Science Page: Why PRRS elimination doesn’t work in some herds

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 an article by Dr. Amber Stricker from Suidae Health and Production, published in

“Over the years, there’s been considerable progress in the development of strategies aimed at eliminating porcine respiratory and reproductive syndrome virus (PRRSV). I define successful PRRSV elimination as the absence of clinical disease in the breeding herd and, more importantly, the absence of the vertical transmission of virus to weaned pigs. Unfortunately, successful PRRS elimination isn’t always achieved in some herds, and I have several experiences that may help answer why.”
Dr. Stricker then compiles six reasons that, in her experience, led to a failure in PRRS elimination:
  • No break in disease cycle or insufficient herd closure
  • Incomplete exposure
  • Compliance problems
  • Holding back pigs
  • New virus introduction
  • Insufficient diagnostics

Take a look at the full article to read more about each of those facors.

A new multidrug resistant Salmonella enterica serotype found in Midwestern swine

Text reproduced from the Center for Infectious Disease Research and Policy (CIDRAP)

A new study by Dr. Julio Alvarez‘s team from the STEMMA laboratory, published  in Clinical infectious Diseases suggests that a Salmonella strain circulating in pigs in the US Midwest is part of an emerging clade from Europe that is resistant to multiple antibiotics and may pose a public health risk.

The strain, Salmonella 4,[5],12:i:-, causes many foodborne disease outbreaks mostly tied to pigs and pork products and is expanding in the United States, according to the report by researchers from Minnesota and the United Kingdom.

The team used whole-genome sequencing to assess the relatedness of 659 S 4,[5],12:i:- isolates and 325 S Typhimurium isolates from various sources and locations in the United States and Europe. They also searched for resistance genes and other virulence factors and, for 50 livestock isolates and 22 human isolates, determined the antimicrobial resistance phenotypes.

The researchers found that the S 4,[5],12:i:- isolates fell into two main clades, regardless of their host or place of origin. Eighty-four percent of the US isolates recovered from 2014 through 2016, including nearly all those from pigs in the Midwest, were part of an emerging clade. This clade carried multiple genetic markers for antimicrobial resistance, including resistance to ampicillin, streptomycin, sulphonamides, and tetracyclines.

In addition, phenotypic (actual) resistance to enrofloxacin and ceftiofur was found in 11 of the 50 tested livestock isolates and 9 of the 22 human isolates. This was accompanied by plasmid-mediated resistance genes.

The authors conclude that S 4,[5],12:i:- strains circulating in Midwestern swine herds “are likely part of an emerging multidrug resistant clade first reported in Europe, and can carry plasmid-mediated resistance genes that may be transmitted horizontally to other bacteria and thus could represent a public-health concern.”

Click on the banner below to access the entire article.

salmonella enterica multidru resistant Alvarez 2017


Salmonella 4,[5],12:i:-, a worldwide emerging pathogen that causes many foodborne outbreaks mostly attributed to pig and pig products, is expanding in the U.S

Whole genome sequencing was applied to conduct multiple comparisons of 659 S. 4,[5],12:i:- and 325 S. Typhimurium from different sources and locations (i.e. U.S. and Europe) to assess their genetic heterogeneity, with a focus on strains recovered from swine in the U.S. Midwest. In addition, presence of resistance genes and other virulence factors was detected and the antimicrobial resistance phenotype of 50 and 22 isolates of livestock and human origin, respectively, was determined.

The S. 4,5,12:i:- strains formed two main clades regardless of their source and geographical origin. Most (84%) of the U.S. isolates recovered in 2014–2016, including those (50/51) recovered from swine in the U.S. Midwest, were part of an emerging clade. In this clade, multiple genotypic resistance determinants were predominant, including resistance against ampicillin, streptomycin, sulphonamides and tetracyclines (ASSuT). Phenotypic resistance to enrofloxacin (11/50) and ceftiofur (9/50) was found in conjunction with the presence of plasmid-mediated resistance genes (qnrB19/qnrB2/qnrS1 and blaCMY-2/blaSHV-12, respectively). Also, higher similarity was found between S. 4,[5],12:i:- from the emerging clade and S. Typhimurium from Europe than with S. Typhimurium from the U.S.

Salmonella 4,[5],12:i:- currently circulating in swine in the U.S. Midwest are likely part of an emerging multidrug resistant clade first reported in Europe, and can carry plasmid-mediated resistance genes that may be transmitted horizontally to other bacteria and thus could represent a public-health concern.

Science Page: Natural and experimentally-induced Senecavirus A infections in boars

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 study from Dr. Matt Sturos from the University of Minnesota, Veterinary Diagnostic Laboratory regarding Senecavirus A in boars.

