How could HIV have crossed species?

It has been known for a long time that certain viruses can pass between species. Indeed, the very fact that chimpanzees obtained SIV from two other species of primate shows just how easily this crossover can occur. As animals ourselves, we are just as susceptible. When a viral transfer between animals and humans takes place, it is known as zoonosis.

Below are some of the most common theories about how this 'zoonosis' took place, and how SIV became HIV in humans:

The 'hunter' theory

The most commonly accepted theory is that of the 'hunter'. In this scenario, SIVcpz was transferred to humans as a result of chimps being killed and eaten or their blood getting into cuts or wounds on the hunter. Normally the hunter's body would have fought off SIV, but on a few occasions it adapted itself within its new human host and became HIV-1. The fact that there were several different early strains of HIV, each with a slightly different genetic make-up (the most common of which was HIV-1 group M), would support this theory: every time it passed from a chimpanzee to a man, it would have developed in a slightly different way within his body, and thus produced a slightly different strain.

“Retroviral transfer from primates to hunters is still occurring even today”

An article published in The Lancet in 2004 4, also shows how retroviral transfer from primates to hunters is still occurring even today. In a sample of 1099 individuals in Cameroon , they discovered ten (1%) were infected with SFV (Simian Foamy Virus), an illness which, like SIV, was previously thought only to infect primates. All these infections were believed to have been acquired through the butchering and consumption of monkey and ape meat. Discoveries such as this have led to calls for an outright ban on bushmeat hunting to prevent simian viruses being passed to humans.

The oral polio vaccine (OPV) theory

Some other rather controversial theories have contended that HIV was transferred iatrogenically (i.e. via medical interventions). One particularly well-publicised idea is that polio vaccines played a role in the transfer.

In his book, The River, the journalist Edward Hooper suggests that HIV can be traced to the testing of an oral polio vaccine called Chat, given to about a million people in the Belgian Congo, Ruanda and Urundi in the late 1950s. To be reproduced, live polio vaccine needs to be cultivated in living tissue, and Hooper's belief is that Chat was grown in kidney cells taken from local chimps infected with SIVcmz. This, he claims, would have resulted in the contamination of the vaccine with chimp SIV, and a large number of people subsequently becoming infected with HIV-1. 5

Many people have contested Hooper's theories and insist that local chimps were not infected with a strain of SIVcmz that is closely linked to HIV. Furthermore, the oral administration of the vaccine would seem insufficient to cause infection in most people (SIV/HIV needs to get directly into the bloodstream to cause infection - the lining of the mouth and throat generally act as good barriers to the virus). 6

In February 2000 the Wistar Institute in Philadelphia (one of the original manufacturers of the Chat vaccine) announced that it had discovered in its stores a phial of polio vaccine that had been used as part of the program. The vaccine was subsequently analysed and in April 2001 it was announced that no trace had been found of either HIV or chimpanzee SIV. 7 A second analysis confirmed that only macaque monkey kidney cells, which cannot be infected with SIV or HIV, were used to make Chat. 8 While this is just one phial of many, it means that the OPV theory remains unproven.

The fact that the OPV theory accounts for just one (group M) of several different groups of HIV also suggests that transferral must have happened in other ways too, as does the fact that HIV seems to have existed in humans before the vaccine trials were ever carried out. More about when HIV came into being can be found below.

The contaminated needle theory

This is an extension of the original 'hunter' theory. In the 1950s, the use of disposable plastic syringes became commonplace around the world as a cheap, sterile way to administer medicines. However, to African healthcare professionals working on inoculation and other medical programmes, the huge quantities of syringes needed would have been very costly. It is therefore likely that one single syringe would have been used to inject multiple patients without any sterilisation in between. This would rapidly have transferred any viral particles (within a hunter's blood for example) from one person to another, creating huge potential for the virus to mutate and replicate in each new individual it entered, even if the SIV within the original person infected had not yet converted to HIV.

The colonialism theory

The colonialism or 'Heart of Darkness' theory, is one of the more recent theories to have entered into the debate. It is again based on the basic 'hunter' premise, but more thoroughly explains how this original infection could have led to an epidemic. It was first proposed in 2000 by Jim Moore, an American specialist in primate behaviour, who published his findings in the journal AIDS Research and Human Retroviruses. 9

During the late 19th and early 20th century, much of Africa was ruled by colonial forces. In areas such as French Equatorial Africa and the Belgian Congo, colonial rule was particularly harsh and many Africans were forced into labour camps where sanitation was poor, food was scarce and physical demands were extreme. These factors alone would have been sufficient to create poor health in anyone, so SIV could easily have infiltrated the labour force and taken advantage of their weakened immune systems to become HIV. A stray and perhaps sick chimpanzee with SIV would have made a welcome extra source of food for the workers.

