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The Organism
The etiological agent (organism)
responsible for Johne's disease is Mycobacterium avium
subspecies paratuberculosis also known as
Mycobacterium paratuberculosis.
Mycobacterium avium
subspecies paratuberculosis or abbreviated MAP for short, is a
slow growing gram positive, facultative intracellular, acid fast
bacillus requiring exogenous mycobactin for growth outside of a
natural host (animal). Mycobactin, a chelating substance, provides a
more efficient mode of transport for required iron through the
organism's cell wall and is used as a supplement in laboratory media
to culture the organism. Outside of a natural host, MAP does not
reproduce (except under laboratory conditions), but can remain viable
(alive) in the environment for an extended amount of time. MAP has
been shown to survive up to a year in the environment: including
stream water, pond water, black soil, and manure slurry, due to its
resistance to heat, cold, and dry conditions. The organism is also
resistant to many forms of disinfectants.
The most common mode of transmission
for MAP into a natural host is by oral ingestion. Once inside a
natural host, the organism readily multiplies and is no longer
dependent on exogenous mycobactin for growth. Researchers believe the
organism obtains its requirement for iron by certain iron binding
proteins within the host's blood stream. Once established in a
natural host, the organism readily multiplies and is intermittently
shed back into the environment via the infected host's feces,
beginning the transmission cycle over again by infecting new hosts.
We have calculated that an infected adult cow can excrete up to 30
trillion live MAP organisms per day.
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Natural Host
and Transmission
Although largely considered a disease of cattle,
sheep, and goats, all ruminants are susceptible. Ruminants are
hoofed, four footed, cud chewing mammals that have a 3 to 4 chamber
stomach. Ruminants also include bison, deer, elk, llamas, and camels,
just to name a few. Paratuberculosis has been recognized as a problem
in wildlife, exotic animals residing in zoological gardens and in
non-conventional domesticated animals.
Models of transmission can occur prenatal (before
birth) or postnatal (after birth).
The most common mode of transmission occurs as
postnatal or after birth. Postnatal transmission most commonly occurs
when the causative organism is ingested, however, certain procedures
such as rectal examination and natural bull servicing practices may
also serve as possible modes of postnatal transmission.
Young animals are considered the most susceptible to
infection. The most common mode of transmission for infection is
through bacteria-laden feces. Ingestion of bacilli occurs most
commonly when young animals nurse on teats, which have been
contaminated with infected feces. In addition to contaminated fecal
ingestion, studies have shown the organism may also be present in colostrum and milk from infected dams and dairy cows. Studies are
currently in progress to evaluate the possibility of MAP surviving
current pasteurization methods.
Prenatal transmission of the causative organism can
occur within an infected cow. The organism (MAP) can disseminate
(spread) from the most commonly infected area, the gastrointestinal
tract, to extraintestinal sites. These sites include, but are not
limited to, the uterus, udder, and supramammary lymph nodes. Prenatal
transmission may also occur via semen from an infected bull during
natural service play and/or artificial insemination.
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