History of Research at the
U.S. Department of Agriculture and Agricultural Research Service
Farms with hog cholera were quarantined.
In the early
years of this century [the 1900s], hog cholera "often swept through the
countryside, causing devastating losses. During the fall months, looking across
the prairies of the Middle West, one could often see smoke ascending from
perhaps a half-dozen farms where pigs dead of cholera were being burned,"
USDA veterinarian C.N. McBryde recalled later...
On January 31, 1978, Secretary Bob Bergland declared the
United States hog cholera free in ceremonies in Washington, D.C. This was 99
years after USDA began hog cholera research and 17 years after the start of a
Federal-State eradication campaign.
Whether hog cholera originated in America or Europe is not
definitely established, but most experts believe it to be native to this
country. Cholera was first reported in the United States in 1833 in southern
Ohio. By 1893, 90 separate areas of infection were known to exist. Outbreaks in
1886, 1887, and 1896 each killed more than 13 percent of the Nation's hogs;
more than 10 percent died during the 19l3 outbreak. The disease was still
costing producers $50 million a year in the early 1960's.
A key research discovery leading to control and eventual
eradication of hog cholera was made in l903. Marion Dorset of USDA's Bureau of
Animal Industry (BAI) demonstrated that hog cholera is caused by an
ultramicroscopic virus, and hogs recovered from the disease are immune for
Almost 20 years earlier, Department scientists thought they
had identified a bacterium resembling the one responsible for typhoid fever as
the cause of hog cholera. But when Dr. Dorset tested a serum from that
bacterium during an 1897 outbreak in Iowa, injected hogs still died of cholera.
He therefore questioned prevailing scientific opinion, and about 6 years later
proved that hog cholera is a virus disease.
USDA scientists have long played a role in developing vaccines for hog cholera.
The first practical preventive
measure, injection of anti-hog-cholera serum and then the virus, was
successfully tested in 1907 by the BAI Field Station near Ames, site of many
later advances in hog cholera research. Dr. Dorset, Dr. McBryde, and W.B. Niles
found that serum from the blood of immune hogs conferred immunity lasting only
a few weeks to other hogs. But injection of the BAI hyperimmune serum plus
injection of virus gave lifelong protection against hog cholera to most
A system of swine sanitation named for McLean County, Ill.,
where BAI developed it in 1927, became a valuable adjunct in immunization
against cholera by this method. The system, developed primarily to prevent
roundworm infestation of hogs, also reduced incidence of filth-borne intestinal
diseases. Serious side effects were produced when hogs with even low-level
intestinal infections were immunized against hog cholera.
Department scientists realized that a control method
involving use of the live virus offered no hope for eradicating hog cholera. So
they began work toward a protective vaccine made with killed virus. Research of
Drs. Dorset and McBryde, and C.G. Cole at Ames culminated in the development of
the crystal violet killed vaccine in 1935. An initial problem of contaminants
in the vaccine was overcome when F.W. Tilley patented a procedure 10 years
later for preparing a consistently sterile crystal violet vaccine.
Scientists long sought reasons why this and later improved
vaccines did not confer immunity to some hogs. The problem was partly solved in
1949 with the discovery of a variant of the hog cholera virus. Antiserum
against both types of virus was needed for protection.
Researchers in Iowa meanwhile had learned much about the
transmission and persistence of the hog cholera virus. They found that the
virus is present in the circulating blood of the sick animal and also in the
various secretions and excretions. The virus remained active throughout the
winter in carcasses of cholera-infected hogs buried in the fall, and unburied
carcasses of infected pigs remained infectious for 11 weeks during cold
Contrary to popular belief at one time, pigeons did not
prove to be carriers of hog cholera virus, although transmission by crows and
buzzards was not ruled out. Extensive experiments at Ames demonstrated
conclusively that the house fly and stable fly are capable of transferring the
cholera virus from sick to well pigs. Prevalence of biting flies and incidence
of hog cholera correlated closely during the years of the study.
Hogs under observation.
After Dr. J.A. Baker, Cornell
University veterinarian, developed a modified live virus vaccine for hog
cholera in 1951, producers faced a dilemma. The live-virus vaccine gave more
effective protection but was a potential source of new infections. Killed-virus
vaccines offered a chance for eradication of hog cholera but did not give as
high a level of protection.
In 19632 years after eradication was mandated by
lawAgricultural Research Service (ARS) regulatory officials prohibited
interstate shipment of virulent virus or of feeder pigs and breeding stock
vaccinated with this virus. Modified live vaccine and killed vaccines continued
in use until banned in 1969. Animal disease regulatory activities were
transferred to USDA's Animal and Plant Health Inspection Service in
The 1961 decision to eradicate hog cholera came during
large-scale field tests conducted by ARS in Florida, Iowa, and Georgia.
The ARS Hog Cholera Research Station at Live Oak, Fla.,
tested modified live-virus vaccines and anti-hog-cholera serum on about 60,000
swine on 1,500 Florida farms. ARS veterinary medical officer M.R. Zinober in
1959 reported that 92.2 percent of the pigs and 89 percent of the vaccinated
herds were adequately protected.
In a 5-year Iowa farm study, ARS veterinary medical officer
J.P. Torrey reported adequate protection for most of 67,558 pigs that received
a single injection of killed hog cholera vaccine 2 weeks before weaning. Two
vaccinations, 2 weeks after weaning and a month later, overcame the inability
of some pigs to develop immunity. No cholera outbreaks took place on 60 farms
cooperating in the study completed in 1963.
No cases of hog cholera were confirmed during a 3-year
eradication test begun in 1961 in Lowndes County, Ga. Ninety-seven percent of
the hogs in the county received two vaccinations during a 2-month period. ARS
scientists said results of these experiments confirmed that killed-virus
vaccines are safe and effective.
As late as the early 1960's, the procedure for diagnosing
hog cholera was expensive and sometimes required several weeks to complete.
Blood from the suspect animal was injected into one free of the disease, it was
observed for clinical signs, and an attempt was made to recover hog cholera
virus. The marked disadvantages of the test precluded its use in a nationwide
Today's hogs are healthier than ever.
In 1963 ARS veterinary medical
researchers W.L. Mengeling, E.C. Pirtle, and J.P. Torrey reported a rapid,
accurate diagnostic test for hog cholera that takes less than a day. In the
test developed at the National Animal Disease Center, Ames, Iowa, a culture
containing tissue from a suspect hog is treated with fluorescent dye that is in
combination with anti-hog-cholera serum. Infected cells retain the dye-serum
and are readily distinguished from noninfected cells under a microscope.
Later research at NADC furnished additional tools for
restricting transmission of hog cholera virus and contributed to successful
Dr. Torrey and W. C. Amtower found that
sodium-o-phenylphenate, a material used for disinfecting hospitals, was
effective and safe for disinfecting farms, trucks, sale barns, and packing
houses where hog cholera virus may be present.
Another study by Dr. J. K. Prather indicated that heating
infected blood or serum to 69· C. and holding this temperature for 30
minutes destroys hog cholera virusand therefore would destroy the virus
in heat-processed meat. Department scientists had earlier found that the virus
survives dry-salt or brine curing of hams taken from cholera-infected
Hog cholera was the most devastating disease of swine in
this country for more than a century. Containment and eventual eradication
required research-based information on the cause and transmission of the
disease as well as methods for diagnosis and prevention. And total eradication
was achieved through close cooperation of scientists, Federal and State
regulatory agencies, producers, and affected industrial
Cholera" was published in the March 1978 issue
of Agricultural Research magazine.
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