Article ID: WMC002363 ISSN 2046-1690 Does Virulence Offer Benefit to the Parasite? Corresponding Author: Dr. Fatik B Mandal, Associate Professor in Zoology, Bankura Christian College, College Road, Bankura, 722101 - India Submitting Author: Dr. Fatik B Mandal, Associate Professor in Zoology, Bankura Christian College, College Road, Bankura, 722101 - India Article ID: WMC002363 Article Type: Review articles Submitted on:22-Oct-2011, 11:12:34 PM GMT Published on: 25-Oct-2011, 12:44:37 PM GMT Article URL: http://www.webmedcentral.com/article_view/2363 Subject Categories:PARASITOLOGY Keywords:Parasite\'s Virulence, Mutation, Co-infection, Adaptive Theory, Host Resistance, Mode of Transmission How to cite the article:Mandal F B. Does Virulence Offer Benefit to the Parasite? . WebmedCentral PARASITOLOGY 2011;2(10):WMC002363 Source(s) of Funding: None Competing Interests: None WebmedCentral > Review articles Page 1 of 9 WMC002363 Downloaded from http://www.webmedcentral.com on 10-Jan-2012, 11:16:46 AM Does Virulence Offer Benefit to the Parasite? Author(s): Mandal F B Abstract resistance leading to the development of more virulent virus strains 6,7. Killing the host may be advantageous for parasites which are transmitted only from dead hosts 8. The Probability of producing virulent variants Parasites and hosts remain locked in a continuous increases with host population size, and crowding and struggle for survival. The co-evolutionary interactions co-mingling of the different host species 9. The between the hosts and parasites influence both the parasite-host relations naturally constitute a host and parasite. Parasite pathogenesis and host co-adaptive evolutionary dance along the defense act in a concert to shape the evolution of pathogenecity threshold, which is likely to be crossed virulence. There are various theories on the evolution due to anthropogenic disturbances 10. With this of virulence in parasite. Host mortality, host resistance, summary of background of our understanding, the host recovery, mutation, co infection, super infection, present communication aims to sum up the topic of host heterogeneity, and mode of transmission have parasitic virulence in the light of modern evolutionary been described for explaining the virulence in parasite. research. The evolution of parasite virulence focuses on the tradeoff between costs and benefits of the parasite Virulence of parasite from the host exploitation and appears to be satisfactory. The interaction between the hosts and parasites is multidimensional, dynamic and Virulence is the reduction in host’s fitness due to exceedingly complex. A number of factors perhaps act parasite infection 11 which is proportional to reduction to shape the level of parasite virulence. Generally, in the reproductive success of the infected host. This virulence is not beneficial to most of the parasites. is not in the parasites interest to severely damage or Virulence attains a maximum level due to the mutation kills its host as this affects the parasite’s fitness. A which is induced by the changing environment. A weak host is the easier prey for parasites than a thorough understanding of pathogenesis including a strong one. Evolution leads to an increase or decrease complete inventory of spatial and temporal expression in virulence, depending on the circumstances 12 of genes by both hosts and parasites, from the time of .Virulence is indicated by the mortality of the infected exposure to the final resolution of infection, would hosts, the average life-span of infected hosts, and the contribute a lot in our understanding of evolution of lethal dose50, LD50. In avirulent parasite, the infected parasite’s virulence. host does not die. Evolutionary change of parasite Introduction occurs due to change in micro-environment provided by the host. Parasites with more opportunities for vertical transmission and reduced opportunities for horizontal transmission often become less virulent 13. The survival of the parasite depends on the conditions The zoonotic transfer of a parasite from one host in which parasites grow and multiply1. Death of the species to another may be highly virulent in the new host often limits the evolutionary success of the host, as observed in 1918 influenza pandemic. Severe parasite. Successful microbes avoid extinction, virulence kills the host before the parasite’s multiply, and leave descendants 2. Myxoma virus transmission to new hosts. Less virulent parasite cause a mild disease in the American rabbit .Simian increases over time14. Immunodeficiency Virus (SIV) does not cause immunodeficiency in its natural hosts 3.Many Trade- off theory Orthomyxo-, Arena-, and Hantaviruses cause asymptomatic infection4. Some parasites are extremely virulent in the novel host. The