Epidemiology and EcologyJ Zimmerman

• PRRS virus is highly infectious (a pig can become infected by exposure to just a few viral particles) but nothighly contagious (is not transmitted from one pig or contaminated surface to another pig very easily).

• PRRS virus can be transmitted vertically (from a mother to her offspring) during gestation. This is known as inutero or “transplacental” infection. The virus can be transmitted through milk but the significance of this is notknown.

• Horizontal infection (from an infected pig to an uninfected pig) is also possible. This may result from exposureto body fluids (semen, blood, oral and nasal secretions), feces, contaminated surfaces or vectors, and possiblythrough the air.

• Vectors that may contribute to transmission of PRRS virus include, needles and syringes, insects, clothing andouterwear, and birds although their significance is unknown.

• Feral pigs can become infected with PRRS virus but their importance in transmission or maintenance of theinfection in an area is unknown.

• PRRS virus can be found under specific conditions in pork meat. However, the ingestion of pork meat is notthought to be important in transmission of the virus.

• PRRS virus is found in nearly all areas of the world where pigs are located.

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Epidemiology and EcologyJ Zimmerman

Introduction

The section summarizes current knowledge regardingthe distribution of PRRS virus in populations anddescribes what we know about how PRRS virus ismaintained in populations and how it interacts withthe environment. This information is the key toPRRS prevention and/or control. Under the bestcircumstances, the basic sciences are able to developeffective tools to use against infectious diseases,especially efficacious vaccines and good diagnosticassays. Ultimately, however, the effectiveapplication of even the best tools science can developmust be grounded in population medicine.

Transmission

Transmission requires that the pathogen successfullyexits an infected host, escape potential threats to itsexistence in the environment, avoid the host defensesystem, and reach the site of replication in a new,susceptible host. Transmission is a process with anassociated probability of success. Intuitively, it canbe recognized that this probability is not fixed, butvaries depending on the stage of infection and thespecific circumstances. For example, the probabilityof transmitting PRRS virus from a pig is greaterduring the acute phase of the infection, when virus isbeing shed in great amounts, than during the chronicstage of infection. Specific circumstances areimportant, as well. For example, infected sows ingestation crates are less likely to transmit virus thanpen-housed sows. We do not currently have goodestimates of the probability of transmission forvarious circumstances and stages of disease, butsome probability of transmission exists as long asinfectious PRRS virus is present in the pig.

PRRS virus is highly infectious, but not highlycontagious. This may sound like a contradiction, butit is not. To be highly “infectious” means thatexposure of the animal to relatively few virusparticles results in transmission. Thus, Yoon et al.(1999) estimated that intranasal or intramuscularexposure to 10 or fewer virus particles weresufficient to produce infection. By comparison,many other diseases of swine considered to be highlyinfectious (such as pseudorabies) require severalthousand virus particles to produce infection."Contagious" means that transmission occurs bycontact with infected animals or virus-contaminatedsurfaces. A highly contagious pathogen transmits

readily from infected to susceptible individuals. Incontrast, producers often comment that it is difficultto intentionally transmit PRRS virus by housingnegative animals with infected animals and that it iscommon to find negative animals within infectedgroups. In 1992, Potter (1994) investigated nineherds that had received infected breeding stock froma single PRRS virus-positive herd and found noserological evidence of transmission. For PRRSvirus, the “highly infectious, but not highlycontagious” viral strategy has been effective becauseinfectious virus is present in carrier animals formonths and because the population density in swineherds essentially guarantees that a circumstance willeventually occur that brings a carrier together with asusceptible animal.

Vertical Transmission

Vertical transmission is defined as transmission fromone generation to the next by infection of the embryoor fetus in utero (within the uterus). Some expertsinclude transmission of infectious agents to offspringin milk as vertical transmission, as well.

Transplacental transmission (transmission of fetusoccurring through passage of the virus across theplacenta) was first reported by Christianson et al.(1992). To date, the cumulative evidence indicatesthat transplacental transmission occurs most readilyduring the third trimester of pregnancy (Benfield etal., 1997, Christianson et al., 1993, Kranker et al.,1998, Lager et al., 1997a, Mengeling et al., 1994),but Molitor et al. (2001) discovered that someisolates are capable of transplacental transmission asearly as 30 days of gestation. Further, Prieto et al.(1996b, 1997a, 1997b) found that inoculation orinsemination of gilts with semen contaminated withSpanish PRRS virus strain 5710 did not interfere withconception and fertilization, but virus was recoveredfrom 20 day-old embryos in 3 of 5 litters, indicatingearly infection and death of embryos. Once infected,immunity against transplacental transmission uponsubsequent challenge with the same strain of thevirus appears to be long term. However, for reasonsunknown at this time, immunity is incomplete againstdifferent strains of the virus. That is, re-challenge ofpreviously infected sows with viruses other than theoriginal virus isolate may result in transplacentaltransmission (Benson et al., 2000, Lager et al., 1999).

