Difference between revisions of "Porcine Reproductive and Respiratory Syndrome"

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−
Also known as: '''PRRS — Blue Eared Pig Disease
+
{{unfinished}}
  
−
==Introduction==
+
Also known as: PRRS, blue eared pig disease
−
Porcine reproductive and respiratory syndrome (PRRS) first appeared in the USA in 1987, although subsequent serologic evidence indicated that it had been circulating for some time prior to being recognised. The disease was seen in Europe in 1990 and reported in Humberside in the UK in 1991. PRRS is caused by an arterivirus, and leads to abortions, mortality and reduced growth, significantly impacting pig welfare and production economics worldwide. This disease is now found in most areas of the world where pigs are raised. Besides Europe and North America, it was identified in China in 1995 and is present in Japan, Vietnam, the Philippines, Malaysia, and Korea among other countries in Asia. Australia, New Zealand, several European countries are currently free of the disease.
+
 
 +
==Description==
 +
 
 +
Porcine reproductive and respiratory syndrome (PRRS) first appeared in the USA in 1987, although subsequent serologic evidence indicated that it had been circulating for some time prior to being recognised. The disease was seen in Europe in 1990 and reported in Humberside in the UK in 1991. PRRS is caused by an arterivirus, and leads to abortions, mortality and reduced growth, significantly impacting pig welfare and production economics worldwide.
  
 
==Aetiology==
 
==Aetiology==
−
The virus that causes PRRS is an arterivirus, within the [[:Category:Arteriviridae|Arteriviridae]] family. The arteriviruses are the only genus contained within the Arterivirdae family, and other members include the agent responsible for [[Equine Viral Arteritis|equine viral arteritis]] and lactate dehydrogenase-elevating virus of mice. The viruses are host-specific and antigenically distinct, and establish persistent infections after invasion.
+
 
 +
The virus that causes PRRS is an aterivirus, within the Arteriviridae family. The arteriviruses are the only genus contained within the Arterivirdae family, and other members include the agent responsible for equine viral arteris and lactate dehydrogenase-elevating virus of mice. The viruses are host-specific and antigenically distinct, and establish persistent infections after invasion.
 +
 
 +
Arteriviruses are enveloped viruses of 45 to 80 nm in diameter with a spherical appearance due to the shape of the envelope. However, the icoshedral nucelocapsid that contains the positive-sense single-stranded RNA genome is actually icosohedral. The lipoprotein envelope has ring-like structures on its surface, but there are no gross surface spikes. Arteriviruses have a tropism for macrophages and endothelial cells, where they reproduce in the cytoplasm. The genome is 13Kb in length, and is infectious itself without the capsid or envelope. 
 +
 
 +
Arteriviruses are inactivated by traetment with chloroform or ether. They are very resistant to freezing, althouth infectivity is in reduces as temperature risees.
  
 
==Transmission and Epidemiology==
 
==Transmission and Epidemiology==
−
The virus is spread by contact with infected pigs, and infection also be transmitted venereally via infected semen. Blood, saliva, urine, milk and semen from infected animals have all been shown to contain PRRS virus. Fomites, such as contaminated needles, boots, overalls and transport vehicles can also transmit PRRS, as can certain species of insects including [[Musca spp.|house flies]]. Aerosol transmission is thought to occur, particularly under conditions of high humidity, low temperatures, and low wind speeds, but this has been difficult to reproduce in the field and experimentally. Once infected, adult animals shed PRRS virus for up to 86 days, and younger, weaned pigs for 157 days. Virus excretion in semen can persist for up to 93 days after infection. This lengthy persistence of infection is an important factor in the maintenance of infection within a herd.
+
 
 +
The virus is spread by contact with infected pigs, and infection also be transmitted by infected semen. Fomites, such as contaminated needles, boots, overalls and transport vehicles can also transmit PRRS, as can certain species of insects including house flies. It has been thought that aerosol transmission may occur, particularly under conditions of high humidity, low temperatures, and low wind speeds, but this has been difficult to reproduce in the field and experimentally. Once infected, adult animals shed PRRS virus for up to 86 days, and younger, weaned pigs for 157 days. Virus excretion in semen can persist for up to 93 days after infection.
 +
 
 +
Transmission of PRRS virus to swine occurs most efficiently
 +
through direct contact. Blood, saliva, urine, milk
 +
and semen from infected animals have been shown to
 +
transmit PRRS virus to susceptible swine (Wills et al.,
 +
1997). Aerosol transmission may occur but has proven
 +
difficult to demonstrate even over relatively short distances
 +
(meters). Transmission by semen, either
 +
through natural service or artificial insemination is well
 +
established (Yaeger et al., 1993).
  
