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  There are advantages and disadvantages to every test. Choosing the right test for the right task is important. There are two widely used tests for Johne's disease in cattle: 1) culture of manure (referred to as fecal culture) to detect the bacteria that causes Johne's disease: M. paratuberculosis, and 2) measurement of the level of antibody in blood produced by the animal in response to M. paratuberculosis infection. The strengths and weakness of each test will be discussed. This information is provided as a supplement to what you will find on the "Diagnosis" page under the "Dairy" area of interest. Readers are strongly urged to visit that part of this website as well. |
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Fecal
culture
Culture
of M. paratuberculosis from domestic cattle manure samples, referred to
as fecal culture, virtually always indicates that the animal from which the manure
sample was taken has Johne's disease (is infected) because this bacterium is not
found in uninfected animals (some rare exceptions to this statement occur but
are not worth mentioning here). In terms of diagnostic accuracy we say that culture
for Johne's disease is 100% specific
meaning there are no false-positives
This is the greatest strength of this assay.
The ability of culture to detect infected
cattle, called sensitivity,
is dependent in the stage of infection of the animal. In the early stages
of infection, few or no M. paratuberculosis
bacteria are shed in the manure. Hence, it is not detectable and thus
the sensitivity of fecal culture in the early stage of Johne's disease
is very low or essentially zero. As the infection progresses, the ability
of culture to detect infected animals goes up. Since the infection usually
progresses as the animal ages, it is a fair generalization to state that
the sensitivity of culture is higher in older animals, i.e. it is more
likely to detect the organism with this test in an infected older animal
that is more likely to be shedding M. paratuberculosis than a
younger animal. Sensitivity of culture in animals less than 2 years old
is so low that most experts say it is a waste of money to culture manure
samples from animals this young (except in herds with a very high infection
rate).
For Johne's disease surveillance of beef cattle, it is frequently recommended
to culture fecal samples from animals older than 3 or even 4 years old. In very
general terms, fecal culture has a sensitivity of 50% for these adult cattle due,
as stated above, to those animals at an early stage of infection that are not
shedding and so are culture-negative. The good news is that such animals have
only a limited chance of spreading the infection. Hence while fecal culture may
be able to detect only half of the truly infected animals, it detects the most
infectious animals - the ones that should be removed from the herd first since
they have the greatest potential of spreading the infection.
The greatest disadvantages of fecal culture are its cost and long time to get
results. Producers can expect to pay $20-$30 per sample for fecal culture and
results will not generally be available for 3 to 4 months due to the extremely
slow growth rate of the organism.
The quality of fecal culture test is dependent on the quality of the culture medium
used and the proficiency of the laboratory personnel. In the U.S. a federal laboratory
administers a proficiency test annually and provides a publicly available list
of those laboratories that have proven their ability to perform fecal culture
for M. paratuberculosis.
For a list of laboratories that have passed
the fecal culture proficiency test for the current calendar year go to
websites listed on our "links"
page. (For suppliers of culture media, visit the sponsors
page of this website)
Blood tests
There are a number of blood tests for Johne's disease. While they all
detect serum antibodies to M. paratuberculosis, there are different
types of laboratory technology used: complement fixation (CF),
agar-gel immunodiffusion (AGID),
and enzyme-linked immunosorbent assay (ELISA)
are three examples. For more information on blood tests sold by sponsors
of this website please visit our sponsor
page.
CF test
The
CF test has historically been the test required most often for pre-shipment testing
of animals traded internationally. Unfortunately, the assay is not well standardized
and each country uses slightly different reagents, methods and interpretation
formulas. In addition, published evaluations of the CF test for Johne's disease
indicate that the sensitivity and specificity are less that those of the other
commercially available tests for Johne's disease. For this reason, most countries
are moving away from use of the CF test in favor of the ELISA. For more about
international testing requirements see the O.I.E. website found on our links
page.
AGID
The
AGID for Johne's disease was developed at the University of Minnesota. It is simple
to perform and fairly easy to interpret. This assay has been commercialized in
the U.S. and is sold under the trade name Rapid Johne's Test (RJT). The AGID performs
best on cattle showing clinical signs of Johne's disease and is primarily used
as a "rule in" diagnostic test on animals showing diarrhea and weight loss. The
sensitivity of AGID is too low to screen healthy looking cattle for evidence of
paratuberculosis. Specificity of the AGID, however, is 100%: no false-positives.
This makes it simple to interpret when the test is positive.
ELISA
ELISA technology
is widely used for diagnosis of many diseases of humans and animals. The advantages
of the absorbed ELISAs for Johne's disease are: 1) low cost: $8-10/sample to herd
owners, 2) speed: results can be in the hands of herd owners within one week,
3) good correlation with fecal culture, 4) ability to use some assays quantitatively
(cows with higher ELISA results are more likely to be infected and more likely
to be shedding M. paratuberculosis in their feces, hence they will be the
most infectious cattle in the herd (see the diagnosis
section under the dairy cattle topic for more details).
The disadvantage of the ELISA is that its sensitivity is somewhat lower than that
of fecal culture. Lower sensitivity means that more M. paratuberculosis-infected
cows will not have made any antibody and so be test-negative by ELISA than would
be test-negative by fecal culture. The same basic association between age and
diagnostic sensitivity exists for blood tests such as the ELISA as was described
for fecal culture (above). Whether this is a critical weakness impeding control
of Johne's disease in beef herds remains to be determined. No controlled field
studies have been done to compare use of fecal culture to ELISA as the sole test
for operation of a Johne's disease control
program.
Future tests
Commercial
companies will likely deliver faster, cheaper and more accurate tests
for Johne's disease in the coming years. However, the tests currently
available are adequate to effectively control this serious and spreading
problem in cattle. Waiting for a better test is the worst course of action
for herd owners.
Two types of new tests for Johne's disease are on the horizon. DNA probes that
can detect the organism in fecal samples in just a few days will likely become
available for routine use very soon. While these sophisticated tests may be somewhat
more expensive, the tradeoff for speed may be worth the cost to some herd owners.
Tests that will accurately detect M.
paratuberculosis infections
in cattle younger than two years also look very promising. These tests
will be based on detection of a cellular immune response in cattle, the
first type of immune response that occurs after infection with M.
paratuberculosis. Early diagnosis
of paratuberculosis will speed control efforts as well as lower the financial
impact of culling infected cattle.
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