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 Testing
individual and bulk milk samples is an economic method that is widely
used in Europe for surveillance, monitoring and control of infectious
diseases in cattle herds and, to a lesser extent, in sheep flocks.
Milk samples are readily available in dairy herds; there are no
additional costs to the diagnostic procedure for drawing blood
samples.1
Bulk milk samples are usually taken from milk tanks sampled from
herds of 20, 50 or more cows. The herd size for bulk-milk testing is
determined by the antibody response of seropositive cows to
disease-causing agents, the intraherd prevalence of antibody and the
sensitivity of the diagnostic test. Since every cow provides a
different amount of milk in the bulk tank, and positive, infected cows
often provide less milk, bulk-milk testing should only be considered
as a herd-screening tool to determine disease prevalence or
maintenance of the negative-herd disease status.
Pooled milk samples have an advantage compared to bulk or tank milk
samples. Milk pools are usually made up in the laboratory by pooling
equal volumes of individual milk samples.
Bulk milk and pooled milk samples can be tested by concentrating
the immunoglobulin fraction,2 however, this method is
generally considered too laborious for routine screening.3
When using milk samples as specimens for diagnostic testing with
ELISA, certain precautions should be considered:
- Samples of milk should be taken after cleaning and drying the tip
of the teat, but antiseptics should be avoided.
- Collect the milk in a clean container after stripping the first
stream of milk from the teat.
- Milk for serological tests should not be frozen, heated or
subjected to violent shaking.4
For more information on how to run milk
samples in ELISA, see the Technical Tip.
Disease control using milk testing
Note: The license status of the IDEXX products
discussed in this article varies by country. Some products are not
available for sale in particular countries, and not all products have
approved milk testing claims in all countries. If you plan to use a
particular product for milk testing, you should first determine
whether that product has an approved claim for milk testing in your
country.
Brucellosis
An extremely efficient means of screening dairy herds for
brucellosis is by testing milk from the bulk tank. Milk from this
source can be obtained inexpensively and more frequently than blood
samples, and is often available centrally at dairies.
The indirect ELISA is the most sensitive and specific test for
milk, and is particularly valuable for the testing of large herds.5
The IDEXX HerdChek* Brucella abortus Antibody Test is an
indirect enzyme immunoassay that uses LPS antigens for the detection
of antibody to Brucella in bovine milk samples. The assay has
user-friendly, ready-to-use, standardized reagents for reliable test
results.
More information >
The IDEXX HerdChek Brucella abortus Antibody Test gives
interpretation guidelines for areas using vaccination to control
brucellosis, and for brucellosis-free or nonvaccinated populations.
The assay has been validated and USDA-approved for herds of up to 1000
animals.6
The IDEXX Brucella abortus test correctly detects the OIE
strong-positive standard as positive when diluted 1:125 in negative
serum and then again 1:10 in negative milk. This translates into a
sensitivity that could detect a strong B. abortus
antibody-positive cow in a pool of 1000 milk samples.
The assay also correctly detects the OIE weak-positive standard
diluted 1:16 in negative serum and then again 1:10 in negative milk
positive, which translates into a sensitivity that could detect a weak
B. abortus antibody-positive cow in a pool of 160 milk
samples.7
When a positive bulk or pool milk test result is obtained, all cows
contributing milk, or preferably all cattle in the herd, should be
tested serologically using milk, serum or plasma samples. Animals
seropositive for brucellosis should be removed from the herd to
eliminate the source of infection.
Conclusion: The HerdChek Brucella abortus
Antibody ELISA is a sensitive, rapid and easy-to-use diagnostic tool
for screening cattle herds for bovine brucellosis in milk and bulk
milk samples.
Enzootic bovine leukosis
The IDEXX HerdChek Anti-BLV Antibody Test is an enzyme immunoassay
for detection of antibody to bovine leukemia virus (BLV), the
causative agent of enzootic bovine leukosis (EBL) in serum and milk
samples.
Note: This test is USDA-licensed for serum only.
