Science Page: Within farm PRRS time-to-stability differences in sow farms in the Midwest

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing a report by the MSHMP team regarding PRRS time-to-stability differences in sow farms.


  • There is significant within farm PRRS time-to-stability variation.
  • Several factors contribute to PRRS time-to-stability variability; however, there is still a significant amount of unexplained variability.
  • The role of within farm management practices and internal biosecurity measures should be further explored.


Porcine reproductive and respiratory syndrome (PRRS) stability is reached when no evidence of infection is observed in wean-age piglets. Sample size to detect PRRS virus in wean-age piglets usually involves blood sampling of 30 piglets, at least four times, 30 days apart (Holtkamp et al., 2011). The cumulative time from the intervention (i.e. whole herd exposure, herd closure) to PRRS stability is usually referred to as time-to-stability (TTS).

Here we summarize differences in TTS in MSHMP participating farms located in the Midwest that have had at least two PRRS outbreaks.


Six systems that are similar in the way they test to classify a herd as stable were selected for inclusion in the study. PRRS outbreaks reported from 2011 to 2017 were used for analysis.

TTS was defined as the time period from the date of outbreak reporting to the date when PRRS stability was reported (last consecutive negative PCR result). To assess the variability in TTS, only farms that had at least two PRRS outbreaks were selected.


Overall, 133 PRRS outbreaks in 53 farms were recorded withtwo, three, four and five outbreaks in 35, 11, 5, 2 farms, respectively. The median TTS standard deviation of PRRS outbreaks within the same farm was 12 weeks (minimum = 0 weeks, maximum=88 weeks).

After accounting for the effect of the intervention using MLV or FVI, the RFLP pattern of the virus associated with the outbreak and previous PRRS outbreaks in the farm, the PRRS time-to-stability correlation of outbreaks in the same farm and system was only 1.2%.

In other words, TTS of two given outbreaks in the same farm were not correlated indicating that TTS within farm is highly variable.


There is a high TTS variability after a PRRS outbreak within the same farm that is not accounted for by the effect of the intervention used, the virus (i.e RFLP), previous PRRS outbreaks in the farm and system.

Science Page: Salmonella monophasic harboring plasmid mediated resistance genes to enrofloxacin and ceftiofur is expanding in swine in the Midwest

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing a report by Dr. Elnekave and the STEMMA lab regarding a multiresistant clad of Salmonella isolated in the Midwest.

Key Points:

  • A genetically distinct clade of Salmonella 4,[5],12:i:- (also referred to as S. monophasic), harboring multiple antimicrobial resistance genes (including to ampicillin,streptomycin, sulfonamides, and tetracyclines) became the predominant S. monophasic type in swine in the U.S. during 2014-2016.
  • Phenotypic resistance to enrofloxacin (fluoroquinolone) and ceftiofur (3rd generation cephalosporin) was present in a proportion these isolates, and whole genome sequencing revealed the presence of the plasmid-mediated genes.
  • These plasmid-mediated resistance genes could potentially transfer horizontally to other microorganisms and augment the problem of antimicrobial resistance to these critically important antibiotics.

S. monophasic emerged globally in the recent years and pig products have been identified as a source in some foodborne outbreaks. The prevalence of S. monophasic, and phenotypic resistance (minimum inhibitory concentration (MIC) above the cut-off value for this bacteria) to enrofloxacin increased in swine clinical samples in the Midwest during 2006 and 2016.

During this period, injectable enrofloxacin was approved by the Food and Drug Administration (FDA) for treatment of swine respiratory disease and colibacillosis in piglets (in 2008 and 2014, respectively); therefore, the objective of the study was to characterize the S. monophasic in swine in the U.S Midwest.

Salmonella genotypic profile
Maximum likelihood tree of S. monophasic collected in the U.S. and Europe during 1991-2016. Tip colors indicate of the period of sample collection: 1991-2009 (red), 2010-2013 (green), 2014-2016 (turquoise) and not available (n.a.; purple). The location of samples collection is indicated by the background color: Europe (red), U.S. (blue) and not available (green). ASSUT= presence of resistance genes against ampicillin, streptomycin, sulfonamides, and tetracyclines. qnr genes – conferring resistance to quinolones.

