Swine Influenza

RICHARD E. SHOPE.

SWINE influenza is an acute, highly contagious, infectious disease caused by the concerted activity of a tiny bacterium, Hemophilus influenzae suis, and a filterable virus.

The disease made its first appearance, as far as anyone knows, in the autumn of 1918, when a great outbreak of severe and highly fatal influenza was prevalent in the human population. Many veterinarians and farmers at the time believed that swine had acquired their influenza infections in the first instance from man. Subsequent laboratory work has tended to support the belief.

At any rate, when the cause of human influenza was discovered in 1933 to be a filterable virus, it was soon found that this virus from man bore a striking similarity to the one earlier discovered in swine influenza. Furthermore, the blood serums of most human beings born before 1918, and hence living at the time of the great 1918 epidemic, contain immune substances (antibodies) specific for the swine influenza virus, whereas serums from people born after 1918 are largely without these specific antibodies.

While the evidence is strong that swine may have acquired their influenza from man, there is nothing to suggest that the swine virus now presents a health hazard for people, because its long sojourn in swine has apparently "fixed" it for that species.

Swine influenza is essentially a disease of the late autumn and early winter months. It seldom appears, as a farm infection, at other times of the year. Its occurrence is largely limited to swine droves in the Middle Western States. It is primarily a herd disease, and single cases within a herd are unknown.

Ordinarily swine influenza begins explosively in October or November, depending on whether the onset of wet, inclement weather has been early or late that year. It frequently appears to start up in many different swine droves in an area almost simultaneously, a characteristic that early gave the disease a reputation for spreading like wildfire. After the initial flareup, new droves sicken in sequence for a period of 6 weeks or 2 months. Then the disease subsides and largely disappears as a farm ailment until the following year.

PERHAPS THE BEST WAY to outline the symptoms ordinarily seen in swine influenza would be to describe a typical outbreak of the disease as it might be encountered on any one of hundreds of Midwestern farms almost any autumn.

We will suppose it is the third day of illness for the hypothetical drove under observation. About 100 shoats are bedded in the runway of the barn. All are lying down, most of them on their sides. A few rest on their bellies, assuming a partly sitting position with the body propped on the forelegs; apparently they are in extreme respiratory distress. All show a rapid, jerky type of breathing. They are obviously very ill; one can walk among them without rousing them from the ground.

Their temperatures may range from 104.5 to 107 F. The animals have shown little interest in food for 3 days, and most are exhibiting early evidence of gauntness. One has died. To a person unfamiliar with the extreme prostration so characteristic of swine influenza, it would seem that many others are doomed. The animals are finally aroused by pulling some of them to their feet.

Most of them begin coughing soon after getting up. The cough is paroxysmal in character, the back being often arched, and is of sufficient violence as sometimes to cause vomiting. When the coughing attacks have passed, the animals stand for a short while in a listless attitude with their heads down and their tails limp. They soon return to their nests.

The owner says that only 4 days ago the drove was apparently in perfect health, was eating ravenously, and was making a nice gain; 3 days ago they appeared listless in the morning and showed little enthusiasm for their food. Practically the entire drove appeared ill. By afternoon most of them were markedly depressed and had taken to their nests in the runway of the barn. The few animals that had eaten in the morning were off feed by afternoon. By the following day the illness was full blown.

Visits to the drove on the fourth and fifth days of illness reveal little change in its condition, except that on the fifth day a second animal is dead. On the sixth day, when a person unfamiliar with the course of swine influenza might have expected to discover most of the herd dead or dying, the animals have left their nests, are up and around, and are hungry once again.

Still coughing severely, they show definite improvement in their condition. Their temperatures are found now to have dropped to normal, ranging from 101 to 103 . Aside from a noticeable gaunt appearance, the pigs seem little the worse for their influenza) attacks. From this point on, recovery is rapid and usually is uneventful, although regain of the weight and condition lost during the illness is slow. The troublesome post-influenzal cough may last for 3 weeks or longer after recovery.

