Hog Cholera

J. P. TORREY.

HOG CHOLERA is a 40-million-dollar annual expense to the 4-billion-dollar swine industry in the United States.

It occurs in every State and in parts of every country where hogs are raised, except those Northern Ireland, Denmark, and Australia, among them that have undertaken vigorous and continuing eradication measures. Reports are that hog cholera broke out in countries where meat from the United States was shipped for use by American Armed Forces.

We have the means to stamp out hog cholera, would we but use them.

THE FIRST AUTHENTIC REPORT of hog cholera in the United States came from Ohio in 1833. The disease spread rapidly. By 1887 it existed in 35 States. It became unusually prevalent at intervals of about 10 years between 1887 and 1926--1887, 1896, 1913, 1926.

The annual loss of hogs that died was placed at 65 million dollars. The average for a 10-year period was not less than 20 million dollars. The indirect costs doubled that figure.

THE CAUSE of hog cholera is a virus, which has a mean diameter of 27 millimicrons. (25 million millimicrons equal 1 inch.) This virus readily passes through Berkefeld and Chamberland filters. It is killed in 30 minutes by heat at 55 C.; in 10 minutes at 60 C.; and in 60 minutes in dried blood at 72 C.

The virus is destroyed by a 2-percent cresol solution in 60 minutes. A 3-percent solution of sodium hydroxide in combination with 2-percent milk of lime kills the virus in 15 minutes.

The virus lives best in an acid solution. The optimum pH for preservation of virulence is 5.0 to 5.5. The virus is quite resistant to phenol, and will survive long periods in a 0.5-percent solution. Phenol is used as a preservative when the virus is produced for immunization purposes. It has been kept in a perfect state of preservation for 7 years at 40 C. It will survive in meat products for months and will live at least 6 months in pickled, salted, and smoked meats. Putrefactive processes destroy the virus in about 5 days, except when it is in the bone marrow, where it survives for at least 15 days.

When it enters the pig's body, the virus passes to the blood stream and develops there. It is therefore a viremia, or blood infection. The blood of a pig becomes infectious within 24 hours after the virus is injected into its body. The urine and feces usually contain the virus within 48 hours. The secretions of the eyes and nose become infectious by the third day.

Rarely do pigs show visible symptoms earlier than the fourth day. Infected pigs therefore can transmit the disease before any symptoms can be seen. The maximum growth of virus is reached in 6 to 8 days.

The virus moves to all parts of the body, primarily in the red blood cells.

The liver and spleen contain more of the virus than other organs do.

Some viruses will cause infection in pigs when 1/5,000,000th of 1 cubic centimeter is injected. Others will not cause disease when less than 1/10,000th of 1 cubic centimeter is injected.

The incubation period the time that elapses between the invasion of the first cells of the body and the first symptoms of the disease is quite variable and depends on the virulence of the virus and the individual resistance of the pig. Symptoms may be observed 3 days after the injection of virus, but sometimes it may be 7 days before the pig is visibly sick.

When susceptible swine are exposed by contact or in other ways, one cannot know the exact time when infection occurs, but visible symptoms are rarely observed within less than 5 or 6 days after such exposure.

HOG CHOLERA may be introduced into a herd in many ways. When the disease is introduced in a small amount of infected material, such as a scrap of raw pork or small amount of dirt on an attendant's shoes, one or two pigs that eat it become sick first. Several days may pass before more animals become sick.

At least 2 days pass before the urine of the first exposed pig contains virus and other pigs eating feed contaminated with the urine will become exposed. The period of incubation must elapse before these pigs show symptoms. Thus when cholera is introduced in a herd, about 7 days will elapse between the time the first sick pig was exposed and other susceptible pigs begin to show symptoms. This characteristic of the disease shows the importance of being concerned when only one pig in a herd becomes sick. Immediate isolation of the first sick pig and early diagnosis, followed by proper immunization, will prevent heavy losses in the herd if the trouble is due to hog cholera.

When hog-cholera virus is introduced into a herd in large amounts, as in a bone brought from the neighboring farm by the farmer's dog or litter thrown from a stock truck, the outbreak will be more explosive, and a number of animals will become sick about the same time. These cases are more difficult to handle and the loss usually is greater.