We used whole genome sequencing to compare S. monophasic isolates collected from livestock in the Midwest with isolates collected from different sources in the U.S. and Europe. We then determined the antimicrobial resistance genotypes and presence of other virulence factors that could help to explain the emergence of this variant.

Salmonella monophasic formed two main genetic clades regardless of source and geographical origin (Figure 1). Most (84%) isolates recovered in the U.S. during 2014-2016, including 50 isolates (out of 51) originating mainly from swine in the Midwest, were part of an emerging clade genotypically resistant to ampicillin, streptomycin, sulphonamides and tetracyclines. In the Midwest samples, phenotypic resistance to enrofloxacin (11 out of 50; 22%) and ceftiofur (9 out of 50; 18%) was found in conjunction with plasmid-mediated resistance genes. This is of particular concern because fluoroquinolones and 3rd generation cephalosporins are often used to treat invasive Salmonella infections in people. Furthermore, because the genes were plasmid borne there is greater likelihood for horizontal transfer of these genes to other bacterial strains.

Science Page: Transmission and survivability of African swine fever virus

Wild boar

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing a literature review on ASF virus transmission and survivability created by Carles Vilalta. Other recent posts on African Swine Fever can be found on this blog.


  • New introductions of ASF to free areas of the disease are usually by uncooked pork fed to pigs.
  • Virus can be inactivated with temperature and low pH.
  • Survivor animals may play a role in the transmission and persistence of the disease.

Further outbreaks of African Swine Fever virus (ASFV) were reported last week in China several miles away from what is thought to be the first outbreak. This geographic dispersal leads us to think about dissemination mechanisms within the country and between countries.


Infected animals will go through a viremic phase and can shed the virus through nasal secretions, feces and urine. Therefore, the main transmission route is oral-nasal, as pigs can be exposed to ASF positive secretions or tissues (i.e. pork products). Indirect transmission can also occur by exposure to contaminated fomites. This virus can also be transmitted by ticks. This vector-borne route becomes relevant when the wild boar
population is present and moves across regions and countries. The common introduction route into ASF free regions is usually through positive pigs transported into the area, or contaminated pork products that are fed to other pigs. ASFV has also been detected in air samples; however, airborne transmission is considered a secondary route of transmission due to the high virus load needed.


Inactivation and persistence

Although ASFV is highly resistant, the virus can be inactivated at pH < 4 and pH >11. Survivability outside the host is heavily related to temperature. For instance, the infectious half-life in urine and feces can range from 3 to 15 days and 4 to 8 days at 37°C and 4°C, respectively. The virus may persist for several weeks or months in frozen, fresh, or uncooked pork, as well as in salted dried pork products. In contrast, ASFV is inactivated at high temperatures (i.e. 70°C cooked or canned hams) and in cured or processed products such as Spanish cured pork products after day 122–140 of curing. Pigs can become persistently infected and the virus can stay viable in their carcasses for up to six months. Therefore, infected carcasses represent a risk to other pigs. More recently, an investigation simulating a trans-Atlantic shipping of ASFV contaminated feed ingredients from Europe proved that viable virus can be recovered after 30 days.

The role of survivor pigs

ASFV recovered and sub-clinically infected pigs become a source of virus to other pigs. This plays an important role in disease transmission and persistence in endemic areas as well as becoming one of the most important routes of transmission into disease-free zones. In-vivo experiments have revealed an infectious period of moderately virulent virus isolates ranging from 20 to 40 days. In another in-vivo transmission study, pigs that had been exposed to ASFV 90 days prior were commingled with naive pigs and the virus was transmitted to naive pigs.

Serological field studies performed in positive regions of Brazil, the Iberian Peninsula, East Africa, Kenya and Uganda revealed that the there was a very low percentage of seropositive animals one year after the outbreak. It was hypothesized that those few seropositive pigs were still carriers and could have been responsible of some of the newer outbreaks.


ASF has a complex epidemiology with different routes of transmission that can involve animals and ticks as direct transmission, and contaminated clothes, tools, and surfaces as indirect transmission. Thus, early detection and intervention of the diseases are key to containing disease spread in absence of an effective vaccine.

