Disease transmission by Arthropods

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Disease transmission by Arthropods:

Disease transmission by Arthropods Dr. P. F. Ayeh-Kumi Dept of Microbiology

Transmission of Infectious Diseases:

Transmission of Infectious Diseases Three main ways 1. Vertical 2. Horizontal 3. Tangential

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Vertical This is hereditary. There is direct transfer of infection from parent to progeny (transovarial in arthropods) Transmission is through ovum, placenta, sperm, milk etc

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Horizontal Other forms of transmission other than the direct transfer from parents. Examples include: respiratory, salivary, fecal-oral, venereal, blood-borne, skin- skin, eye-eye and zoonosis

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Tangential This refers to the kind if transmission that occurs when pathogens shuttle between vectors, vertebrates (reservoir host) and humans

Transmission of disease by arthropods:

Transmission of disease by arthropods There are two main ways: a) Mechanical transmission (Carriers) b) Biological transmission (Vectors)

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Biological transmission 1. Arthropods act as Vectors: 2. The arthropods form an integral part of the life cycle of disease causing organisms. In which case the organisms undergo a) Multiplication b) Development c) or Both in the arthropod

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Chain of events in biological transmission Events leading to successful transfer of pathogen by vector from one host to another. Involves links which when interrupted prevent successful transfer of the infection.

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First link Involves the developmental cycle of the pathogen within the body of an appropriate host. This link could be broken by medicinal treatment e.g. Malaria

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b) Second link This involves the arthropod’s access to infection. This is based on feeding habits, manner of ingestion of food, the structure of mouthparts of the arthropods

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c) Third link This represents the progress of development of the pathogen within the body, stomach or other organs of The vector.

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d) Fourth link Represents the manner in which the infective microorganism leaves the vector and enters the next susceptible host to cause an infection. This in effect is the mechanism of infection that may include

Life cycle of Malaria parasites:

Life cycle of Malaria parasites

Biological transmission:

Biological transmission Is the most common method of pathogen development among arthropod associated diseases of vertebrates. This may be divided into three types 1. Propagative 2. Cyclo-propagative 3. Cyclo-developmental

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Propagative The organisms undergo no cyclical changes but multiply in the body of the vector. This is a term used to describe bacterial and viral multiplication in vectors

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Examples of Propagative Type The plague bacilli ( Pasturella pestis ) multiply in the stomach of fleas. Epidermic typhus ( R. prowezeki ) Yellow fever/Dengue fever (viruses) Louse borne relapsing fever ( Borrellia recurrentis )

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Cyclo-propagative In this case the organisms undergo cyclical changes or cyclic changes and multiplication in the body of the vector. E.g changes in protozoa

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Examples of Cyclo-propagative The malaria parasite (Plasmodium sp). Trypanosomiasis ( Trypanosoma brucei gambiense ) Trypanosomiasis ( Trypanosoma cruzi ) Leishmaniasis ( Leishmania donovani )

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Cyclo-developmental This refers to the case where the Organisms undergo cyclic changes but do not multiply in the body of the vector. Examples include microfilariae developing from L1 to L3 without multiplication. Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, Loa loa

Life cycle of W. brancrofti:

Life cycle of W. brancrofti

Factors that determine the effectiveness of a vector:

Factors that determine the effectiveness of a vector Pathogen receptivity Host specificity Longevity Frequency of feeding Mobility Numbers Physiology and behavioral plasticity

Biological transmission:

Biological transmission Vector Pathogen Disease Victim Phlebotomus fly Leishmania spp Leishmaniasis humans Anopheles spp. Plasmodium spp. Malaria humans Chrysops spp. Loa loa Loiasis humans Simulium spp. Onchocerca volvulus Onchocerciasis humans

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Mechanical transmission 1. Arthropods act as Carriers: 2. The arthropods carry disease- causing organism mechanically from subject to subject

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Bacteria, protozoa and eggs of helminthes may be transported mechanically by arthropods in many ways. They may adhere to the hairy legs, wings and mouthparts or gain entrance into their bodies. This may ultimately find itself in the alimentary canal or body cavity, salivary glands and muscle of the arthropod.

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Transmission is by deposition of pathogen in susceptible places as the lips, eyes, ears, and wounds or following bites by the contaminated mouthparts The pathogen may be regurgitated into the blood stream when a blood sucking arthropod feeds

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Simple inoculation during subsequent feeding may be affected by piercing sucking mouthparts which have been contaminated with blood pathogens (e.g. housefly) Organisms may be discharged with faeces and ultimately rubbed into an abraded skin as in the case of Chagas disease

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Organisms may also be deposited on the skin when the vector is crushed (rocky mountain spotted fever) The vector e.g. blood sucking flies and mosquitoes may also transfer eggs of other arthropods. These are disseminators of human botfly eggs

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Organisms may also be accidentally swallowed whole Additionally arthropods may breed or feed in/on excrement, become contaminated and ultimately transfer their burden to vegetables, meat or water. The agents of Cholera, amoebiasis and typhoid fever may be transmitted in this manner.

Mechanical transmission:

Mechanical transmission Vector Pathogen Disease Victim House fly, Cockroaches Entamoeba histolytica amebic dysentary Humans Cockroaches Toxoplasma gondii toxoplasmosis humans, cats Tabanidae (triatomid bug) Trypanosoma spp. trypanosomiasis Humans

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