Key points

  • Naturally-infected boars have been documented to shed Senecavirus A (SVA) RNA in semen for up to three months after exhibiting vesicular disease.
  • Experimentally-infected boars shed SVA RNA in semen for up to three weeks post-inoculation.
  • The majority of experimentally-infected boars did not exhibit clinical signs or develop apparent lesions.
Senecavirus A in boars
Testis of boar naturally-infected with Senecavirus A. Bright red areas indicate positive signal for SVA by in-situ hybridization.

“This update shows that SVA RNA is shed in semen from both naturally-infected and experimentally-inoculated boars. The prolonged shedding of viral RNA in semen and the presence of SVA RNA in the testes and tonsils of the naturally-infected boars for up to three months are concerning findings and raises the possibility of persistent infection in boars. While the duration of shedding in semen for the experimentally-infected boars was considerably shorter than for the naturally-infected boars, the fact that all contemporary-strain boars had PCR-positive semen on at least one collection indicate that shedding in semen is a repeatable phenomenon and shedding occurred in some boars which did not exhibit clinical signs or develop vesicular lesions. It is currently unknown whether semen from infected boars can serve as a source of infection if used to inseminate susceptible females.”


Longitudinal study of Staphylococcus aureus colonization and infection in a cohort of swine veterinarians in the United States

In this study conducted by Dr. Sun during her PhD under the direction of Dr. Peter Davies, 66 swine veterinarians were sampled via nasal swabs every month for 18 months. Swabs were first cultured to detect Staphylococcus aureus and to determine the strains susceptibility (or resistance) to methicillin, and then were characterized by spa typing and MultiLocus Sequence typing (MLST). Click on the banner below to read the publication in open-access.

Sun longitudinal study Saureus in swine veterinarians 2017

S. aureus monthly prevalence ranged from 58% to 82% and MRSA ranged between 6% and 15%. Those results were higher than the ones for the US population. 52% of the swine veterinarians were classified as persistent carriers, meaning that their samples came back positive for S.aureus at least 80% of the time, 47% were intermittent carriers. 21% of the veterinarians were true persistent carriers meaning that the same spa type of S.aureus was recovered each time the sample was positive.

Staphylococcus aureus positive samples repartition
Number of veterinarians based on the carrier index (number of samples positive for S.aureus divided by number of samples taken per veterinarian)

Whole genome sequencing showed that strains isolated at the beginning and at the end of the study were similar genetically. Comparing spa types, 83% of all isolates belonged to a type also present in swine.



People working with pigs are at elevated risk of harboring methicillin resistant S. aureus (MRSA) in their nose, which is attributable to occupational exposure to animals harboring livestock adapted S. aureus. To obtain insight into the biological nature of occupationally related nasal culture positivity, we conducted a longitudinal study of 66 swine veterinarians in the USA.


The study cohort resided in 15 US states and worked predominantly with swine. Monthly for 18 months, participants self-collected nasal swabs and completed a survey to report recent exposure to pigs and other animals; the occurrence of work related injuries; and any relevant health events such as skin and soft tissue infections or confirmed staphylococcal infections. Nasal swabs were cultured using selective methods to determine the presence of MRSA and methicillin susceptible S. aureus (MSSA), and isolates were characterized by spa typing and MLST.


Prevalences of S. aureus (64%, monthly range from 58 to 82%) and MRSA (9.5%; monthly range from 6 to15%) were higher than reported for the US population (30% and 1.5% respectively). Predominant spa types were t034 (ST398, 37%), t002 (ST5, 17%) and t337 (ST9/ST398 13%), a distribution similar to that found in a concurrent study in pigs in the USA. Veterinarians were classified into three groups: Persistent carriers (PC, 52%), Intermittent carriers (IC, 47%) and Non-carriers (NC, 1%). Persistent carriage of a single spa type was observed in 14 (21%) of participants, and paired (first and last) isolates from PC subjects had minor genetic differences. Swabs from PC veterinarians carried higher numbers of S. aureus. Among IC veterinarians, culture positivity was significantly associated with recent contact with pigs.


Exposure to pigs did not lead to prolonged colonization in most subjects, and the higher numbers of S. aureus in PC subjects suggests that unknown host factors may determine the likelihood of prolonged colonization by S. aureus of livestock origin. Exposure to S. aureus and persistent colonization of swine veterinarians was common but rarely associated with S. aureus disease.

Science Page: Multiple influenza viruses circulate in growing pigs during epidemic events

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 study from Dr. Andres Diaz and the Torremorell lab.

