Wildlife disease agents can be transmitted vertically or horizontally. Vertical transmission refers to transmission of a disease agent from parent to offspring. Horizontal transmission refers to transmission of a disease agent from animal to animal, independent of their parental relationship. Horizontal transmission is the most common method of disease spread. It can result from several actions, including:
• skin to skin contact between members of the same species;
• airborne transmission of droplets containing disease agents;
• contact with secretions and excretions, including residual fecal materials;
• contact with genital and sexual materials;
• discharge from lesions;
• contact with infected carcasses;
• ingestion of contaminated water and food;
• transmission by other species.
Here we discuss only the latter means of disease transmission, transmission by other species, and specifically by arthropods. Because another species is involved in the transfer from host to host, it is considered to be a form of indirect transmission . Not surprisingly, direct transmission is defi ned as the transfer of a disease agent (pathogen) from one host to another without the involvement of another species. Transmission of an infective stage of a disease agent to a host can occur in various ways. Passive transmission occurs when the host is contaminated or infected accidentally through ingestion of food, water, or an infected arthropod; this occurs with many nematodes. Active transmission occurs when the disease agent actively penetrates the bodies of their host after gaining contact with them; this occurs with hookworms. Finally, inoculative transmission occurs when a vector such as a mosquito injects the disease agent into the new host during the process of blood feeding, as occurs with the protozoa causing malaria.
The importance of arthropod transmission as a means of disease spread varies greatly among diseases. For some wildlife diseases it is the only means or most important form of transmission, but for other diseases it is less important or arthropod transmission does not occur. It is useful to note that transmission of disease agents by arthropods is not completely independent of the other routes of horizontal transmission. Arthropods can be associated with food and carrion, for example, and wildlife could contract a disease from an insect or by feeding on contaminated food. Nevertheless, because an arthropod (serving as a ‘ vector ’ ) is involved in the transmission or transport process, such diseases are called vector - borne.
Vectors differ in their ability to acquire disease agents and to infect hosts ( vector competence ). Insects usually are short - lived, and acquire and transmit disease agents quickly, with the incubation period in the vector lasting perhaps a few days. Insects may be quite mobile, and typically take many small blood meals. In contrast, ticks are more long - lived, and incubation may require months. Ticks lack wings, so they are much less mobile, and normally take few but large blood meals.
Differences in vectorial capacity or vector competence are due to several factors. There are genetic variants among a single species of disease agent, for example, that differ in their ability to be acquired or transmitted. The vectors also differ in susceptibility to oral infection and effi ciency of transmission, population structure (density, longevity, etc.), host preference, and geographic distribution. Also, the vertebrate hosts differ in susceptibility, which may be manifested in the ability of the host to develop concentrations adequate to infect the vector, population structure (availability of susceptible stages), immune status (prior exposure may confer immunity), and overlap in space and time with the vectors. Often, the suitability of a host animal to produce adequate concentration of the disease agent, or the ability of the disease agent to replicate in the vector, determines the ability of the vector to acquire and transmit the agent. Disease concentration thresholds seemingly exist, especially with mosquitoes, below which transmission does not occur. Thus, certain hosts or vectors are more important in disease transmission cycles. With tick vectors, however, the concentration of disease agent within the host seems to be less important, as ticks can infect one another while feeding together on the same host. Such ‘ co - feeding ’ infection occurs with only minimal or incomplete systemic infection, and usually the ‘ donor ’ and ‘ recipient ’ ticks must be feeding in proximity.
