HERD HEALTH                                       PIH-99


Mycobacteriosis (Tuberculosis) in Swine

Janet B. Payeur, USDA/NVSL, Ames, Iowa
J. Glenn Songer, University of Arizona

John R. Cole, Jr., University of Georgia
Ralph F. Hall, University of Tennessee
John and Teri Peters, Twin Falls, Idaho
Charles O. Thoen, Iowa State University

Introduction and History

     Mycobacteriosis, also called tuberculosis, is found in about
0.4%  of  all hogs slaughtered under Federal inspection (based on
United States Department of Agriculture [USDA], Food  Safety  and
Inspection  Service  [FSIS], records for 1990) and probably costs
the swine industry between $5.1 and $6.3 million  annually.  This
is  not  a  large  amount  compared  to  losses  from other swine
diseases. Although there are relatively few infected  herds,  the
economic  losses  can be devastating to those producers that have
the disease in their herds.

     Mycobacteriosis in pigs bears almost no resemblance  to  the
disease  in cattle or humans. The disease in pigs has no apparent
affect on the health of  the  animal.  Apparently  it  cannot  be
transmitted  to  man,  either by direct contact or by eating pork
products. Lack of transmission of the disease from  pigs  to  man
cannot be proven, however. Thus, meat inspection regulations for-
mulated in l972 call for special handling of carcasses  in  which
evidence  of  mycobacteriosis  is found. Economic losses occur to
the swine industry because of these regulations.

     Tuberculosis has been largely eliminated in cattle and poul-
try.  Tuberculin  testing  of cattle with subsequent slaughter of
reactors, and in some cases depopulation  of  entire  herds,  has
lowered the prevalence of the disease to about 0.001% (FSIS, 1990
records) in slaughter cattle. Similarly, the poultry industry has
changed  to  all-pullet  flocks  and  has  essentially eliminated
tuberculosis. Elimination of older birds has  been  an  effective
control  measure  in  this industry. The rate of condemnation for
tuberculosis is 0.0001% in light fowl (FSIS, 1990 records).

     It has been assumed by many that eradication of tuberculosis
from cattle and chickens would automatically lead to its eradica-
tion in swine. This has not been the case, however, and  mycobac-
terial  infections  in  swine remain a problem for pork producers


     Historically, the primary agents of mycobacterial disease in
swine  are  the  same  as  those  causing  tuberculosis  in other
animals. The disease is transmitted through animal to animal con-
tact.  Early  in the 20th century when tuberculosis in cattle and
man was more  prevalent,  mycobacteriosis  in  swine  was  either
Mycobacterium  bovis  or  M.  tuberculosis.  By l925, however, M.
avium, the cause of tuberculosis in birds, began  to  occur  more
frequently  in  swine.  The  most common serotypes of M. avium in
pigs are 1, 2, 3, 4, 6, and 8.  Outbreaks of disease due to these
serotypes appear to have originated in soil, contaminated litter,
usually sawdust, wood shavings or straw and contact from infected
fowl.  Today  isolation  of mycobacteria other than M. avium from
swine, is uncommon (less than 4%;  National  Veterinary  Services
Laboratories  [NVSL]; 1990). From 1985 to 1990, the NVSL isolated
M. bovis from swine only 4 times.  In each case these  pigs  were
on the same premises with M. bovis-infected cattle.

     The public health significance of Mycobacterium  avium  com-
plex  infections  in man is now recognized. The most common sero-
types of M. avium complex reported from  patients  with  acquired
immune  deficiency syndrome (AIDS) are serotypes 1, 4 and 8. They
are among the most virulent of the known serotypes and  are  most
likely environmental in origin. Since M. avium and other mycobac-
teria abound in the environment and therefore occur in  food  and
drinking water, it is not surprising that they are present in the
human alimentary tract. Thus the alimentary tract  could  be  the
source of frequently disseminated M. avium infections in patients
with AIDS.

     Rhodococcus equi infection in swine also produces  a  local-
ized  lesion  that resembles mycobacteriosis microscopically. The
earliest reports of R. equi infection in pigs  were  made  during
the  l930's, and isolation of the organism has been reported fre-
quently since then. Rhodococcus equi is common in the soil of hog
pens,  and  infection with this organism occurs about as often in
swine with or without mycobacteriosis. The importance of R.  equi
infections to the swine industry is unknown.

     In summary,  although  other  bacteria  can  cause  diseases
resembling  swine  mycobacteriosis,  M.  avium is responsible for
nearly all reported cases (96%; NVSL, 1990) in countries with  M.
bovis  eradication  programs. There is no evidence that swine are
the source of M. avium infections in man. The environment is  the
most likely source for both man and animals.


     Pigs usually become infected with M. avium by ingesting  the
organism.  After  ingestion,  the organism penetrates the wall of
the pharynx near the tonsils or the wall of the  small  intestine
and  becomes  localized  in  the  mandibular and mesenteric lymph
nodes respectively. Small areas of  infection  develop  in  these
lymph nodes, and the organisms rarely escape these initial sites.
As a result, the health and condition of the infected  pigs  usu-
ally  are not affected, and it is often impossible to establish a
clinical diagnosis of mycobacteriosis in these animals.  Mycobac-
terium avium also has been isolated from the lymph nodes of swine
that were negative to skin tests, presented  no  lesions  in  any
tissue, and had no signs of illness.

