Vector-Borne Disease

Vector-Borne Disease Transmission

There are two main types of pathogen transmission by vectors, known as internal transmission (sometimes called mechanical transmission) and external transmission. Internal transmission means that a pathogen is carried inside a vector. This can occur as biological transmission, in which the pathogen passes through a necessary stage in its life cycle inside the vector host, which it could not do inside a different host organism. An example of a pathogen that experiences biological transmission is Plasmodium, the infectious pathogen that causes malaria. Internal transmission may also occur as harborage transmission, in which the pathogen remains in the same form and life stage inside the vector as when initially entering the vector. The plague bacterium, Yersinia pestis, is a harborage transmission pathogen because it does not change morphologically when inside fleas, the common vectors that transmit plague. External transmission occurs when a pathogen is carried on the body surface of the vector. When it lands on a human, the vector passively transmits the pathogen to its new host. An example of a pathogen that is transmitted by external transmission is Shigella, which is carried on the legs of flies.

Vector-Borne Pathogens

The list of vector-borne pathogens is long and despite improvements in insect control, understanding of disease, and sanitary conditions of large populations of humans, many of these pathogens are still endemic or epidemic in some parts of the world today. Malaria has beleaguered humans for centuries if not millennia; however, the causative agent Plasmodium wasn't identified until 1880 by French army surgeon Charles Louis Alphonse Laveran. There are multiple species of Plasmodium that cause malaria, some more infectious than others, with Plasmodium falciparum being the most common in occurrence. When a female mosquito bites a human, Plasmodium sporozoites are transmitted into the bloodstream, introducing the pathogen to its new, human host. The sporozoites travel through the blood to the liver where they enter cells, mature into schizonts, undergo asexual reproduction, and cause the liver cell to rupture, releasing hundreds of merozoites. Some species of Plasmodium will lay dormant in the liver for long periods of time (sometimes years) before maturing and rupturing cells, whereas others will cause liver cell rupture within 2 weeks of initial infection. Merozoites released into the blood enter red blood cells where they rapidly reproduce, causing cell rupture and release of Plasmodium that may [Page 1074]be in different life-cycle stages. Some may be immature trophozoites that will enter new red blood cells, mature into schizonts, and lead to the release of merozoites, resulting in a continuous proliferation of Plasmodium within the human host, sometimes resulting in a chronic case of malaria. Other Plasmodium that are released from rupturing blood cells are gametocytes in the sexual stage that cause sporogonic development in mosquitoes. It is these gametocytes that the female mosquito ingests in her blood-meal from humans, allowing the life cycle of Plasmodium to continue. This period of development typically occurs in mosquitoes of the genus Anopheles. During this time, gametocytes (male and female) begin the sporogonic stage in the mosquito's stomach, leading to the production of sporozoites that migrate to the mosquito's salivary glands, facilitating their transfer to a human host when the mosquito ingests another blood-meal. This stage of the Plasmodium life cycle highly depends on temperature in that it requires a minimum temperature to be initiated and will stop if temperature becomes too high. Moisture levels also greatly influence the success of pathogen replication. This temperature and moisture dependence is reflected in the greater occurrence of malaria in tropical and subtropical climates that encourage optimal mosquito body temperature and life span and provide sufficient moisture for mosquito breeding, allowing the pathogen to flourish.

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