NCSU Extension Swine Husbandry
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Jun, 2004 . Volume 27, Number 05
Dr. Robert Desrosiers of St. Hyacinthe, Quebec, delivered the prestigious Howard Dunne Memorial Lecture at the annual meeting of the American Association of Swine Veterinarians in Des Moines, Iowa, in March. His full lecture on "Epidemiology, Diagnosis, and Control of Swine Diseases" is available in the proceedings. Below are his comments on the aerosol transmission of porcine reproductive and respiratory syndrome virus (PRRSv).
—W. E. Morgan Morrow
In 1991, a meeting on PRRS was organized in Brussels, Belgium, which drew scientists and researchers from around the world. One of the conclusions of this meeting was the following: "The main means of spread of PRRS are the farm-to-farm movement of pigs and airborne dispersion. The first outbreaks in a population frequently have been due to the movement of infected pigs, but airborne transmission appears to be responsible for much of the subsequent neighborhood spread."1
Robertson2 reported the means by which the virus was thought to have spread in the first 100 reported cases in Great Britain. Aerosol was the most important with 63 percent of the cases. He also reported the same tentative conclusions for 81 cases that occurred in Belgium in 1991 and for which airborne spread was considered to have been involved in 69 percent of the cases (Belgians talk more about "neighborhood infection" than "airborne spread").
Vannier3 described the progression of the virus in the French Cotes d'Armors from October 1991 to May 1992. He concluded that only aerosol transmission over a short distance could explain the diffusion observed, which occurred during the cold season. In The Netherlands, Komijn et al.4 stated that one of the reasons why the disease initially spread so fast in that country was that the direction of the wind and the weather conditions during the winter months favored the spread of airborne virus. The number of outbreaks thought to have been caused by PRRS went from 10 between December 17, 1990, and January 13, 1991, to 1,019 between January 14 and March 10, 1991. The presence of the virus in Denmark was identified for the first time in March 1992 and was suspected of having been introduced from Germany via the airborne route.5
An epidemiological study was conducted in Denmark in 1996-97.6 Seventy-three case herds and 146 control herds were included. One of the conclusions was that biosecurity measures did not prevent infection of the herds, and the authors suggested that the virus spread from neighboring herds by aerosol. They used a formula to quantify the risk of neighborhood exposure. In their model, a herd located 300 meters from an infected farm was 45 times more likely to become infected than a same-size farm that had no contaminated neighbor within 3 kilometers (km).
In another epidemiological study conducted in Denmark, this time for the period 1994-98, a total of 344 genetic herds were evaluated.7 The average annual incidence of infection was found to be about 8 percent. The analysis indicated that the risk of infection increased with the pig density of PRRS-positive neighbors, but decreased with distance from them. The authors concluded that there was a predominant feature of locally spread PRRS virus in Danish pig herds, probably mainly via airborne transmission.
In North America, six different studies or field investigations reported transmission of the virus by the so-called "area spread."8-13 In each of these studies, sequencing of strains suggested that what appeared to be the same strains were found, within a relatively short period of time, in neighboring farms without a simple explanation as to how they were introduced onto these farms. In five of six studies, the authors referred to the possibility of aerosol involvement in the area spread, but it is understood that there were other possible transmission means, such as insects.8,9-13 It should be noted, however, that in the epidemiological study conducted in Quebec by Larochelle et al.,12 about 75 percent of the total PRRS strains identified (226) were from cases submitted in autumn and winter, and particularly from November through April, a time of the year during which, in most of the cases, insect involvement was unlikely. The same is true for two of the field investigations referred to above, where cases of area spread occurred in the cold season.11,13 In one of these, six Quebec farms developed PRRS within a month, in November-December. 13 All were relatively close—from 100 meters to about 2 km from each other. Three were independent producers, and three were owned by the same company. Four of the farms used negative replacement animals, as well as semen from a PRRS-negative boar stud. Strains from these four farms were sequenced, and all were found to be 99.5 percent homologous or more, using ORF 5.
This is not to say that aerosol transmission is the only possible cause of area spread of this virus. In another one of these field investigations, 12 backyard pigs were thought to be responsible for the contamination of a negative sow herd located half a mile away.14,15 One would think that even though it has been shown that as few as 20 virions are enough to infect a pig by intranasal inoculation, 12 pigs might not generate an aerosol capable of infecting other pigs at such a distance. Without disregarding aerosol in that particular case, which occurred during the summer, transmission by other means—insects, for example—should remain on the list of possibilities. It should be mentioned as well that if only 12 pigs located half a mile away were really responsible for this indirect transmission, might not trucks or vans containing hundreds of pigs that could be viremic and shedding virus also constitute sources of contamination?