Dr. Carole Simard, from the Canadian Food Inspection Agency,
reported preliminary data obtained with the HerdChek Anti-BLV ELISA
for serum and milk samples.8 Her data indicates that the
IDEXX BLV ELISA, using milk rather than blood samples, might be used
to diagnose bovine leukosis in dairy cattle. The test is sensitive
enough to diagnose BLV in pools of sera (blood) and milk, individual
milk, and bulk tanks.9
In Switzerland, Rosskopf, et al. found that the HerdChek Anti-BLV
(Milk) had an agreement of 99.8% when testing milk samples, as
compared to another commercially available BLV-ELISA.10
More information >
Conclusion: Testing milk samples for antibody to
BLV using the HerdChek Anti-BLV (Milk) ELISA could be a cost-efficient
screening for BLV infection in herd cattle.
Bovine viral diarrhea virus
(BVDV)
The HerdChek BVDV Antibody Test is an enzyme immunoassay for
detection of BVDV antibodies in bovine serum, plasma and milk samples.
The assay detects antibody to BVDV type I and type II strains, and
shows a good correlation to virus neutralization assays.
More information >
Note: The HerdChek BVDV Antibody Test has not been
approved for use in the U.S., and is available from IDEXX Europe only.
BVDV is highly infectious and transiently infects cattle, causing
seroconversion usually 2–3 weeks post-infection. However, in
herds with persistently BVDV-infected (PI) cattle, the percentage of
BVDV antibody-positive cattle is very high, which can be detected in
milk and bulk milk samples by using the HerdChek BVDV Antibody ELISA.
Testing milk and bulk milk samples for antibodies to BVDV can be a
useful tool for initial screening for disease prevalence and for
maintenance of the negative disease status in BVDV-free herds.
Kuhne, et al. compared three commercially available indirect BVDV
ELISAs to a neutralization test using serum and milk samples. They
found the IDEXX HerdChek BVDV Antibody ELISA detected seroconversion
as early as 14 days post-infection, and was better at detecting BVDV
II compared to the other assays tested. The same authors tested milk
samples compared to a neutralization test, and found a sensitivity of
100% for the IDEXX HerdChek BVDV Antibody ELISA vs. 92% and 96% for
two other BVDV Antibody test kit suppliers.11
Note: In cattle herds using BVDV vaccination,
vaccine-induced antibodies may interfere with antibodies from
transient BVDV infection. Therefore, disease control based on antibody
monitoring in serum and milk should be done only in nonvaccinated
herds.
Figures A and B: BVDV antibody-positive individual
milk samples from Austria (Figure A) and other European countries
(Figure B) were diluted in negative bulk milk samples. Depending on
the amount of antibody to BVDV in the individual milk sample,
antibodies could be detected in dilutions from 1:10 up to 1:80.
More information >
Conclusion: Persistently infected cattle are a
major cause of BVD transmission. In immune-competent cattle, BVDV is
highly infectious, causing transient infection and seroconversion of
most cattle in the herd. In cattle herds having persistently infected
animals, the percentage of antibody-positive lactating cows is
generally very high. This should indicate the possible utility of the
IDEXX BVDV Antibody ELISA for detection of BVDV herd infection in bulk
milk samples from 50 cows (a size that is commonly used for testing).
Neospora caninum (N.
caninum)
The IDEXX HerdChek Anti-Neospora caninum Antibody Test Kit
is an enzyme immunoassay for the detection of antibody to N.
caninum in bovine serum. The assay can be used for
epidemiological studies and for management decisions to attempt to
reduce the prevalence of neosporosis in cattle herds.
More information >
Note: This test is USDA-licensed for serum only.
Recently, Schares, et al. evaluated the IDEXX assay for the use in
milk samples. Using a milk dilution of 1:2 and a cutoff of S/P 0.261
determined by TG-ROC analysis, they found that the IDEXX Anti-Neospora
caninum Antibody Test Kit had a sensitivity and specificity of
90%, testing 791 paired milk and serum samples. They concluded that
the test could be a potentially valuable tool to assess the herd
status with regard to abortion caused by N. caninum.12
Use this link to download
the article.
Conclusion
The HerdChek Anti-Neospora Test is used worldwide to test
cattle for infection with Neospora caninum, a major cause of
abortion in cattle. Recent scientific data suggest that the test could
potentially be used for milk samples to test for neosporosis in cattle
herds.
