On episode #271 of the science show This Week in Virology, the TWiV crew discusses two reports on viruses that might have crossed kingdoms, from plants to honeybees and from plants to vertebrates.
You can find TWiV #271 at www.twiv.tv.
9 February 2014
7 February 2014
Virulence, the capacity to cause disease, varies markedly among viruses. Some viruses cause lethal disease while others do not. For example, nearly all humans infected with rabies virus develop a disease of the central nervous system which ultimately leads to death. In contrast, most humans are infected with circoviruses with no apparent consequence. Is there a benefit for a virus to be virulent?
One explanation for viral virulence is that it facilitates transmission. However, a comparison of infections caused by two enteric viruses, poliovirus and norovirus, does not support this general view. Both viruses infect the gastrointestinal tract and are spread efficiently among humans by fecal contamination. However, norovirus infection causes vomiting and diarrhea, while poliovirus infection of the intestine is without symptoms (the rare invasion of the nervous system, and subsequent paralysis, is an accidental dead end). Both viruses have successfully colonized humans for many years, so why does only one of them cause gastrointestinal tract disease?
Two recent studies of bacterial virulence provide some clues about the evolution of virulence. In one a commensal strain of Escherichia coli was serially propagated in the presence of macrophages, which are cells of the immune system that take up and destroy the bacteria. After many such passages, bacterial clones were isolated that escape phagocytosis and killing by macrophages. These clones had also acquired increased pathogenicity in mice. In other words, the genetic changes that allowed the bacteria to evade the immune response also lead to increased virulence.
In another example of evolution to virulence, it was found the the bacterium Pseudomonas aeruginosa can sense the presence of competing gram-positive bacteria because the latter shed the cell wall component peptidoglycan. In response to this molecule, P. aeruginosa secretes proteins that kill the other bacteria. These secreted proteins also make the bacterium more virulent in a host – in their absence, the bacteria are less virulent. In other words, P. aeruginosa damages its host in an attempt to remove nearby bacterial competitors.
In both bacterial examples, virulence can be viewed as collateral damage: the consequence of evading the immune response, or killing off competitors. Being virulent was not the primary goal. This explanation for bacterial virulence is straightforward and compelling: virulence is not directly selected for during evolution but comes along for the ride. Can it be applied to viruses?
All eukaryotic viruses must encode at least one protein that antagonizes host immune responses, otherwise they would be eliminated. These immune evasion proteins are certainly virulence factors: in general, when they are deleted or altered, the capacity of the virus to cause disease in a host is reduced. Like bacterial virulence, viral virulence might be collateral damage incurred by having to evade immune responses. This hypothesis is attractive but seems overly simplistic. If the ubiquitous and benign circoviruses did not evade host responses, then they would be eliminated from the human population.
The reasons why some viruses are virulent and others are not remain elusive. It is possible to reduce viral virulence by mutation, but this type of experiment does not reveal why viruses cause disease. The inverse experiment would be more informative: to select from a population of avirulent virus those that can cause disease. The results of such an experiment would help to identify the selection pressures that allow viruses to evolve to virulence.
2 February 2014
28 January 2014
Porcine epidemic diarrhea arrived in the United States in the spring of 2013. The disease, caused by a coronavirus, was first identified in the United Kingdom in 1971, and has subsequently spread throughout Europe and Asia. The disease is a concern for the swine industry because it is associated with high case fatality ratios in suckling pigs.
Porcine epidemic diarrhea virus is a member of the coronavirus family, which also includes the SARS and MERS coronaviruses (CoV). Before 2013 the virus had not been isolated in North America. It was detected on a farm in Iowa in May and subsequently spread to 22 states. It is estimated that between 1-4 million pigs have died of the disease in the US. Recently the virus was found on a Canadian pig farm in Middlesex County, where it most likely arrived from the US.
PEDV can infect pigs of all ages, but is most serious in nursing pigs in which the clinical symptoms are the most severe. The disease is characterized by acute vomiting and watery diarrhea which in nursing pigs leads to dehydration and frequently death. There is no treatment for the disease other than rehydration; no antiviral drugs are available, but a vaccine was developed in 2013. The virus does not infect humans.
Sequence analysis of genomes from three US isolates of PEDV indicate that they are most closely related to a virus isolated in 2012 in Anhui, China. These data suggest, but do not prove, that the US PEDV originated from China. How the virus might have arrived from that country is a matter of speculation. The virus is believed to move from farm to farm on trucks that are used to carry pigs, as well as on contaminated boots and clothing. Many farms observe strict biosecurity procedures in which trucks are properly washed, disinfected, and heated to inactivate the virus. However these procedures cost money and take time, and may be bypassed in some cases. If older pigs are asymptomatically infected, they might be transported to other farms and spread the virus. Once in a farm, stopping spread of the virus is difficult: it is spread by fecal-oral contamination.
