Permissive vaccines and viral virulence

chicken farmA permissive vaccine prevents disease in the immunized host, but does not block virus infection. Would a permissive vaccine lead to the emergence of more virulent viruses?

This hypothesis is based on the notion that viruses which kill their hosts too quickly are not efficiently transmitted, and are therefore removed by selection. However a vaccine that prevents disease, but not viral replication in the host, would allow virulent viruses to be maintained in the host population. It has been suggested that in this scenario, viruses with increased virulence would be selected if such a property aids transmission between hosts.

On the surface this hypothesis seems reasonable, but in my opinion it is flawed. One problem is that increased transmission might not always be associated with increased virulence. The more serious flaw lies in making anthropomorphic assessments of what we think viruses require, such as concluding that increased viral transmission is a desired trait. Our assumptions fail to recognize the main goal of evolution: survival. Evolution does not move a virus along a trajectory aimed at perfection. Change comes about by eliminating those viruses that are not well adapted for the current conditions, not by building a virus that will fare better tomorrow. All the viruses on Earth today transmit well enough, or they would not be here; yet some kill their hosts clearly much faster than others. The fact is that humans have little understanding of what drives virus evolution in large populations. Our assumptions of what constitute the selective forces are usually tainted by anthropomorphism.

This long preamble is an introduction to a series of findings which are purported to support the idea that permissive vaccines (the authors call them ‘leaky’ and ‘imperfect’ vaccines but I dislike both names because they imply defects) can lead to the selection of more virulent viruses. The subject of the paper is Marek’s disease virus (MDV), a herpesvirus that infects chickens. MDV is shed from feather follicles of infected chickens and is spread to other birds when then inhale contaminated dust. Vaccines have been used to prevent MDV infection since the early 1970s. These vaccines prevent disease, but do not block viral replication, and vaccinated, infected birds can shed wild type virus. The virulence of MDV has been increasing since the 1950s, initially from a paralytic disease, to paralysis and death. The authors wonder if the use of permissive Marek’s vaccines has lead to the selection of more virulent viruses.

To address their hypothesis, the authors inoculate vaccinated or unvaccinated chickens with a series of MDV isolates that range from low to high virulence. Unvaccinated chickens inoculated with the most virulent MDV died within a week and shed little virus. In contrast, most vaccinated birds survived infection with virulent viruses, and shed virus for the length of the experiment, 56 days.

A transmission experiment was done to determine if shed virus could infect other birds. The authors infected vaccinated or unvaccinated birds and asked if sentinel, unvaccinated chickens became infected. Unvaccinated birds died within 10 days after infection with virulent MDV, and did not transmit infection. In contrast, vaccinated birds survived at least 30 days, and co-housed sentinel animals became infected and died.

The experiments are well done and the conclusions are clear: more virulent Marek’s disease viruses replicate longer in vaccinated than unvaccinated chickens, and can be readily transmitted to other chickens. But these results do not prove that more virulent MDV arose because of permissive vaccines. Nor do the results prove in general that leaky vaccines lead to selection of more virulent viruses. The results simply show that a vaccine that does not prevent replication will allow transmission of virulent viruses.

To prove that vaccinated chickens can allow the selection of more virulent viruses, vaccinated chickens could be infected with an avirulent virus, and the shed virus collected and used to infect additional, vaccinated birds. This process could be repeated to determine if more virulent viruses arise. While the results of this gain-of-function experiment would be informative, they would be done in a controlled laboratory setting which would not duplicate all the selective forces present on a poultry farm.

The authors note that most human vaccines do prevent replication of infecting virus. They do not mention the one important exception: the Salk poliovirus vaccines. People who are immunized with the Salk vaccine can be infected with poliovirus, which will then replicate in the intestines, be shed in the feces, and transmitted to others. This behavior has been well documented in human populations, yet the virulence of poliovirus has not increased for the 60 years during which the Salk vaccine has been used.

I do not feel that these experimental results have general implications for the use of any animal vaccine. It is unfortunate that the work has been covered in many news sources with the incorrect implication that vaccines may be responsible for the emergence of more virulent viruses.

A retrovirus makes chicken eggshells blue

Araucana eggWhen you purchase chicken eggs at the market, they usually have white or brown shells. But some breeds of chicken produce blue or green eggs. The blue color is caused by insertion of a retrovirus into the chicken genome, which activates a gene involved in the production of blue eggs.

The Araucana, a chicken breed from Chile, and Dongxiang and Lushi chickens in China lay blue eggs. Blue eggshell color is controlled by an autosomal dominant gene: eggs produced by homozygote chickens are darker blue than those from heterozygotes. The gene causing blue eggshell color is called oocyan (O) and was previously mapped to the short arm of chromosome 1.

To further refine the location of the O gene, genetic crosses were performed using molecular markers on chromosome 1. The O gene was then located in a ~120 kb region which contained four genes. Only the SLCO1B3 was expressed in the uterus of Dongxiang chickens that produce blue eggs; it was not expressed in chickens that produce brown eggs.

Sequence analysis of the SLCO1B3 revealed that an endogenous avian retrovirus called EAV-HP has inserted just upstream of the gene. This insertion places a promoter sequence in front of the SLCO1B3 gene. As a consequence, the SLCO1B3 gene is transcribed. In chickens that produce brown eggs, no retrovirus is inserted before the SLCO1B3 gene, and no mRNA encoding the protein is produced.

The retrovirus insertion has occurred at different positions in the Chilean and Chinese chicken genomes. This observation indicates that the insertion arose independently during breeding of chicken strains several hundred years ago to produce blue egg layers. The chicken genome contains multiple copies of endogenous retroviruses, which can duplicate and move to other locations. We can assume that a random insertion upstream of the SLCO1B3 gene was selected for by breeding procedures that were aimed at producing blue egg-laying chickens.

The SLCO1B3 gene encodes a membrane transporter protein that mediates the uptake of a wide range of organic compounds into the cell. The blue eggshell color is produced by deposition of biliverdin on the eggshell as it develops in the uterus. Biliverdin is one component of bile salts, which are transported by SLCO1B3, providing a plausible hypothesis for the role of the protein in making blue eggshells.

Blue eggshell color is another example of the important roles that retroviruses have played in animal development. One other is the help provided by retroviruses in producing the placenta of mammals. Not all retroviral insertions are beneficial – integration next to an oncogene can lead to transformation and oncogenesis.

TWiV 116: Cocaine, colonies, and chickens

hive and feeding bottlesHosts: Vincent RacanielloAlan Dove, and Rich Condit

On episode #116 of the podcast This Week in Virology, Vincent, Dickson, Alan, and Rich review an adenovirus-based vaccine strategy against drug addiction, a field trial of RNAi to prevent Israeli acute paralysis virus infection in honeybees, and suppression of avian influenza transmission in transgenic chickens.

Click the arrow above to play, or right-click to download TWiV #116 (64 MB .mp3, 89 minutes).

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Links for this episode:

Weekly Science Picks

Dickson – Brian Deer’s investigation of the Wakefield and MMR vaccine
Rich –
Photographic periodic table of the elements
Alan – Year of the bat (site one and site two)
Vincent – EteRNA (NY Times article)

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