Rabies in animals
Rabies is a viral zoonotic neuroinvasive disease which causes inflammation in the brain and is usually fatal. Rabies, caused by the rabies virus, primarily infects mammals. In the laboratory it has been found that birds can be infected, as well as cell cultures from birds, reptiles and insects. Animals with rabies suffer deterioration of the brain and tend to behave bizarrely and often aggressively, increasing the chances that they will bite another animal or a person and transmit the disease. Most cases of humans contracting the disease from infected animals are in developing nations. In 2010, an estimated 26,000 people died from rabies, down from 54,000 in 1990.
Stages of disease
Three stages of rabies are recognized in dogs and other animals.
- The first stage is a one- to three-day period characterized by behavioral changes and is known as the prodromal stage.
- The second stage is the excitative stage, which lasts three to four days. It is this stage that is often known as furious rabies due to the tendency of the affected animal to be hyperreactive to external stimuli and bite at anything near.
- The third stage is the paralytic or dumb stage and is caused by damage to motor neurons. Incoordination is seen due to rear limb paralysis and drooling and difficulty swallowing is caused by paralysis of facial and throat muscles. This disables the host's ability to swallow, which causes saliva to pour from the mouth. This causes bites to be the most common way for the infection to spread, as the virus is most concentrated in the throat and cheeks, causing major contamination to saliva. Death is usually caused by respiratory arrest.
Bat-transmitted rabies occurs throughout North and South America but it was first closely studied in Trinidad in the West Indies. This island was experiencing a significant toll of livestock and humans alike to rabid bats. In the 10 years from 1925 and 1935, 89 people and thousands of livestock had died from it—“the highest human mortality from rabies-infected bats thus far recorded anywhere.”
In 1931, Dr. Joseph Lennox Pawan of Trinidad in the West Indies, a Government Bacteriologist, found Negri bodies in the brain of a bat with unusual habits. In 1932, Dr. Pawan discovered that infected vampire bats could transmit rabies to humans and other animals. In 1934, the Trinidad and Tobago Government began a program of eradicating vampire bats, while encouraging the screening off of livestock buildings and offering free vaccination programs for exposed livestock.
After the opening of the Trinidad Regional Virus Laboratory in 1953, Arthur Greenhall demonstrated that at least eight species of bats in Trinidad had been infected with rabies; including the common vampire bat, the rare white-winged vampire bat, as well as two abundant species of fruit bats: the Seba's short-tailed bat and the Jamaican fruit bat.
Recent data sequencing suggests recombination events in an American bat led the modern rabies virus to gain the head of a G-protein ectodomain thousands of years ago. This change occurred in an organism that had both rabies and a separate carnivore virus. The recombination resulted in a cross-over that gave rabies a new success rate across hosts since the G-protein ectodomain, which controls binding and pH receptors, was now suited for carnivore hosts as well.
Rabies is rare in cats. In the United States between 200-300 cases are reported annually. Cats that have not been vaccinated and are allowed access to the outdoors have the most risk for contracting rabies as they may come in contact with rabid animals. The virus is often passed on during fights between cats or other animals and is transmitted by bites, saliva or through mucous membranes and fresh wounds. The virus can incubate from one day up to over a year before any symptoms begin to show. Symptoms have a rapid onset and can include unusual aggression, restlessness, lethargy, anorexia, weakness, disorientation, paralysis and seizures. Vaccination of felines (including boosters) by a veterinarian is recommended to prevent rabies infection in outdoor cats.
Despite natural infection of rabbits being rare, they are particularly vulnerable to the rabies virus; rabbits were used to develop the first rabies vaccine by Louis Pasteur in the 1880s, and are continued to be used for rabies diagnostic testing. The virus is often contracted when attacked by other rabid animals and can incubate within a rabbit for up to 2–3 weeks. Symptoms include weakness in limbs, head tremors, low appetite, nasal discharge, and death within 3–4 days. However, there are currently no vaccines available for rabbits. The National Institutes of Health recommends that rabbits be kept indoors or enclosed in hutches outside that do not allow other animals to come in contact with them.
Rabies has a long history of association with dogs. The first written record of rabies is in the Codex of Eshnunna (ca. 1930 BC), which dictates that the owner of a dog showing symptoms of rabies should take preventive measure against bites. If a person was bitten by a rabid dog and later died, the owner was fined heavily.
Three stages of rabies are recognized in dogs. The first stage is a one- to three-day period characterized by behavioral changes and is known as the prodromal stage. The second stage is the excitative stage, which lasts three to four days. It is this stage that is often known as furious rabies due to the tendency of the affected dog to be hyperreactive to external stimuli and bite at anything near. The third stage is the paralytic stage and is caused by damage to motor neurons. Incoordination is seen due to rear limb paralysis and drooling and difficulty swallowing is caused by paralysis of facial and throat muscles. Death is usually caused by respiratory arrest.
