HERD HEALTH PIH-103
PURDUE UNIVERSITY. COOPERATIVE EXTENSION SERVICE.
WEST LAFAYETTE, INDIANA
Trichinosis
Authors:
K. Darwin Murrell, USDA/ARS Peoria, Illinois
George T. Woods, University of Illinois
LeRoy G. Biehl, University of Illinois
Reviewers:
Ray Gamble, USDA/ARS, Beltsville, Maryland
Peter M. Schantz, CDC, Atlanta, Georgia
Trichinosis has been a stigma to the consumption of pork for
years. A recent study indicates that a trichina-safe pork supply
would increase consumer confidence and pork consumption, and
result in additional income to pork producers. The National Pork
Producers Council is studying trichinosis and methods of provid-
ing trichina-safe pork to consumers. The Council has set a goal
of a trichina-safe pork supply by 1987.
Trichinosis is a disease of man and other animals caused by
a tiny parasitic worm, Trichinella spiralis. Humans may be
infected by eating the meat of infected domestic pigs or occa-
sionally the meat of wild bears, wild pigs, or walruses. A number
of wild animals are known to be infected. Over the last decade,
between 100 and 150 human cases per year are reported in the
United States. One study indicated 73.2% of the human cases were
attributed to pork products.
The number of human cases of trichinosis has declined
dramatically in the United States in the last 40 years, but the
infection rate remains the highest of any developed country in
the world. Recent surveys indicate the national infection rate in
swine is about 0.125% per year. The infection rate in swine in
Germany is 0.00003%, 0.0008% in Russia, and 0.0% in Denmark. With
approximately 89 million hogs slaughtered each year in the United
States, this means there are about 110,000 infected hogs per
year. If 360 meals are obtained from one hog carcass, approxi-
mately 40,000,000 potential servings of infected pork are pro-
duced each year in the United States. Some investigators estimate
that there are 100,000 to 300,000 human exposures per year in the
United States but 99% of resulting infections are subclinical. In
Europe, a major factor in reducing the incidence of swine trichi-
nosis has been the adoption of specific trichina inspection pro-
cedures at the slaughter houses.
The major importance of trichinosis in swine is the danger
of human exposure resulting in possible clinical disease. Accord-
ing to a study by the Department of Energy, the purely economic
benefits to the swine industry of a reduction in trichinosis
would be an estimated $449 million dollar increase in revenue per
year, an increase in pork exports by one-third, an increase in
domestic pork consumption of 2%, and increased confidence of con-
sumers in trichina-safe pork.
Life Cycle
Swine and wild animals are the reservoirs of trichinosis.
Swine are usually infected by consuming viable trichinae larvae
in pork scraps found in uncooked garbage and by meat from
infected carcasses of swine, rats, and other carnivorous wildlife
(Figs. 1, 2, and 3).
After the ingestion of infected meat, the larvae are dig-
ested free of the muscle cyst, enter the small intestine, and
within four to six days develop into sexually mature adults. The
adults give birth to larvae that migrate through the intestinal
wall into the circulatory system. When they come into contact
with skeletal muscle, the larvae invade the muscle and by 17-21
days after infection become mature encysted larvae. Once again,
the life cycle is complete, and the trichinae are in the infec-
tive stage. Encysted larvae can survive in putrefying meat for
long periods. It has been estimated that 25-30% of the total
number of muscle larvae present in an infected pig carcass are in
the hams and 20% are present in the shoulder cuts. Apparently,
swine naturally infected with trichinosis do not show clinical
effects. In experimental infections with large numbers of larvae,
however, rear paralysis and systemic reactions have been
reported.
Treatments
No routine treatment for infected swine before slaughter has
been developed that will clean the animals of trichina cysts. In
human infections, thiabendazole and other supportive treatments
are used. Similar drugs have been used experimentally in swine
and have been found effective, primarily against the adult worms
in the intestine and less effective against muscle larvae.