Science Page: An overview of African Swine Fever

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing a fact sheet regarding African Swine Fever, followed by a map of the current outbreak situation.


  • Recent outbreak of African swine fever in china may have influence in the global trade market of pork during the following months.
  • Prevention focusing on imports and international movements is the best strategy in absence of a vaccine.
  • Rapid diagnostics and culling are key components of an effective eradication.

After the recent outbreak of African swine fever in China and the implications for international trade, swine health, and production we thought it would be a good idea to review the characteristics of the disease.


African swine fever (ASF) ranks third as a potential risk that could threaten the US swine industry in the swine disease matrix, from the Swine Health Information Center (SHIC). ASF is a highly contagious disease that causes hemorrhages in pigs. It is caused by a DNA virus from the Asfaviridae family. It affects pigs, warthogs, and European and American wild boars.


Clinical signs vary depending on the virulence of the virus. Severe infections can cause up to 100% mortality in 2Ͳ7 days with high fever as the main characteristic. Other relevant clinical signs are bleeding (nose or rectum), diarrhea, redness of ear, abdomen, or leg skin, respiratory disorder, loss of appetite and depression. Moderately virulent strains cause less intense symptoms as the beforehand mentioned but mortality can still range between 30-70%. ASF can also be found in a chronic form with loss of weight, discontinuous fever, respiratory signs, skin ulcers and arthritis.


Appearance of clinical signs and high mortality rates may trigger suspicion of ASF but confirmation has to be done through laboratory test. Differential diagnosis includes classical swine fever (CSF), high pathogenic porcine reproductive and respiratory syndrome (HPͲPRRS), swine erysipelas, septicemic salmonellosis and porcine dermatitis nephropathy syndrome (PDNS).
Diagnostic techniques include detection of antibodies in serum or the etiologic agent in different tissues (blood, spleen, lymph nodes, tonsil and kidney). Isolation, PCR,Haemadsorption test and Antigen detection by fluorescent antibody test are the techniques for the virus identification.


The warthog is the main reservoir of the disease and it transmits form pig to pig through a soft tick. Wild boars and other wild pigs can also carry and spread the disease. Domestic pigs usually become infected through direct contact with sick pigs or eating pig meat containing ASF virus. Also indirect spread can occur through contaminated vehicles, premises, equipment or clothes.


No treatment or vaccines are available at this point. Therefore the best strategies are implement strategies to avoid the introduction of the virus is to focus on import policies and movement of vehicles and people from infected countries. Rapid diagnosis and culling are the key features of a successful eradication program along with surveillance, movement controls, cleaning, and disinfection of the affected premises.

OIE African Swine Fever map.png
Map of the current ASF outbreaks. Source: OIE


Since the disease landed in Georgia in 2007 ASF has made steady progress through Europe.Latvia, Lithuania, Poland and more recently Hungary are the last countries that reported the presence of the disease in Europe. The outbreak occurred in one of the most swine dense regions China, relatively close to the Korean peninsula. Three other cases have been reported to date. The effects of the outbreak will probably shape the global
trade of pork in the following months.


Science Page: An Overview of Porcine Astrovirus

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing a disease sheet on porcine astrovirus by Drs. Arruda and Schwartz.

Key points:

  • Further research is needed in all areas of the virus in order to better understand, treat, and prevent Astrovirus.
  • Astrovirus is a public health concern in humans as it is implicated in foodborne illnesses and has zoonotic potential.
  • Porcine Astrovirus may play a role in enteric disease, and has been associated with neurological disease.

Porcine Astrovirus (PoAstV)  is a nonenveloped RNA virus with 5 different strains present in U.S herds. It has been detected in both healthy and diseased pigs, so more research is needed to determine the clinical implications of a PoAstV infection. Recently a U.S swine production system reported PoAstV-associated neurological disease. In the sow farm 100% of pigs affected with disease died, while in the growing-finishing farms case-mortality rate was 75%. Signs exhibited by affected animals included paralysis, ataxia, paresis, and knuckling, which eventually progressed into lateral recumbency.