Rabies can be contracted in horses if they interact with rabid animals in their pasture, usually being bitten on the muzzle or lower limbs. Signs include aggression, incoordination, head-pressing, circling, lameness, muscle tremors, convulsions, colic and fever. Horses that experience the paralytic form of rabies have difficulty swallowing, and drooping of the lower jaw due to paralysis of the throat and jaw muscles. Incubation of the virus may range from 2–9 weeks. Death often occurs within 4–5 days of infection of the virus. There are no effective treatments for rabies in horses. Veterinarians recommend an initial vaccination as a foal at three months of age, repeated at one year and given an annual booster.
Experimental studies of rabies infection in the Virginia opossum have shown the importance of the mode of transmission. Opossums became infected when exposed to air-borne virus but were found to be fairly resistant to intramuscular inoculations. The aerosol transmission of rabies in opossum was investigated following the death from rabies of two men who had visited the Frio Caves, Texas, and did not remember any direct contact with bats.
Rabies cases in the Virginia opossum are spillover cases from other wildlife species such as bats, skunks and the raccoon epizootic in the eastern United States. Cases have been reported across the United States from California to New York. In New York state, laboratory confirmed rabies cases in opossums occurred five of the ten years between from 1989 to 1999. Recently, from 2010 to 2014, rabid Virginia opossums were reported in New York City and the states of New Jersey, Maryland, and Virginia.
In the U.S., there is currently no USDA-approved vaccine for the strain of rabies that afflicts skunks. When cases are reported of pet skunks biting a human, the animals are frequently killed in order to be tested for rabies. It has been reported that three different variants of rabies exist in striped skunks in the north and south central states.
Humans exposed to the rabies virus must begin post-exposure prophylaxis before the disease can progress to the central nervous system. For this reason, it is necessary to determine whether the animal, in fact, has rabies as quickly as possible. Without a definitive quarantine period in place for skunks, quarantining the animals is not advised as there is no way of knowing how long it may take the animal to show symptoms. Destruction of the skunk is recommended and the brain is then tested for presence of rabies virus.
Skunk owners have recently organized to campaign for USDA approval of both a vaccine and an officially recommended quarantine period for skunks in the United States.
Under normal circumstances, wild wolves are generally timid around humans, though there are several reported circumstances in which wolves have been recorded to act aggressively toward humans. The majority of fatal wolf attacks have historically involved rabies, which was first recorded in wolves in the 13th century. The earliest recorded case of an actual rabid wolf attack comes from Germany in 1557. Though wolves are not reservoirs for the disease, they can catch it from other species. Wolves develop an exceptionally severe aggressive state when infected and can bite numerous people in a single attack. Before a vaccine was developed, bites were almost always fatal. Today, wolf bites can be treated, but the severity of rabid wolf attacks can sometimes result in outright death, or a bite near the head will make the disease act too fast for the treatment to take effect.
Rabid attacks tend to cluster in winter and spring. With the reduction of rabies in Europe and North America, few rabid wolf attacks have been recorded, though some still occur annually in the Middle East. Rabid attacks can be distinguished from predatory attacks by the fact that rabid wolves limit themselves to biting their victims rather than consuming them. Plus, the timespan of predatory attacks can sometimes last for months or years, as opposed to rabid attacks which end usually after a fortnight. Victims of rabid wolves are usually attacked around the head and neck in a sustained manner.
Monkeys, like humans, can get rabies, however they do not tend to be a common source of rabies. Monkeys with rabies tend to die more quickly than humans. In one study, 9 of 10 monkeys developed severe symptoms or died within 20 days of infection. Rabies is often a concern for individuals travelling to developing countries as monkeys are the most common source of rabies after dogs in these places.
Other small mammals
The most commonly infected terrestrial animals in the U.S.A. are raccoons, skunks, foxes, and coyotes. Any bites by such wild animals must be considered a possible exposure to the rabies virus.
Most cases of rabies in rodents reported to the Centers for Disease Control and Prevention in the U.S.A. have been found among groundhogs (woodchucks). Small rodents such as squirrels, hamsters, guinea pigs, gerbils, chipmunks, rats, and mice and lagomorphs like rabbits and hares are almost never found to be infected with rabies and are not known to transmit rabies to humans.
Transport of pet animals between countries
Rabies is endemic to many parts of the world, and one of the reasons given for quarantine periods in international animal transport has been to try to keep the disease out of uninfected regions. However, most developed countries, pioneered by Sweden, now allow unencumbered travel between their territories for pet animals that have demonstrated an adequate immune response to rabies vaccination.
Such countries may limit movement to animals from countries where rabies is considered to be under control in pet animals. There are various lists of such countries. The United Kingdom has developed a list, and France has a rather different list, said to be based on a list of the Office International des Epizooties (OIE). The European Union has a harmonised list. No list of rabies-free countries is readily available from OIE.[original research?]
In recent years, canine rabies has been practically eliminated in North America and Europe due to extensive and often mandatory vaccination requirements. However it is still a significant problem in parts of Africa, parts of the Middle East, parts of Latin America, and parts of Asia. Dogs are considered to be the main reservoir for rabies in developing countries.
However, the recent spread of rabies in the northeastern United States and further may cause a restrengthening of precautions against movement of possibly rabid animals between developed countries.
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