled at 5 to 6 years old. Without any clear signs of disease until it's too late, she lingers in the flock spreading the virus. Costing you more in feed year-round as well as orphan lamb feed and labor during lambing season."

- Dr. Rosie Busch
Veterinary Medicine Extension
UC Davis

OPP, caused by the Small Ruminant Lentivirus (SRLV), is transmitted horizontally from infected sheep/goat to uninfected sheep/goat via exposure to body secretions containing the virus. This could include colostrum/milk, blood, respiratory secretions, feces, or semen. After an incubation period of up to two years, symptoms of OPP include thin ewes with decreased oxygen capacity (often phrased as "lungers"), decreased vigor, and signs of progressive pulmonary disease. Retroviral mastitis is a common subclinical feature too. It is often described as "hard bag" among producers in relation to an udder that feels firm and has little to no milk production from an otherwise healthy-looking udder [1]. Mastitis is a health and welfare concern for the breeding ewe as well as nursing lambs. The most recent NAHMS study of the sheep industry in the United States indicated that mastitis was one of the leading reasons for culling breeding ewes and was second to respiratory disease for antibiotic use in sheep operations. Additionally, producers indicated starvation as the second most common cause of lamb mortality, which can commonly be related to udder issues. The American Sheep Industry Association, Inc. reported in 2016 that producers identified mastitis as one of the highest disease management challenges in breeding ewes. Mastitis is a flock disease issue with substantial economic impact, second only to internal parasites. However, in lieu of its importance to the industry, few papers characterizing mastitis in ewes in the eastern United States have been published. OPP-infected ewes are found to produce lambs that weigh approximately twelve pounds less at weaning and require an increased amount of milk replacer and labor[2]. This is due to the decrease in milk production as the ewe ages. Symptoms of OPP-infected sheep are often like those with bacterial pneumonia or mastitis. Additionally, subclinical OPP infection can predispose ewes to secondary bacterial pneumonia or mastitis. Both effects lead to an elevated use of antibiotics in OPP-infected flocks.  A recent study at the USDA MARC in Nebraska found that 70–90% of new infections occur by adult-to-adult transmission, and only 15 to 25% of infections occur from the dam to the lamb in the time before weaning [3]. This suggests that transmission from dam to lamb through the milk is not the major route for OPP transmission [2]. It's not unusual to find negative lambs born to positive-testing ewes and vice versa.

Disease Management

Ultimately, you cannot manage what you don't measure. Eradication of OPP is a venture that requires planning and commitment. It's important to collect a random sample of various ages when screening. Moreover, animals should have been in the flock for at least two years. If the virus is found, explore all options with your veterinarian before taking your next steps. Multiple serological tests are necessary to confirm OPP status and eradicate the disease if that`s your goal. Testing for the disease, culling any seropositive animals, retesting in six months, then yearly, and any new incoming animals is an ideal management practice. The OPP Society offers numerous management strategies online [4] which producers of all sizes can follow to build a plan that offers them the best prospect of success by minimizing losses due to culling and lack of longevity in the ewe flock. When animals are tested for the first time, a partial flock screening (see Table 1) can reduce costs for commercial flocks. Genetics may also be a potential avenue for OPPV mitigation. Haplotype variants within the genetic marker TMEM154 impact lifetime susceptibility to OPPV infection in naturally exposed ewes [5]. This SNP marker test is available through Superior Farms, Inc. Flock54 panel and Neogen. GeneCheck offers a PCR test.

Table 1: Sample size recommendation set by the OPP society for flock screening of OPP presence. [4]

Flock Size (Sample size)

• <30 (test all animals)
• 30 (26)
• 40(31)
• 50 (35)
• 60 (38)
• 70 (40)
• 80 (42)
• 90 (43)
• 100 (43)
• 120 (47)
• 140 (48)
• 160 (49)
• 180 (50)
• 200 (51)
• 250 (53)
• 300 (54)
• 350 (54)
• 400 (55)
• 450 (55)
• 500 (56)
• 600 (56)
• 700-800 (57)
• 1000 (57)
• 2000 (58)

Research Summary

Recent results on OPP transmission routes are relatable to the industry in Pennsylvania. The 2001 National Animal Health Monitoring System (NAHMS) report for sheep described 36.4% of all flocks and 24.2% of all the sheep in the U.S. were infected with the OPP virus. In the Eastern region, 23.5% of flocks and 6.1% of sheep were positive for OPP. Additionally, 36.3% of farm flocks in this study had one or more animal test positive for OPP, and 17.1% of all sheep tested from farm flocks were positive. >80% flock positivity is not uncommon. While one in three operations (31.5%) participating in the same study reported never having heard of OPP. Currently there are no studies that identify the seroprevalence or risk factors associated with OPP status in Pennsylvania sheep flocks. We seek to utilize a serological survey to establish a baseline for the presence of OPP in the state and distribute further outreach materials detailing how to handle positive cases and improve maternal flock health with a specific emphasis catering to management settings in Pennsylvania. We also intend to take tissue samples on the same animals serologically surveyed to provide you some information on your animals` genetic susceptibility. Recruitment for this project is ongoing and additional information can be found by contacting Austin Brown, (aqb5615@psu.edu or 814-865-1718). We also encourage you to take part in the survey found at this QR code or by contacting Austin.

Conclusion

Ewes must be able to produce enough milk to support a higher lambing rate. Better disease management contributes to the American Sheep Industry`s objective to increase the national lambing percentage to 150% or 1.5 lambs per ewe. It is of utmost importance that maternal characteristics contributing to desirable weaning weights are a crucial selection criterion to be proactively pursued to maximize genetic potential.

References

1. Plummer Paul J. and Plummer Cassandra, Diseases of the Mammary Gland, Sheep and Goat Medicine (Second Edition), 2012
2. American Association of Small Ruminant Practitioners. "Ovine Progressive Pneumonia." Aasrp.org, June 2021.
3. Freking, Brad. "USMARC Update - Home - American Sheep Industry Association." sheepusa.org, U.S. Meat Animal Research Center, Jan. 2020, 
4. OPP Society, "OPP Testing." 9 April 2022.
5. Murphy, Thomas W., Et al., Effect of TMEM154 E35K variant (haplotypes 1 and 3) on the incidence of ovine lentivirus infection and ewe productivity during lifetime exposure. Journal of Animal Science, Volume 99, Issue 11, November 2021, doi.org/10.1093/jas/skab304
6. Richards, Isabel. "Udder Health in Ewes: Mastitis, Udder Scores, and Management." OSU Sheep Team, 28 February 2022.
7. Lacasta, Delia. "Selected Respiratory Tract Diseases of Sheep in Spain." Flock and Herd Case Notes, Flock & Herd, Apr. 2013.
8. Heaton M.P. and Laegreid W.W. 2006. USMARC, Clay Center, NE unpublished results.