HERD HEALTH PIH-99
PURDUE UNIVERSITY. COOPERATIVE EXTENSION SERVICE.
WEST LAFAYETTE, INDIANA
Mycobacteriosis (Tuberculosis) in Swine
Authors
Janet B. Payeur, USDA/NVSL, Ames, Iowa
J. Glenn Songer, University of Arizona
Reviewers
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
today.
Etiology
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.
Pathogenesis
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.
Epidemiology
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-
losis.
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%.
Diagnosis
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
rare.
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
considered.
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
birds.
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.
Summary
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-
tion.
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)
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