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Vertical transmission has been shown to be apotential means of biosecurity breech. Dewey et al.(2000) reported the case of 350 pigs imported intoCanada from the Netherlands in May 1999. The pigswere caesarian-derived, raised in isolation until theywere 2 to 3 weeks of age, then shipped to Canada.Upon arrival, they were quarantined in a federallycontrolled facility for 3 months, then moved to a farmisolation facility. In August, tonsil biopsies werecollected from 26 animals, of which 13 were positivefor PRRS virus. Eleven of the samples showedgreater than 99 percent homology to Lelystad virus(the European strain of the virus, not found in Canadaat that time). The only possible conclusion is thatthese animals were infected in utero.

It is suspected that neonatal pigs may becomeinfected after birth by exposure to infected dams, butthe mechanism(s) of transmission has not beendemonstrated. PRRS virus has been detected in milkunder experimental conditions (Wagstrom et al.,2000) and transmission to neonates is assumedpossible, but unproven. Transmission by contact ofneonates with virus-contaminated oronasal secretionsfrom the dam may occur, but is also unproven.

Horizontal Transmission

Direct transmissionDirect transmission involves the immediate transferof an infectious agent by physical contact with theinfected individual or by contact with virus-contaminated material from an infected individual.PRRS virus transmission most commonly occurs bydirect transmission, i.e., close contact betweenanimals (nose-to-nose) or by exposure tocontaminated body fluids (semen, virus-taintedblood, or perhaps mammary secretions). Socialbehavior and the character of pig-to-pig interactionsare important in direct transmission, particularly theaggressive behavior associated with establishing asocial order within a group. Typically, fightinginvolves slashes or bites in the shoulders, neck, andhead and results in the exchange of blood and saliva.Bierk et al. (2001) associated transmission withaggressive behavior between carrier sows and naivecontacts. Non-aggressive behavior that results inexchange of blood and saliva, i.e., tail-biting and ear-biting, may also function in transmission. Thefrequency of these behaviors is related to facilities,management system, group sizes, group stability,mixing, and other population factors (Hafez andSignoret, 1969, Whittemore, 1998).

Indirect transmission (fomites, arthropods,aerosols)By definition, indirect transmission meanstransmission by an intermediate vehicle, such asinanimate objects or substances (e.g., water, food),living carriers (insect, bird, wildlife vectors), oraerosols.

Fomites - As previously discussed, PRRS virus isshed at low levels in a variety of secretions andexcretions and is inactivated in the environment at arate that depends on the ambient temperature andmoisture conditions. Under warm dry conditions,inactivation is relatively rapid (Pirtle and Beran,1996). Otake et al. (2002a) showed that PRRS viruswas present on workers' coveralls, boots, and handsfollowing 60 minutes of contact with acutely infectedpigs. However, elementary sanitation procedures,such as changing coveralls, changing boots, andwashing hands, was sufficient to stop transmission.Under experimental conditions, Dee et al. (2002a,2002b) showed that PRRS virus could be movedeasily on fomites in the field under winter conditions,but to a much lesser degree during warm weather.

Contamination of instruments and medications withbody fluids from PRRS virus-infected animals canresult in transmission. This includes instrumentsused for ear notching, tail docking, teeth clipping, ortattooing, as well as and needles, syringes,medications, and biologics. Recently, Otake et al.2002b confirmed needle-borne transmission of PRRSvirus under experimental conditions. Preventing thetransmission of PRRS virus via commonly usedinstruments will require awareness of the risk andstrict adherence to procedures that preventtransmission.

Insects - Insects are well recognized for their role inthe transmission of a variety of infectious agents.Insects may serve as mechanical vectors, in which theinfectious agent is carried either internally orexternally (body surface) to the next susceptibleindividual, or biological vectors, meaning that thepathogen replicates in the insect prior to transmissionto the host. Research in insect-borne infections ischallenging because the interactions betweenvertebrate host, insect, pathogen, and environmentare often highly complex.