Scientific publications relating to Porcine Astrovirus are rare. The majority of information, however, supports fecal-oral as the main route of transmission. Some reports have shown PoAstV to retain infectivity in ground water for extended periods of time and can survive up to 3 hours in water with a p.H of 4.0. There is currently no vaccine available for this disease. The large antigenic diversity and high mutation rate are the biggest challenge for vaccine development. Diagnosis is typically made via PCR.

The major concern with Astrovirus is the zoonotic potential. Human Astrovirus is easily transmitted through contaminated food and water and causes moderate gastroenteritis in infants. Human-to-pig transmission is suspected due to the detection of human-porcine recombinant viruses. Pig-to-human zoonosis has not been reported, but Astroviruses can rapidly mutate, so it may be only a matter of time before a zoonotic strain emerges.

Further research into pathogenesis and vaccine development is crucial to prepare for a possible zoonotic outbreak. 

— Blog post written by Joseph Thurston.

Science Page: Actinobacillus pleuropneumoniae: a case of suspected lateral transmission (Part 2: outbreak investigation)

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing the second part of a report regarding an Actinobacillus pleuropneumoniae outbreak in the Midwest, across 3 systems and 5 farms.

If you missed part 1, you can find it here.

Key Points:

  • Communication between veterinarians and farm managers can help unravel patterns that might seem unique in one system.
  • Even though the source of APP was not determined outbreak investigation can help to find common links between sources.

A series of Actinobacillus pleuropneumoniae (APP) outbreaks involving five farms belonging to three different production companies were reported. Serotype 8 was confirmed as the source of the clinical signs in all the cases. The outbreak started with the two southernmost located farms (Company A Farm 1 and Company B Farm), followed by Company C (Farm 1) four weeks later. The distance among these growing pig sites ranged from 0.6 to 8.3 miles and the region where they are located can be considered as a high hog density area (Picture 1).

APP farm locations
Map of the farms involved in the APP outbreak

Common links between several sites were revealed after conversations among veterinarians and production managers. The main transmission route for this bacterium is introducing APP carrier pigs. In this case, it can be easily ruled out as these are sites that flow independently.

Other possibilities include indirect transmission through fomites and aerosol. Although these production companies do not share employees or tools they do have a common link in that some did share the same rendering company which could have been servicing other sites that were APP positive. As for manure removal, companies do not use the same manure removal company. One company did have the same individual doing the manure removal procedure at one site while breaking and then proceeded to the next one. Airborne transmission has been suggested as another possibility and after preliminary wind direction analyses during the outbreak dates it was inconclusive.

Science Page: Actinobacillus pleuropneumoniae: a case of suspected lateral transmission (Part 1: diagnostics)

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing the first part of a report regarding an Actinobacillus pleuropneumoniae outbreak in the Midwest, across 3 systems and 5 farms.

Key points:

  • Actinobacillus pleuropneumoniae can significantly contribute to increase the costs of the growing period by increasing mortality and antimicrobial treatments.
  • All-in all-out of the affected sites accompanied with standard cleaning and disinfection procedures may suffice to ensure elimination of the bacteria.

A series of outbreaks with a sudden increase in mortality in growing pig herds located in Northwest Iowa were reported beginning in late October and early November.

Five wean-to-finish farms belonging to three different production companies were affected by a sudden onset (within 12-36 hours) of lethargy, respiratory distress and septicemia across hundreds of pigs. Clinical signs quickly spread through the sites and mortality rapidly increased with pigs having foamy bloody nasal discharge. Post-mortem examination revealed acute pleuritis and severe necrotizing bronchopneumonia. In all cases, Actinobacillus pleuropneumoniae (APP) was cultured from multiple sections of fresh lung. The APP isolate from each case was submitted to the University of Montreal for serotyping and it was confirmed to be serotype 8.

Each veterinarian intervened by rapidly mass injecting the growing herd with antibiotics suggested from the antibiotic susceptibility test together with either in-feed or water medication. Mortality rates for each site are shown on the figure below.

AAP associated mortality rates

The estimated cost of APP for each of these outbreaks was $30-$35/pig, considering treatment costs and a $2/pig cost for each 1% mortality.

Each site was completely emptied of pigs, washed and disinfected following a standard procedure. Sites were reloaded with new groups of pigs that have remained free of clinical signs associated with APP.