Pseudorabies

Pseudorabies virus (PRV) is a viral disease in swine that is endemic in most parts of the world. It is caused by porcine herpesvirus 1 and is also known as Aujeszky's disease, and in cattle as mad itch. PRV is considered to be the most economically important viral disease of swine in areas where hog cholera has been eradicated.

History
The earliest reports of a disease suspected to be pseudorabies were in 1813 in the United States, describing a condition in cattles characterized by severe itching and called mad itch. In 1902 A Hungarian veterinarian, Aládar Aujeszky, isolated PRV from a dog, ox, and cat and showed that it caused the same the disease in swine and rabbits. The name pseudorabies came from the symptoms similar to rabies that it caused in rabbits.

Disease overview
The virus is shed in the saliva and nasal secretions of infected swine and is spread through oral or nasal contact. Aerosolization of the virus and transmission by fomites also may occur. The virus may potentially survive for seven hours in humid air and spread up to two kilometers. Furthermore, 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.

There are many secondary hosts of pseudorabies, including dogs, cats, cattle, rats, and horses, but not humans. Secondary hosts are infected through direct contact with swine, rats, and other infected secondary hosts and eating infected uncooked pork. Diagnosis is made through an ELISA test. A vaccine is available for swine. There are eradication programs in the United States and the United Kingdom. In 2004 the commercial swine population of the United States was declared free of pseudorabies, but the disease remained in feral pig populations.

Symptoms
Swine are usually asymptomatic, but PRV 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 percent, but it is less than 10 percent in pigs between one and six months of age. 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 cats the disease is so rapidly fatal that there are usually no symptoms. Any infected secondary host generally only lives two to three days.

Applications in Neuroscience
PRV is a powerful tool used in neurobiology that can be employed to analyze neural circuits in the central nervous system (CNS). The Bartha strain of PRV is an attenuated form developed in 1961, and is employed as a retrograde transneuronal tracer. PRV-Bartha is transported to a neuronal cell body via its axon where it is replicated and dispersed throughout the cytoplasm and the dendritic tree. PRV-Bartha is able to cross synaptic gaps into the axons of only synaptically connected neurons, thereby propagating the virus in the retrograde fashion. Using temporal studies and/or genetically engineered strains of PRV-Bartha, second, third, and higher order neurons may be identified in the neural network of interest.