 
==Pathogenesis==
 
==Pathogenesis==
−
Once PRRS virus gains entry to the host, primary replication occurs in regional or mucosal macrophages such as in the tonsil and the nasal and respiratory epithelium. After around twelve hours, viraemia occurs and the virus is disseminated systemically. The PRRS virus then secondarily replicates in the monocytes and macrophages in a wide variety of tissues. Other cells, such as fibroblasts, endothelial cell and smooth muscle may acquire PRRS infection, but only cells of the macrophage lineage have been shown to unequivocally support replication. The tropism for macrophages impairs the cellular immune response, and mucosal surfaces are also damaged. Viraemia persists for around 4-6 weeks in most infections, but lymphoid tissues can remain infected for extended periods of time: following experimental inoculation, PRRS virus has been detected for up to 225 days. Both cell mediated and humoral immune responses are induced by PRRS virus infection, and persistence of infection occurs despite this.
+
 
 +
Once PRRS virus gains entry to the host, primary replication occurs in regional or mucosal macrophages such as in the tonsil and the nasal and respiratory epithelium. After around twelve hours, viraemia occurs and the virus is disseminated systemically.  
 +
Secondary replication of PRRS virus occurs in monocyte/
 +
macrophages in many tissues. While other cell
 +
types, including fibroblasts, endothelial cells, seminiferous
 +
epithelium and smooth muscle, have been reported
 +
to be PRRS virus-positive by in situ hybridization and/or
 +
immunohistochemistry, only cells of macrophage lineage
 +
have been unequivocally shown to support PRRS
 +
virus replication (Sur et al., 1997).
 +
In most swine, viremia resolves within 4-6 weeks following
 +
infection. However, PRRS virus can persist in
 +
lymphoid tissues, such as tonsil, for extended periods of
 +
time and be transmitted to susceptible swine (Zimmerman
 +
et al., 1992). PRRS virus has been detected for up
 +
to 225 days following inoculation, though the proportion
 +
of positive animals appears to drop off between 80 and
 +
100 days (Albina et al., 1994; Allende et al., 2000; Wills
 +
et al., 2003). Persistence occurs in the face of neutralizing
 +
and other antibodies, and cell mediated immune
 +
responses, as indicated by gamma-interferon producing
 +
cells. Studies indicate that a low T cell response
 +
contributes to prolonged PRRS clearance (Xiao et al.,
 +
2004). Persistence is of epidemiologic significance
 +
since persistently infected animals are capable of transmitting
 +
PRRS virus to susceptible pigs via direct contact
 +
(Albina et al., 1994). This is likely an important mechanism
 +
by which PRRS virus is maintained within, and
 +
transmitted between, swine herds.
  
 
==Diagnosis==
 
==Diagnosis==
−
In the field, suspicion of PRRS is based on clinical signs of reproductive failure and high levels of neonatal mortality. Farm records may provide useful information for confirming reproductive failure.
 