The role of insects in the transmission of PRRS virushas recently become an active area of investigation.Otake et al. (2002c) reported detection of PRRS virusin mosquitoes captured on a farm undergoing a PRRSoutbreak. Subsequently, they demonstrated

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mechanical transmission under experimentalconditions in 2 of 4 attempts by allowing mosquitoesto feed on viremic pigs (pigs that had PRRS viruscirculating in their blood). Otake et al. (2002d) alsoexamined the transmission of PRRS virus byhouseflies. By abrading the skin of infected andrecipient pigs so as to provide access to blood,houseflies that fed on infected pigs were able tomechanically transmit virus to recipient pigs. In aseparate study, house flies were allowed to feed on aviremic pig, then were held at 27° C (81° F) andtested for the presence of PRRS virus over time.PRRS virus could be detected in the flies zero and 6hours after feeding but later samples (12, 24, 48, 72,and 96 hours) were negative.

Overall, the current data indicate that flies andmosquitoes are mechanical vectors of PRRS virusunder experimental conditions. Whether PRRS virusis actually an insect-borne disease will requireadditional experimental and field studies. If it isproven that certain insect species participate in thetransmission of PRRS virus in any one region, it willneed to be established that the appropriatevector/host/environment relationships are in placeelsewhere in the world where pigs are raised.

Aerosol transmission - Airborne virus was onceconsidered the primary route of PRRS virustransmission, with aerosol transmission suspected tooccur over distances of up to 20 kilometers (Anon,1991). Since PRRS virus is present in the upperrespiratory tract and oropharyngeal area of infectedpigs for an extended period, this was not anunreasonable hypothesis. Wills et al. (1997b),however, found that transmission by direct contactoccurred much more readily than transmission acrossa space of up to one meter. Other workers reportedsimilar results. Torremorell et al. (1997)demonstrated transmission over a distance of onemeter in one of two attempts. Likewise, Lager andMengeling (2000) successfully transmitted PRRSvirus in one of two attempts from infected pigs tosusceptible pigs via a tube 8-cm in diameter and 50cm in length. Otake et al. (2002e) reportedtransmission over a distance of 2.5 meters frominfected animals to susceptible pigs sharing the sameair space, but aerosols emitted from exhaust fans overdistances of one to 30 meters did not transmit PRRSvirus to sentinel pigs. The one exception to thispattern of poor transmissibility via aerosols is areport by Kristensen et al. (2002). In three trials,approximately 50 acutely infected pigs transmittedPRRS virus over a distance of one meter toapproximately 50 negative pigs when 1, 10 or 70percent of air was exchanged.

Available data suggest that the minimum infectiousdose via aerosol exposure is probably low (Yoon etal., 1999). If the threshold exposure dose is low,however, then the experiments described abovesuggest either that infected pigs aerosolized very littlevirus or that aerosolized virus was rapidly inactivatedunder the environmental conditions under whichmost of the experiments were conducted.

Factors of undetermined significance intransmissionDifferences among virus isolates, age of pig at timeof infection, stress, bacterial or viral co-infections,diet, and host genetic factors, should be included onthe list of factors of undetermined significance intransmission. In addition, the following are factorswith a possible, but uncertain, role in PRRS virustransmission.

Alternate hosts – It is possible that a wildlife specieswas the original source of the virus, therefore, one ormore alternate host species may exist. Theidentification of alternate hosts is important becauseof their potential role in transmitting PRRS virusbetween herds (area spread) and the possibility thatan unrecognized species might serve as a source ofnew strains of PRRS virus.

Feral swine are susceptible to PRRS virus infection,but the occurrence of infection in free-ranginganimals is relatively rare. Overall, the data suggestthat PRRS virus did not originate in feral swine, butmoved from domestic swine into the feral population.Feral swine generally live in small groups of fewerthan 20 individuals (Hafez and Signoret, 1969), apopulation probably not sufficiently large to maintainthe infection indefinitely. Nevertheless, in areaswhere feral swine interact with domestic swine, feralswine might serve as a source of PRRS virus.

