Parasite Manipulation of Vector Behaviour

Parasite Manipulation of 13

Vector Behaviour

J.G.C. Hamilton and H. Hurd

Centre for Applied Entomology and Parasitology, School of Life Sciences,

Keele University, Keele, Staffordshire ST5 5BG, UK

Introduction

One of the major differences between a parasitic and a free-living lifestyle

is the requirement that a parasite must periodically pass from host to host.

This transmission phase of its life cycle may involve active host-seeking

stages or passive transfer via ingestion of the current host by the next one.

However, many parasites dwelling in the circulatory system or super-

ficially, in the skin, utilize a third mode of transfer, that of vector trans-

mission. In this case a haematophagous, or blood-sucking, arthropod

transports the parasite to another host. Many micro- and macroparasites

of medical or veterinary importance, such as arboviruses, bacteria,

rickettsia, protozoa and nematodes, are carried between hosts in this way

(Lehane, 1991).

The parasite may have very transient contact with its transport

vehicle, by passing from one host to the next on contaminated vector

mouth-parts or by being ingested but passing through the gut unchanged

and exiting in contaminated faeces. This type of transmission is known

as ‘mechanical’. Alternatively, ‘biological’ transmission occurs when

the parasite undergoes a distinct life-cycle phase within the vector and

engages in a parasitic association with it. This can just involve a phase

of parasite growth and differentiation, as seen in filarial worms, such

asWuchereria bancrofti, where exsheathed microfilariae penetrate the

midgut and undergo two moults within the thoracic flight muscles

(Kettle, 1990). Alternatively, pathogens such as the plague bacillus,

Yersinia pestis, and many of the arboviruses multiply in the vector but

do not change in form. Finally, the malaria parasites and many of the

trypanosomes and leishmanias both multiply and change to new forms

within the vector.

CABInternational2002.The Behavioural Ecology of Parasites
(eds E.E. Lewis, J.F. Campbell and M.V.K. Sukhdeo) 259