Invertebrates, particularly insects, act as vectors of the most debilitating diseases in many already socially and economically compromised populations of the developing world. The diseases vectored include malaria, sleeping sickness, Chagas’ disease, leishmaniasis, lymphatic filariases and river blindness, dengue and yellow fever, and schistosomiasis. Only the latter is transmitted by non-insectan invertebrates, i.e. snails of the genus Biomphalaria. Hurd (2003) recently pointed out that of the ten most important tropical diseases affecting the poorer nations, eight of these are transmitted by invertebrate vectors (Table 1). In addition, and often underestimated in importance since they are probably second only to mosquitoes as vectors of human infectious diseases (Parola & Raoult, 2001), are the ticks. In fact, in the USA ticks transmit more vector-borne diseases than any other vector (US Centers for Disease Control and Prevention, 1999). The diseases transmitted by ticks include Lyme borreliosis, tick-borne encephalitis, ehrlichiosis and babesiosis (Gratz, 1999).
The reason that insects are particularly widespread vectors undoubtedly reflects the success of this group, occupying almost every habitat on earth in vast numbers. In addition, their power of flight, their extraordinarily well developed sense organs and their haematophagous habit have made them ideal vehicles for transmitting human blood-borne diseases. Since many insects live in places infested with pathogens and parasites they can only do so due to the extreme efficiency of their immune defences so that any invading parasite must counteract these defences to survive.
Thus, most parasites are not simply passively transported from human to human by their vectors, but interact intimately with their invertebrate hosts, usually undergoing significant biochemical and molecular modifications to survive, differentiate and multiply in their vectors.