A number of species have been examined and foundnot to be susceptible to PRRS virus, including mice,rats (Hooper et al., 1994), and guinea pigs (J.Zimmerman, unpublished data). Likewise, Wills etal. (2000b) found no evidence of PRRS virusreplication in cats, dogs, mice, opossums, raccoons,rats, skunks, house sparrows, or starlings. Inaddition, 30 dogs and 5 deer captured in suburbanareas in Kanagawa Prefecture (Japan) were negativefor PRRS virus antibodies (Neagari et al., 1998).Zimmerman et al. (1997) reported that some avianspecies, mallard ducks in particular, were susceptibleto PRRS virus. Mallards exposed to PRRS virus indrinking water shed virus in feces and virus was

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recovered from fecal samples collected from 8 of 20ducks 39 days after exposure. In a secondexperiment, mallard-to-mallard transmission wasdemonstrated by infecting ducks with feces fromducks shedding PRRS virus. Swine were shown tobe susceptible to the mallard-derived virus.However, subsequent workers have been unable toreplicate these experiments (F. Osorio, personalcommunication). At this juncture, the issue of non-swine host species is unresolved.

PRRS virus in pig meat (cannibalism) - Questionsregarding the ability of infectious PRRS virus topersist in pork or pork products quickly surfacedamong international trading partners in the early1990s. Understandably, PRRS virus-free countrieshad no wish to introduce the virus through theimportation of virus-contaminated pork products if itwas determined that ingestion of pork meat was animportant route of transmission.

Several studies have evaluated the presence of PRRSvirus in meat. Bloemraad et al. (1994) reported thatvirus was present, although at low titer, in muscletissue collected from viremic pigs and that thequantity of virus was only slightly affected by storagefor up to 48 hours at 4º C (39º F). In a researchsetting, Magar et al. (1995b), inoculated 6-month-oldpigs with PRRS virus and muscle tissue sampleswere collected at 7 and 14 days post inoculation.Virus was isolated from samples collected 7 dayspost inoculation but samples collected 14 days afterinoculations were negative. In a slaughterhousestudy, muscle tissue samples were collected from 44carcasses from herds known to be PRRS viruspositive but no virus could be isolated. Theinvestigators concluded that meat does not retaindetectable amounts of PRRS virus and thattransmission of virus through pork is unlikely.Overall, the data suggest that virus is present at lowlevels, or not at all, in the meat of market-aged swineand that the risk of transmission to swine by theconsumption of pig meat is low.

Transmission Within Herds

Once infected, PRRS virus tends to circulate within aherd indefinitely. Investigators have reportedisolation of virus from nursery pigs up to 2.5 yearsafter the initial PRRS outbreak (Joo and Dee, 1993,Stevenson et al., 1993). However, spontaneouselimination of PRRS virus from commercial herdshas been reported, but the circumstances under whichthis occurs are not well defined (Freese et al., 1993).The key components appear to be persistent PRRSvirus infection in clinically normal carrier animals

and the continual introduction of susceptible animalseither through birth or purchase (Wills et al., 1997c).In a typical scenario, the virus is perpetuated by acycle of transmission from dams to pigs either inutero or post partum, or by commingling susceptibleanimals with infected animals in later stages ofproduction. In neonatal pigs, maternal antibodiesmay provide some protection from infection.However, the degree of protection is not very wellcharacterized and appears to be of short duration.Under conditions in which susceptible and infectiouspigs are mixed, e.g. at weaning, a large proportion ofthe population may quickly become infected. Deeand Joo (1994) reported 80 to 100 percent of pigs inthree swine herds were infected by 8 to 9 weeks ofage and Maes (1997) found 96 percent of markethogs sampled from 50 herds to be positive.However, the pattern of infection in PRRS virus-endemic herds often deviates from this description ofrapid, uniform spread. Within infected herds, markeddifferences in infection rates between groups, pens,or rooms of animals are often observed. Houben etal. (1995) found transmission to vary even withinlitters, with some littermates becoming positive asearly as 6 to 8 weeks and other individuals as late as10 to 12 weeks of age. In some cases, litters of pigsreached 12 weeks of age, the end of the monitoringperiod, and still remained free of PRRS virusinfection. Thus, it is possible for animals inendemically infected herds to escape infection for anextended period of time. Dee et al. (1996) concludedthat the presence of susceptible animals in breedingherds provided a mechanism to maintain persistentviral transmission in chronically infected farms.

Albina et al. (1994) described the mechanisms thatallow PRRS virus to persist in infected farms as,

1. Incomplete infection of the susceptiblepopulation during the acute phase.

2. Introduction of new susceptible pigs in theform of replacement breeders.

3. A persistent viral infection in individualpigs with the potential to excrete virusunder certain conditions, such as animalgrouping, farrowing or weaning.

4. A rapid decrease in maternal immunity,with young pigs becoming susceptible toinfection or re-infection several monthslater.