  
−
The differential diagnoses for PRRS include those of respiratory and reproductive signs. Respiratory differentials include [[Swine Influenza|swine influenza]], [[Enzootic Pneumonia - Pigs|enzootic pneumonia]], ''[[Haemophilus parasuis]]'' infection, porcine respiratory coronavirus and [[Porcine Circoviruses|porcine circovirus]]-associated disease. Reproductive differentials include [[Porcine Parvovirus|porcine parvovirus]], [[Porcine Enterovirus|porcine enterovirus]], and [[Suid Herpesvirus 1|Aujesky's disease]].
+
===Clinical Signs===
  
−
===Clinical Signs===
+
There appears to be two distinct clinical phases to PRRS: reproductive failure and post-weaning respiratory diseases. When sows are infected, period of acute disease is seen which is characterised by lethargy, inappetance and pyrexia. Severe infections may also be associated with vestibular signs, and death of up to 10% of the sow stock can occur. Reproductive failure is results in increased numbers of stillborn piglets, mummified foetuses, abortions, premature farrowings and weak piglets. Lactating sows often display anorexia and agalactia, which leads to a rise in preweaning mortality. Signs are also seen in the piglets themselves, including a "thumping" respiratory pattern which on post-mortem examination is revealed to be associated with ls a severe, necrotising, interstitial pneumonia. Oedema may be seen around the eyes, and there is considerable variation in the size of piglets within litters.
−
[[Image:Blue Eared Pig.jpg|thumb|righ|200px|A pig with PRRS, showing blue ears. Source: Wikimedia Commons; Author: Dingar (2007)]]
 
−
There appears to be two distinct clinical phases to PRRS: reproductive failure and post-weaning respiratory diseases. When sows are infected, a period of '''acute disease''' is seen which is characterised by '''lethargy, inappetance and pyrexia'''. Occasionally, the snout, ears and vulva can appear blue. These signs are generally mild, but occasionally severe infections can occur which are associated with vestibular signs, and death of up to 10% of the sow stock. The reproductive aspects of the syndrome are seen when sows become infected in the last trimester of pregnancy. '''Reproductive signs''' include increased numbers of stillborn piglets, mummification, abortions, premature farrowings and the birth of weak piglets. Lactating sows often display anorexia and agalactia, which leads to a rise in preweaning mortality. Signs are also seen in the piglets themselves, including a "thumping" respiratory pattern which on post-mortem examination is revealed to be associated with a severe, necrotising, interstitial pneumonia. Oedema may be seen around the eyes, and there is considerable variation in the size of piglets within litters.
 
  
−
Post-weaning performance is also affected, with daily live weight gain reduced by up 85%. Mortality in growing pigs can be increased by up to 25% when PRRS is present in the herd. Lethargy and a loss of appetite is seen in weaned pigs, as well as a failure to thrive. Breathing is laboured and there may be obvious respiratory distress. The skin can appear red and blotchy, with a rough hair coat.
+
Post-weaning performance is also affected, with daily live weight gain reduced by up 85% and increases in mortality by up to 25%. Lethargy and a loss of appetites is seen as in adult pigs, and weanling fail to thrive. Breathing is laboured and there may be obvious respiratory distress. The skin can appear red and blotchy, with a rough hair coat.
  
−
Secondary infections commonly occur concurrently to PRRS. Bacteria previously reported to be associated with PRRS include ''[[Streptococcus suis]]'', ''[[Escherichia coli]]'', ''[[Salmonella]] choleraesuis'', ''[[Haemophilus parasuis]]'' and ''[[Mycoplasma hyopneumoniae]]''. Viral infections such as porcine respiratory coronavirus and [[Swine Influenza|swine influenza]] virus are also possible.
+
Outbreaks of the reproductive form of PRRS reportedly last between one and four months, depending on the initial health status on the pigs and the management of the outbreak. However, the post-weaning respiratory phase can become chronic, creating major production problems.
  
−
Outbreaks of the reproductive form of PRRS reportedly last between one and four months, depending on the initial health status of the pigs and the management of the outbreak. However, the post-weaning respiratory phase can become chronic, creating major production problems.
+
Secondary infections commonly occur. Bacteria previously reported to be associated with PRRS include ''Streptococcus suis'', ''Escherichia coli'', ''Salmonella choleraesuis'', ''Haemophilus parasuis'' and ''Mycoplasma hyopneumoniae''. Viral infections such as porcine respiratory coronavirus and swine influenza virus are also possible.
  
