What Goes Around Comes Around, Part 2
The Nipah virus is a good example of the interconnection between environmental encroachment and emerging infectious disease. The story of Nipah goes something like this. In 1997, some 10 million acres of tropical rainforest in Sumatra and Borneo found themselves on fire, in the service of the cattle and palm oil industry. This, quite predictably, resulted in a mass migration of fruit bats. Some of these ended up in mango trees that were in close proximity to some rather massive pig farms, near the Malaysian village of Sungai Nipah. The bats dribbled mango fruit and sprayed urine into the pig pens. Sadly, the saliva and urine both contained a virus. The pigs became sick first, developing a ghastly cough,
followed by internal hemorrhaging. Neurological symptoms— spasms and paralysis— were quickly followed by death. Humans became sick soon after and displayed similar symptoms: high fever, headaches, convulsions, and then death. Case fatality rate was estimated to be somewhere in the range of 40-75%. The outbreak lasted 7 months, spreading all over the Malaysian peninsula, and reaching as far as northern Australia.
This story combines several important elements. First, there is the environmental disturbance— the destruction of the rainforest— engineered by us. In engineering this, we managed to introduce the animal reservoir of a virus to another species, pigs, which also provided a suitable environment for the virus. The pigs then acted as intermediate hosts, passing the virus on to us. Merely carrying a virus is not enough for the pigs to qualify as intermediate hosts. For this, we needed to do something else: eat them. In short, we engineered the introduction that made the pigs into hosts— in the sense of carriers— of the virus. And we engineered the circumstances in which these hosts could become specifically intermediate hosts— links in the transmission chain that led from fruit bats to us.
The Nipah virus is a paramyxovirus, a cousin of measles – but far more deadly. (Although one should remember the incredibly deadly character of measles when it first emerged in the 8/9th centuries as a mutant form of the rinderpest virus that we acquired from goats and sheep). After Malaysia, Nipah moved on to Bangladesh and, there, outbreaks have been recorded in most years since 2001. There are several things that are concerning about this. There are the cases of human-to-human transmission, which are becoming quite frequent. Indeed, in Bangladesh, human-to-human transmission now accounts for one-third of cases. In one instance, the virus was even acquired from a corpse, which suggests a quite high level of infectivity. Then, there is the case fatality rate of up 75%, which speaks for itself. Just as worrying is the evident level of mutation the virus is capable of undergoing. The Malaysian strain of Nipah presents quite differently from the Bangladesh strain. In Malaysia, only 14% presented with a cough. In Bangladesh, 62% of patients presented with cough. In Malaysia, only 6% of patients had abnormal chest radiographs. In Malaysia, 69% of patients had abnormal chest radiographs, demonstrating substantial pulmonary damage. These differences in the way the strains present may be responsible for the difference in human-to-human transmission rates in Malaysia and Bangladesh. At present, even in Bangladesh, Nipah is still a stage 3 pathogen, with limited human-to--human transmission, and an estimated R0 of 0.48 However, it is clear that Nipah is a highly mutable virus. It is conceivable that one of the numerous strains of Nipah circulating in Bangladesh has already achieved a R0 of greater than 1 (S. Luby, ‘The pandemic potential of Nipah virus’, Antiviral Research 100, 1, 2013, 38-43).