5. Lack of protective immunity, or variableperiods of active immunity, in infectedpigs.

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Transmission Between Herds

Frequently, elimination of PRRS virus from herds inswine dense areas is commonly followed by re-introduction of the virus weeks or months later. Theintroduction of virus into a herd in the absence of anyapparent animal or human contact is termed "areaspread." Under most circumstances, the source of thevirus is unproven. Possible sources for considerationinclude the introduction of infected animals orcontaminated fomites onto the premises, or spreadvia insects, aerosols, water, or non-porcine hosts.

Dee (1992) was the first to recognize the primary roleof infected animals in herd-to-herd transmission.Following outbreaks of PRRS in late 1990, Dee(1992) reported that, of 10 farms surveyed, 8 hadpurchased breeding stock from the same source. Theinterval from arrival of the stock to appearance ofclinical signs ranged from 3 to 24 days. In a regionalPRRS virus control program in France, Le Potier etal. (1997) estimated that 56 percent (66 of 118) ofherds acquired the infection through infected pigs, 20percent (23 of 118) through infected semen, 21percent (25 of 118) through fomites/slurry, and 3percent (4 of 118) through unidentified sources. InIllinois, Weigel et al. (2000) noted that the purchaseof semen or boars was associated with increased risk,whereas isolation of gilts after purchase wasassociated with decreased risk of infection. Otherinvestigators have also noted that proximity toinfected herds increased the risk of acquiring PRRSvirus. For example, in Denmark, it was observed thatthe risk of a herd becoming PRRS virus-positiveincreased with the density of PRRS virus-positiveneighboring herds, but decreased with distance fromthem (Zhuang et al., 2002). In France, Le Potier etal. (1997) found that 45% of herds suspected to havebecome infected through area spread were locatedwithin 500 meters (0.3 miles) of the suspected sourceherd and only 2% were more than one kilometer fromthe initial outbreak.

Using a molecular approach to the problem of areaspread, Goldberg et al. (2000) evaluated the genesequences from 55 field isolates collected in Illinoisand eastern Iowa with the objective of determiningwhether the genetic similarity of PRRS virus isolatesreflected their geographical proximity. Somewhatsurprisingly, they found that the genetic similarity ofisolates did not correlate with their geographicaldistance and, on that basis, concluded that PRRSvirus was most commonly introduced into herdsthrough animals or semen, as opposed to mechanismsassociated with spread from neighboring herds.

Recent publications by Dee et al. (2002a, 2002b)have demonstrated the ease with which PRRS viruscan be moved between farms on commonplaceequipment and objects common to swine farms, e.g.,styrofoam semen coolers, metal toolboxes, plasticlunch pails, and cardboard boxes, especially whenwet and cold. It is highly unlikely that clean-appearing and ordinary objects would be recognizedas the source of the introduction days or weeks later,when the infection became apparent.

Overall, the cumulative information suggests that therisk of introducing PRRS virus can be reduced bycarefully monitoring the PRRS virus status ofreplacement livestock and boar studs that supplysemen for artificial insemination and byimplementing procedures to avoid the introduction ofPRRS virus on contaminated fomites.

Summary

PRRS virus is found in most areas of the world.Within infected countries, 60 to 80 percent of herdsare typically infected, with in-herd prevalence highlyvariable. Estimates of prevalence are complicated bythe use of MLV vaccines in most parts of the world.MLV vaccines have been available since 1994 andantibodies against vaccine virus are not easilydifferentiated from antibodies against PRRS virusfield strains. Population density has a marked effecton the prevalence of PRRS within herds and regions.Even within the same area, larger herds tend to havehigher in-herd prevalence than smaller herds.

Swine are susceptible to PRRS virus by severalroutes of exposure, including intranasal,intramuscular, intraperitoneal, oral, and vaginal.Exposure to 10 or fewer PRRS virus particles byintranasal, intramuscular, and probablyintraperitoneal routes results in infection (Yoon et al.,1999).

Infection of susceptible animals results in theshedding of virus in saliva, nasal secretions, urine,semen, and perhaps feces, with shedding occurringsimultaneously from many sites at low levels orperhaps intermittently. Pregnant susceptible femalesinoculated in late gestation have been shown to shedvirus in mammary secretions (Wagstrom et al. 2001).The infection is a chronic, persistent infectionwhereby virus replicates in susceptible cells ofinfected individuals for several months. Shedding ofPRRS virus in secretions and excretions results inenvironmental contamination and creates thepotential for transmission via fomites. The virus isgenerally short-lived in the environment and is

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quickly inactivated by drying, but it can remaininfectious for an extended time under specificconditions of temperature, moisture, and pH. Dee etal. (2002a, 2002b) illustrated that PRRS virus couldbe moved easily on fomites in the field under winterconditions but to a much lesser degree during warmweather. Standard disinfection and sanitationprocedures are effective against the virus, but theymust be studiously applied.

