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John Hermon-Taylor Department
of Surgery, St. George's Hospital Medical School, London SW17 ORE,
U.K.
Correspondence to: Professor John Hermon-Taylor, Department
of Surgery, St. George’s Hospital Medical School, London SW17 ORE,
U.K.
Fax. 020-8725-3594
Email. jhermon@sghms.ac.uk
Received: 2000-07-05 Accepted: 2000-08-01
Subject
headings: mycobacterium avium;
paratuberculosis; Crohn’s disease; immunologic tests;
immunocompetence; mycobacterium avium subspecies paratuberculosis
Hermon-Taylor J. Mycobacterium avium subspecies paratuberculosi s in
the causation of Crohn’s disease. World J Gastroentero,
2000;6(5):630-632
Mycobacterium avium subspecies paratuberculosis (MAP), originally
called Johne
’s bacillus was first described from Germany in 1895 as the cause
of a chronic inflammatory disease of the intestine in a cow. As the
20th century progressed, clinical and sub- clinical MAP infection in
farm animals in Western Europe appeared to become more prevalent.
Among the early reviews available are the excellent ones prepared by
Doyle[1] from the Veterinary Laboratory, Weybridge, UK,
and Riemann and Abbas[2] from the University of
California, Davis. In general, the response on farms to the
appearance of clinical Johne
‘s disease was to cull infected animals. This practice over the
course of the 20th century may have exerted a selection pressure on
MAP favouring the emergen
ce of strains which can infect animals for years without necessarily
causing clinical disease. In the latter part of the 20th century the
incidence of clinical disease due to MAP in some areas of Western
Europe and North America appeared to decrease. The problem which
confronts these regions now is subclinical MAP infection of domestic
livestock throughout Western Europe
and North America and the emergence of wildlife reservoirs including
those in rabbits and their predators[3]. In the United
States and Canada the herd
prevalence of MAP infection is reported in the range 21%-54%[4-8].
In Western Europe the herd prevalence lies in the same range,
although a recent serological study of bulk-tanked milk from 900
dairy herds in Denmark reported
that 70% of herds tested positive for MAP infection[9].
What is beyond doubt is that MAP is widespread in our domestic
animals.
Subclinically
infected dairy cows secrete MAP in their milk. It is one of the ways
the organism passes from infected parent to offspring when the calf
may be most susceptible. MAP is more robust than M. bovis or M.
tuberculosis
and the destruction of all viable MAP by exposure to current
pasteurisation con
ditions of 72℃
for 15 seconds is not assured. In on-going research in the
Department of Food Science, University of Belfast, N. Ireland, small
slow-growi
ng, mycobactin-dependent, IS900 PCR positive colonies of MAP have
been cultured from abou
t 3% of retail units of pasteurised cows’ milk, so far tested. In
the U.K. what is also beyond doubt is that the human population is
being exposed to MAP i
n retail milk supplies. These organisms accumulate particularly in
the ileocolonic regions of the intest
ine where they may remain for years and not cause clinical disease.
This situation is similar in principal to the widespread exposure of
human popul
ations in Europe, North America and elsewhere, to M. bovis before
the intro
duction of milk pasteurisation and the tuberculin testing of dairy
herds introdu
ced i
n the middle third of the 20th century. With MAP, only those
individuals with a
particular inherited or acquired susceptibility may go on eventually
to develop
clinical disease.
Infected animals excrete MAP onto pastures. Wildlife
reservoirs
contribute to environmental contamination. The problems now being
caused by MAP, differ from those previously caused by M.bovis,in
that MAP can survive
for long periods in the environment. Rains falling on contaminated
land will w
ash MAP into ground and river waters. Although much research needs
to be carried
out in this area, it is probable that MAP in the environment is
taken up into o
rganisms such as amoebae in which they can survive. This may allow
them to repli
cate, to increase their resistance to biocides and potentially
acquire a phenoty
pe which is more pathogenic for humans. Water abstracted from rivers
and lakes contaminated with MAP may convey these organisms to human
populations. MAP arri
ving at domestic outlets in high dilution may accumulate in biofilms
lining hou
sehold water systems. Either in the food chain therefore, or in
water supplies,
it is inevitable that humans sharing the same geographic areas with
animals which are extensively infected, will be exposed to these
pathogens.
The
question as to whether MAP may also cause disease in humans has its
origins in a proposition first published in 1913 [10]. We
have recently prepared a detailed analysis of this complex issue
which is in general poorly understood, even by medical specialists
in the field of chronic i
nflammatory diseases of the intestine[11]. It is now
known that MAP can
cause chronic inflammation of the intestine in a very broad range of
animals inc
luding large and small ruminants, monogastrics such as dogs and
pigs, and so far at least four types of sub-human primates. MAP
shows a well defined tissue tropism and will end up causing chronic
inflammation of the intestine even if administered experimentally by
subcutaneous, intravenous or intraperitoneal routes. The
histopathological features of MAP disease in animals ranges from one
in which millions of typical ZN-positive MAP are visible
microscopically in the inflamed intestine to the other extreme where
no MAP can be seen at all, but there is chronic granulomatous
inflammation. This is just what leprosy does in humans. One of the
properties of MAP which has retarded our understandin
g of the problems it is causing is that it may be very difficult to
culture in the laboratory. Patient work in many laboratories has,
however, shown that MAP can be grown in conventional culture from
about 5% of people with Crohn’s disea
se, but not from normal people. Cultures have had to be incubated
for months or years before any growth identifiable by conventional
means, becomes visible.