 
===Laboratory Tests===
 
===Laboratory Tests===
−
The most frequently used diagnostic tests are an [[ELISA testing|ELISA]] and an indirect [[Immunofluorescence|fluorescent antibody test]]. In order for meaningful results to be generated, serum should be collected from around 20 exposed animals in the herds. Both of the ELISA and the IFAT tests measure antibodies against PRRS virus. Anti-PRRS specific IgM can be detected within 7 days of infection, and IgG can be detected within 14 days. Titres peak about 5–6 weeks after infection, and can persist for up to 144 days while circulating virus is present. Once virus ceases to circulate, antibody levels quickly drop. If titres are particularly high, exposure may be recent and the sampled population could currently be shedding virus.
+
The most commonly used tests are the ELISA or the indirect fluorescent antibody test. These tests measure IgG antibodies to PRRS virus. They cannot measure the level of immunity in an animal or predict whether the animal is a carrier. Titers are detected within 7-10 days after infection and can persist for up to 144 days. High titers may indicate recent exposure, and viral shedding may be occurring within the sampled population. Tests for PRRS virus include PCR, virus isolation, and immunohistochemistry. Recently, nucleic acid sequencing of the open reading frame 5 region of the virus has become commercially available, and has proved to be an excellent tool for epidemiologic investigations in the field to confirm similarity between isolates recovered from different sites.
  
−
Tests for the PRRS virus itself also exist. These include RT-PCR and virus isolation. For virus isolation, buffy coat, serum, lung, lymph nodes, spleen and tonsils are the specimens of choice. The virus is incubated with swine pulmonary alveolar macrophages, and cytopathic effect are seen in around four days. RT-PCR can be performed on whole blood, buffy coat and clarified homogenates of the tissues appropriate for virus isolation. The test is particularly useful for examination of mummified or aborted litters. Immunohistochemistry is also possible, and nucleic acid sequencing of the 5' end of the virus has recently become commercially available. This is useful for comparing isolates recovered from different sites in order to facilitate epidemiological investigations.
+
==Pathology==
  
−
===Pathology===
+
Gross necropsy lesions are minimal in the uncomplicated respiratory form of PRRS, but interstitial pneumonitis is a consistent histopathologic finding. There are no gross or histopathologjc lesions noted in aborted or stillborn fetuses.
−
Although PRRS virus is capable of infecting many organs of the pig, gross lesions are usually only observed in the respiratory and lymphoid tissues. Both gross and microscopic lesions are most marked in neonatal and young weaned pigs.
 
  
−
After an uncomplicated PRRS infection in finishing pigs, gross lesions can vary from unremarkable to severe. In severe disease, lungs are mottled red and tan, and fail to collapse properly. The cranioventral lobes are worst affected. Lymph nodes appear moderately to severely enlarged, tan in colour and may be haemorrhagic with some strains of virus. Most PRRS virus-infected pigs are co-infected with one or more secondary pathogens, so pathology can vary quite widely depending on these. Microscopically, interstitial pneumonia is the typical finding. There is a mononuclear infiltration of the alveolar walls with type 2 pneumonocyte proliferation, and necrotic debris and macrophages accumulate in the alveolar spaces. In lymph nodes, follicular hyperplasia and necrosis may be seen. Non-suppurative meningoencephalitis and choroiditis is commonly present, but severity is highly variable.
+
Microscopic lesions of the fetus
 +
and placenta include umbilical vasculitis, interstitial
 +
pneumonia, myocarditis and pulmonary arteritis but are
 +
inconsistent and not diagnostic for PRRS (Lager and
 +
Ackermann, 1994; Lager and Halbur, 1996; Rossow et
 +
al., 1996b). There may be residual effects of PRRS
 +
virus infection in sows manifest as a reduced conception
 +
rate (Lowe et al., 2006).
  