The coronavirus family is divided into four genera: Alphacoronavirus, Betacoronavirus, Gammacoronavirus and Deltacoronavirus. Several human coronaviruses that cause common cold-like illness, as well as PEDV, are alphacoronaviruses. SARS-CoV and MERS-CoV are betacoronaviruses. Viruses in the alphacoronavirus and betacoronavirus genera are believed to have originated from bats, while birds might have been the origin of viruses in the other two genera. Analysis of the PEDV genome sequence indicate that the 5’-untranslated region is similar to that of a bat coronavirus. Based on this information it has been suggested that PEDV originated from a cross-species transfer of a bat alphacoronavirus into pigs.
National Hog Farmer is not usually on my reading list, but it has a good summary of porcine epidemic diarrhea.
26 January 2014
21 January 2014
Robert Herriman, co-founder of The Global Dispatch, recently started a radio show called Outbreak News This Week. Robert calls the show “Your source for all the news about worms and germs”. He covers the latest news and information about infectious diseases and often includes interviews with expert guests. The show can be heard live Saturday mornings at 7:30 am EST on on The Tan Talk Radio Network: 1340 AM WTAN Clearwater, 1350 AM WDCF Dade City and 1400 AM WZHR Zephyrhills. You can also listen online.
I have been a frequent guest on Robert’s Outbreak News This Week, most recently this past Saturday, when we had a broad-ranging conversation about influenza virus. We also managed to squeeze in a few words about my favorite virus, poliovirus, and India’s success in remaining polio-free for three years. Listen below.
19 January 2014
18 January 2014
Viruses inspire many different types of art, but I was unaware of the number of people who make viruses out of fiber!
This made me wonder how many people knit viruses, so I searched Ravelry for ‘virus’. Here are some of the interesting creations I found.
There are also bacteria, such as this collection (with some viruses) from Clare:
You can find more by searching for ‘microbe’ at Ravelry (login required), where you’ll also find the patterns to reproduce these wonderful creations. Microbes are clearly inspiring and fascinating to fiber artists!
Do you make fiber viruses? If so let me know and we can include a photograph here.
14 January 2014
13 January 2014
Three years ago today, on 13 January 2011, the last case of poliomyelitis was reported in India. This achievement represents a remarkable turnaround for a country where control of the disease had for years been extremely difficult. As recently as 2009 there were 741 confirmed cases of polio caused by wild-type virus in India. Being polio-free for three years is certainly a cause for celebration, but not for becoming complacent. Immunization efforts in India must not decline, because wild-type and vaccine-derived polioviruses continue to circulate and pose a threat to any unimmunized individual.
Wild polioviruses – those that have always been circulating in nature – continue to cause disease in Afghanistan and Pakistan, two countries close to India. Pakistan reported 58 polio cases in 2012, and 85 so far in 2013; for Afghanistan the numbers are 37 and 12. But distant countries can also transmit polio: recent outbreaks in the Horn of Africa and in Syria originated in Nigeria and Pakistan, respectively.
Perhaps a greater threat are vaccine-derived polioviruses. The Sabin poliovirus vaccines, which have so far been the mainstay of the polio eradication effort, comprise infectious viruses that are taken orally. Upon replication in the intestinal tract, the vaccine strains confer immunity to infection, but they also revert and become capable of causing paralysis. Such vaccine-derived polioviruses circulate and can cause outbreaks of polio. Because India has been using Sabin poliovirus vaccines intensely for many years, there is no doubt that vaccine-derived polioviruses are circulating in that country. If polio vaccine coverage drops, there will be outbreaks of polio caused by vaccine-derived strains. Even if wild polioviruses disappeared from the globe, as long as Sabin vaccines are used, vaccine-derived polioviruses will circulate. The solution to this conundrum is to switch to Salk’s inactivated poliovirus vaccine and wait for the Sabin-derived strains to disappear. This switch is now part of the WHO’s eradication plan (it wasn’t always), but it will not be easy: Salk vaccine must be injected, and therefore requires trained health care personnel; administering Sabin vaccine requires no special skills. But we cannot simply stop immunizing with Sabin vaccine – that is a recipe for outbreaks of polio.
According to the World Health Organization, being free of wild polio for three years means that the virus is probably no longer endemic in India. However, WHO does not certify individual countries as polio-free; rather it declares a WHO region polio-free when all countries in the Region have not reported a case of wild polio for 3 years in the face of highly active surveillance. The Americas, the Western Pacific, and European regions have been declared polio-free by WHO. India is part of the South-East Asia region, which also includes Bangladesh, Bhutan, Democratic People’s Republic of Korea, Indonesia, Maldives, Myanmar, Nepal, Sri Lanka, Thailand, and Timor-Leste, none of which have reported a case of polio for 3 years. WHO will decide in March whether to declare this region polio-free. That would leave the regions of Africa and the Eastern Mediterranean as the last known reservoirs of wild poliovirus.
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