Johne’s Disease in Sheep

Johne’s disease (pronounced YO–nees) is a contagious, chronic, and usually fatal infection that affects

primarily the small intestines of ruminants. Johne’s

disease (JD) is caused by a resistant species of bacteria belonging to the same family as tuberculosis

and leprosy and is found all over the world.

Concern about JD among U.S. owners of ruminant livestock is increasing due to the economic

impact of the disease on individual producers and the

possible impact on international marketing.

Individual Flock Owners Should Be

Concerned

JD can be an economic drain. Flocks with high rates

of infection may lose up to 10 percent of their adult

ewes due to wasting (loss of body condition). The

disease can require early culling of ewes, on average

at 2.5 to 4 years of age. Initially, JD may show up in

older ewes, but as the disease becomes more prevalent in the flock, younger animals begin to show clinical signs. The disease can also cause decreased

milk and carcass yield and affect sales of replacement and breeding stock.

What Is JD?

JD, or paratuberculosis, is a chronic bacterial infection caused by Mycobacterium avium subspecies

paratuberculosis (MAP). There are several strains of

MAP. The agent seems to change depending on the

species it infects. For example, the C or cattle strain

in bison has different culture requirements, making it

grow slower and be more difficult to culture than the

same strain from cattle. The S or sheep strain has

been very difficult to culture, and an optimal method

for detection has yet to be found for U.S. strains in

sheep.

In sheep with JD, the intestines become thick and

less efficient at absorbing nutrients. Affected sheep

continue to eat but lose weight and “waste away.”

Although the disease causes diarrhea in cattle, less

than 20 percent of sheep show diarrhea. In up to 70

percent of sheep, the disease may remain at subclinical levels, where individual animals never show signs

of the disease but shed the agent in their feces and

infect other sheep and contaminate the environment.

Johne’s Disease and Humans

JD is not currently known to be transmissible to

humans. However, Crohn’s disease in people resembles JD in ruminates. One theory about the cause of

Crohn’s disease is that, in genetically susceptible

people, some cases of Crohn’s disease are incited by

MAP. Evidence for this theory is not conclusive, and

uncertainty about potential risk to public health from

this organism persists. Crohn’s disease is not known

to occur more often in people exposed to infected

animal manure. MAP has been isolated from a few

humans with Crohn's disease, as have numerous

other bacteria and viruses.

Recent research from the U.S. Department of

Agriculture’s (USDA) Agricultural Research Service

indicates that commercial pasteurization does inactivate 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. Because of continued potential public-health concerns relating to this

disease, animal-production industries must give JD

more attention.

Other Factors That Cause Wasting in

Adult Sheep

The JD diagnosis often mimics other diseases such

as caseous lymphadenitis abscesses, dental disease,

ovine progressive pneumonia, scrapie, nutritional

problems, parasitism, and chronic infections of the

lung, liver, or kidney. In MAP-infected sheep, feces

can demonstrate high counts of parasite eggs. If an

individual sheep appears to have a parasite problem

in a flock with a good deworming program, Johne’s

should be ruled out as the underlying infection actually causing the wasting.

How Sheep Get Infected

JD is a disease producers “buy into.” It usually enters

flocks via an infected but outwardly healthy animal

that is releasing MAP into the environment through

its feces. Lambs are more susceptible than adult

sheep, but age resistance can be overcome with

higher doses or prolonged exposure. In cattle, MAP

has been shown to be passed to the unborn calf in

utero, and MAP can be found in colostrum and milk.

The same is probably true for sheep. MAP is resistant to heat, desiccation, UV light, freezing, and disinfectants and can survive in manure in pastures and

pond water for up to 11 months.

Factsheet 

Veterinary Services May 2006

APHIS 
How the Disease Progresses in Infected

Sheep

As with many infectious diseases, there are several

stages of JD. Stage 1 is silent infection—usually seen

in animals less than 1 year old that show no signs of

the disease. At this stage, JD is not detectable by any

tests.

Stage 2 is subclinical disease. The sheep with

Stage 2 JD shows no signs but may be shedding the

agent in its feces. Few sheep disclose JD in this

stage.

Stage 3 is clinical disease. The sheep with Stage

3 is losing weight but eating well. At this time, a blood

test can detect some infected sheep.

Stage 4 is advanced clinical disease. The Stage

4 sheep is weak and emaciated, shedding large numbers of the organism in its feces. Sheep will not survive once the disease has progressed to this stage.