PRRS virus transmission most commonly occurs bydirect transmission, i.e., close contact betweenanimals or by exposure to contaminated body fluids(semen, virus-tainted blood, or perhaps mammarysecretions). The aggressive behavior associated withestablishing a social order within a group thatinvolves slashes or bites in the shoulders, neck, andhead and results in the exchange of blood and salivaand transmission of PRRS virus. Indirecttransmission by fomites, vectors, or aerosols mayalso occur. Of these, transmission via instrumentsand medications contaminated with body fluids fromPRRS virus-infected animals is the most important.This includes instruments used for ear notching, taildocking, teeth clipping, or tattooing, as well as andneedles, syringes, medications, and biologics. Recentresearch has shown that flies and mosquitoes arecapable of mechanical transmission of PRRS virus

under experimental conditions (Otake et al., 2002c,2002d). Establishing whether PRRS virus is aninsect-borne disease will require additionalexperimental and field studies. Aerosol transmissionis also still an open question. Results of pig-to-pigaerosol transmission experiments have been mixedand essential information (the quantity of virusexcreted by pigs and the rate of inactivation ofaerosolized virus) is missing.

The ability of PRRS virus to establish persistentinfections in animals is the primary challenge tosuccessful prevention and control programs.Establishing and maintaining herd immunity in theface of persistent infection is problematic becausevaccines that induce long-term protective immunityagainst heterologous isolates and eliminate or reducevirus shedding are not yet available. Finally, ifelimination is achieved, herds are vulnerable to re-infection with PRRS virus through the introduction ofsubclinically-infected animals or by area spread.This scenario is reminiscent of other infectiousagents, i.e., classical swine fever virus (hog cholera)or African swine fever, which have been successfullycontrolled and/or eliminated in the past throughcoordinated regional efforts.

References

Albina E, Madec F, Cariolet R, Torrison J. 1994. Immune response and persistence of the porcine reproductive andrespiratory syndrome virus in infected pigs and farm units. Vet Rec 134:567-573.

Anon. 1991. The new pig disease: Conclusions reached at the seminar. In: The new pig disease. Porcinereproductive and respiratory syndrome. A report on the seminar/workshop held in Brussels on 29-30 April1991 and organized by the European Commission Directorate General for Agriculture, pp. 82-86.

Benfield DA, Christopher-Hennings J, Nelson EA, et al. 1997. Persistent fetal infection of porcine reproductiveand respiratory syndrome (PRRS) virus. Proceedings of the American Association of Swine Veterinarians, pp.455-458.

Benson JE, Yaeger MJ, Lager KM. 2000. Effect of porcine reproductive and respiratory syndrome virus (PRRSV)exposure dose on fetal infection in vaccinated and nonvaccinated swine. Swine Health and Production8(4):155-160.

Bierk MD, Dee SA, Rossow KD, et al. 2001. Transmission of porcine reproductive and respiratory syndrome virusfrom persistently infected sows to contact controls. Can J Vet Res 65:261-266.

Bloemraad M, de Kluijver EP, Petersen A, et al. 1994. Porcine reproductive and respiratory syndrome: temperatureand pH stability of Lelystad virus and its survival in tissue specimens from viraemic pigs. Vet Microbiol42:361-371.

Christianson WT, Choi CS, Collins JE, et al. 1993. Pathogenesis of porcine reproductive and respiratory syndromevirus infection in mid-gestation sows and fetuses. Can J Vet Res 57:262-268.

Christianson WT, Collins JE, Benfield DA, et al. 1992. Experimental reproduction of swine infertility andrespiratory syndrome in pregnant sows. Am J Vet Res 53:485-488.