The salient features of the outbreak, which may be summarized as characteristic of swine influenza, are the sudden onset, with most of the animals sickening almost simultaneously; the extreme prostration exhibited by the affected swine; the persistence of the illness without abatement for a period of about 5 days; and the remarkably sudden and unexpected cessation of the illness, almost by crisis, on about the sixth day. As one observer has said, "the patient begins to recover about the time death is expected." The 2-percent mortality assigned to the hypothetical drove I described is about the overall figure generally given for swine influenza.

THE POSTMORTEM findings in a pig with influenza are quite characteristic. In an animal sacrificed on the third or fourth day of illness, the trachea and bronchi will be found to contain a thick, sometimes frothy, mucus exudate, usually in copious amount. The lungs will show a patchy pneumonia, purple in color in contrast to the white nonpneumonic lung, involving the smaller anterior lobes.

The line of demarcation between the pneumonic and nonpneumonic areas is sharp. Ordinarily the large posterior lobes are not involved. The lymph nodes lying along the lower part of the trachea are greatly enlarged and very juicy. The lining of the stomach is reddened. Ordinarily the remaining organs and tissues are normal in appearance. In fatal cases the findings at autopsy are similar to those just described, except that the pneumonia seen in the anterior lobes of the lungs is apt to be extensive and the large posterior lobes are distended, bloody, and filled with fluid.

Animals fully recovered from swine influenza are ordinarily immune for life and their blood serums contain antibodies capable of destroying the influenza virus. These antibodies can be tested for in the laboratory, and their presence is sometimes used for diagnostic purposes.

SWINE INFLUENZA, though highly contagious, would die out and disappear forever at the end of an epidemic if its causative virus did not have some mechanism for lasting through the 9- month period during which the disease disappears as a farm infection.

To take care of this, nature has provided the organism with an ingenious mechanism for survival which involves lungworms (thin, white worms present in the bronchi at the bases of the lung) and earthworms. The eggs laid by the female lungworms in the lung of a pig sick of influenza contain virus. The eggs are coughed up, swallowed, and eventually passed in the feces.

Earthworms eating this manure ingest the lungworm eggs, and the eggs hatch inside the earthworm. The baby lungworms develop through several stages and finally imbed themselves in the hearts and gizzards of the earthworm. There they remain until the earthworm harboring them is rooted out and devoured by a pig.

The baby lungworms burrow through the lining of the pig's intestine and, after a period of wandering, finally reach the lung, where they locate in the basal bronchi and develop to adults. These lungworms have carried the influenza virus throughout the whole of their cycle but in a latent, noninfectious form. Because of this, the pigs acquiring the lungworms do not immediately come down with influenza. They remain normal to all appearances until such a time as they are exposed to an unusual set of weather conditions, in which they become wet and chilled.

In some manner, not at present understood, this chilling renders the latent virus present in the lungworms in the pig's respiratory tract active, and it infects the animal. After a short period of usually 2 or 3 days, the pig shows beginning signs of illness and comes down with influenza.

Ordinarily the incidence of infective lungworms in a drove is high. That accounts for the onset of the disease in a large proportion of the herd simultaneously. The few pigs not infected at this time acquire their influenzas by contact with the sick animals. The weather condition responsible for provoking a swine influenza infection in one drove is apt to be operative over a large area, thus accounting for the appearance of the disease in many droves in an area at the same time.

There is little in the line of specific treatment that proves beneficial in swine influenza. Bedding the animals down in a dry, nondrafty, warm place and leaving them completely undisturbed is probably the best procedure. They should, of course, have free access to all the clean water they might care to drink. Being febrile, they are frequently thirsty.

Although a vaccine prepared from the swine influenza virus would probably be effective, none was commercially available in 1956.

The matter of lungworm control by rearing pigs on cement or pasturing them in areas where the earthworms are free of lungworm larvae could be of value in preventing swine influenza, but is not practical on most farms.

RICHARD E. SHOPE is a member of the Rockefeller Institute for Medical Research, New York. He was trained as a physician, but his scientific work has dealt entirely with diseases of animals. He has worked with, swine influenza, hog cholera, pseudorabies, the filterable rabbit tumors, and cattle plague.