An introduction to parasitic virulence

Trichoptera caddis flies 1 clade [c. Parasitoid wasp Potter waspan idiobiont, building a mud nest; she will provision it with paralysed insects, on which she will lay her eggs; she will then seal the nest and provide no further care for her young Within the Hymenoptera, parasitoidism evolved just once, and the many described [d] species of parasitoid wasps [27] represent the great majority of species in the order, barring those like the antsbeesand Vespidae wasps that have secondarily lost the parasitoid habit. The parasitoid wasps include some 25, Ichneumonoidea22, Chalcidoidea5, Vespoidea4, Platygastroidea3, Chrysidoidea2, Cynipoideaand many smaller families.

Above I have provided an outline of the theoretical basis for understanding the evolution of parasite virulence. I have however also suggested that what will occur during actual virulence evolution is going to be dependent on specific circumstances.

Though it is not within the scope of this book to provide numerous detailed examples of parasite virulence evolution, nonetheless in this section I delve into a handful of illustrative examples, beginning with relatively simple systems and progressing toward increasing complexity.

These examples include, in order, the virulence displayed by phages against host bacteria, the virulence displayed by viruses against other viruses i. Subsequently I consider pathogen evolution from a genomics perspective as An introduction to parasitic virulence as parallels between cancer progression and parasite virulence evolution.

Phage Virulence Phages illustrate a number of generalizations pertaining to parasite virulence, plus a few unique perspectives of their own. The most important of these perspectives may be in distinguishing the concept of virulence in terms of absolute host impact versus rapidity of host impact, plus in defining the concept of host itself which here I will define as equivalent to a bacterial culture, which I justify in the following two paragraphs.

In addition, in terms of phage virulence it is relatively simple to appreciate how bypassing a usual transmission mechanism can result in the evolution of greater parasite virulence.

I begin, though, with a brief introduction to the history of phage virulence since otherwise my subsequent discussion will only confuse those who have greater familiarity with phages.

Phages have been known to science for approximately years Abedon et al. In other instances phage biology seems to have diverged from the precepts adhered to by biologists in general. The concept of phage virulence is perhaps the best example of this latter tendency, though this was not always so.

Phage virulence, originally, seems to have been a concept that was little different from the common idea that it may be measured in terms of host health. For early phage biologists, however, the bacterial culture, perhaps more so that the bacterium itself, was viewed as the phage host.

Thus, phage infection can be viewed as a culture "infection", and the relative health of that culture a function of the inherent virulence of the added phages. Some phages when added had a negligible effect on a culture e. These phages could be described effectively as avirulent.

Other phages, by contrast, gave rise to dramatic drops in culture turbidity. Those latter phages could be described as virulent. In between there exist various phages that have negative impacts on cultures, though impacts that are less dramatic than culture-wide lysis.

Such phages display an intermediate level of virulence. An easy measure of phage virulence involved and still involves the addition of a certain quantity of phages to a certain quantity of bacteria, which is then followed for signs of virulence, especially culture-wide lysis.

These assays actually have a time component since they involve, effectively, a race between bacterial growth to stationary phase at which point most phages are no longer active and phage growth to sufficient densities to infect and then lyse most the bacteria present.

Those phages which could still lyse cultures starting from lower phage densities could be described as more virulent. This is because they have a greater potential to effect culture-wide lysis relative to phages displaying lower virulence.

Certain phages also exist that do not lyse bacteria, i. Especially in the latter case, the word virulent has come to describe phages that are obligately lytic, that is, unable to display either lysogenic or chronic infections, versus these other phage varieties.

Within the context of potential to achieve culture-wide lysis, this description is reasonably accurate. Nonetheless, for those who are familiar with phage biology may be confused by the idea that the concept of phage virulence might have something to do with concepts of parasite virulence in general.

It does, especially historically, and I apologize for that confusion.Moreover, a high parasite reproductive rate, and therefore higher virulence, are favored if the host has a short life span due to external factors, if the parasites are likely to be extinguished before transmission (e.g., by antibiotic treatment), or if the opportunity for infection of new hosts rapidly increases, as during disease epidemics.

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Send questions or comments to doi. The initial amount of pathogens required to start an infection within a susceptible host is called the infective dose and is known to vary to a large extent between different pathogen species.

Sabouraud Dextrose Agar (SDA) is a selective medium primarily used for the isolation of dermatophytes, other fungi and yeasts but can also grow filamentous bacteria such as Nocardia. Moreover, a high parasite reproductive rate, and therefore higher virulence, are favored if the host has a short life span due to external factors, if the parasites are likely to be extinguished before transmission (e.g., by antibiotic treatment), or if the opportunity for infection of new hosts rapidly increases, as during disease epidemics.

Optimal virulence is a concept relating to the ecology of hosts and parasites. One definition of virulence is the host's parasite-induced loss of fitness. The parasite's fitness is determined by its success in transmitting offsprings to other hosts.

At one time, the consensus was that over time, virulence moderated and parasitic relationships evolved toward symbiosis. This view has been challenged.