Emerging diseases and immunity

[mynah]

In 2007 a new strain of cold virus, an adenovirus named Ad14, infected an unknown number of people. At least 140 in the USA died of it, including young, previously healthy people, according to an article in the 6 September 2008 edition of NewScientist. By 2008, it had spread around the world – but by then the symptoms were no worse than those of any other cold.

In 2002 a coronavirus made a species jump from bats to human, resulting in SARS, causing prolonged respiratory symptoms in humans, who often died after weeks or months of suffering. By 2004 the disease had almost run its course, and those who did get infected generally suffered only mild illness.

Emerging diseases are often devastating when they first infect humans. A form of avian influenza, the Spanish flu of 1918, raged round the world, leaving millions dead in its wake before it finally subsided. Unlike most epidemic diseases, it was more likely to kill young, healthy adults in their prime than children and middle-aged sufferers.

The Spanish flu never really disappeared: Most of us have probably suffered infection, stayed in bed to recuperate for a day or two, and resumed our lives without getting too ill.

The explanation given for diminished virulence is usually that humans tend to build up antibodies and acquire at least partial immunity to new diseases, rendering infections relatively harmless. With global travel, infections spread round the world fast, but immunity is acquired at equal speed. In previous centuries, epidemics tended to be localized, but there were a few devastating exceptions: The Black Death was brought to Europe from Asia by merchant ships, Columbus and his successors decimated native Americans with European diseases (and took back syphilis as a souvenir that was much more lethal then than now), and the Spanish flu accompanied WWI soldiers to their native lands. In all cases the hapless recipients of the new germs were settlements of people who rarely traveled far from home, and whose immune systems experienced these viruses and bacteria as totally foreign.

It makes sense – but to me, it only does so to a point. I rarely get colds, and have not had one in the past two years. Does that mean that, when Ad14 rocks along, I’ll get really, really ill, or worse? If not, what confers immunity to those unaffected by the initial outbreak, and if I have been exposed, what prevented me from succumbing to severe illness the first time my body encountered the virus?

[Cedars]

Originally Posted by mynah

It makes sense – but to me, it only does so to a point. I rarely get colds, and have not had one in the past two years. Does that mean that, when Ad14 rocks along, I’ll get really, really ill, or worse? If not, what confers immunity to those unaffected by the initial outbreak, and if I have been exposed, what prevented me from succumbing to severe illness the first time my body encountered the virus?

Some peoples immune systems are more likely to correctly detect an invader.

There is also the factor of exposure to something similar enough in the past (as your spanish flu comments relate) which may make you less suceptable to something related but not quite the same as the new variation/mutation.

[modest]

Originally Posted by mynah

The Spanish flu never really disappeared: Most of us have probably suffered infection, stayed in bed to recuperate for a day or two, and resumed our lives without getting too ill.

Interesting. Where did you hear this?

Originally Posted by mynah

it was more likely to kill young, healthy adults in their prime than children and middle-aged sufferers.

Monkeys recently infected with resurrected Spanish flu died with symptoms of Cytokine storm which explains why young, adult, healthy people suffered worse. It's an overreaction of the immune system - healthy immune systems doing the most damage.

~modest

[mynah]

Extract from an article I wrote for the South African Encyclopedia (available online, but unfortunately not without subscription) a few years ago:

Apart from the Black Death - the bubonic plague pandemic that ravaged Europe and Asia in the 1340s – no epidemic yet recorded spread so fast and affected such a number of people.

Although the pandemic reached Spain early and was given much publicity in that country, the devastating strain had its origins elsewhere. The movements of large numbers of soldiers who participated in World War One would no doubt have contributed greatly to the spread of the disease in an era before civil aviation.

In the spring of 1918 an army cook was the first person to get influenza at Fort Riley, a military base in the US state of Kansas. Within two days, 522 soldiers were affected. The base bred and slaughtered its own pigs and poultry, and it is possible that the cook was infected while working with meat of afflicted animals.

The initial cases were not unusually severe – but in August 1918 the virulent strain emerged almost simultaneously in France, the United States and Liberia. From the beginning it was clear that this no ordinary flu: Sufferers would often be too weak to walk within hours of becoming ill. Shortly afterwards the faces of many would take on a bluish tinge and, as they struggled for breath, bloody froth would bubble from their mouths. Millions of people died within a day of two after the first symptoms appeared. In the US the pandemic reached its peak in October 1918, and elsewhere the disease spread like wildfire. Almost 140 000 deaths were recorded in South Africa before the last cases occurred in 1919.

The Spanish flu pandemic was remarkable in a number of ways.

Mortality rates in influenza and most other infectious diseases typically form a U-shaped curve when plotted against age on a graph, indicating that elderly and very young patients are at greatest risk of dying from the disease. The Spanish flu, however, yielded a W-shaped curve, as young adults were more likely to die than those just younger or older. At the time the deaths were often noted to be due to “unusually severe pneumonia” – but it is now believed that sufferers mostly died as a result of cytokine storm. Healthy young adults were most at risk because of the efficiency of their immune systems.

Another characteristic of the pandemic was the rate at which it spread and the high percentage of people affected: The largest inhabited area unaffected by disease was an island in the estuary of the Amazon River. Mortality varied greatly from area to area: Although the average figure of around 2,5 percent in the industrialised countries was considerably higher than that usually reported for flu, the pandemic was not nearly as devastating there as in remote areas where the disease had never been recorded before: Among some Eskimo communities in Alaska a third or more of those who were infected died, and it is believed that entire villages in the Third World disappeared off the face of the earth.

