Emerging disease and zoonoses #9--the emergence of Nipah virus

I've touched on the broad issue of "cultural practices" that can lead to emerging diseases in a few posts already (such as farming practices and cockfighting and avian influenza, or petting zoos and bacterial diseases), but today I'm going to go into a bit more detail with the specific example of the emergence of Nipah virus in Malaysia.

Nipah is a paramyxovirus, in the same family as measles and mumps. The virus was first recognized in 1999, during an encephalitis outbreak in Malaysia that had begun the previous September. The vast majority of the cases had occupational exposure to pigs, leading to the recognition that humans had acquired the virus from the infected animals. In addition to encephalitis, Nipah causes an influenza-like illness after a 4-18 day incubation period; the fatality rate has been approximately 50% during outbreaks (resulting in 105 deaths). The virus does not appear to be transmissible between humans (or only minimally so). Studies conducted following the outbreak found that the reservoir of the virus appears to be several species of fruit bats.

Where did this virus come from? The history presents an interesting story of a number of factors that converged to bring the virus into the human population. First, the pathogenic agent must be present in a reservoir that comes into contact with other species. Second, environmental and logistical factors--including cultural practices--must allow for or facilitate this contact. This is what happened in Malaysia in 1998-9.

The Nipah outbreak originated in the Ipoh area, in the western part of the country. This is an area of intensive pig farming, where thousands of pigs are housed in close areas. (The "index farm" where the Nipah outbreak originated had approximately 30,000 pigs). Fruit trees are also frequently present on these pig farms (several of which had been built near rainforests--home to the fruit bats), and the pig barns are open to the environment. The fruit trees attracted fruit bats--and their passenger Nipah viruses--and the open barns resulted in fruit bats being in close contact with large populations of swine (and other animals--horses, cats, and dogs that were housed nearby or roamed the area were also found to be infected). The swine then acted as amplifiers, spreading the virus amongst their own populations and subsequently to humans.

This alone wasn't enough for a large, ongoing outbreak, though. To add to the mess, breeding stock was traded in Ipoh--so pigs were moving from that area out to other parts of the country, bringing their Nipah infections with them. There were also no effective measures in place to prevent the spread of such a disease, should it originate--representing a failure on the part of farmers to expect and diagnose an emerging disease. Additionally, when the disease was finally recognized, sales of swine actually increased initially, as breeders tried to get rid of their infected stock. This was partially due to cultural disrespect for the biosecurity regulations put in place by government (and mostly ignored by the area farmers), and partly, of course, human nature to put one's own interest above that of others.

On top of the lax measures, even farms that had been certified as "biosecure" (generally meaning they practiced good animal and farm hygiene) sometimes acted illegally as a clearinghouse for infected animals. Swine were supposed to be branded in order to track where they were coming from, but during the outbreak, these practices were ignored, and many infected swine were branded with the label of the "biosecure" farm and sold, highlighting the need for external auditing in addition to self-regulation.

Finally, many of the farming practices themselves led to an increased risk of disease transmission from swine to human. The factors that were found to be most risky included feeding pigs; close handling of pigs (especially those who were sick or dead); assisting in breeding pigs; assisting in the birth of piglets; and medicating pigs. Much of this was done by people who weren't specifically trained in these areas, and certainly weren't trained to carry them out in such a manner as to reduce the possibility of disease transmission.

Again, this is an example from Malaysia, which may seem far removed from those of us in the U.S. or Europe, but we deal with many of the same practices here as well. Some potential for disease transmission may be reduced simply by wearing gloves, especially when performing procedures that involve a lot of body fluids (such as birthing). And when a pathogen does enter the population--as it surely will again--people need to be ready to take measures to minimize the spread to other animals. Governments need to have effective biosecurity measures in place, and need to be able to enforce them. Farmers, too, need to be compensated for the loss of their animals, in order to minimize the rush-to-sell mentality that may allow them to save their own skins, but will put many others at risk. And lest anyone read this as an indictment of farmers, it's not, but there are certain realities that must be dealt with in these types of cases. The Nipah outbreak had a huge economic impact. Approximately a million pigs were culled, hundreds of farms were closed, and thousands of jobs were lost due to the infection, at an estimated cost of $450 million USD.

Still, disease wise, Nipah has been fairly small potatoes. H5N1 has surpassed it now in number of deaths, and another significant outbreak of the virus has not occurred. Perhaps it will fade into obscurity, a footnote in history. Even if it does, the lessons from the outbreak should not be forgotten--and will be just as applicable tomorrow as they are today.

(Image from http://www.wildlifetrust.org/news/2005/images/pigs.jpg)


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Very interesting.

I have never heard of that disease before. It really opens my eyes to the problems within farms such as people hiding their chickens to avoid them being culled. Not to mention in Africa I heard today a man was arrested for eating a dead H5N1 chicken. Most of them do not agree with the preventive measures being taken because they have not seen anyone sick with the disease and they can't afford to lose the chickens. Its really scary.

Thank for this--I also had not heard of this disease even though it has killed nearly as many as H5N1 with a 50% fatality rate. Also thank you very much for this whole series!

An important issue regarding farming practices is the transformation of low-pathogenic to high-pathogenic strains of viral disease. The conditions for this to occur appear to exist in large confinements, at least as far as avian flu is concerned. This is due, not only to sheer numbers of very closely confined animals, but because of nutritional and immunological stress common in animal 'factories'.

An important factor in the early spread of H5N1 was Infectious Bursal Viral Disease (IBVD), though WHO/FAO/et al have given no acknowledgement of this. This is an old, mild disease of chickens that became more virulent with the advent of modern chicken production (ie there is an infamous 'Delaware strain'). It is usually not fatal but causes lowered production and, especially, lowers immune response, making vaccines less effective and infection more likely. The IBVD virus was directly implicated by researchers in the early Hong Kong outbreaks of H5N1. Did anyone discuss these issues in the conference?

Ron, I've not heard of IBVD previously--interesting. How was it a factor in the early spread--just by compromising the immune system? I didn't see anything in PubMed on its connection to H5N1 either.

Tara, there is one cultural factor (in my opinion the major cultural factor) that you missed out.

Malaysia is a Muslim country, and the eating of pork is taboo for the Muslim Malay majority. The pig farms in Ipoh produce pork for the ethnic Chinese minority, and are something of a blind spot for the regulators, since they are effectively a niche business. I don't think the Malaysian government provided compensation to pig farms due to the pork taboo, which was the primary reason for the farmers rushing infected stock to market.

On a personal note, my grandmother lives in a little village just outside Ipoh, and sells dry goods in the local market. The pork butchers at the market effectively went out of business, because no one (even the ethnic Chinese) ate pork for about a year after that. The same thing is happening with chicken now.

Thanks! I hadn't even thought about that part of it. Just once again illustrates how dealing with such diseases takes a broad knowledge of not only the epidemiology and pathogenesis of the disease, but also the cultural practices of the people and ecology of the organism.

I'm from Singapore, just south of Malaysia, and I remember how we were affected by this disease, especially since before the outbreak almost all our pork was imported daily from Malaysia.

Pork imports were stopped, laws were changed necessitating that only frozen and chilled pork be sold at wet markets, and if I recall correctly, all our pork is now imported from Indonesia. H5N1 affected us as well; it is horrible when you go months without eating eggs, as the only available eggs are horribly expensive and imported from Australia and Brazil.

Which goes to show that disease in one country can have economic repercussions elsewhere, especially for a country that depends so much on imports for dietary staples.