II. AS TRANSMITTERS OF PATHOGENIC ORGANISMS

Arthropods are also important to veterinary medicine because they can be involved in the transmission of pathologic organisms. There are several types of transmission that can occur and there are several methods by which organisms can be transmitted.

1. TYPES OF TRANSMISSION.
   1. Mechanical. When pathogenic organisms are carried passively by an arthropod. This can occur by attachment to body parts of the arthropod or by contaminated mouth parts of blood feeding arthropods.
   2. Biological. When transmission of an organism takes place as a result of a required association with an arthropod. Specific changes in the organism take place including various morphological, physiological, or developmental changes before the organism is infective for the new host. These changes are classified as:
   • cyclico-developmental. Developmental changes in the organism but no changes in numbers. Examples include canine heartworm in mosquitoes.
   • cyclico-propagative. Developmental changes in the organism as well as increases in numbers of organisms. Examples include malaria in mosquitoes.
   • propagative. When no development or changes occurs for the organism but increases in numbers occur. Examples include plaque in fleas.

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2. METHODS BY WHICH ARTHROPODS TRANSMIT ORGANISMS.
   • Bites. When the organism is introduced as a result of blood feeding activities.
   • Mechanical transfer. When the organism is transmitted passively as a result of either contact or feeding activities.
   • Transstadial. Transmission of the organism between stages in the life cycle of the arthropod. (Infection picked up by one stage and maintained through the next stage so that transmission is
   • more likely to occur.).
   • Transovarian. Transmission of an organism from female to offspring via the egg.
   • Fecal. Transmission of an organism via feces rather than through bites or contact.

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3. ARTHROPODS AS INTERMEDIATE HOSTS FOR PROTOZOA AND HELMINTHS.

   Many arthropods serve as intermediate hosts for various parasitic helminth organisms. Examples include orbatid mites as intermediate hosts for various cestode infections.

4. ARTHROPODS AS RESERVOIRS FOR DISEASE AGENTS.

   These animals play an important role in the natural distribution of arthropod-borne diseases and maintaining their existence in nature when the disease is not exhibited in the host population. The true reservoir animal suffers little or no ill effects from the organism but is essential in maintaining it.