others coexist with their host? Two prime factors determine parasitic

virulence: the manner in which the parasite is transmitted, and the

evolutionary history of the parasite and its host. Parasites which

have colonized a new host species tend to be more virulent than

parasites which have coevolved with their hosts. Parasites which are

transmitted horizontally tend to be more virulent than those

transmitted vertically. It has been assumed that parasite-host

interactions inevitably evolve toward lower virulence. This is

contradicted by studies in which virulence is conserved or increases

over time. A model which encompasses the variability of parasite-host

interactions by synthesizing spatial (transmission) and temporal

(evolutionary) factors is examined. Lenski and May (1994) and Antia et

al. (1993) predict the modulation of virulence in parasite-host

systems by integrating evolutionary and transmissibility factors.

Why do certain parasites exhibit high levels of virulence within

their host populations while others exhibit low virulence? The two

prime factors most frequently cited (Esch

hypothesis (Esch and Fernandez 1993), in which parasitic virulence

host defenses and parasitic self-regulation (Esch and Fernandez 1993,

time, favoring an equilibrium level of virulence (Lenski and May

their host, heartworm (Dirofilaria immitis) and malaria (Plasmodium

ingestion by sheep (Esch and Fernandez 1993, Combes 1991). Infection

potential for compromise in such a system. Snails (Biomphalaria

and v is the rate of recovery of infected hosts. Parasite populations

hosts, while accumulation of rabies virus in the salivary glands

Clinical Textbook for Veterinary Technicians, Second Edition.

subsets of the host population, driving themselves to extinction along

Hotez, P. J. and Pritchard, D. I. 1995. Hookworm Infection. Scientific

TRANSMISSION AND THE DIRECTION OF MODULATION

Some common words found in the essay are:
Esch Fernandez, Sin Nombre, MODULATION Herre's, Virulence- ABSTRACT, Cheney Hibler, Keymer Read, HIV Gibbons, Rabies Lyssavirus, Read Harvey, VIRULENCE Incongruent, et al, host density, parasitic virulence, fernandez 1993, esch fernandez 1993, esch fernandez, lenski 1994, host population, host mortality, antia et al, al 1995, al 1993, et al 1993, et al 1995, toft et al,
Approximate Word count = 3092
Approximate Pages = 12 (250 words per page double spaced)