Rabies

H. W. SCHOENING.

RABIES, or hydrophobia, is caused by a filterable virus a type of infective agent that can pass through certain filters that retain ordinary bacteria and cannot be seen through ordinary microscopes. The virus is found in the saliva of affected animals. The disease is produced by the bite of a rabid animal or by contact with the saliva of a rabid animal. The bite makes a wound in which the virus in the saliva is deposited.

The disease is primarily one of the dog, although many species, including people, are susceptible to infection.

Rabies has been reported in the cat, cow, horse, mule, sheep, goat, hog, wolf, fox, coyote, hyena, skunk, monkey, deer, antelope, camel, bear, elk, polecat, squirrel, hare, rabbit, rat, mouse, jackal, badger, marmot, woodchuck, porcupine, weasel, hedgehog, mongoose, gopher, raccoon, the owl, hawk, chicken, pigeon, stork, and several species of bats.

When rabies is once controlled in dogs its importance from an economic or public-health standpoint will be greatly reduced. When a disease becomes established in a wild species, however, a serious situation develops, and strenuous efforts must be made to control it in the species affected.

Rabies in foxes became rather widespread after 1945 in certain States, and considerable livestock losses occurred when rabid foxes attacked farm animals, particularly cattle.

The number of cases of rabies in all species between 1938 and 1955 was 146,627 a staggering total. The financial loss to the country runs into large figures and the constant hazards to human beings and animals call for strong national action to control and eventually eradicate the disease.

The period of incubation the time between exposure to infection and the first appearance of symptoms of the disease may be as short as 2 weeks or as long as many months. Most cases develop within 3 months.

All animals or persons bitten do not develop the disease. The proportion of persons who contract the disease after being bitten by rabid dogs and who were not treated has been estimated at 15 percent.

From 35 to 45 percent of the dogs, 40 percent of the horses, 36 percent of the hogs, and 25 to 30 percent of the cattle bitten by rabid animals contract the disease. Whether an individual animal contracts the disease depends in part on the location and size of the wound, the amount of bleeding produced, and other conditions. In general, the nearer the bite is to the central nervous system and the deeper the wound, the greater is the danger of infection. If the hemorrhage resulting from the bite is profuse, the possibility exists that the virus will be washed out of the wound. Also, exact information is not available on whether the virus of rabies is constantly present in the saliva of a rabid animal and whether it varies in both quantity and virulence.

After it is deposited in the wound, the virus remains latent for an extremely variable period, which depends on the size, location, and depth of the wound and the amount of virulent saliva introduced. The virus follows the course of the nerves to the spinal cord and along the spinal cord to the brain before the symptoms appear. The period between the bite and the appearance of the first symptom may be 14 to 285 days.

The bite of a dog may be infectious at least 3 days before the dog manifests symptoms of rabies. In one case in the Pasteur Institute at Athens, Greece, infection was found to be present in the saliva 8 days before the dog showed signs of the disease.

A dog can transmit rabies through a bite only if it is rabid. A normal dog cannot transmit the disease. Rabid foxes and other wildlife, through their attacks upon animals, have been responsible for outbreaks of rabies.

A FEW INSTANCES of finding rabies virus in the milk or udder tissue of lactating animals have been reported. Research workers in different parts of the world have investigated the subject at various times, but most have failed to find the virus in milk or udder tissue.

Scientists in the Department of Agriculture conducted some experiments on this point in 1934. They inoculated the virus of rabies into the tongues of two milking cows. Samples of milk were taken at frequent intervals, and milk from the udder tissues was collected at death. Samples were inoculated into the brains of rabbits. The results were negative. A calf and four pigs were fed the milk taken daily from the two cows from the time of exposure to the virus to the time of death from rabies. The mucous membrane of the mouth of two of the pigs and the calf was scarified to make minute wounds, but in no case did the animals show any harmful effects from the consumption of the milk. Rabies virus in the form of a brain emulsion also was added to milk and fed to four hogs. Abrasions were produced mechanically on the mucous surfaces of the mouths of the animals. In no case did the animals show any evidence of rabies. It appears, therefore, from the information available, that although the virus of rabies at times may be found in milk, such chances are remote.

From a practical standpoint, there seems to be little danger in consuming milk from even a rabid animal. Milk secretion usually is considerably diminished at the clinical appearance of the disease, so that it is quite unlikely that the animal's milk has been consumed after symptoms have appeared. Nevertheless, the milk from a rabid or suspected cow should be condemned as unfit for consumption.