Johne's disease
(paratuberculosis)
IDEXX HerdChek M.pt. test is an enzyme-linked immunoassay
for the detection of antibody to Mycobacterium paratuberculosis
(M.pt.), the causative agent of paratuberculosis (Johne's
disease). IDEXX M.pt. ELISAs are approved for serum and
plasma samples and widely used in the U.S., Europe, Australia and
other countries. View
more information about the M.pt. tests.
The IDEXX M.pt. tests are not approved for milk testing,
but currently, IDEXX is investigating the potential use of milk
samples. Because a certain percentage of cattle will have low antibody
levels in the blood, and even lower amount of antibody in the milk,
the detection of positive animals from milk seems difficult. Because
of the low intraherd prevalence of cattle seropositive for Johne's
disease (mostly 5–25%), testing bulk milk would be unlikely to be
of diagnostic value.
Infectious bovine
rhinotracheitis (IBR)
Testing of milk and bulk milk samples for antibodies to bovine
herpesvirus I (BHV-1), the causative agent of IBR, is widely used in
veterinary diagnostic laboratories. Automated systems allow a rapid
and reliable processing of those samples.13
IDEXX offers three IBR ELISA test kits (HerdChek Anti-IBR, HerdChek
Anti-IBRgE and HerdChek IBRgB) that have been validated and approved
for milk testing in the U.S. and/or other countries.
More information >
Note: The HerdChek Anti-IBR test is USDA-licensed
for serum and milk. The HerdChek Anti-IBRgE and HerdChek IBRgB are
available from IDEXX Europe only.
For the HerdChek Anti-IBR, Rosskopf et al. found in a study
performed in Switzerland, an agreement of 99.8% testing milk samples
compared to another commercially available IBR-ELISA .14
In the Netherlands, IBR control on IBRgE deleted-marker-vaccinated
cattle populations is performed on milk and bulk milk samples.
Wellenberg, et al. reported that the IDEXX IBRgE test had a
sensitivity of 96% testing milk samples from marker-vaccinated and
IBR-infected herds.15
The same group of scientists evaluated the IDEXX IBRgE test for the
detection of antibodies to BHV-1 in bulk milk samples. On 500 bulk
milk samples from BHV-1-negative herds from Denmark, the specificity
of the IBRgE test was 100%. The sensitivity of the IBRgE test on 69
BHV-1-positive herds was 88.4%. The authors concluded the HerdChek
IBRgE ELISA to be a suitable tool to detect herds positive in bulk
samples when 10–15% of the animals within the herd had become
BHV-1-gE-seropositive.16
Additionally, Frankena et al. found, in another study performed in
the Netherlands using an IBRgB ELISA, that at an average herd size of
45 dairy cows, the introduction of a positive animal onto a negative
farm often remains undetected.17
Note: ELISAs, including the IBRgE-ELISA, are
increasingly used for the detection of antibodies in (bulk) milk
samples, but have some limitations. A negative bulk milk test
indicates that not more than 20% of the adult milking herd has
antibodies to BHV-1. Many individual seropositive cows have a milk
antibody titer less than 1/5. Consequently, it is not possible to
declare a herd to be free from BHV-1 infection on the basis of bulk or
pooled milk tests, and a negative bulk milk test should be followed up
with individual serum samples from all cattle in the herd. For general
surveillance purposes, bulk-milk tank tests can give an estimate of
BHV-1 prevalence in an area or country. These should be supplemented
by serum testing (individual or pooled) from nonmilking herds.18
Conclusion: All three IDEXX IBR tests, the
HerdChek Anti-IBR, Anti-IBRgE and IBRgB ELISAs, have been demonstrated
in independant studies to be useful to detect antibody to BHV-1 in
milk and bulk milk samples. In approved countries, these tests are
suitable tools to assess the IBR herd status. However, the tests
should not be used to detect single IBR-positive animals in commonly
used bulk-milk test sizes of 50 dairy cows.
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In
Ruminant Testing
When using
whole milk, the samples should be centrifuged or allowed to stand
undisturbed overnight at 4°C to allow separation of the cream
layer. The samples intended for the assay should be drawn from below
the cream layer because cream in the ELISA plate has the potential
to cause unreliable results. Samples that have curdled should not be
used.