USDA Regulations

     Lesions (granulomas) in the lymph nodes of infected pigs are
found at slaughter. These granulomas are small abscesses that are
detected by repeatedly slicing the  lymph  nodes  with  a  knife.
Before  l972, tissues with lesions were trimmed and discarded. In
l972, a new USDA regulation required all carcasses found to  have
2  isolated lesions of mycobacteriosis to be cooked at 170o  F for
30 minutes. For example, if one lesion is found in the mandibular
lymph node and one near the mesenteric lymph nodes, a carcass was
classified "passed for cooking"  or  PFC.  This  step  was  taken
because  tissues  of infected swine were suspected as a potential
source of infection  for  humans.  Carcasses  processed  in  this
manner  lose  most  of their commercial value, and the additional
labor in cooking is  an  added  expense.  Also,  many  processing
plants have no facilities for cooking, and the carcasses are sub-
sequently condemned.  When lesions are found  in  only  one  site
such  as  the head or small intestines, the affected part is con-
demned and the carcass is passed without restriction.

     During l979 and l980, studies were conducted at the National
Animal  Disease  Center  to  determine  the  temperature and time
needed to effectively eliminate mycobacteria from edible  tissues
and  meat products during processing. When wieners were processed
at 150o  F for at least 10 minutes, 99.9%  of  added  mycobacteria
were  killed.  Subsequently,  the  FSIS  of  the  USDA proposed a
revised set of processing guidelines  for  PFC  carcasses.  Since
most  M.  avium-infected  pigs  are  PFC  (14,997  in  1990, FSIS
records) and few are condemned (3,437  in  1990,  FSIS  records),
these  proposed changes to lower the temperature for cooking car-
casses would virtually eliminate the mycobacteriosis problem  for
pork  producers  and  packers. However, they have not been imple-
mented because of anticipated bad publicity for government  agen-
cies and the pork industry.


     Because diagnosis of mycobacteriosis in the live  animal  is
usually  impossible, the prevalence of the disease must be deter-
mined from post-mortem findings  by  meat  inspectors.  The  pre-
valence  of  lesions  was  about  0.4%  in hogs slaughtered under
Federal inspection in 1990. The  actual  infection  rate  may  be
higher  since  mycobacteria can be cultured from lymph nodes with
no visible lesions and because some lesions  may  go  undetected.
Moreover,  since  infection  with  R. equi may be misdiagnosed as
that caused by M. avium, the reported rate of mycobacteriosis may
be higher than the actual rate.

     Tuberculous chickens may continue to be a primary source  of
infection  for swine, although other environmental sources may be
more significant. Garbage feeding is a possible, but  infrequent,
means  of  spreading  swine mycobacteriosis. Improper handling of
chicken wastes fed to swine also may allow  transmission  of  the
disease.  Soil  and water are other possible reservoirs of infec-
tion for pigs. Pathogenic mycobacteria may survive for more  than
4 years in soil and litter contaminated by chickens with tubercu-

     Studies have shown that sawdust or wood  shavings  used  for
bedding  may  be  a  source of mycobacteriosis in swine. Mycobac-
terium avium is often found in samples of sawdust and wood  shav-
ings  where  it  survives  for long periods. The mycobacteria may
multiply under proper  conditions  of  moisture  and  temperature
which  could  explain  the  seasonal occurrence of the disease in
some herds.  Seasonal changes may produce less  favorable  condi-
tions  for  survival of organisms in wood shavings and thus cause
the infection rate to decrease.

     The presence of infected sites in the intestinal  wall  with
subsequent pig-to-pig transmission probably is due to shedding of
mycobacteria in the feces.  Mycobacterial  infections  of  lungs,
mammary  glands, and uterus also may occur with the potential for
transmission of organisms from these sites. Thus, the addition of
infected  breeding stock could introduce the disease into a herd,
and transmission from infected sows to their litters may maintain
the disease within a herd.

     Many species of wild birds are infected with M.  avium.  The
disease has been transmitted to sparrows and pigeons either caged
or associated with domestic poultry. Prevalence of the disease in
starlings may be as high as 5%.


     Infection in pigs exposed to M. avium is usually limited  to
the  lymph  nodes  of the head and the digestive tract and rarely
spreads to other locations.  Diagnosis of mycobacteriosis by phy-
sical examination of the live pig is usually impossible.

     Visual examination of infected sites at  slaughter  can  not
differentiate  lesions  of  mycobacteriosis  from those caused by
other microorganisms or conditions; a confirmed diagnosis  should
be  based  on microscopic examination, isolation, and identifica-
tion of mycobacteria from these sites.

     Diagnosis of mycobacteriosis in swine on  a  herd  basis  is
important  and  usually  depends  on  detection of infected lymph
nodes from pigs at slaughter. When mycobacteriosis has been  con-
firmed  by  microscopic  and bacteriologic examinations, the pro-
ducer should work with  a  veterinarian  to  determine  potential
sources  of the infection and alter management practices to elim-
inate them if possible.