Prevention and Control
Experimental vaccines are being studied in pigs but are not
available. At this time, management practices are the only tools
available to producers to eliminate trichinosis from their market
animals. Producers should practice the following trichina-
preventive measures:
o Observe all garbage feeding regulations. If garbage is fed,
feed only well-cooked garbage, including household scraps
(212o F. for 30 min.).
o Practice stringent rodent control. Rats may be important
sources in some swine herds.
o Avoid exposing dead pig or wild animal carcasses to live
hogs. Do not throw wild game carcasses or parts to hogs or
domestic pets.
o Ensure that hog carcasses are properly buried, incinerated,
or sent to a rendering plant.
o As often as possible, construct effective barriers between
hogs and wild animals.
Pork and meat from all wild mammals should be thoroughly
cooked before human consumption. Official federal and state meat
inspection programs require that all processed pork products that
may be eaten without additional cooking be heated to at least
137o F. to assure destruction of any trichinae larvae that may be
present. A recent USDA study indicated an increased chance of
survival of trichinae in microwave cooking. Uneven cooking with
cold spots in the microwave oven may cause some areas of fresh
pork not to reach 137o F. and thus any live trichina would per-
sist. This has brought on a wave of anxiety about trichinosis by
the public. However, recent research indicates that pork can be
prepared safely in the microwave if an oven cooking bag is used
in the cooking procedure. To allow a margin of safety the USDA
recommends fresh pork be cooked to 170o F.
Fresh pork less than 6 in. thick can be rendered safe if
frozen to 5o F. (-17o C.) for 20 days, -10o F. (-23o C.) for 10
days, or -20o F. (-29o C.) for 6 days. Dry curing, which is the
interaction of salt and drying for relatively long periods, will
devitalize trichina cysts, if proper time and temperature rela-
tionships are established.
Hamburger ground in a grinder not properly cleaned following
grinding raw pork, or hamburger that has pork added illegally,
may transmit the disease to humans if it is insufficiently
cooked.
Trichina-Safe Pork
As a part of the National Pork Producers Council's
Trichina-Safe Pork in the U.S. by 1987 program, samples would be
collected from all swine at slaughter and tested for trichinosis.
Infected herds could then be traced back to the farm of origin if
a national swine identification program is enacted. The
Trichina-Safe Committee of the NPPC has recommended adoption of
such a swine identification program.
Since low-dose irradiation (30,000 rads) is sufficient to
inactivate encysted trichinae, feasibility of use of this pro-
cedure on pork carcasses after slaughter also is being studied.
Eradication and certification of trichina-safe pork will
open up new markets for the swine industry at home and abroad. A
strong educational campaign of producers and consumers is neces-
sary to remove this currently held stigma from pork.
REV 12/91 (7M)
Figure 1.Source:Zimmerman, W., and Zinter, D. H.S.M.H.A. Health
Reports 86 (Oct. 1971): 937-945. (Adapted from and used with per-
mission)
LIFE CYCLE OF TRICHINELLA SPIRALIS
1. INFECTED MEAT EATEN BY HOST
2. MEAT FIBERS AND CYST WALLS ARE DIGESTED WHICH...
3. RELEASE TRICHINAE... WHERE IN SMALL INTESTINE DEVELOP INTO...
4. SEXUALLY MATURE ADULTS... WHICH MATE
5. FEMALE GIVES BIRTH TO YOUNG TRICHINAE (1,000-1,500) IN MUCOSA
OF INTESTINE
6. YOUNG TRICHINAE TRAVEL BY WAY OF INTESTINAL LYMPHATICS TO
BLOOD CIRCULATION TO...
7. MUSCLES OF HOST WHERE THEY ENCYST
Figure 2.Sylvatic cycle, representing the transmission of trichi-
nosis in nature, independent of man. ``Carnivores and
scavengers'' include fox, bear, rat, walrus, hyena, wildcats, and
many others. In the case of human infection, the source would be
called game meat, rather than carrion, and the infection would
represent an offshoot of the cycle. Original diagram from W. C.
Campbell, ``Epidemiology I. Modes of transmission.'' In Tri-
chinella and Trichinosis, edited by W. C. Campbell. (New
York:Plenum Press, 1983): 425-444. (Adapted from and used with
permission)
Figure 3.Domestic cycle, the predominant source of human trichi-
nosis.
(Illustration courtesy of Dr. K. D. Murrell)
% Figures are available in hard copy
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