Rhinopneumonitis
A Microbial Biorealm page on the genus Rhinopneumonitis
Classification
Virus; Incertae sedis; Incertae sedis; Herpesvirales; Herpesviridae ; Varicellovirus; Equine Herpesvirus
Description and significance
Rhinopneumonitis, also called rhino, is an inflammation of the mucosa of the nasal cavities and lungs in horses. It is caused by the equine herpes virus and only affects horses. Almost all adult horses are infected due to natural exposure after birth. After infection, the virus hibernates in the lymph nodes of the respiratory system and the horse is infected for the rest of its life. Whether or not the horse exhibits symptoms of the disease depends on the strength of the immune system. The immune system keeps the virus in check and if it is busy with other diseases or problems affecting the horse, the virus can escape its confinement. Rhino is similar to the human herpes virus in this way because of its capability of latency and relapse. Symptoms of the repiratory disease caused by the virus include: fever, hacking cough, nasal discharge, enlarged lymph nodes in the neck, and stocking up and heat in the lower legs. Respiratory disease is the most common illness caused by the virus, but the virus can also cause abortion in brood mares. Vaccinations against herpes viruses are not very effective because the virus lives in the horse and affects its immune system. However, vaccinations are most important for brood mares to protect them against viral abortion. Minimal protection is vaccinating once a year. To prevent respiratory disease in young horses requires administration every 2-3 months and to prevent abortion, mares should be vaccinated three times during gestation.
Genome structure
The Rhinopneumonitis virus is composed of large, 150 kb, double-stranded DNA genomes that are enclosed within a protein capsid and surrounded by a delicate lipid envelope containing a dozen different glycoproteins. The virus's chromosome is circular. Because the envelope is so delicate, the survival of the virus in the environment is very limited and makes them highly susceptible to destruction by common disinfectants. EHV-1 and EHV-4 strains of rhino contain 76 homologous, co-linear genes.
Cell structure, metabolism & life cycle
The viral DNA of rhinopneumonitis is enclosed in the central nucleocapsid, which is composed of 162 hollow capsomeres arranged as a shell. The nucleocapsid is surrounded by a flexible lipid membrane that contains virus-encoded glycoprotein.
The life cycle of rhinopneumonitis is 1. infection, 2. establishment of latency, 3. periodic reactivation from latency, and 4. respiratory shedding with transmission to new hosts. The latent carrier state is essential the maintenance and spread of the the virus. Latency is a reversible state, so the virus can deactivate and reactivate when production deems fit. Transmission of the virus is efficient and outbreaks can spread rapidly in dense horse populations.
Ecology (including pathogenesis)
Rhinopneumonitis only affects horses. It usually enters its host via the respiratory tract and infects its primary target tissue, the respiratory mucosal epithelium. Infection is acquired by close physical contact with an infected horse that is actively shedding the virus into its respiratory secretions, i.e. nasal discharge. Transmission by snorting directly onto horses, stall walls, or fences is especially efficient. Indirect transmission occurs by contamination of human hands, feed equipment, etc. The incubation period for signs of respiratory infection after natural exposure ranges from 2-5 days. A horse infected for the first time can shed the virus for up to 14 days and peak shedding occurs during the first days after the appearance of nasal discharge. The severity of the disease is influenced by the horse's state of health, stress, infection history, immune system, and virulence level of the strain.
Interesting feature
Rhinopneumonitis can cause neurological symptoms in affected horses. The virus attacks the spinal cord and causes weakness, incoordination and collapse. This leads to inflammation in the spinal cord, which causes paralysis od the bladder and cystitis. Current vccinations can not provide protection against this form of disease caused by the virus.
References
http://www.petplace.com/horses/rhinopneumonitis/page1.aspx
Allen, G.P. "Respiratory Infections by Equine Herpesvirus Types 1 and 4". Equine Respiratory Diseases, P. Lekeux (Ed.) 2002. http://www.ivis.org/special_books/Lekeux/allen/IVIS.pdf