Going back to aerosol per se, PRRS virus is probably the organism over which the aerosol debate has been the hottest. There seems to be ample field evidence suggesting the possibility that PRRS virus can be transmitted by aerosol. The doubt has come mainly from the fact that reproducing aerosol transmission of this pathogen over short distances has been, at least in some of the experiments, either difficult or impossible to demonstrate. These experiments were largely described at this and other meetings in the past, so I will not go over them. In Denmark, though, Kristensen et al.14 were able to readily infect negative pigs when 70, 10, or even as low as 1 percent of the air getting into their closed unit came from another unit, distanced by one meter, containing infected pigs. Finally, a recent study by Dee15 looked at the possibility for the virus to be isolated in the air of a tube at different distances from its production point. Conducted during the winter, the study showed that the virus could be isolated at all distances tested, including the longest one, 150 meters, and one of two pigs exposed to infected aerosol over this longest distance became infected.
References
1. Anonymous. Porcine reproductive and respiratory syndrome (PRRS). Proc Seminar on Porc Reprod and Resp Syndr. Brussels, Belgium. 1991:Conclusions, 2.
2. Robertson IB. Porcine reproductive and respiratory syndrome (blue-eared pig disease): Some aspects of its epidemiology. Proc Soc Vet Epi Prev Med. Edinburgh, Scotland. 1992:24-37.
3. Vannier P. Concepts generaux sur la transmission des maladies infectieuses entre les elevages porcins et la persistance des agents infectieux au sein des elevages. Proc Jour Recher Porc France. 1993:321-328.
4. Komijn RE, van der Sande WJH, van Klink EGM. Report on the epidemiology of PRRS in The Netherlands. Proc Seminar on Porc Reprod and Resp Syndr. Brussels, Belgium. 1991:8-12.
5. Mortensen S, Madsen K. The occurrence of PRRS in Denmark. AASP newsletter. 1992:4:48.
6. Mortensen S, Stryhn H, Sogaard R, Boklund A, Stark KDC, Christensen J, Willeberg P. Risk factors for infection of sow herds with porcine reproductive and respiratory syndrome (PRRS) virus. Prev Vet Med. 2002:53:83-101.
7. Zhuang Q, Barfod K, Wachmann H, Mortensen S, Willeberg P. Serological surveillance for PRRS in Danish genetic pig herds and risk factors for PRRS infection. Proc IPVS. Ames, Iowa. 2002:2:231.
8. Lager KM, Mengeling WL, Wesley RD. Evidence for local spread of porcine reproductive and respiratory syndrome virus. J Swine Health Prod. 2002:10:167-170.
9. Mohr MF, Rossow KD. Upper porcine reproductive and respiratory syndrome (PRRSv) sequences identified in four Minnesota swine herds. Proc AAVLD. 2002:42.
10. Daniels, CS. Area spread of PRRS virus from a small population of backyard pigs. Proc AASV Pre-Conference Seminar on: Preventing and Controlling PRRSV: Mission Impossible? Orlando, Florida. 2003:29-33.
11. Torrison J, Rossow K, Olson S. Molecular evidence of area spread of PRRS virus among neighboring swine farms. Proc Int Symp Swine Dis Erad. St. Paul, Minnesota. 2001:89-91.
12. Larochelle R, D'Allaire S, Magar R. Molecular epidemiology of porcine reproductive and respiratory syndrome virus (PRRSv) in Quebec. Virus Res. 2003:96:3-14.
13. Desrosiers R. Aerosol spread of the PRRS virus: Why it is too early to say no. Int Pigletter. 2002:22:21-23.
14. Kristensen CS, Botner A, Angen O, Sorensen V, Jorsal SE, Takai H, Barfod K, Nielsen JP. Airborne transmission of A. pleuropneumoniae and PRRS virus between units. Proc IPVS. Ames, Iowa. 2002:1:272.
15. Dee, S. The "Alaskan pipeline": A new model for evaluating aerosol transmission of swine pathogens. Int Pigletter. 2003:23:15-18.
—Adapted by Morgan Morrow, from Robert Desrosiers, DVM, Dipl ABVP, Howard Dunne Memorial Lecture, as presented at the American Association of Swine Veterinarians Annual Meeting, March 6-9, 2004, Des Moines, Iowa.
Life is full of challenges, and in today's world we seem to suffer from information overload. To me, it seems we have more and more information about things we don't really care for and less and less critical information about things that require an informed decision. This is particularly noticeable when issues become controversial and people's livelihoods and core beliefs are challenged.