“SIV could easily have infiltrated the labour force and taken advantage of their weakened immune systems”

Moore also believes that many of the labourers would have been inoculated with unsterile needles against diseases such as smallpox (to keep them alive and working), and that many of the camps actively employed prostitutes to keep the workers happy, creating numerous possibilities for onward transmission. A large number of labourers would have died before they even developed the first symptoms of AIDS, and those that did get sick would not have stood out as any different in an already disease-ridden population. Even if they had been identified, all evidence (including medical records) that the camps existed was destroyed to cover up the fact that a staggering 50% of the local population were wiped out there.

One final factor Moore uses to support his theory, is the fact that the labour camps were set up around the time that HIV was first believed to have passed into humans - the early part of the 20th century.

The conspiracy theory

Some believe that HIV is a 'conspiracy' or that it is 'man-made'. A recent survey carried out in the US for example, identified a significant number of African Americans who believe HIV was manufactured as part of a biological warfare programme, designed to wipe out large numbers of black and homosexual people. 10 Many say this was done under the auspices of the US federal 'Special Cancer Virus Program' (SCVP), possibly with the help of the CIA. Linked in to this theory is the belief that the virus was spread (either deliberately or inadvertently) to thousands of people all over the world through the smallpox inoculation programme, or to gay men through Hepatitis B vaccine trials. While none of these theories can be definitively disproved, the evidence given to back them up is usually based upon supposition and speculation, and ignores the clear link between SIV and HIV or the fact that the virus has been identified in people as far back as 1959.

When?

During the last few years it has become possible not only to determine whether HIV is present in a blood or plasma sample, but also to determine the particular subtype of the virus. Studying the subtype of virus of some of the earliest known instances of HIV infection can help to provide clues about the time it first appeared in humans and its subsequent evolution.

Four of the earliest known instances of HIV infection are as follows:

A plasma sample taken in 1959 from an adult male living in what is now the Democratic Republic of the Congo. 11

A lymph node sample taken in 1960 from an adult female, also from the Democratic Republic of the Congo. 12

HIV found in tissue samples from an American teenager who died in St. Louis in 1969. 13

HIV found in tissue samples from a Norwegian sailor who died around 1976. 14

A 1998 analysis of the plasma sample from 1959 suggested that HIV-1 was introduced into humans around the 1940s or the early 1950s. 15

In January 2000, the results of a new study 16 suggested that the first case of HIV-1 infection occurred around 1931 in West Africa. This estimate (which had a 15 year margin of error) was based on a complex computer model of HIV's evolution.

However, a study in 2008 17 dated the origin of HIV to between 1884 and 1924, much earlier than previous estimates. The researchers compared the viral sequence from 1959 (the oldest known HIV-1 specimen) to the newly discovered sequence from 1960. They found a significant genetic difference between them, demonstrating diversification of HIV-1 occurred long before the AIDS pandemic was recognised.

The authors suggest a long history of the virus in Africa and call Kinshasa the “epicentre of the HIV/AIDS pandemic” in Central Africa. They propose the early spread of HIV was concurrent with the development of colonial cities, in which crowding of people increased opportunities for HIV transmission. If accurate, these findings imply that HIV existed before many scenarios (such as the OPV and conspiracy theories) suggest.

What about HIV-2? When did that get passed to humans?

Until recently, the origins of the HIV-2 virus had remained relatively unexplored. HIV-2 is thought to come from the SIV in Sooty Mangabeys rather than chimpanzees, but the crossover to humans is believed to have happened in a similar way (i.e. through the butchering and consumption of monkey meat). It is far rarer, significantly less infectious and progresses more slowly to AIDS than HIV-1. As a result, it infects far fewer people, and is mainly confined to a few countries in West Africa.

In May 2003, a group of Belgian researchers published a report 18 in Proceedings of the National Academy of Science. By analysing samples of the two different subtypes of HIV-2 (A and B) taken from infected individuals and SIV samples taken from sooty mangabeys, Dr Vandamme concluded that subtype A had passed into humans around 1940 and subtype B in 1945 (plus or minus 16 years or so). Her team of researchers also discovered that the virus had originated in Guinea-Bissau and that its spread was most likely precipitated by the independence war that took place in the country between 1963 and 1974 (Guinea-Bissau is a former Portuguese colony). Her theory was backed up by the fact that the first European cases of HIV-2 were discovered among Portuguese veterans of the war, many of whom had received blood transfusions or unsterile injections following injury, or had possibly had relationships with local women.