Dee S, Deen J, Rossow K, et al. 2002a. Mechanical transmission of porcine reproductive and respiratory syndrome

33

virus throughout a coordinated sequence of events during cold weather. Can J Vet Res 66:232-239.Dee S, Deen J, Rossow K, et al. 2002b. Mechanical transmission of porcine reproductive and respiratory syndrome

virus throughout a coordinated sequence of events during warm weather. Can J Vet Res (in press).Dee SA, Joo HS, Henry S, et al. 1996. Detecting subpopulations after PRRS virus infection in large breeding herds

using multiple serologic tests. Swine Health and Production 4(4):181-184.Dee SA, Joo HS. 1994. Prevention of the spread of porcine reproductive and respiratory syndrome virus in

endemically infected pig herds by nursery depopulation. Vet Rec 135:6-9.Dee SA. 1992. Investigation of a nationwide outbreak of SIRS using a telephone survey. American Association of

Swine Practitioners Newsletter 4:41-44.Dewey C, Charbonneau G, Carman S, et al. 2000. Lelystad-like strain of porcine reproductive and respiratory

syndrome virus (PRRSV) identified in Canadian swine. Can Vet J 41:493-494.Freese WR, Joo HS, Simonson RR. 1993. A potential spontaneous elimination of porcine endemic abortion

respiratory syndrome (PEARS) virus spread in a commercial swine herd. Proceedings of the International PigVeterinary Society Congress, p. 66.

Goldberg TL, Hahn EC, Weigel RM, Scherba G. 2000. Genetic, geographical and temporal variation of porcinereproductive and respiratory syndrome virus in Illinois. J Gen Virol 81:171-179.

Hafez ESE, Signoret JP. 1969. The behavior of swine. In: The Behavior of Domestic Animals. Hafez ESE (ed).The Williams and Wilkins Company, Baltimore, Maryland, pp. 349-390.

Hooper CC, Van Alstine WG, Stevenson GW, Kanitz CL. 1994. Mice and rats (laboratory and feral) are not areservoir for PRRS virus. J Vet Diagn Invest 6:13-15.

Houben S, van Reeth K, Pensaert MB. 1995. Pattern of infection with the porcine reproductive and respiratorysyndrome virus on swine farms in Belgium. J Vet Med [B] 42:209-215.

Joo HS, Dee SA. 1993. Recurrent PRRS problems in nursery pigs. Proceedings of the Allen D. Leman SwineConference, pp. 85-86.

Kranker S, Nielsen J, Bille-Hansen V, Bøtner A. 1998. Experimental inoculation of swine at various stages ofgestation with a Danish isolate of porcine reproductive and respiratory syndrome virus (PRRSV). VetMicrobiol 61:21-31.

Kristensen CS, Bøtner A, Angen Ø, et al. 2002. Airborne transmission of A. pleuropneumoniae and PRRS virusbetween pig units. Proceedings of the International Pig Veterinary Society Congress, 1:272.

Lager KM, Mengeling WL, Brockmeier SL. 1997a. Homologous challenge of porcine reproductive and respiratorysyndrome virus immunity in pregnant swine. Vet Microbiol 58:113-125.

Lager KM, Mengeling WL, Brockmeier SL. 1999. Evaluation of protective immunity in gilts inoculated with theNADC-8 isolate of porcine reproductive and respiratory syndrome virus (PRRSV) and challenge-exposed withan antigenically distinct PRRSV isolate. Am J Vet Res 60:1022-1027.

Lager KM, Mengeling WL. 2000. Experimental aerosol transmission of pseudorabies virus and porcinereproductive and respiratory syndrome virus. Proceedings of the American Association of Swine Practitioners,pp. 409-410.

Le Potier M-F, Blanquefort P, Morvan E, Albina E. 1997. Results of a control programme for the porcinereproductive and respiratory syndrome in the French "Pays de la Loire" region. Vet Microbiol 55:355-360.

Maes D. 1997. Descriptive epidemiological aspects of the seroprevalence of five respiratory disease agents inslaughter pigs from fattening herds. Epidémiol Santé Anim 31-32:05.B.19.

Magar R, Robinson Y, Dubuc C, Larochelle R. 1995b. Evaluation of the persistence of porcine reproductive andrespiratory syndrome virus in pig carcasses. Vet Rec 137:559-561.

Mengeling WL Lager KM, Vorwald AC. 1994. Temporal characterization of transplacental infection of porcinefetuses with porcine reproductive and respiratory syndrome virus. Am J Vet Res 55:1391-1398.

Molitor T, Papenfuss T, Joo HS. 2001. Hormonal profiles in sows infected with atypical porcine reproductive andrespiratory syndrome virus (PRRSV), A mechanism of virus-induced abortion. NPPC Project #98-020. 2000Research Investment Report. National Pork Producers Council, PO Box 10383, Des Moines, Iowa 50308.

34

Neagari Y, Sakai T, Nogami S, et al. 1998. [Incidence of antibodies in raccoon dogs and deer inhabiting suburbanareas]. Kansenshogaku Zasshi 72:331-334.