Birds, such as pigeons and sparrows, and insects have been suspected of spreading hog cholera, but experimental evidence has failed to incriminate them.

Man is most to blame for the spread of the virus. Improper handling and use of the virus used for vaccinating pigs may start an outbreak of cholera. Men who visit sales barns, stockyards, slaughterhouses, and rendering plants and do not disinfect their shoes or truck and change clothing when they return to the farm may spread the disease.

The virus is spread within a herd by contact with sick animals or material infected with excrements from a sick animal. Animals recently vaccinated with virulent virus may spread the disease, especially if they show some reaction to the vaccination. Such animals should never be mixed with susceptible swine.

THE VIRUS of hog cholera differs from most other infectious agents in its relation to the white cells of the blood. Most infections, especially bacterial infections, cause an increase of white blood cells. The function of white corpuscles is to pick up infecting or invading particles and to try to carry them from the body. Hog-cholera virus has the opposite effect on white corpuscles. It destroys the corpuscles and apparently stops the production of them. The number of white cells in the normal pig may vary between 7,000 and 30,000 per cubic centimeter. Any number below 7,000 is subnormal, and the condition is called a leucopenia.

This characteristic of hog cholera can be used as a diagnostic agent, but it cannot be relied upon if the count is higher. An animal may be infected with cholera and another disease at the same time. The cell count might then be high and a wrong diagnosis would be made.

The American strains of the virus are more virulent than the European strains. They differ also from the Canadian viruses.

The American strains seem to vary in virulence among themselves from time to time. Because there is no known serological method of differentiating the viruses, they have been considered the same virus. Heavy death losses in 1949 and 1950 in swine after vaccination with certain serials of viruses and serums indicated, however, that the viruses were different.

After extensive investigation, C. N. Dale and other scientists of the Department of Agriculture learned that the viruses differed from regular viruses in their antigenic properties ability to produce immunizing properties--in a serum. The immunizing properties produced by a regular virus would not protect against the irregular viruses unless the amount of serum was greatly increased. These viruses are called variant viruses.

Since this discovery, variant viruses have been isolated from sick pigs that were not vaccinated with serum and virus. Variant viruses may have been involved in post-vaccination losses for a long time, but the cause of the losses may not have been determined.

The differences between variant viruses and regular viruses are: Variant viruses have low antigenic properties and therefore will not make good hyperimmune serum or vaccine. Hyperimmune serum made from regular viruses will not protect pigs against variant viruses in the same dosage as is used for regular viruses. The minimum lethal dose of variant viruses is greater than the dose of regular viruses. The variant characteristics are not stable, and after several passages in pigs the characteristics are the same as regular viruses. The variant characteristics are retained when the virus is injected into susceptible pigs simultaneously with subprotective doses of immune serum.

Hog-cholera virus has been propagated in swine tissues implanted on the chorio-allantoic membrane of chick embryos.

It has been cultivated more successfully in tissue cultures that employ various swine tissues, such as lymph node, spleen, kidney, testicle, and choroid plexus. More recent methods used only spleen as a culture medium.

THE SYMPTOMS of hog cholera differ in different hogs and in different herds, depending on the strength of the virus and the ability of the animal to resist the disease attack. Because of the variations, the disease is said to exist in two forms. In the acute, or severe type, the hogs sicken and die quickly. In the chronic, or less severe type, the hogs may be sick for weeks or months before they succumb or (in a few cases) recover.

At first the pig's temperature quickly rises to 105 to 107 F. The temperature continues high for a few days. Then it gradually drops. The temperature may go to subnormal if the pig lingers for a time.

A few pigs (or possibly only one) sicken at first. They refuse to come for feed with the herd but remain in the nest. They may appear to be cold and shivery, and their backs may be arched when they are driven from the bed. The sick hogs become gaunt or tucked up in the flank as the disease progresses. They have a weak, stilted, staggering gait; the weakness is most noticeable in the hind legs. Constipation is commonly present in the early days, but it may be followed by a diarrhea.

A red or purplish discoloration of the skin on the belly, ears, and inner surface of the legs may happen in some cases, but not all. Because the same discoloration may be found in other diseases, it cannot be a diagnostic symptom.

The eyes in the early stages may have a watery discharge. Later this becomes thicker and gums the eyelids shut. The eyes may be congested.