The viral strain, which would later be named H1N1, showed the characteristics of a virus that had undergone species jumping. The high rate of contagion, prevalence of the disease and extraordinary virulence had the hallmark of a foreign virus that had found its way around the human immune system, and against which humans had little resistance. Variants of H1N1 are still circulating among humans – but because most people have developed a degree of resistance, as well as the fact that vaccines and antiviral drugs are now available, the virus is not in the least as threatening as in 1918.

It was long assumed that H1N1 originated in pigs, which are susceptible to a similar strain. In 2005, however, tests on the frozen body of a woman who died of the Spanish flu in Alaska proved that this disease was a form of avian influenza. It is, however, possible that the virus was first assembled in a pig that was infected by both avian and human flu viruses. On the other hand, the original mixing vessel may have been a human, and it could be that pigs were infected later by contact with humans.

[modest]

Originally Posted by mynah

The viral strain, which would later be named H1N1, showed the characteristics of a virus that had undergone species jumping. The high rate of contagion, prevalence of the disease and extraordinary virulence had the hallmark of a foreign virus that had found its way around the human immune system, and against which humans had little resistance. Variants of H1N1 are still circulating among humans – but because most people have developed a degree of resistance, as well as the fact that vaccines and antiviral drugs are now available, the virus is not in the least as threatening as in 1918.

Spanish flu is a strain of H1N1, just like half of all flu viruses today are a strain of H1N1. N1N1 existed before spanish flu, and obviously still exists now. Nevertheless, Spanish flu is extinct in the wild - completely extinct.

People don't still get Spanish flu, yet have an immunity to it as you say 2 posts ago. A person infected today would presumably be just as affected as someone infected in 1918.

~modest

[mynah]

Perhaps - but it would be very hard to either prove or disprove that H1N1 circulated in humans before 1918. The virulence of the disease suggests a species jump - although the partial immunity of some populations may be due to exposure to not greatly dissimilar strains. As it were, H1N1 never caused a severe epidemic again: Even the Asian flu epidemic of 1957 that involved a double antigenic shift to H2N2 was not as serious, nor was the Hong Kong flu epidemic of 1968 (single antigenic shift to H3N2). The avian flu outbreak that started in Asia in 1997 was caused by a species jump. H5N1 had been identified in birds decades before, but had never been seen in humans until then. Unlike the Spanish flu it was marked by low infectivity but very high mortality in those affected - a typical situation in the early stages of a species jump.

[Cedars]

Originally Posted by mynah

The avian flu outbreak that started in Asia in 1997 was caused by a species jump. H5N1 had been identified in birds decades before, but had never been seen in humans until then. Unlike the Spanish flu it was marked by low infectivity but very high mortality in those affected - a typical situation in the early stages of a species jump.

I dont know if I would call whats happened with H5N1 an authentic specie jump yet. Seems its poor handling of the poultry that is infecting people with a few cases of possible human to human transmission.

[modest]

Originally Posted by mynah

Perhaps - but it would be very hard to either prove or disprove that H1N1 circulated in humans before 1918. The virulence of the disease suggests a species jump

Yeah, you've got a good point there. In fact, I went to school a stone's throw from Ft. Riley (Kansas State).

I personally think the severity of an epidemic or pandemic has as much to do with the mutations of the virus as the immunity of those infected. Why, for example, was the 1951 epidemic so much worse in Liverpool as the rest of England? Could the people of Liverpool lacked an immunity that the rest of the country had? I don't think so. The only reasonable explanation would seem to be with the virus - some subtle mutation as it spread.

Death rates in Liverpool were higher in '51 than from Spanish flu in '18 & '19. Had the '51 H1N1 strain kept it's virulent nature as it spread out of the heart of England it's easy to imagine a pandemic as bad as during World War 1.

Then again, there are examples like Russian flu of '77 which was clearly lessened in virulence with anyone who was exposed to a similar strain in '47 - '57. So, immunity is, of course, a factor.

I just hope we're not ruling out the possibility that H1N1 could make itself very deadly again. An unrelenting pandemic for which there is no natural immunity does not seem impossible. If Spanish flu got out of the lab...

I guess, at least we have vaccinations now

~modest

[mynah]

Originally Posted by Cedars

I dont know if I would call whats happened with H5N1 an authentic specie jump yet. Seems its poor handling of the poultry that is infecting people with a few cases of possible human to human transmission.

It depends on how you define "species jump". Some would say a species jump had taken place once a disease had been transmitted from the host species to another species for the first time: In this article, for instance rabies due to Australian bat lyssavirus is listed among diseases that had undergone a species jump, even though only two human cases (both transmitted through bat bites) have been recorded. One should also keep in mind the stringent measures some countries took to limit the spread of avian flu: Vietnam, for instance, culled its entire farmed bird population and started anew with imported hatchlings.

The WHO defined three stages and six phases in the development of a flu pandemic: By their definition, avian flu had reached stage two (pandemic warning period) and was entering phase 4 ("Minor outbreaks of limited distribution from human to human occur, although such outbreaks are highly localised. This appears to indicate a viral strain that is not yet well equipped to infect humans.") when the tide was turned.

[mynah]

I personally think the severity of an epidemic or pandemic has as much to do with the mutations of the virus as the immunity of those infected. Why, for example, was the 1951 epidemic so much worse in Liverpool as the rest of England? Could the people of Liverpool lacked an immunity that the rest of the country had? I don't think so. The only reasonable explanation would seem to be with the virus - some subtle mutation as it spread.

Strange, indeed, especially as it did not carry the signature of a pandemic due to an entirely new strain (high mortality in young adults due to immune system overreaction and cytokine storm). The fact that the epidemic started in Liverpool may have something to do with it, but the rate at which the epidemic spread makes it unlikely that the virus lost potency through mutation in so many directions in such a short time. Perhaps there was some elusive co-factor at work, possibly in the same way that HIV infection now turns otherwise innocuous infections deadly?