Where?

The question of exactly where the transfer of HIV to humans took place, and where the 'epidemic' officially first developed has always been controversial. Some have suggested that it is dangerous to even try to find out, as AIDS has frequently been blamed on an innocent person or group of individuals in the past. However, scientists remain keen to find the true origin of HIV, as most agree it is important to understand the virus and its epidemiology in order to fight it.

So did it definitely come from Africa?

Given the evidence we have already looked at, it seems highly likely that Africa was indeed the continent where the transfer of HIV to humans first occurred (monkeys from Asia and South America have never been found to have SIVs that could cause HIV in humans). In May 2006, the same group of researchers who first identified the Pan troglodytes troglodytes strain of SIVcpz, announced that they had narrowed down the location of this particular strain to wild chimpanzees found in the forests of Southern Cameroon 19. By analysing 599 samples of chimp droppings (P. T. troglodytes are a highly endangered and thus protected species that cannot be killed or captured for testing), the researchers were able to obtain 34 specimens that reacted to a standard HIV DNA test, 12 of which gave results that were virtually indistinguishable from the reactions created by human HIV. The researchers therefore concluded that the chimpanzees found in this area were highly likely the origin of both the pandemic Group M of HIV-1 and of the far rarer Group N. The exact origins of Group O however remain unknown.

HIV Group N principally affects people living in South-central Cameroon, so it is not difficult to see how this outbreak started. Group M, the group that has caused the worldwide pandemic, was however first identified in Kinshasa, in the Democratic Republic of Congo. It is not entirely clear how it transferred from Cameroon to Kinshasa, but the most likely explanation is that an infected individual travelled south down the Sangha river that runs through Southern Cameroon to the River Congo and then on to Kinshasa, where the Group M epidemic probably began.

Just as we do not know exactly who spread the virus from Cameroon to Kinshasa, how the virus spread from Africa to America is also not entirely clear. However, recent evidence suggests that the virus may have arrived via the Caribbean island of Haiti.

Why is Haiti significant?

The AIDS epidemic in Haiti first came to light in the early 1980s, at around the same time that cases in the USA were being uncovered. Following the discovery of a number of Haitians with Kaposi's Sarcoma and other AIDS-related conditions, medical journals and books began to claim that AIDS had come from Haiti, and that Haitians were responsible for the AIDS epidemic in the United States.

These claims, which were often founded on dubious evidence, fuelled pre-existing racism in the US and many Haitians suffered severe discrimination and stigma as a result. A large number of Haitian immigrants living in the US lost their jobs and were evicted from their homes as Haitians were added to homosexuals, haemophiliacs and heroin users to make the 'Four-H Club' of groups at high risk of AIDS. 20

The emotionally-charged culture of blame and prejudice that surrounded HIV and AIDS in the early years meant that it soon became politically difficult to present epidemiological findings in a neutral and objective way. For many years the link between Haiti and the US epidemic was therefore dropped as a subject.

“Tracing HIV’s origins remains a politically sensitive exercise ”

In March 2007 however, it returned to the public eye at the Fourteenth Conference on Retroviruses and Opportunistic Infections (CROI) in Los Angeles. A group of international scientists presented data based on complex genetic analysis of 122 early samples of HIV-1, group M, subtype B (the most common strain found in the USA and in Haiti) showing that the strain had probably been brought to Haiti from Africa by a single person in around 1966; a time when many Haitians would have been returning from working in the Congo. 21

Genetic analysis then showed that subtype B spread slowly from person to person on the island, before being transferred to the US, again probably by a single individual, at some point between 1969 and 1972. A paper published in October 2007 by Worobey and colleagues gave a 99.7% certainty that HIV subtype B originated in Haiti before passing to the US. 22

It is possible that HIV had entered the US several times before subtype B took a firm hold (which would explain the infection of the St. Louis teenager in the early to mid-1960s), but it was the late 1960s / early 1970s transfer that is believed to be responsible for the widespread epidemic seen in the US today. Once the virus had established itself in the gay community, it would have spread fairly rapidly (anal intercourse carries a very high transmission risk), with transmission occurring within and between the US and Haiti, and internationally, until the original route taken by the virus was largely obscured.

Dr Michael Worobey, lead researcher in the study, claimed that his data was not intended to place any blame on Haiti, or on Central Africans, and stressed that none of the people who first transmitted HIV would have been aware they were infected. His work still received strong protests from one Haitian delegate at the CROI conference however, demonstrating the extent to which tracing HIV’s origins remains a politically sensitive exercise.

Date: 2016-01-03; view: 913