Johne’s Disease in Cattle
Q. What is Johne’s disease, and what animals can
get it?
A.
Johne’s (pronounced “Yo-nees”) disease is an
infectious bacterial disease of livestock that primarily
affects the intestinal tract. Johne’s disease should be
considered a herd problem as well as an individual
animal problem.
Cattle, sheep, and goats are most commonly
affected. The disease has been reported in several
species of wild ruminants, both captive and free rang-
ing. In addition, a few reports of isolated cases in non-
ruminants (including nonhuman primates) have been
reported, but none of the affected species are believed
to be sources for Johne’s disease in cattle. Some
studies have cultured the microbe from, or detected its
genetic components in, humans. However, the signifi -
cance of these fi ndings as they relate to any human
disease has yet to be determined.
Q. What causes Johne’s disease?
A.
Johne’s disease is caused by the bacterium
Mycobacterium avium
subspecies
paratuberculosis
and is often referred to by the acronym MAP. MAP is a
distant relative of
Mycobacterium bovis
which causes
tuberculosis (TB) in humans and animals, but MAP
does not cause TB.
MAP bacteria grow and multiply inside the cells of
an animal’s immune system and are excreted in the
feces, and to a lesser extent in milk and saliva. When
the microbe is excreted, it can contaminate soil or
water. Outside the host animal, the organism multi-
plies poorly—if at all—but it can survive more than
a year in the environment because of its resistance
to heat, cold, and drying. The primary cause of the
spread of Johne‘s disease is contact with the feces or
saliva of an infected animal.
Q. What are the signs of Johne’s disease?
A.
Because of the slow, progressive nature of the
infection, the signs of Johne’s disease may not show
up until years after initial infection. When they fi nally
occur, the signs are long-lasting diarrhea and weight
loss despite good appetite. Affected cattle do not
generally have a fever. Some infected animals appear
malnourished and weak; others just have chronic diar-
rhea. Signs of Johne’s can easily be confused with
those of several other diseases. In an infected cow or
heifer, noticeable signs commonly start within a few
weeks after a stressful event, like calving.
Q. What causes the signs of disease?
A.
The bacteria are taken up by specialized cells in
the part of the small intestine called the ileum, where
nutrients are absorbed from feed. As the body tries
to rid itself of these bacteria, the immune response
causes a thickening of the intestinal lining, preventing
it from functioning normally. This leads to poor absorp-
tion of nutrients and eventual diarrhea. As a result,
although animals may be feeling and eating well, they
begin to lose weight gradually.
Q. Is Johne’s disease fatal?
A.
Yes, once clinical signs of the disease appear, the
animal will not recover and will continue to deteriorate
until it dies.
Q. Is there a cure for Johne’s disease?
A.
No, a cure has not yet been found.
Q. How can I tell if my herd is infected?
A.
Some animals may be infected, appear normal,
and be culled before they show any clinical signs. In
such cases, herd owners may never realize that their
herd is infected.
Beyond disease signs like diarrhea and weight
loss, producers should pay close attention if herd pro-
duction is going down or it is not as high as it should
be, especially in 3- to 6-year-old cows. In attempting to
fi nd the cause of low-herd production, owners should
test several ill-appearing animals for Johne’s disease.
In other herds, owners who see one or more cows
with diarrhea or weight loss should consider Johne’s
disease as a possible cause. Environmental samples
from cattle congregating areas can also be cultured to
determine if MAP, the bacteria that causes Johne’s, is
present on the premises.
Q. How can some cattle be infected with MAP, yet
not show signs?
A.
Infectious diseases, including Johne’s disease,
typically pass through four stages. Stage I is initial
infection. The animal is infected but not showing signs
of disease. It may be shedding small numbers of
microbes into the environment that are not detectable
by diagnostic tests.
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In Stage II, the infection is progressing and the ani-
mal still does not show any clinical signs. Neverthe-
less, the disease-causing organism may be excreted in
large numbers—enough to infect other animals nearby.
Infection is detectable by fecal culture or DNA probe
techniques but not often by blood tests.
In Stage III, a sick animal shows the early signs of
disease, and many types of diagnostic tests can detect
infection. Animals in Stage IV show obvious clini-
cal disease signs, readily recognizable by the trained
observer and detectable by diagnostic tests. It may
take 2 or more years for Johne’s disease to progress
through all of these stages in an individual animal.
A single herd affected by Johne’s disease may
have animals in each of the four stages of disease.
For each animal showing obvious signs of Johne’s
disease (Stage IV), 5 to 15 times as many animals
at various stages of infection may be present but not
showing signs.
Q. What are common sources of MAP bacteria?
A.
The most common source is feces, or manure.
Ingestion of food tainted by manure containing the
microbe is the most common way animals become
infected. Depending on conditions, the MAP organism
can remain alive in the environment for over a year
while protected in fresh manure.
MAP typically enters a herd via an infected but
healthy-looking animal in Stage I of the disease. As
the disease progresses in that animal, the frequency
and number of bacteria being excreted into the envi-
ronment increase. Every day, billions of Johne’s
microbes may be excreted from an animal in Stage III
or IV of the disease.
Another source of infection is milk from infected
dams. The likelihood of MAP being excreted in milk
from infected females increases as the disease pro-
gresses. Studies suggest that 36 percent of cows with
Johne’s in Stage III and IV could have MAP in their
colostrum. In beef herds, where calves remain with
their mothers and nurse daily, the chance for trans-
mission of the infection through colostrum and milk is
high. MAP bacteria may be excreted directly through
the mother’s milk, and they can be present on the
outside of teats from feces contamination.
Prenatal exposure may be a source of infection
for calves. Becoming infected before birth is possible
for a fetus if its mother is in the late stages of Johne’s
disease. Studies have shown that, in dams with Stage
III or IV of the disease, 8 to 40 percent of fetuses are
infected while still in the womb. The risk of infection for
fetuses from mothers in Stages I and II of the disease
is, however, low.
Pond water and pastures contaminated with
infected feces are other potential sources of infection.
Q. Can humans get Johne’s disease?
A.
The organism that causes Johne’s disease is not
currently known to cause disease in humans, but it
has been detected in humans with Crohn’s disease,
as have numerous other bacteria and viruses. The
symptoms of Crohn’s disease in humans are similar to
the signs of Johne’s disease in ruminants. However,
no defi nitive evidence is available proving MAP causes
Crohn’s disease. A few publications have shown MAP
to be an opportunistic pathogen in people with
compromised immune systems.
Research from the U.S. Department of Agriculture’s
(USDA) Agricultural Research Service indicates that
commercial pasteurization inactivates MAP bacteria in
milk. However, some researchers still have concerns
about MAP in undercooked meat, unpasteurized milk
products, and water as potential sources of exposure.
While MAP remains largely an animal health issue, the
risk of human exposure through contaminated food
sources creates a quality assurance concern in milk
and meat products.
Q. Where can I obtain more information on Johne’s
disease?
A.
For more information on Johne’s disease, contact:
Dr. Michael Carter
National Center for Animal Health Programs
USDA
-
APHIS
-
Veterinary Services
4700 River Road, Unit 43
Riverdale, MD 20737
Telephone: (301)
851-3510
E-mail: michael.a.carter@aphis.usda.gov
Or, visit the APHIS Web site a
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