Many of the deadliest and most feared diseases - from AIDS and influenza to smallpox and zoster (shingles) - as well as some of the most common have been viral. What causes viral emergence? Most new or emerging viruses are the result of changes in traffic patterns that give viruses new highways. It seems human actions often precipitate viral emergence. Deforestation, war, and agricultural and food production practices are some of the general factors most often blamed nowadays.
So-called ‘new’ viruses most probably derive from existing viruses; in general, viruses of today have ancestors and relatives. However, as viruses show great variety, and many of the viruses of gravest concern mutate rapidly and unpredictably, it is not always possible to trace their ancestry with any great confidence. A ‘new virus may be genuinely new or a major evolutionary variant arising from genetic processes such as mutation or recombination. Their rate of mutation is so high that controlling them, or predicting their behaviour, is well-nigh impossible. High mutation rate means that no RNA (ribonucleic acid) virus population is a single entity, but rather a ‘quasi species’. Introduction of a virus from one species to another and dissemination from a smaller to a larger population are also among the basic mechanisms by which viruses emerge.
The emerging viruses are surfacing from ecologically damaged parts of the earth. When viruses come out of an ecosystem, they tend to spread in waves through the human population. Among the most notorious are: Lassa, Rift Valley, Oropuche, Rocio, Q. Guanarito, VEE, Monkeypox, Dengue, Chikunganya, the Hanta viruses, Machupo, Junin, the rabies like strains Mokola and Duvenhage, LeDantec.
AIDS: Acquired Immunodeficiency Syndrome
Human immunodeficiency virus (HIV) is a more difficult case compared to, for example, influenza, itself a great killer - the influenza pandemic of 1918-9 in China caused some 20 million fatalities. We do not know the origin of HIV, but a probable primate origin is often suggested, and appears highly plausible. HIV-2 (one of the major strains of HIV) may be a mutant that jumped into humans from an African monkey known as the sooty mangabey, perhaps when monkey hunters or trappers touched body tissue. HIV-1 (the other strain) may have jumped into man from chimpanzees - perhaps when hunters butchered chimpanzees.
HIV attacks the type of lymphocytes known as helper T-cells that stimulate the activity of B lymphocytes that produce antibodies. After an HIV infection sets in, helper T-cells begin to decline in number and the person becomes more susceptible to infections.
The AIDS virus mutates rapidly and constantly. It is a hyper-mutant, a shape-shifter, spontaneously altering its character as it moves through individuals. It mutates even in the course of one injection.
The drug AZT has been found to be helpful in prolonging the lives of AIDS patients. Also, it has been shown that administration of Interleukin-2 and AT-538 can prevent viral replication in cells in the early stages of the infection.
Marburg and Ebola viruses were recently elevated to family status as the Filoviridae. The name, ‘thread viruses’, is based on their morphology. These viruses made their appearance in the 1960s and 1970s in the form of frightening nosocomial and occupational outbreaks, initially among polio vaccine production workers in contact with Ugandan green monkeys and their kidney tissues, then in independent hospital epidemics of devastating proportions during 1976 in the Sudan and Zaire. Mortality from these infections can range to nearly 90%, that is 90% of infections, not just of clinical illnesses.
The incubation period of a filovirus in a human being, is from 3 to 18 days during which time the number of virus particles climbs steadily in the bloodstream. Then the suffering begins.
Marburg was the first filovirus to be discovered. It erupted in a factory producing vaccines using kidney cells from African green monkeys. Thirty-one people caught the virus. Seven infected persons died in pools of blood in just two weeks. This fatality rate of one in four makes Marburg an extremely lethal agent, yet it is the mildest of the filoviruses. No vaccine is available.
The next indication of the existence of filoviruses came from an extraordinary pair of epidemics in 1976. In the Sudan the initial focus was a cotton factory in Nzara, and several of the earliest recognized cases worked there in a single room. Travel to nearby Maridi introduced the virus into the medical care system, with subsequent latrogenic dissemination that devastated the Maridi hospital. In this outbreak, 150 out of 280 infected people died.
In 1979, it recurred in Nzara, Sudan. The index case had worked in the same room in the cotton factory identified in the earlier epidemic. Twenty-two out of 34 infected patients died. When it first erupted, the medical stuff had used dirty needles which facilitated the spread of infection. Again, no vaccine is available.
Two months after the start of the Sudan emergence in 1976, an even more lethal filovirus emerged. The Ebola Zaire, a new strain was nearly twice as lethal as Ebola Sudan. The fatality rate of Ebola Zaire is 9 out of 10. Staff at a hospital which had been at the epicenter of the epidemic were almost all wiped out: 13 out of 17 of the doctors and nurses died. The disease erupted more or less simultaneously in fifty-five villages near the headwaters of Ebola River.
This epidemic was clearly dependent on the use of unsterilized needles and syringes for its major dissemination between villages, although multiple later generations of cases occurred among family contacts without any defined exposure route.
Ebola is distantly related to measles, mumps and rabies. It is also related to certain pneumonia viruses, to the parainfluenza virus which causes colds in children, and to the respiratory syncytial virus, which can cause fatal pneumonia in a person who has AIDS.
Ebola kills a great deal of tissue while the host is alive. It triggers a creeping, spotty necrosis that spreads through all the internal organs. The liver bulges up and turns yellow, begins to liquefy and then it cracks apart. The kidneys become jammed with blood clots and dead cells, and cease functioning. As the kidneys fail, the blood becomes toxic with urine. No vaccine is available.
Ebola does in ten days what it takes AIDS ten years to accomplish. In principle Ebola-like viruses could spread out all over the world in just one month and a half. Richard Preston, the author of Hot Zone argues that AIDS may be the first step in a natural process of clearance. What is being cleared is human beings: the earth’s immune system, so to speak, has recognized the presence of our species and is starting to kick in and signal its potential to rid itself of an infection by human beings. There can be no excuse for failing to reflect radically on the way we manage our economies, our societies and our lifestyles. We have no absolute right of tenure of this planet, nor does our collective intelligence or our science and technology afford a reliable immunity from vulnerability, whether as individuals or as a species. We do need to be fearful perhaps, though not to panic - so that we redress the balance in favour of humility in our attitudes and activities, so that we become more caring and careful in using our God-given faculties to prevent the worst before it happens.
- MADER, S. S. (1992) Human Biology, WCB Group.
- PRESTON, R. (1993) Hot Zone, Random House, New York.
- MORSE, S. 5. (1993) Emerging Viruses, Oxford University Press, New York.
- DOWDLE, W. (1993) ‘The Origins of Plagues’, Science, 261 (September 17).
- GRIFFIN, B. E. (1994) ‘Live and Let Live’, Nature, 368 (March 3).