Key points

  • The diversity of influenza A viruses in growing pigs is dynamic
  • Influenza A viruses can replicate as a swarm of viruses that are identical, closely related to each other (>99%), or clearly distinct (H1 vs. H3 subtypes)
  • Influenza A viruses of the same genotype can re-infect pigs within a short period of time.
132 3-week old piglets selected at weaning and placed in a wean-to-finish farm were sampled weekly for 15 weeks (n=2080 samples). Samples were tested by RT-PCR and the complete genome of influenza was obtained from 93 samples using next generation sequencing.
Two epidemic waves of IAV infection were detected with 3 distinct viral groups (VG swarms) found (VG1, VG2 and VG3). An H1 gamma (VG1) dominated the first outbreak, an H3 (VG3) dominated the second outbreak and an H1 beta (VG2) was only recovered when none of the two other viruses dominated.
The complete version of this study can be found online in open-access.

Influenza Herd-Level Prevalence and Seasonality in Breed-to-Wean Pig Farms in the Midwestern United States

The Torremorell lab is continuing to explore swine influenza epidemiology in this recent publication from Dr. Fabian Chamba Pardo in Frontiers in Veterinary Science. After showing that multiple genome constellations of similar and distinct influenza viruses co-circulate in pigs, the group is now presenting new data about influenza herd-level prevalence in the Midwest, and how it is influenced by seasons. Click on the banner below to read the entire research article.

Influenza seasonal prevalence Midwest herds Chamba 2017

60 sow farms from a single Midwestern production system were enrolled in this study. Between one and seven oral fluid samples were collected at each farm weekly and meteorological data (air temperature and relative humidity) was compiled from stations located from the farms.

Swine herd level prevalence Chamba 201728% of submissions had at least one influenza positive result. All farms tested positive at least once during study period. Herd-level prevalence ranged from 7% to 57% as show in the figure above. Prevalence was low in summer, rose during fall, and peaked twice in both early winter (December) and late spring (May). August was the month with the lowest prevalence. Influenza herd-level prevalence was higher when both mean outdoor air temperature and air humidity were lower.

The most common clades identified were H1 delta 1, H1 gamma 1, and clusters H3 IV A  and H3 IV B. Furthermore, 21% of the farms had 3 different influenza genetic clades circulating during the study period and 18% had 2.


Influenza is a costly disease for pig producers and understanding its epidemiology is critical to control it. In this study, we aimed to estimate the herd-level prevalence and seasonality of influenza in breed-to-wean pig farms, evaluate the correlation between influenza herd-level prevalence and meteorological conditions, and characterize influenza genetic diversity over time. A cohort of 34 breed-to-wean farms with monthly influenza status obtained over a 5-year period in piglets prior to wean was selected. A farm was considered positive in a given month if at least one oral fluid tested influenza positive by reverse transcriptase polymerase chain reaction. Influenza seasonality was assessed combining autoregressive integrated moving average (ARIMA) models with trigonometric functions as covariates. Meteorological conditions were gathered from local land-based weather stations, monthly aggregated and correlated with influenza herd-level prevalence. Influenza herd-level prevalence had a median of 28% with a range from 7 to 57% and followed a cyclical pattern with levels increasing during fall, peaking in both early winter (December) and late spring (May), and decreasing in summer. Influenza herd-level prevalence was correlated with mean outdoor air absolute humidity (AH) and temperature. Influenza genetic diversity was substantial over time with influenza isolates belonging to 10 distinct clades from which H1 delta 1 and H1 gamma 1 were the most common. Twenty-one percent of farms had three different clades co-circulating over time, 18% of farms had two clades, and 41% of farms had one clade. In summary, our study showed that influenza had a cyclical pattern explained in part by air AH and temperature changes over time, and highlighted the importance of active surveillance to identify high-risk periods when strategic control measures for influenza could be implemented.

Science Page: Uterine prolapses trend in production sow herds

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 study from Dr. Carmen Alonso and collaborators at Elanco.

Objectives of the study:

The objectives of the study, were: 1) to analyze the trends in prolapses of sows from 2012 to 2016, and 2) to evaluate the role of management practices, production parameters, and PRRS and PED disease status as covariates in the trend analysis of uterine sow prolapses.

Key points:

  • Uterine prolapse primarily affects sows around parturition and is still defined by an uncertain list of verified etiologies.
  • Since early 2013, swine companies have been experiencing an increase in the incidence of uterine prolapses in their herds.
  • Understanding the trends and potential risk factors would be crucial to improve the economics and welfare of the affected sow farms.

Uterine prolapses significant variables Alonso Results from this study indicate that the percentage of prolapsed sows has consistently increased every year (significant from 2014-2016) as a percentage of total deaths with the incidence being higher during the winter months and the lowest during the summer months. Total born, the use of toxin binder, assistance during farrowing, and PED health status had an association to sow deaths with prolapse per sows farrowed.

Click here to read the entire report on Uterine prolapses trends.