JD is a herd problem. For every clinical case,

there may be 10-15 subclinically infected sheep in the

flock. This phenomenon is referred to as the “Johne’s

iceberg.” The number of observed cases is just the tip

compared to the number of subclinical (or incubating)

animals in the flock.

How To Determine If Your Sheep Are

Infected With MAP

To determine whether your sheep are infected with

MAP, you will need to combine history, clinical signs,

and test results.

Tests include

• Fecal or tissue culture (usually from tissues 

obtained at necropsy). Culture of the sheep strain

has been difficult, detecting less than 12 percent 

of infected sheep. New methods using liquid 

media that will greatly improve the sensitivity of 

cultures are currently being validated.

• Tissue histology. The presence of acid-fast 

bacteria and lesions typical of JD found on tissues

obtained at necropsy can help with the diagnosis.

• Blood tests. The agar gel immunodiffusion 

(AGID) test is currently the blood test of choice for 

diagnosing JD in sheep. This test can be used to 

diagnose disease in individual animals or to 

screen a flock of sheep for JD. The test works 

best in sheep at Stage 3 and 4 (finding 85–100 

percent of infected animals) and has a low rate of 

false-positive reactions. Most of these false 

positives are cross-reactions due to caseous 

lymphadenitis infection. Electroimmunosorbent 

assay (ELISA) tests for JD in cattle have been 

adapted for diagnosing JD in sheep. However, 

the use of ELISA tests is also limited due to cross

reactivity to caseous lymphadenitis.

Do I Need To Test All My Sheep?

Flock screening using targeted testing can determine

if you have a problem in your flock. One-quarter of

the flock is tested, beginning with sheep that have the

lowest body conditioning scores. Targeted testing

saves on costs while involving the animals most likely

to be infected.

Sheep positive on the AGID test should have their

status confirmed with fecal culture or tissue culture

because false-positive AGID reactions can occur.

Until fecal culture for sheep strain becomes available

in the United States, histology is the definitive or confirmatory test. Alternatively, the AGID could be

repeated in 8 to 10 weeks. Most cross-reactive antibodies will disappear in that period of time.

Most MAP-infected AGID-positive sheep do not

revert to a negative status. If the AGID test discloses

a positive result, confirmation by histology should still

be pursued to rule out other sporadic infections, such

as Mycobacterium avium.

What If I Don’t Think My Sheep Have JD?

Consider testing your flock, perhaps with the screening method above. If your sheep test negative, take

steps to protect your flock’s status. Make sure your

lambs kept for replacement do not get exposure to

adult manure, even at shows and on trailers. Before

buying new sheep, inquire about the JD status of the

source flock. Ask that the 25 percent of the flock with

the lowest body-condition scores be tested before

purchasing replacement animals from that flock.

What If My Sheep Do Have JD?

There is no treatment for JD, and a vaccine for sheep

is not available in the United States. (Vaccines currently available in other countries do not prevent new

infections but do reduce shedding of MAP into the

environment.) However, flock-cleanup plans can

reduce the prevalence of infection in your flock and

eventually eliminate the disease. Work with your flock

veterinarian to develop a flock-cleanup plan specific to

your operation, abilities, and goals.

Management changes alone can decrease the

prevalence of JD infection in a flock and reduce associated losses. These changes include using milk

replacer and reducing exposure of lambs to adult

manure. Flock cleanup is often possible with management changes and institution of a test-and-cull

program. Prevalence reduction can be achieved in

several years, but complete cleanup may take 7 years

or longer due to the chronic nature of the disease and

difficulty in diagnosing animals infected at subclinical

levels.
Plans To Address JD in U.S. Sheep

The U.S. sheep industry is concerned about JD and

has begun to develop a JD test-negative program

through the efforts of the United States Animal Health

Association’s Small Ruminant Committee. USDA is

working with several other agencies and universities

to develop and validate the technology to culture the

S strain of MAP.

For any JD-reduction program to be successful,

all laboratories testing animals for MAP must be performing at the same level to provide increased confidence in the test results. Laboratories can currently

be approved to conduct tests for JD in cattle by passing a check test from USDA’s National Veterinary

Services Laboratories (NVSL). NVSL plans to offer

such testing for sheep in the future.

For More Information

If you would like to know more about JD control and

prevention, contact

Michael A. Carter, D.V.M, M.P.H

National Johne’s Disease Control Program 

Coordinator

USDA–APHIS–Veterinary Services

4700 River Rd., Unit 46

Riverdale, MD 20737

(301) 734–4914 

e-mail: michael.a.carter@aphis.usda.gov

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