Caprine Arthritis Encephalitis
Caprine arthritis encephalitis (CAE), commonly known as 'big knee', is caused by a lentivirus or 'slow' virus associated with nervous disorder (encephalomyelitis) in kids and slowly-developing disease syndromes in older goats.
Symptoms can vary markedly between animals with most well managed goats showing no obvious clinical signs.
The virus occurs mainly in improved dairy goat breeds and is spread primarily via infected colostrum and milk. Control programs have been conducted in many countries but CAE is still causing problems in dairy goat populations world-wide, including within NSW.
CAE is a concern in terms of market access and animal welfare. It also causes significant production losses through mastitis, ill-thrift, arthritis, pneumonia, ascending paralysis and arthritis, pneumonia, ascending paralysis and encephalitis in kids. Caprine retrovirus is an OIE listed disease.
Clinical signs
The disease occurs when the monocytes carrying the latent virus mature and multiply as macrophages in different body 'target' organs – mammary gland, lungs, tendons sheaths, joints and nervous tissue.
The clinical signs that can occur in an infected goat are mainly due to the body's reaction against the virus infected cells. Clinical signs include:
viral mastitis, with an increase of white blood cells in the milk and a decreased resistance against mastitis. ‘Hard udder’ (indurative mastitis) can occur.
arthritis in any joints, most notably the carpal joints (big knee), due to inflammation of the tendon sheaths and joint lining (synoviae),
pneumonia following inflammation in the lung tissue, and
neurological disease due to inflammation in the brain and its covering (meningitis/encephalitis) or inflammation in the spinal cord, resulting in progressive paralysis.
Kids less than 6 months of age are more likely to show neurological disease.
The majority of CAE infected goats do not show any outward clinical signs, although viral mastitis may be present in does.
Clinical signs can arise in a previously sub-clinical goat if the goat is exposed to stressful situations such as poor nutrition and overcrowding. Well managed infected goats may never express clinical signs.
Spread
The main spread of the virus between goats is through the ingestion of infected milk by kids or adults. Adult goats can also become infected by exposure to infected milk droplets during milking.
The virus can also be spread by respiratory secretions, saliva and tears when goats are kept in close quarters.
Transfer sometimes occurs by blood on gear such as vaccination needles, tattooing equipment, dehorners or foot/hair shears, or through exposure to open wounds.
Venereal spread in semen and in utero spread to kids are less likely but can occur.
The virus usually enters a clean property in an infected goat. The goat may or may not be antibody positive for CAE at the time of blood testing because of the delay between exposure to the virus and the development of antibodies.
Zoonotic risk
People drinking milk from infected does can develop antibodies to the CAE virus. There is no evidence that this contact has resulted in persistent viral infection.
There is a strong cross reactivity between surface glycoproteins on the CAE virus and the HIV virus. It's been postulated that the false positive reactions to HIV in some people may be due to previous exposure to the CAE virus in goat milk.
CAE risk factors
The least-risk herd for CAE is a closed herd (no introductions) where there have been at least two whole herd tests with negative blood results at minimum 12 months interval. CAE accredited herds and a number of non-accredited herds fulfil this criteria.
A low to moderate risk herd is a herd with a history of negative testing that buys in goats (open herd). Goats bought from CAE accredited herds or from closed herds with a good testing history would pose a low risk. Goats bought from herds with no testing history present a moderate to high risk (even if the purchased goat is tested).
A high risk herd is a herd with no testing history in the last 12 months on all goats over 12 months of age (including goats that had previously been tested negative or were sourced from tested herds), regardless of introduction policy.
Low risk factors include:
extensive management where single doe/kid(s) units predominate and where goat to goat contact is minimal (commercial fibre and meat goats),
the use of pasteurised milk to feed kids (heated to greater than 56°C for a minimum of 10 minutes),
strict hygiene in the use of common equipment or handling between goats.
High risk factors include:
feeding pooled milk to kids and/or adult goats,
exposure to secretion contaminated feed or water,
keeping goats in close confinement,
sharing equipment between goats without sterilisation,
persons handling a number of goats (especially udder or mouth) from different sources,
untested goats being milked as a common group,
co-mingling of goats from different farms at one site, e.g. at shows and sales.
Shows, sales and any other venues where goats from multiple sources congregate for a period of time present a significant transmission risk. The risk increases with the period the goats are kept in close confinement.
CAE control
Detecting sub-clinically infected goats is the key to preventing CAE spread.
Infected goats are detected by serological testing. The most accurate test is the ELISA although some countries still use the less specific AGID test.
Repeated blood testing during a 12 month period will detect the majority of infected goats (a very small number may take longer to seroconvert). The CAE status of goats should be determined in goats 12 months and older. Adult does should not be tested in the period from one month either side of kidding as inconsistent results could occur. No goat should be tested within one month of any vaccination.
Certification for CAE status
Dairy goat herds are at greater risk of being infected with CAE because both kids and adults are managed intensively. Knowledge of the testing history of the adult herd (especially within the last 12 months), and assessment of the herd management as low risk, provide confidence when certifying for CAE status.
Where the herd is not accredited and there is no history of testing within the previous 12 months, or where the herd management is medium to high risk or unknown, all relevant adult animals over 12 months of age should be blood tested. This may include a subset of the adult herd (for example, a show or exhibition group). Any ELISA positive animal indicates that infection could be present in the herd and the CAE status of the herd is positive.
By Department of Primary Industries NSW - Last updated 16 November 2012
