Infectious Diseases: Transmission

A number of major diseases such as malaria, leishmaniasis, trypanosomiasis, the various forms of filariasis, bacteria (recurrent fever, typhus, plague, tularaemia, bartonellae, etc...) as well as arboviruses are transmitted by bloodsucking arthropods: mosquitoes, biting flies, sandflies, bugs, lice, fleas, ticks, mites. Sometimes this involves purely human diseases (e.g. malaria), but often there is a zoonotic cycle in nature between arthropods and vertebrate animals.

Transmission of pathogens from insect to man can occur in various ways:

• mechanical transmission, comparable to sharing a dirty needle. This can occur in rapid repetitive blood meals of mobile insects on different hosts, e.g. the host reacts to the pain caused by the bite and interrupts the insect’s feeding. The hungry insect will soon try to bite a second host and infect him via the blood of the first host which is still sticking to its mouthparts. This sort of transmission, however, is rare, e.g. tularemia spread by tabanid flies.

• biological transmission, in which the pathogenic organism either (1) reproduces in the vector (e.g. plague, arboviruses), (2) undergoes maturation before it becomes infectious (e.g. river blindness), (3) both reproduces and undergoes maturation (e.g. malaria, sleeping sickness).

How precisely does the vector transport the organism? Haematophagous arthropods have fine mouthparts with which they attempt to puncture a narrow blood vessel. Similar vessel-feeders or solenophages include mosquitoes, lice, bedbugs and many fleas. Other arthropods tear the skin and its capillaries and drink from the pool of blood that then forms. These are the pool-feeders or telmophages (simulids, sandflies, Culicoides sp.). Pathogens can be introduced into the wound via the saliva or by regurgitation ("vomiting") of intestinal contents via the mouthparts. South American assassin bugs defaecate during bloodsucking and the liquid excreta can contain pathogenic organisms. A similar system is found in lice (epidemic typhus and trench fever). Some ticks secrete a liquid between their legs ("coxal fluid") which can contain organisms. Borrelia recurrentis is only transmitted when the vector, the body louse, is crushed.

Category: Medicine Notes