−
There are no gross lesions noted in aborted or stillborn foetuses, but histopathology inconsistently reveals interstitial pneumonia, myocarditis and pulmonary arteritis. However, these changes are neither specific nor diagnostic for PRRS.
+
Microscopically, interstitial pneumonia
 +
with mononuclear septal infiltration, type 2 pneumonocyte
 +
proliferation and accumulations of necrotic
 +
debris and macrophages in alveolar spaces is evident.
 +
Follicular hyperplasia and necrosis may be present in
 +
lymph nodes. Nonsuppurative meningoencephalitis and
 +
choroiditis is commonly present, with highly variable
 +
severity
  
−
==Treatment and Control==
+
==Control==
−
There is currently no known effective treatment for PRRS. Non-steoidal anti-inflammatory drugs have been used in an attempt to reduce fever, and appetite stimulants to counteract inappetance and poor weight gain. However, these appear to have minimal benefit. Antibiotics may be used to counteract secondary bacterial infections.
+
Currently, there are no effective treatment programs for acute PRRS. Attempts to reduce fever using NSAID (aspirin) or appetite stimulants (B vitamins) appear to have minimal benefit. The use of antibiotics or autogenous bacterins to reduce the effects of opportunistic bacterial pathogens have also been reported; however, results have been mixed.
−
 
+
Prevention of infection appears to be the primary means of control. Understanding the PRRS status of replacement gilts and boars, as well as proper isolation and acclimatization of incoming stock are critical measures to prevent viral introduction. Pigs should be retested on arrival at the isolation facility and 45-60 days later, before entry to the herd. Elimination of existing infection by multisite production and segregated early weaning has also been described. While these strategies have had some success, the longterm risks of reinfection appear high. Prevention of viral spread by nursery depopulation has been described. This is successful when virus transmission is not occurring in the sow herd (usually 12-18 mo after initial outbreak), but the nurseries and growing/finishing pigs are still infected. All nursery pigs are removed from the farm to be finished elsewhere. The nurseries are then aggressively washed and disinfected and left empty for 7-14 days, after which they can be used normally. The technique has successfully eliminated PRRS virus from several herds, in which pigs have remained seronegative (for >1 yr) to market age, and production in the nurseries has improved, both in growth rate and mortality.
−
As treatment for PRRS is ineffective, prevention is very important. In herds where PRRS is not present, bought-in stock and semen should be PRRS-free, and any animals joining the herd should be quarantined on-site before being introduced to the cohort. Pigs should be tested on arrival to the unit, and again 45-60 days later before they join the main herd. In herds where PRRS is endemic, breeding gilts may be exposed to adult sows before they become pregnant in order to induce immunity. Existing infection can also be eliminated by multisite production and segregated early weaning, but the long-term risk of reinfection is high. Complete depopulation, disinfection and repopulation can also be successful, as can test-and-removal strategies.  
+
Commercial vaccines, both modified live and killed, have been licensed and have been effective in controlling outbreaks and preventing economic losses.
−
 
+
Recently, eradication of PRRS has been demonstrated to be possible on an individual farm basis. Methods such as whole herd depopulation-repopulation, test and removal, and herd closure have been documented as effective methods for eliminating PRRS virus from endemically infected herds. Unfortunately, a number of eradication efforts have failed due to the introduction of new isolates through unidentifiable routes.
−
Both modified live and killed vaccines are available. These are effective in controlling outbreaks and preventing economic losses.
 
−
 
 
−
{{Learning
 
−
|powerpoints = [[E-Lecture:Porcine Reproductive and Respiratory Syndrome]]
 
−
|literature search = [http://www.cabdirect.org/search.html?rowId=1&options1=AND&q1=Porcine+Reproductive+and+Respiratory+Syndrome&occuring1=title&rowId=2&options2=AND&q2=&occuring2=freetext&rowId=3&options3=AND&q3=&occuring3=freetext&publishedstart=2000&publishedend=yyyy&calendarInput=yyyy-mm-dd&la=any&it=any&show=all&x=41&y=7 Porcine Reproductive and Respiratory Syndrome publications since 2000]
 
−
|full text = [http://www.cabi.org/cabdirect/FullTextPDF/2006/20063124592.pdf ''' What's new with PRRS and SIV from the diagnostic lab.''' Rossow, K.; The North American Veterinary Conference, Gainesville, USA, Large animal. Proceedings of the North American Veterinary Conference, Volume 20, Orlando, Florida, USA, 7-11 January, 2006, 2006, pp 330-331, 1 ref.]
 
