08/06/2026
CAE & OPP - Part 4: “But She Looks Perfectly Healthy”
A goat or ewe tests positive for CAE or OPP.....
She looks great, active, and maintaining excellent body condition. Her feet and legs look good. Her udder appears normal. She raises healthy kids or lambs and remains one of the most productive animals in the herd/fock.
So the producer naturally asks:
“How can she have CAE or OPP? There's nothing wrong with her.”
The answer is one of the most important things to understand about small-ruminant lentiviruses:
Being infected is NOT the same as appearing sick.
Most sheep and goats infected with these viruses remain largely asymptomatic (don't display symptoms of illness)... sometimes for their entire lives. They may never develop the dramatic arthritis, respiratory distress, neurologic disease, or severe wasting you might commonly hear about with CAE and OPP.
- They're still infected.
- They are still persistent carriers.
- They may still transmit the virus to other animals.
- Infection, disease and clinical illness -
We often use these words as though they mean the same thing, but they describe different stages of the process.
- Infection means the virus has entered the animal and established itself within its cells.
- Disease means the infection is creating abnormal changes or damage within the body.
- Clinical illness means that damage has progressed far enough to produce signs we can see from the outside.
Bottom line... An animal can be infected without appearing ill.
It can even have microscopic tissue damage without showing anything obvious during an ordinary examination.
By the time visible signs develop, the infection may have been present for years... and often has been.
- The animal isn't "lying to us" -
The healthy-looking positive animal isn't a "false picture" of disease.
It's the NORMAL picture of a lentivirus infection.
These viruses survive by moving slowly and persisting quietly. They don't need to make every infected animal visibly sick. In fact, an animal that remains active, productive, and socially connected actually provides the virus with more opportunities to spread than an animal that quickly becomes severely ill.
Look at it from the virus's perspective:
- The virus benefits from time.
- Time to nurse offspring.
- Time to share airspace.
- Time to stand at the feeder.
- Time to enter the milking string.
- Time to move between farms.
A visibly sick animal attracts attention fast, and that's not how this virus has been so successful.
- A positive test does not predict the animal’s future -
A confirmed positive result tells us the animal has developed detectable evidence of infection.
It does not tell us exactly what will happen next.
Some infected goats eventually develop chronic arthritis, often involving the carpal joints. Others may develop a firm, poorly productive udder, chronic pneumonia, progressive weight loss or, less commonly, neurologic disease. Merck notes that "clinical signs are observed in only a minority of infected goats during their lifetime".
"Clinically affected sheep are most often recognized by progressive pneumonia, exercise intolerance, weight loss, or a hard, poorly productive udder. Neurologic disease may also occur, but many infected sheep remain outwardly normal".
We can't just look at a positive animal and reliably predict:
- Whether it will ever become clinically ill
- Which organ system will be affected
- How quickly disease might progress
- How much virus it may shed
- Which exposed animals will become infected
A positive result identifies infection.
It is not a countdown clock.
- Why does one animal become sick while another doesn't? -
Clinical disease is most often influenced by several interacting factors:
- Viral strain
- Amount and frequency of exposure
- Age at infection
- Genetics
- Immune response
- Stress
- Nutrition
- Concurrent disease
- Housing and ventilation
- Length of time infected
That's why two animals living in the same herd can have very different outcomes.
- One may develop swollen knees and progressive lameness.
- Another may develop a hard udder.
- A third may slowly lose weight.
- A fourth may remain apparently healthy for life.
They can all carry the same general category of infection while expressing it differently.
- “She has never had a problem” isn't a test result -
Livestock producers are trained to value observation.
That is usually a strength.
We notice appetite, posture, movement, rumen fill, body condition, breathing and behavior. Those observations often help us identify disease before laboratory results are available.
Lentiviruses challenge that skill because the absence of visible signs tells very little about infection status.
At the end of the day:
- You can't reliably identify CAE or OPP carriers by looking at them.
- You can't feel every infected animal’s knees and declare the herd negative.
- You can't examine udders and assume every normal udder belongs to an uninfected animal.
- You can't watch a flock walk across the pasture and determine which sheep are carrying OPP.
Clinical observation can HELP identify animals already showing disease. Butt can't identify every infected animal.
I've heard many people say... "I can tell".... ummmm no you can't.
That requires testing.
- Production can decline before the animal looks sick -
Not all effects of infection are dramatic (and they usually aren't).
An animal doesn't have to become severely lame or visibly short of breath before the infection begins to matter economically.
The changes may be subtle:
- A ewe begins trailing slightly behind the flock.
- A doe’s milk production gradually declines.
- An udder feels firmer than expected but is not hot or painful.
- An animal loses condition despite appearing to eat normally.
- A mature goat becomes stiff but is dismissed as simply "getting older".
- A ewe repeatedly raises lighter lambs.
None of those observations proves/disproves CAE or OPP.
Many other conditions can produce the same signs.
But... that's exactly why lentivirus-associated losses can go unrecognized. Slow changes are easily explained away as age, genetics, parasites, nutrition, previous injury, or the dreaded and very scientific..."bad luck".
I rarely see sheep or goats reach the dramatic pictures shown in a veterinary textbook.
They often simply become less productive.
- The best-looking animal may still be the source -
This is where emotion enters the discussion.
The positive animal may be:
- A favorite doe
- A proven mother
- A valuable donor
- A champion
- The foundation of a bloodline
- An animal that has never appeared sick
The virus doesn't care about any of this, and deep down in places many people don't talk about.... this is a huge issue and reason the virus is such a problem.
*Healthy carriers are why these diseases persist*
- Positive does not mean worthless!-
A positive animal is not automatically suffering.
- It's not poisonous.
- It doesn't need to be treated as though standing near it creates an emergency.
And... a positive result does NOT dictate one identical decision for every farm.
The "appropriate" response depends on the producer’s goals, facilities, prevalence within the herd, ability to separate groups, and willingness to accept continued risk.
- Some producers may cull positive animals.
- Others may maintain a completely separate positive group.
- A heavily infected herd may choose a gradual replacement program rather than immediate removal.
What's not reasonable is pretending the result means nothing because the animal looks healthy.
At the end of the day.... believe the test before you believe the appearance.... enough said.
Up Next: What should you do with a positive animal -culling, separation, and realistic control options-