virulence decline in The evolution of parasite virulence is based on a course of time as observed in smallpox, measles, and trade-off between the advantage of within-host 2. influenza infections in Indian populations In replication and the disadvantage of such replication on 5, Australian populations of European rabbits the host survival 15,16. Hypotheses on parasite virulence are initially introduced myxoma virus strain was lethal with the conventional wisdom, adaptive theory, and case mortality of > 99 percent. In subsequent years, non-adaptive theory. Changes in transmission rate the virulence was declined .Later, rabbits developed WebmedCentral > Review articles Page 2 of 9 WMC002363 Downloaded from http://www.webmedcentral.com on 10-Jan-2012, 11:16:46 AM lead to changes in virulence suggesting the existence new hosts. This trade-off between the infection rate of underlying trade-off. Strong inhibition of host and the duration of infection determine the optimal immunity by the parasite reduces the parasite parasite infectivity and duration of infection. A shorter clearance rate from the host, increase the host’s death infection period is associated with high virulence of the rate and in such case the parasite tend to be relatively parasite. Parasite evolution with this trade-off is the more virulent. As the parasite gains the immediate backbone of the adaptive theory. Anderson and May 15 benefit of escaping before the subsequent proposed a theoretical framework for the evolution of consequences of immune manipulation 17, parasites. immuno-modulatory or escape mechanisms may be favoured in spite of their virulence. Some progress on Host mortality mechanistic coupling between transmission and virulence has been made 18. For example, transmission may occur through spores after host’s death. An increase in the parasite-independent host mortality Autographa californica, a nucleopolyhedro virus rate should lead to selection of parasites that kill their attacks caterpillars. After ingestion, the virus invades hosts rapidly and are more virulent 18. Shorter the midgut cells 19, replicates and spreads until more life-span of the host often leads to a shorter infection than 1010 polyhedra fill the host. Ultimately, the virus period that in turn selects for rapidly replicating produces chitinase and cathepsin that together parasite. A long host life-span leads to selection of breakdown the host cuticle. The host liquefies at death; slower replicating parasites with low virulence. These release the viruses which are taken up by other hosts. predictions have been tested with variable success 24. Many terminally killing pathogens have similar life In multiple infections , high virulence is evolve at low histories20. natural host mortality rate 25 as the long duration of infection increases the probability of an already In mammals; anthrax (Bacilus anthracis) is a case of infected host to be super-infected with a more virulent mechanistic coupling 21. In the late infection, bacterial parasite strain forcing the existing parasites to density in the host grows to high levels and the high become more virulent. Reduction in the mortality of concentration of anthrax lethal toxin knocks out the transmission stages allows parasites to compensate immune system by destroying the macrophages. for increased virulence and maintain infections in Anthrax oedema toxin causes the production of a high population previously too small to sustain them. amount of cAMP in host cells, which disrupts the flow Importantly, increase in the size of host population of ions and cellular functions leading to host death. usually leads to increase in the incidence of parasite Transmission occurs primarily by spores which are population26. released after the death of the host. B. cereus , although closely related to B. anthracis does not Host recovery/host resistance cause severe virulence 22 .B. cereus transmits through oral-faecal route, colonizes the intestine, and causes 23 diarrhoea .From an evolutionary perspective, In plants and invertebrates, host resistance is often mechanistic coupling between transmission and defined as inability of the parasite to infect the host 27. extreme virulence strongly shapes the life history of In vertebrates, host resistance is often the host ability 18. parasites In the first major transmission episode to mount an effective immune response to clear the parasite grows to maximum density and maximize the infection. An increase in the recovery rate should opportunities for successful transmission, select for more virulent parasites28 as a higher consequently severely damages the host, and the recovery rate leads to a shorter duration of infection chance of subsequent transmission rate is low. forcing parasites to evolve