−
}}
 
  
 
==Links==
 
==Links==
  
 
*[http://www.thepigsite.com/diseaseinfo/97/porcine-reproductive-respiratory-syndrome-prrs The Pig Site: Porcine Reproductive and Respiratory Syndrome]
 
*[http://www.thepigsite.com/diseaseinfo/97/porcine-reproductive-respiratory-syndrome-prrs The Pig Site: Porcine Reproductive and Respiratory Syndrome]
−
*[http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/54100.htm The Merck Veterinary Manual - Porcine Reproductive and Respiratory Syndrome]
 
  
 
==References==
 
==References==
Line 63: Line 112:
 
#Merck & Co (2008) '''The Merck Veterinary Manual (Eight Edition)''', ''Merial''.
 
#Merck & Co (2008) '''The Merck Veterinary Manual (Eight Edition)''', ''Merial''.
 
#Done, S and White, M (2003) Porcine respiratory disease and complexes: the story to date. ''In Practice'', '''25(7)''', 410-417.
 
#Done, S and White, M (2003) Porcine respiratory disease and complexes: the story to date. ''In Practice'', '''25(7)''', 410-417.
−
#OIE (2008) [http://www.oie.int/downld/Doc_OIE/PRRS_guide_web_bulletin.pdf PRRS: the disease, its diagnosis, prevention and control.] ''Report of the OIE ad hoc Group on Porcine Reproductive and Respiratory Syndrome''.
+
#OIE (2008) PRRS: the disease, its diagnosis, prevention and control. ''Report of the OIE ad hoc Group on Porcine Reproductive and Respiratory Syndrome''.
−
#Laegreid, W (2006) Porcine Reproductive and Respiratory Syndrome. ''Proceedings of the Annual Meeting of the American College of Veterinary Pathologists and American Society for Veterinary Clinical Pathology''.
+
#Laegreid, W(2006) Porcine Reproductive and Respiratory Syndrome. ''Proceedings of the Annual Meeting of the American College of Veterinary Pathologists and American Society for Veterinary Clinical Pathology''.
−
#OIE (2011) [http://www.oie.int/doc/ged/D13986.PDF Porcine reproductive and respiratory syndrome-General Disease Information Sheets] ''Porcine Reproductive and Respiratory Syndrome''.
 
−
 
 
  
−
{{review}}
 
 
[[Category:Arteriviridae]]
 
[[Category:Arteriviridae]]
−
[[Category:Pig Viruses]][[Category:Respiratory Diseases - Pig]][[Category:Reproductive Diseases - Pig]]
+
[[Category:Pig]]
−
[[Category:Brian Aldridge reviewing]][[Category:Respiratory_Viral_Infections]]
+
[[Category:To_Do_-_Lizzie]]
 +
[[Category:Respiratory_Viral_Infections]]

Revision as of 18:49, 27 August 2010


 This article is still under construction.


Also known as: PRRS, blue eared pig disease

Description

Porcine reproductive and respiratory syndrome (PRRS) first appeared in the USA in 1987, although subsequent serologic evidence indicated that it had been circulating for some time prior to being recognised. The disease was seen in Europe in 1990 and reported in Humberside in the UK in 1991. PRRS is caused by an arterivirus, and leads to abortions, mortality and reduced growth, significantly impacting pig welfare and production economics worldwide.

Aetiology

The virus that causes PRRS is an aterivirus, within the Arteriviridae family. The arteriviruses are the only genus contained within the Arterivirdae family, and other members include the agent responsible for equine viral arteris and lactate dehydrogenase-elevating virus of mice. The viruses are host-specific and antigenically distinct, and establish persistent infections after invasion.

Arteriviruses are enveloped viruses of 45 to 80 nm in diameter with a spherical appearance due to the shape of the envelope. However, the icoshedral nucelocapsid that contains the positive-sense single-stranded RNA genome is actually icosohedral. The lipoprotein envelope has ring-like structures on its surface, but there are no gross surface spikes. Arteriviruses have a tropism for macrophages and endothelial cells, where they reproduce in the cytoplasm. The genome is 13Kb in length, and is infectious itself without the capsid or envelope.