     Tuberculin skin testing has been used for the study of swine
mycobacteriosis.   The  amount of tuberculin used and the site of
injection have varied depending on the investigator.  The  recom-
mended method for a tuberculin skin test in swine is an intrader-
mal injection of 0.1 ml avian PPD in the dorsal  surface  of  the
ear.  The  response  to  injection  is read and recorded 48 hours
later.  Positive reactions usually include swelling and  redness,
and  they  may vary in size and intensity. Hemorrhage and ulcera-
tion may occur at the injection site.

     The reliability of the tuberculin test when used on  indivi-
dual  pigs  has  been questioned. The tuberculin test can be used
successfully as a herd test although false positive and  negative
reactions  occur. No other tests for diagnosis in the live animal
are currently in general use.

Prevention and Control

     Control of mycobacteriosis in swine is difficult because  no
vaccine  is available, and the preventive use of drugs or antibi-
otics in feeds is either illegal or of unknown value.

     Preventing the disease in noninfected herds is  more  effec-
tive than trying to eliminate the disease from infected herds. It
is important not to mix swine and poultry  production  activities
on  the  same farm. Feeding uncooked garbage, unpasteurized milk,
or other materials that might contain viable mycobacteria to pigs
must   be  avoided.  Breeding  stock  should  be  purchased  from
mycobacteriosis-free herds (those in which no lesions of tubercu-
losis  are  found in slaughter pigs). This measure is less impor-
tant since transmission of mycobacteriosis from  pig  to  pig  is

     Efforts should be made to prevent all contact  between  hogs
and wild birds. The potential for transmission of mycobacteriosis
from infected wild birds to pigs is probably slight but  must  be

     Hogs should not be housed in old  poultry  buildings  unless
they  have first been thoroughly cleaned and disinfected. The use
of woodshavings for bedding, especially in  farrowing  buildings,
should  be eliminated. Some producers have used woodshavings with
no problems, but others have been forced out of business  because
of  mycobacteriosis. Woodshavings should be kept dry at the sawm-
ill and on the farm and  protected  from  contamination  by  wild

     There are few options for eliminating  mycobacteriosis  from
infected  herds.   First,  producers  may depopulate the herd and
then repopulate with stock from mycobacteriosis-free herds.  Lit-
tle  is  known  about  decontamination  of  infected  soil  since
mycobacteria can survive in  this  environment  for  at  least  4
years.   To  avoid  such  problems,  concrete lots should be used
whenever possible.  Concrete  surfaces  and  equipment  including
farrowing  crates  and feeders must be disinfected with a phenol-
based disinfectant such as AmphylO or a 2  to  3%  cresylic  acid
solution.  Quaternary ammonium disinfectants such as RoccalO will
not kill mycobacteria. Mycobacteriosis will recur if  the  source
of  infection cannot be effectively decontaminated or if replace-
ment stock is not separated from the source.

     Second, potentially infected gilts can be kept  to  increase
the  herd  size  or replace older breeding stock. Because lesions
caused by M. avium usually disappear with age, older sows usually
pass standard inspection.

     Third, producers may choose to  endure  the  6-month  period
until  all  exposed  pigs  have been slaughtered if the source of
infection such as infected bedding can be found and eliminated.

     Mycobacteriosis increases the need for mandatory identifica-
tion of slaughter hogs. When mycobacteriosis is diagnosed, a pro-
ducer is free to send the hogs  to  slaughter  through  a  public
market  and  force  the  packer  and other producers to share the
economic loss. The ability to trace hogs with mycobacteriosis  to
the herd of origin would help solve this problem.


     1.   Swine mycobacteriosis causes an annual loss of  between
          $5.1  to $6.3 million to the pork industry and is found
          in 0.4% of all swine slaughtered under Federal  inspec-

     2.   Economic loss is not from death or illness of pigs  but
          from  the loss of carcasses passed for cooking and con-
          demned for mycobacteriosis.

     3.   The primary cause of swine mycobacteriosis is M. avium,
          the cause of tuberculosis in poultry.

     4.   The source of infection for pigs may be infected  poul-
          try  or  other birds, uncooked garbage, the environment
          (soil,  woodshavings,  water),   and   possibly   other
          infected pigs.

     5.   Clinical diagnosis of mycobacteriosis in the  live  pig
          is  usually not possible. The occurrence of the disease
          in a herd is best diagnosed by observing probable sites
          of  infection in lymph nodes at slaughter and examining
          these lesions microscopically and bacteriologically.

     6.   Practical immunization and drug therapy  are  currently
          not available. The disease is best prevented by careful
          management of swine feed sources and the environment.

     Reference to products in this publication is not intended to
be  an  endorsement to the exclusion of others which may be simi-
lar. Persons using such products assume responsibility for  their
use in accordance with current directions of the manufacturer.

REV 12/91 (7M)


Cooperative Extension Work in  Agriculture  and  Home  Economics,
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