Transmission dynamics within biological vector populations

Different forms of transmission also occur within biological vector populations and can be very important in maintaining a source of infection to veterinary species. Two major forms of maintenance of disease agents occur within vectors: transstadial and transovarial. In transstadial transmission, infection with a pathogen is maintained in the vector as it develops between life stages. For example, a tick vector infected as a larvae with Borrelia burgdorferi, the causative agent of Lyme disease, will maintain the infection when it next molts to the nymph and then the adult stage. Transovarial transmission is a form of vertical transmission in which the female vector passes the infectious agent through her eggs to the next generation. For example, eggs passed by an adult female Boophilus tick that was infected with Babesia bigemina will hatch infected larvae.

Transovarial transmission can be very important in maintaining a source of infection to animals. For example, transovarial transmission is able to maintain vesicular stomatitis virus in sand fly and black fly vector populations. Vesicular stomatitis virus causes a vesicular disease in swine, horses, cattle, and humans which may be difficult to differentiate from other vesicular diseases of animals including foot and mouth disease, swine vesicular disease, and vesicular exanthema of swine.

Vesicular lesions on snout and hoof of pig.

An outbreak of vesicular stomatitis in the southwestern US in 1982 was at first expected to subside upon the death of adult flies following the first winter frost. At the time transovarial transmission of vesicular stomatitis virus had not yet been demonstrated in black fly and sand fly populations. The outbreak unexpectedly continued throughout the winter. This continuance was attributed to the movement of infected animals and the exposure of uninfected animals to contaminated objects, however, transovarial transmission may have contributed to the overwintering ability of the virus.

Previous Page | Menu | Next Page