Arteriviruses are inactivated by traetment with chloroform or ether. They are very resistant to freezing, althouth infectivity is in reduces as temperature risees.

Transmission and Epidemiology

The virus is spread by contact with infected pigs, and infection also be transmitted by infected semen. Fomites, such as contaminated needles, boots, overalls and transport vehicles can also transmit PRRS, as can certain species of insects including house flies. It has been thought that aerosol transmission may occur, particularly under conditions of high humidity, low temperatures, and low wind speeds, but this has been difficult to reproduce in the field and experimentally. Once infected, adult animals shed PRRS virus for up to 86 days, and younger, weaned pigs for 157 days. Virus excretion in semen can persist for up to 93 days after infection.

Transmission of PRRS virus to swine occurs most efficiently through direct contact. Blood, saliva, urine, milk and semen from infected animals have been shown to transmit PRRS virus to susceptible swine (Wills et al., 1997). Aerosol transmission may occur but has proven difficult to demonstrate even over relatively short distances (meters). Transmission by semen, either through natural service or artificial insemination is well established (Yaeger et al., 1993).

Pathogenesis

Once PRRS virus gains entry to the host, primary replication occurs in regional or mucosal macrophages such as in the tonsil and the nasal and respiratory epithelium. After around twelve hours, viraemia occurs and the virus is disseminated systemically. Secondary replication of PRRS virus occurs in monocyte/ macrophages in many tissues. While other cell types, including fibroblasts, endothelial cells, seminiferous epithelium and smooth muscle, have been reported to be PRRS virus-positive by in situ hybridization and/or immunohistochemistry, only cells of macrophage lineage have been unequivocally shown to support PRRS virus replication (Sur et al., 1997). In most swine, viremia resolves within 4-6 weeks following infection. However, PRRS virus can persist in lymphoid tissues, such as tonsil, for extended periods of time and be transmitted to susceptible swine (Zimmerman et al., 1992). PRRS virus has been detected for up to 225 days following inoculation, though the proportion of positive animals appears to drop off between 80 and 100 days (Albina et al., 1994; Allende et al., 2000; Wills et al., 2003). Persistence occurs in the face of neutralizing and other antibodies, and cell mediated immune responses, as indicated by gamma-interferon producing cells. Studies indicate that a low T cell response contributes to prolonged PRRS clearance (Xiao et al., 2004). Persistence is of epidemiologic significance since persistently infected animals are capable of transmitting PRRS virus to susceptible pigs via direct contact (Albina et al., 1994). This is likely an important mechanism by which PRRS virus is maintained within, and transmitted between, swine herds.

Diagnosis

Clinical Signs

There appears to be two distinct clinical phases to PRRS: reproductive failure and post-weaning respiratory diseases. When sows are infected, period of acute disease is seen which is characterised by lethargy, inappetance and pyrexia. Severe infections may also be associated with vestibular signs, and death of up to 10% of the sow stock can occur. Reproductive failure is results in increased numbers of stillborn piglets, mummified foetuses, abortions, premature farrowings and weak piglets. Lactating sows often display anorexia and agalactia, which leads to a rise in preweaning mortality. Signs are also seen in the piglets themselves, including a "thumping" respiratory pattern which on post-mortem examination is revealed to be associated with ls a severe, necrotising, interstitial pneumonia. Oedema may be seen around the eyes, and there is considerable variation in the size of piglets within litters.

Post-weaning performance is also affected, with daily live weight gain reduced by up 85% and increases in mortality by up to 25%. Lethargy and a loss of appetites is seen as in adult pigs, and weanling fail to thrive. Breathing is laboured and there may be obvious respiratory distress. The skin can appear red and blotchy, with a rough hair coat.

Outbreaks of the reproductive form of PRRS reportedly last between one and four months, depending on the initial health status on the pigs and the management of the outbreak. However, the post-weaning respiratory phase can become chronic, creating major production problems.

Secondary infections commonly occur. Bacteria previously reported to be associated with PRRS include Streptococcus suis, Escherichia coli, Salmonella choleraesuis, Haemophilus parasuis and Mycoplasma hyopneumoniae. Viral infections such as porcine respiratory coronavirus and swine influenza virus are also possible.

