Transmission and pathogenesis of viral infections: The key to developing control programs (Proceedings)
Transmission of Viruses
Direct transmission requires that animals be in close, intimate contact because the animal-free state of the virus must be very short in order to effect a successful transmission of viable virus. Direct transmission of viral infections can be controlled by isolation of infected animals and by immunization of susceptible individuals. The mechanisms for direct transmission of viruses are via aerosol, direct contact, and sexual transmission; it is difficult to separate aerosol transmission clearly from direct contact.Generally, viruses that are transmitted via aerosol are more stable viruses and are able to survive for longer periods of time outside of the host's bodily environment; conversely, viruses that are transmitted by direct intimate contact or sexually are more unstable and require rapid and immediate transmission in order to maintain their viability (infectivity). Enveloped viruses generally require close direct contact, or arthropod vectors, to effect transmission.
Vectors serve as intermediaries between the infected and the susceptible animals. Vectors are either biological or mechanical. Requisite for insect vector transmission is a high level viremia in the infected mammalian animal, without which the vector would be less likely to be infected during a blood meal on an infected animal.
Biological Vectors. Biological vectors support the replication of the virus, with concomitant amplification, before transmission to the new host. Bluetongue virus is transmitted by a biological insect vector. Vector control is, however, the single most important preventative measure.
Mechanical Vectors. Two types of mechanical vectors are of concern: inanimate vectors (fomites) and animate vectors including ticks, flies and other animals. Animate (insect) vectors may also acquire a virus during a blood meal which is then transmitted to a new host without replication within the vector.
Control and management mechanisms applicable to mechanical vectors are generally those of hygiene and disinfection. Therefore, the clinician should be aware of the significance of single-use materials, i.e., syringes and needles, endo-tracheal tubes, palpation sleeves, and thorough disinfection of equipment between animals and premises. As with biological vectors, control of disease requires control and elimination of the vector(s).
Genetic (Vertical) Transmission
Genetic transmission occurs the least often of the mechanisms discussed but is of importance for oncornavirus, herpesvirus, and orbivirus infections. Genetic transmission is the transmission of virus from one generation to successive generation via the germ cell(s). Genetic transmission may occur by the simple attachment or association of an infectious virus or the viral genome to the germ cell. The genetic transmission of IBR and blue tongue viruses in semen has been demonstrated; these viruses have been shown to be absorbed to the sperm cell. Oncornavirus genome may be incorporated into the germ cell genome.