The same position should be taken with regard to the meat from a rabid or suspected animal. The disposition of farm animals that have been exposed to the bites of a rabid dog but do not show signs of the disease also presents a problem. The International Rabies Conference in Paris in 1927 made the following recommendation on this point: "Animals bitten by rabid animals, whether treated or not after the bite, should not be butchered between the eighth day and, at the very least, the end of the third month following the bite."

IN DOGS the first symptom of rabies may be a change in behavior. The animal may become restless and excitable. A friendly dog may become irritable and snappy, and a dog that ordinarily is less amiable may become friendly. Later it may have a tendency to wander and may disappear for a day or two, returning exhausted and considerably emaciated. The dog may seek dark corners and hide. At times the bark might change in tone.

Later on the dog develops partial paralysis and has difficulty in drinking, although it may make efforts to lap water. It staggers around until complete paralysis sets in. Because the virus attacks the brain and spinal cord and sets up degenerative changes, the symptoms--excitability, convulsions, and paralysis can be correlated with changes in the central nervous system. The inability to swallow results from paralysis of the muscles of the throat.

The term "hydrophobia" means fear of water, but its application to the disease in dogs appears to be a misnomer, because affected dogs show no fear of water. The use of the term probably has its origin in the fact that affected dogs and human beings have been observed to develop convulsions through their unsuccessful efforts to drink. Even the mere thought of drinking on the part of human beings has been responsible for convulsions, and a dread of water or drinking becomes established in many patients.

In the furious form of the disease, the animal is aggressive. It snaps at objects placed before it and will attack dogs and people. It may attack the bars of a cage with such vigor as to break its teeth. The dog's tendency to roam, its restlessness, and its inclination to bite spread the disease widely.

Rabies may take a number of forms, and the symptoms I described are those of a typical case. Many animals affected with rabies do not exhibit these symptoms. The symptoms may be more or less masked and may perhaps be manifested eventually only by paralysis.

Paralytic symptoms are the outstanding feature in the dumb form of rabies. The dog is not vicious. It has no tendency to bite or roam. It is not excitable. In fact, it may be the opposite.

The outstanding clinical feature of dumb rabies is paralysis of the lower jaw, or dropped jaw. The animal's mouth stays open an inch or more. It can be closed with the hands, but the dog cannot close its own jaw.

Many times this is mistakenly thought to be due to a bone in the throat, and many persons have exposed themselves to the virus by examining the mouth and throat for a bone. Usually a dog with a bone stuck in its mouth or throat makes repeated efforts with its paws or otherwise to remove the bone, but in dumb rabies the animal makes no motion about the head with its paws. An animal with a dropped jaw should be viewed with suspicion. No examination should be made of the throat. The animal should be taken immediately to a veterinarian for diagnosis.

Also in the dumb form of rabies the animal shows evidence of paralysis of the hindquarters and forequarters Within a few days. It eventually becomes completely paralyzed and dies.

The course of the disease in both the furious and the dumb forms is usually short, and the animal dies in 3 to 7 days.

CATS that have rabies generally hide under the furniture or in a dark corner. There they may die unobserved in a day or two. As a rule, however, the disease in cats implies danger for human beings. The rabid cat becomes bellicose. From its dark corner it may suddenly attack animals or persons, especially children, jumping up to the face and inflicting severe wounds with its teeth and claws. In the violence of this attack it frequently bites itself.

The rabid cat nearly loses its voice and is able only to mew hoarsely.

Later it loses its appetite, has difficulty in swallowing, becomes emaciated, and succumbs within several days with symptoms of paralysis.

CATTLE are susceptible to both furious and dumb rabies. The former is the more common. A sharp distinction cannot always be drawn between the two, however, as the furious type usually merges into the dumb type because of the paralysis that always appears before death. In typical cases of dumb rabies, the paralysis occurs at the beginning of the attack and remains until the animal dies.

The first signs are loss of appetite, stopping of the secretion of milk, great restlessness, anxiety, manifestations of fear, and a change in disposition. There follows in a day or two a stage of excitation or madness, which is indicated by increasing restlessness; loud bellowing, with a peculiar change in the sound of the voice; violent butting with the head and pawing the ground; and an insane tendency to attack other animals, although the desire to bite is not so marked in cattle as in dogs.