Laboratory Tests

The most commonly used tests are the ELISA or the indirect fluorescent antibody test. These tests measure IgG antibodies to PRRS virus. They cannot measure the level of immunity in an animal or predict whether the animal is a carrier. Titers are detected within 7-10 days after infection and can persist for up to 144 days. High titers may indicate recent exposure, and viral shedding may be occurring within the sampled population. Tests for PRRS virus include PCR, virus isolation, and immunohistochemistry. Recently, nucleic acid sequencing of the open reading frame 5 region of the virus has become commercially available, and has proved to be an excellent tool for epidemiologic investigations in the field to confirm similarity between isolates recovered from different sites.

Pathology

Gross necropsy lesions are minimal in the uncomplicated respiratory form of PRRS, but interstitial pneumonitis is a consistent histopathologic finding. There are no gross or histopathologjc lesions noted in aborted or stillborn fetuses.

Microscopic lesions of the fetus and placenta include umbilical vasculitis, interstitial pneumonia, myocarditis and pulmonary arteritis but are inconsistent and not diagnostic for PRRS (Lager and Ackermann, 1994; Lager and Halbur, 1996; Rossow et al., 1996b). There may be residual effects of PRRS virus infection in sows manifest as a reduced conception rate (Lowe et al., 2006).

Microscopically, interstitial pneumonia with mononuclear septal infiltration, type 2 pneumonocyte proliferation and accumulations of necrotic debris and macrophages in alveolar spaces is evident. Follicular hyperplasia and necrosis may be present in lymph nodes. Nonsuppurative meningoencephalitis and choroiditis is commonly present, with highly variable severity

Control

Currently, there are no effective treatment programs for acute PRRS. Attempts to reduce fever using NSAID (aspirin) or appetite stimulants (B vitamins) appear to have minimal benefit. The use of antibiotics or autogenous bacterins to reduce the effects of opportunistic bacterial pathogens have also been reported; however, results have been mixed. Prevention of infection appears to be the primary means of control. Understanding the PRRS status of replacement gilts and boars, as well as proper isolation and acclimatization of incoming stock are critical measures to prevent viral introduction. Pigs should be retested on arrival at the isolation facility and 45-60 days later, before entry to the herd. Elimination of existing infection by multisite production and segregated early weaning has also been described. While these strategies have had some success, the longterm risks of reinfection appear high. Prevention of viral spread by nursery depopulation has been described. This is successful when virus transmission is not occurring in the sow herd (usually 12-18 mo after initial outbreak), but the nurseries and growing/finishing pigs are still infected. All nursery pigs are removed from the farm to be finished elsewhere. The nurseries are then aggressively washed and disinfected and left empty for 7-14 days, after which they can be used normally. The technique has successfully eliminated PRRS virus from several herds, in which pigs have remained seronegative (for >1 yr) to market age, and production in the nurseries has improved, both in growth rate and mortality. Commercial vaccines, both modified live and killed, have been licensed and have been effective in controlling outbreaks and preventing economic losses. Recently, eradication of PRRS has been demonstrated to be possible on an individual farm basis. Methods such as whole herd depopulation-repopulation, test and removal, and herd closure have been documented as effective methods for eliminating PRRS virus from endemically infected herds. Unfortunately, a number of eradication efforts have failed due to the introduction of new isolates through unidentifiable routes.

Links

References

  1. Wise, D J and Carter, G R (2005) A Concise Review of Veterinary Virology, IVIS.
  2. Merck & Co (2008) The Merck Veterinary Manual (Eight Edition), Merial.
  3. Done, S and White, M (2003) Porcine respiratory disease and complexes: the story to date. In Practice, 25(7), 410-417.
  4. OIE (2008) PRRS: the disease, its diagnosis, prevention and control. Report of the OIE ad hoc Group on Porcine Reproductive and Respiratory Syndrome.
  5. Laegreid, W(2006) Porcine Reproductive and Respiratory Syndrome. Proceedings of the Annual Meeting of the American College of Veterinary Pathologists and American Society for Veterinary Clinical Pathology.
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