It is important to understand the nature of those who come to the fore in these debates. Advocates often initiate the discussion with an impassioned plea to right some newly recognized wrong, outlining the case and providing information with which to sway an audience. Then the other side in the battle is marshaled, and very soon both sides bring out their scientists. Brave souls then must sort through the advocates' claims and the scientists' facts. Who has the time to do this kind of analysis? Whom do you believe?
For instance, advocates "know" that feeding antibiotics to food animals is wrong because it leads to a buildup of antibiotic-resistant human pathogens, with consequent unnecessary human suffering and death. By contrast, a scientist may believe there is a wide range of resistance patterns in both humans and food animals but may not believe there is sufficient evidence to accept that the presence of antibiotic resistance in humans is caused by the development of antibiotic resistance in animals. In other words, a bad day for scientists is when they find out that what they think they have proven turns out to be wrong, whereas a bad day for advocates is when they find out that had they done SOMETHING, they could have prevented an (inter)national disaster.
Further complicating the picture for people trying to decide is that often the scientist and the advocate are the same person, either choosing to advance their scientific findings or using their scientific background and knowledge to advance what they believe is right (as opposed to actually having the data to prove their position is the correct one). And that is the difference between a scientific approach and the tack taken by an advocate. When deciding what to do, scientists are full of self-doubt and provisos; on the other hand, advocates see a clear path and believe you are a fool for not seeing it as clearly as they.
Pity the poor decision-makers trying to struggle through this morass of information. The problem is that they must make a decision (even if it is to do nothing), and they can't afford the luxury of waiting to be 95 percent confident that it is the correct decision. That is a luxury reserved for the scientists.
One of the biggest jobs for decision-makers is sorting through myriad sources and formulating a decision best suited to the current opportunity. In that process, personal experiences are invaluable, but often of limited scope, and, therefore, carry an inherent bias.
Newspaper journalists and those in the popular press can be valuable sources of information, but, of course, their vested interest is in selling their publications; and we all know that sensationalism sells. Consider the fear of death generated among consumers when BSE (bovine spongiform encephalopathy or "mad cow" disease) was discovered in Washington state in December 2003, in contrast to the ongoing (therefore mundane) loss of life from human influenza. From October 2003 to January 6, 2004, a total of 93 influenza-associated deaths among children younger than 18 years old were reported to the Centers for Disease Control and Prevention. In contrast, no one has yet died in the United States from eating BSE-contaminated beef. Of course, they may, and that is where the scientists' data are invaluable. You can usually trust scientists, but you must listen carefully. Are they speaking as scientists, or are they advocating a position they passionately believe in? Of course, scientists, as people, have a right to be advocates, provided they make the distinction clear. If they don't make that distinction (and few do), you have to examine their claims critically to determine if they have the data to support their position. If you don't want to be caught with your pants down, you must pay more attention to advocates because they "know" what the future holds, and they just might be right.
Complicating the decision about whom to trust is a disturbing trend—the privatization of information and the dependence of scientists on research money from vested interests (e.g., drug companies). Conflicts of interest arise in the government arena, and for readers of Swine News, the role of the U.S. Department of Agriculture in protecting the public from BSE while still serving the cattle industry is a prime concern. While a potential conflict of interest is often obvious when large public institutions are involved, a conflict of interest when a privately funded scientist is involved can be difficult to discover. Disclosure by the scientist in such situations is the only ethical, moral, and responsible course.
Part of the difficulty for the decision-maker is that many don't know how to actually make a decision. In business, the science of decision making is an emerging discipline, and many still mistrust it. Daniel Kahneman, who shared a Nobel Prize in 2002 for his work in the field of economics, has validated many of the principles we intuitively understand about decisions-making. One is that when left to their own devices, decision-makers are usually overly optimistic (What do you mean the stock market won't keep going up?). This is not to say that optimism is bad, because it can become a self-fulfilling prophecy … but, it is risky behavior. Another principle is that a fresh idea can assume overriding importance and cloud the decision-maker's mind when a more experienced manager would be more skeptical and make a more determined effort to establish the idea's true value. Alternatively, too much familiarity and experience also can be detrimental when they lead decision-makers stubbornly to refuse to acknowledge the obvious.
Perhaps Kahneman's most important finding is that most people spend proportionally too much time on small decisions and not enough time on big ones. His findings show that some companies put as much effort into planning a Christmas party as into considering a strategic merger.
In summary, the decision-maker has a difficult job sorting through the chaff that litters the public forum today. The only solution is for an individual to understand his biases and the forces at play—the vested interests, the passion of the advocates and their fear of doing nothing, the hesitancy of the scientists and their fear of being wrong—and bravely come to a decision. Recognizing that one does not have to be 95 percent confident that the decision is right can be a great relief.
—W.E. Morgan Morrow
Last modified June 8, 2004.