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Diagnostic Services
There are a number of
direct and indirect diagnostic tests available for the detection of
Johne's disease. Direct testing involves finding the causative
organism (MAP) in fecal, milk, or tissue samples. Indirect testing
involves detecting immunological responses (most commonly the
production of antibodies) in the host animal to the invading organism
(MAP). Both forms of diagnostic tests, direct and indirect, are
measured in their ability to differentiate between infected and
non-infected animals, also referred to as the test's sensitive and
specificity.
The sensitivity of a
certain diagnostic test is its ability (based proportionally) to test
positive for an animal, which is truly positive or infected. The
specificity of a certain diagnostic test is its ability (based
proportionally) to test negative for an animal, which is truly
negative or not infected. Unfortunately, there are many factors or
variable, which can effect a test's reported sensitivity and
specificity.
The most commonly used and
inexpensive indirect testing services include
Serum Elisa (Enzyme
Linked Immunosorbent Assay) and
AGID
(Agar Gel Immunodiffusion) test. These blood
test allow for the detection of circulating antibodies to MAP from
samples taken within the host animal's blood stream.
Another form of indirect
testing is the Milk
ELISA. Similar to the Serum ELISA, the Milk ELISA is able to
detect antibodies to MAP, however, the antibodies are detected in milk
samples. Although not a true diagnostic test, nor is its intended
purpose, the Milk ELISA is an inexpensive screening tool used
primarily to monitor the status of a diary herd through individual
testing or through "Kinetic Bulk Tank Testing". The Kinetic Bulk Tank
Testing can be performed on a bulk tank sample containing milk from up
to 50 individuals. The dairy owner can screen and monitor the status
of the herd semi-annually or annually by looking for any increased
titers (results) per bulk tank sample. If increased titers are
detected during consecutive samplings, then the herd owner can
determine if any new animals have recently been added to the herd,
which might be possible carriers for Johne's disease. If there were
no new additions to the herd, then "Escalation Testing" would be
recommended to determine the true source of the infection from within
the herd. Escalation testing would involve the testing of all animals
contributing to the particular bulk tank sample in question. The
recommended diagnostic test for escalation testing would include the
serum ELISA and fecal culturing of all animals for that particular
bulk tank sample. In most instances, usually more than one animal is
infected if increased titers are observed without the addition of any
new animals.
Direct testing services
test for the presence of the causative organism, MAP. The industrial
"Gold Standard" continues to be
fecal culture. Fecal
culture involves the processing of fecal samples and the culturing for
MAP on specialized laboratory media containing
Mycobactin J for 8 to
13 weeks. A combination of colony morphology, microscopic evaluation
of acid fast staining and mycobactin dependency are used to determine
a positive result. The use of PCR/DNA probe may also be used to
confirm a positive result. Standard fecal culturing has a reported
specificity of 100% and a sensitivity of 40%. There are no false
positives in fecal culturing. Since an infected animal may be
intermittently shedding the organism through feces, a negative fecal
culture result does not rule out the possibility of infection.
Routine testing is highly recommended. States with an active Johne's
status and/or control program require the use of the serum ELISA and
fecal culturing for producer participation.
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| Programs
Various state an federal agencies, universities,
research facilities and private industries are actively involved and
working together in the support of a proposed draft of a Uniform
Program Standards for the Voluntary Bovine Johne's Disease Status and
Control Program to control and eventually eradicate Johne's disease
within the United States. To start, the program is centered towards
the cattle industry (dairy and beef), but may eventually include other
ruminants such as goats and sheep. Individual states are in the
process of developing or have developed programs (based on the Uniform
Program Standards) aimed towards controlling and possible eradicating
Johne's disease from within their borders. To date, there is no
single diagnostic test that is 100% accurate in distinguishing between
all infected or all non-infected animals. Therefore, diagnostic
services currently used in states with an active Johne's status or
control program require the combination of serum ELISA and fecal
culturing of animals along with an effective herd health management
program.
You can contact your local or state veterinarian to
obtain vital information concerning Johne's disease and whether your
state has a Johne's program already in place or is in the process of
developing a program.
Producers and local veterinarians must be proactive
in dealing with Johne's disease. Due to slow progression of this
disease, a herd owner (not routinely testing) may think he or she may
not need to worry about Johne's in their herd. Unfortunately,
clinical signs for an infected animal may not be evident for several
years (2 to 6 years). During this subclinical time, the infected
animal is shedding large numbers of the bacteria (MAP) through its
feces contaminating pastures, surface water, and feed lots. This
massive shedding of the bacteria (MAP) has the potential of infecting
the remaining herd, causing Johne's disease to be considered a whole
herd issue, not just an individual animal problem. It is estimated
that for every positive animal showing clinical signs for Johne's
disease in the herd, there may be as many as 15 to 25 other animals
that are likely infected.
Ultimately, livestock owners who are involved in
Johne's programs and have achieved and/or maintained the status of a
Johne's test negative herd have and will continue to benefit for their
choice to participate. A Johne's test negative herd is a more
productive and profitable herd: with increased production,
reproduction, and higher sale value at auction. Buyers are more
likely to purchase and pay top dollar for animals from an established
Johne's test negative herd than from a herd of unknown status. The
time to act is now.
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