Otake S, Dee SA, Jacobson L, et al. 2002e. Evaluation of aerosol transmission of porcine reproductive andrespiratory syndrome virus under controlled field conditions. Vet Rec 150:804-808.

Otake S, Dee SA, Rossow KD, et al. 2002a. Transmission of porcine reproductive and respiratory syndrome virusby fomites (boots and coveralls). J Swine Health Prod 10(2):59-65.

Otake S, Dee SA, Rossow KD, et al. 2002b. Transmission of porcine reproductive and respiratory syndrome virusby needles. Vet Rec 150:114-115.

Otake S, Dee SA, Rossow KD, et al. 2002c. Mechanical transmission of porcine reproductive and respiratorysyndrome virus by mosquitoes, Aedes vexans (Meigen). Can J Vet Res 66:191-195.

Otake S, Dee SA, Rossow KD, et al. 2002d. Evaluation of mechanical transmission of porcine reproductive andrespiratory syndrome virus by houseflies, Musca domestica (Linnaeus). Vet Rec (in press).

Pirtle EC, Beran GW. 1996. Stability of porcine reproductive and respiratory syndrome virus in the presence offomites commonly found on farms. J Am Vet Med Assoc 208:390-392.

Potter RA. 1994. Non-transmission of porcine reproductive and respiratory syndrome virus by seropositive pigsfrom an infected herd. Vet Rec 134:304-305.

Prieto C, Sanchez R, Martin Rillo S, et al. 1996b. Exposure of gilts in early gestation to porcine reproductive andrespiratory syndrome virus. Vet Rec 138:536-539.

Prieto C, Suárez P, Simarro I, et al. 1997b. Transplacental infection following exposure of gilts to porcinereproductive and respiratory syndrome virus at the onset of gestation. Vet Microbiol 57:301-311.

Prieto C, Suarez P, Simarro I, et al., 1997a. Insemination of susceptible and preimmunized gilts with boar semencontaining porcine reproductive and respiratory syndrome virus. Theriogenology 47:647-654.

Rowson KEK, Mahy BWJ. 1975. Lactic dehydrogenase virus. Virology Monographs, p. 100.Stevenson GW, Van Alstine WG, Kanitz CL, Keffaber KK. 1993. Endemic porcine reproductive and respiratory

syndrome virus infection of nursery pigs in two swine herds without current reproductive failure. J Vet DiagnInvest 5:432-434.

Torremorell M, Pijoan C, Janni K, et al. 1997. Airborne transmission of Actinobacillus pleuropneumoniae andporcine reproductive and respiratory syndrome virus in nursery pigs. Am J Vet Res 58:828-832.

Wagstrom EA, Chang C-C, Yoon K-J, Zimmerman JJ. 2001. Shedding of porcine reproductive and respiratorysyndrome (PRRS) virus in mammary secretions of sows. Am J Vet Res 62:1876-1880.

Weigel RM, Firkins LD, Scherba G. 2000. Prevalence and risk factors for infection with porcine reproductive andrespiratory syndrome virus (PRRSV) in swine herds in Illinois (USA). Vet Res 31:87-88.

Whittemore C. 1998. The Science and Practice of Pig Production (2nd edition). Blackwell Science, Ltd., Oxford,pp. 146, 153, 260.

Wills RW, Osorio FA, Doster AR. 2000b. Susceptibility of selected non-swine species to infection with PRRSvirus. Proceedings of the American Association of Swine Practitioners, pp. 411-413.

Wills RW, Zimmerman JJ, Swenson SL, et al. 1997b. Transmission of porcine reproductive and respiratorysyndrome virus by direct close or indirect contact. Swine Health and Production 5(6):213-218.

Wills RW, Zimmerman JJ, Yoon K-J, et al. 1997c. Porcine reproductive and respiratory syndrome virus: Apersistent infection. Vet Microbiol 55:231-240.

Yoon K-J, Zimmerman JJ, Chang C-C, et al. 1999. Effect of challenge dose and route on porcine reproductive andrespiratory syndrome virus (PRRSV) infection in young swine. Vet Res 30:629-638.

Zhuang Q, Barfod K, Wachmann H, et al. 2002. Serological surveillance for PRRS in danish genetic pig herds andrisk factors for PRRS infection. Proceedings of the International Pig Veterinary Society Congress 2:231.

Zimmerman JJ, Yoon K-J, Pirtle EC, et al. 1997. Studies of porcine reproductive and respiratory syndrome (PRRS)virus infection in avian species. Vet Microbiol 55:329-336.