Pseudorabies

Species of virus

Aujeszky's disease, usually called pseudorabies in the United States, is a viral disease in swine that is endemic in most parts of the world. It is caused by Suid herpesvirus 1 (SuHV 1). Aujeszky's disease is considered to be the most economically important viral disease of swine in areas where classical swine fever (hog cholera) has been eradicated. Other mammals, such as cattle, sheep, goats, cats, dogs, and raccoons, are also susceptible. The disease is usually fatal in these animal species. Research on SuHV 1 in pigs has pioneered animal disease control with genetically modified vaccines. SuHV 1 is now used in model studies of basic processes during lytic herpesvirus infection, and for unravelling molecular mechanisms of herpesvirus neurotropism.

In 1902, a Hungarian veterinarian, Aladár Aujeszky, demonstrated a new infectious agent in a dog, ox, and cat, and showed it caused the same disease in swine and rabbits. In the following decades the infection was found in several European countries, especially in cattle, where local intense pruritus (itching) is a characteristic symptom. In the United States, a well known disease in cattle called "mad itch" was concluded to be in fact Aujeszky's disease.

The virus is shed in the saliva and nasal secretions of swine infected by the respiratory route. Aerosolization of the virus and transmission by fomites also may occur. The virus may potentially survive for seven hours in humid air, and it may survive on well water for up to seven hours, in green grass, soil, and feces for up to two days, in contaminated feed for up to three days, and in straw bedding for up to four days. Diagnosis is made mainly by virus isolation in tissue cultures, or through ELISA or PCR tests. Vaccines are available for swine (ATCvet codes: inactivated, live, plus various combinations). The infection has been eradicated in a number of European countries. In the United States, the domestic swine population in 2004 was declared free of Aujeszky's disease, though the infection still remains in feral pig populations.

Respiratory infection is usually asymptomatic in pigs more than two months old, but it can cause abortion, high mortality in piglets, and coughing, sneezing, fever, constipation, depression, seizures, ataxia, circling, and excess salivation in piglets and mature pigs. Mortality in piglets less than one month of age is close to 100%, but it is less than 10% in pigs between one and six months of age. Pregnant swine can reabsorb their litters or deliver mummified, stillborn, or weakened piglets. In cattle, symptoms include intense itching followed by neurological signs and death. In dogs, symptoms include intense itching, jaw and pharyngeal paralysis, howling, and death. In susceptible animals other than swine, infection is usually fatal, and the affected animals most often show intense pruritus in a skin area. Pruritus in Aujeszky's disease is considered a phantom sensation as virus has never been found at the site of pruritus.

The epidemiology of Aujeszky's disease varies with the pathogenicity or virulence of the virus strain involved. This is best illustrated by the development of the severity of the disease in Denmark, where import of swine had been forbidden for decades up to 1972. Before 1964 only genital strains were spread, but then respiratory strains appeared, which subsequently were spread rapidly over the country, mainly by the trade of animals. In the late 1970s more virulent strains developed. The disease in swine became much more severe, outbreaks of respiratory disease in cattle rose dramatically, and the infection was spread airborne to other swine herds. The higher virulence of these virus strains was associated with a certain ability to create syncytia (cell fusion) in tissue cultures (syncytial virus strains). Comprehensive restriction fragment pattern analyses of virus DNA have documented that the more virulent strains had not been introduced from abroad but had developed in two steps from the original Danish strains. The correlation between high virulence of virus strains and syncytium formation in tissue cultures was confirmed by examinations of isolates from other countries. This second step in the severity development of the disease in Denmark caused the decision to eradicate. New outbreaks after the eradication of the indigenous infection by the end of 1985 were all caused by foreign highly virulent, syncytial strains introduced by airborne transmission from Germany. Swine (both domestic and feral) are usual reservoirs for this virus, though it does affect other species. Aujeszky's disease has been reported in other mammals, including brown bears, and black bears, Florida panthers, raccoons, coyotes, and whitetail deer. In most cases, contact with pigs or pig products was either known or suspected. Outbreaks in farm fur species in Europe (mink and foxes) have been associated with feeding contaminated pig products. Many other species can be experimentally infected. Humans are not potential hosts. Cattle have been found to be infected either by the respiratory or the vaginal route (iatrogenic cases disregarded). Primary infection of mucous membranes of the upper respiratory tract is associated with head pruritus, while lung infection results in chest pruritus. Vaginal infection of bovines, which regularly show pruritus of the hindquarters, has been found to be associated with a concurrent genital infection in swine on the same premises, and investigations have evidenced that the vaginal infection of cattle had been sexually transmitted by man from infected sows. Genital infection in swine herds has been closely correlated with the use of boars from boar centres for natural service of sows.

SuHV 1 can be used to analyze neural circuits in the central nervous system (CNS). For this purpose the attenuated (less virulent) Bartha SuHV 1 strain is commonly used and is employed as a retrograde and anterograde transneuronal tracer. In the retrograde direction, SuHV 1 Bartha is transported to a neuronal cell body via its axon, where it is replicated and dispersed throughout the cytoplasm and the dendritic tree. SuHV 1 Bartha released at the synapse is able to cross the synapse to infect the axon terminals of synaptically connected neurons, thereby propagating the virus; however, the extent to which non synaptic transneuronal transport may also occur is uncertain. Using temporal studies and/or genetically engineered strains of SuHV 1 Bartha, second, third, and higher order neurons may be identified in the neural network of interest.