MAP
is very similar to other organisms of the M.avium complex (MAC) to
which we are all exposed, so that immunological tests for MAP
infection in humans using crude extracts of laboratory cultured
organisms usually report no difference be
tween Crohn’s disease and normal people. Recent studies have
however shown that if specific targets on MAP are carefully
selected, highly significant differences in immune recognition can
be demonstrated between Crohn’s disease and normal people. An
important example of this has come from recent research at UCLA
which showed that the blood of 9 out of 10 people with Crohn’s
disease
contained IgA which recognised a mycobacterial protein richly
expressed on MAP called HupB[12]. HupB is identical to
the laminin receptor used by M. leprae to enter Schwann cells round
nerves causing the neural inflammation so characteristic of leprosy[13].
Neural inflammation is long known to be a specific feature in the
inflamed gut in Crohn’s disease[14] and antibodies to
the chronic inflammatory diseaseassociated
autoantigen pANCA, cr
oss-react with HupB.
In 1990, after spending 20 months
carefully optimising sample processing and exp
erimental procedures, we began a study which revealed the presence
of MAP DNA in about two thirds of people with Crohn’s disease
using IS900 PCR[15].
It was also present in the intestine of 12% of normal people, which
is just wha
t would be expected to occur in a totally exposed population. Since
then there have been 18 peer reviewed publications using a variety
of experimental methods,
9 of which reported the presence of MAP in CD gut some or most of
the time and
9 found MAP hardly ever or not at all[11]. Similar
inconsistencies have
occurred in the results of DNA tests applied to other chronic
inflammatory disea
ses such as TB. Apart from some obvious methodological errors, the
reasons for the uncertainty are the low abundance of MAP in Crohn’s
disease intestine and its extraordinarily tough
protease-resistant phenotype. Recent research by Dr Saleh Naser and
colleagues from the University of Central Florida [cited
in 11
], using improved liquid cultures and IS900 PCR on their
centrifugal pellets, has demonstrated MAP in 86% of surgically
resected Crohn’s disease gut.The same authors have also
demonstrated MAP in the centrifugal pellets of breast
milk from each of 2 mothers with Crohn’s disease who had recently
given birth,
but not in the breast milk of 5 normal women. Work in our own lab by
Jun Cheng and Tim Bull, using much improved methods, is currently
reporting Chinese MAP in 69% of Chinese surgical path blocks from
Crohn’s disease patients in China, and in 14% of path blocks of
normal intestine from Chinese people. In our view these studies
clearly demonstrate the presence of this chronic enteric pathogen in
a substantial majority proportion of humans with chronic
inflammation of the intestine of the Crohn’s disease type.
Although the incidence of Crohn
‘s disease in China is currently much lower than in Western Europe
or North Ame
rica, these recent studies in humans strongly suggest that Chinese
people are exposed to these pathogens and that action may need to be
considered at this early stage to limit the more extensive
development of disease in humans.
It
has long been known that infections due to non-tuberculous
mycobacteria in immunocompetant people, particularly those caused by
MAC, are usually resistant to standard anti-tuberculous drugs. These
organisms can prevent the drugs penetrating the microbial cell and
can rapidly develop mutations which confer drug resistance. Lasting
resolution of MAP infections in animals using standard
anti-mycobacterial treatment has never been convincingly
demonstrated, and much
the same outcomes have resulted from a similar treatment approach in
humans with Crohn’s disease[11]. MAP are however more
susceptible to some newer drugs which are man-made chemical
modifications of natural streptomyces
antibiotics such as rifabutin and clarithromycin. These agents also
have the advantage of being concentrated within macrophages where
MAP in infected animals
and in humans almost certainly resides. Our own studies from 1992[16],
supported by the work of Dr. Tom Borody in Sydney, Dr. Ira Shafran
at the Unive
rsity of Central Florida, and by recent work from the North of
England[17]
have shown that a substantial proportion of people with active
Crohn’s disea
se will go into remission with healing of the intestine which is
sometimes lasti
ng, when treated with combinations of these drugs. A randomised
controlled trial of this treatment was initiated in Australia in
September 1999.
A
question which is frequently asked is how can so few MAP cause so
much chronic
inflammatory disease? To answer this we must allow our thinking to
escape from the immobilising presumption that it must be like TB, in
which a major factor in the disease process is a direct
immunological response to cell wall component
s. MAP in animals with the paucimicrobial form of the disease and in
humans, does not have a classical mycobacterial cell wall. MAP
colonising immunoregulato
ry cells like macrophages almost certainly causes an immune
dysregulation. Together with defects in the integrity of the
overlying mucosa, much of the disease itself is caused by an
exaggerated immunological response to leakage into the gut wall of
bacteria and food residues normally confined to the lumen. Clinical
improvement
can be achieved by suppressing or modulating the immune system, by
reducing the allergic com ponent and altering the enteric flora with
elemental diets, and by treatment with antibiotics such as
ciprofloxacin and metronidazole. Without killing the underlying
causative pathogens however, the benefit which follows such
treatments is rarely lasting.
The
present analysis of the MAP problem suggests that particularly in
Western Europe and North America, we are challenged by a public
health issue of substantial proportions for which a range of
remedial measures are needed. These measures include conditions of
pasteurisation of retail milk which do ensure the destruction of all
viable MAP. We need to use the improved culture systems available
together with modern molecular methods to ensure our domestic
livestock are free of sub-clinical infection. We need to test water
supplies to make sure they are clean. In high incidence areas, we
need to make Crohn’s disease reportable so we have accurate data
to monitor the effect of these measu
res on the overall problem. We need to make a rapid increase in the
volume and intensity of research in the field, to sequence the MAP
genome and to develop preventative vaccines for animals and
therapeutic vaccines for humans.
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