Silent invaders: CAE & OPP

By Emily Nogay, VMD, MS, Ohio State University

Silent killers can come in all shapes and sizes. In small ruminants, the viruses caprine arthritis and encephalitis (CAE) and ovine progressive pneumonia (OPP) can go by many names. Commonly, they are referred to as small ruminant lentiviruses and fall into the category of wasting diseases, meaning they can cause progressive weight loss. These viruses are silent invaders because once they infect an animal, they can incorporate their own genetic material into the animal’s genome, making them hidden from the animal’s immune system and allowing the infection to persist for life.

Transmission of these lentiviruses is through another infected animal. The virus is shed through coughing, saliva, blood, colostrum, milk, and semen. Inanimate objects or surfaces generally do not transmit these viruses, as they cannot survive for very long in the environment. For the most part, CAE affects goats, and OPP affects sheep; however, they can rarely infect the opposite species. OPP is more commonly spread through respiratory droplets between adult animals, while CAE does the same; it is also commonly spread to kids through colostrum.

Once infected, both viruses remain for life and can slowly attack different organs. The animal can remain symptom-free yet still spread the infection to others. Both viruses target many tissues, and clinical signs depend on which organ is most affected. Symptoms for both viruses can include a hard infection of the mammary gland, decreased milk production, weight loss, difficulty breathing, arthritis, swollen joints, and even death. Inflammation of the brain can also occur in younger animals with CAE. Even in animals that do not develop these symptoms, herd production is greatly affected. The American Association of Small Ruminant Practitioners quotes that OPP-infected ewes produce approximately 12 pounds less lamb weight at weaning compared to noninfected ewes (AASRP, 2021).

Many people know the harms of these viruses; however, there is a common misconception that they are rare. A 2001 study conducted through the National Animal Health Monitoring System (NAHMS) found that 36.4% of randomly selected flocks in the United States had at least one OPP-positive animal, and 24.1% of all animals tested were positive. In the same study, more than 23% of operations in Ohio, Pennsylvania, and Virginia were found to be infected with OPP (Marshall, 2003). Less information is available on CAE; however, an older study estimated that CAE can be found in up to 81% of dairy goats in the United States (Crawford, 1981). A recent study of Midwestern goat herds found that 45% of sampled goats were positive (Jones, 2014).

Unfortunately, these diseases have no treatment options, and infection is for life. With no treatment, the main goal becomes biosecurity: keeping the virus out of your herd and removing it if it is already present. Identifying positive animals requires a blood test looking for antibodies. The good news is that the test is fairly accurate; however, antibodies can take several months to develop, which means multiple tests may be required to know if an animal is truly negative. By testing a portion of your herd twice yearly, you are more likely to find positive animals over time. Testing new additions to the herd is also necessary to ensure these viruses are not introduced to your herd. New animals require multiple tests to ensure they are truly negative, and a complete history of the animal is important as vaccines for other diseases can sometimes cause false positives. Be sure to discuss testing strategies with your veterinarian, as every herd requires a different protocol to reach herd-specific goals.

Specifically for OPP, a gene has been identified as protective against the disease. Animals that have two copies of the TMEM154 gene (one from the mom and one from the dad) are less likely to develop clinical signs. This gene does not eliminate the virus but rather allows the animals to live better with it. A blood test is available to determine whether an animal is a carrier of this gene.

Small ruminant lentiviruses are common, and even when they do not create overt clinical signs, they can negatively affect a herd. Each virus can cause many different disease manifestations and can spread in many ways. The majority of positive animals do not show clinical signs for years, if at all, allowing the virus to spread to many animals. There are several aspects to consider when controlling these viruses, and each herd requires different testing and elimination strategies, making it necessary to discuss the best approach with your veterinarian.  Small ruminant lentiviruses are silent invaders that can become deadly if the proper biosecurity protocols are not in place. Be sure to keep your herd healthy and protected against these silent killers!