Ontario Animal Health Network (OAHN) Poultry Expert Network Quarterly Producer Report

Runting Stunting Syndrome (RSS): Why is it so difficult to diagnose?

Since early 2016 there have been chicken broiler flocks identified with clinical signs including low body weights, inability to meet 7-day target weights, small chick size, and poor uniformity (Figure 1). Postmortem findings primarily involve thin-walled, dilated duodenum and jejunum filled with watery content (Figure 2). Pancreatic atrophy may also be detected. Samples sent for further testing in the US identified astrovirus in the affected small intestines leading to the conclusion that this clinical presentation could be defined as Runting Stunting Syndrome (RSS). However, it should be noted that not all flocks showing clinical signs of runting and stunting will have RSS. Chickens are bred to grow extremely fast, therefore, any management factor(s) or early disease could potentially cause a general clinical sign of runting and stunting in a flock. Other rule outs include bacterial infection, inclusion body hepatitis, reovirus, RSS, stress, etc. Diagnosis of RSS in different geographic regions could involve different combinations of viruses and other factors, therefore, diagnosis relies on postmortem lesions and histopathology.

dead chickens from runting and stunting
Figure 1: Stunting in broilers (Photo: Dr. Jess Walkey)
Inside of dead chicken from runting and stunting
Figure 2: Thin walled, watery intestines (Photo: Dr. Jess Walkery)

On histopathology, depending on when tissues are collected, lesions may be well developed and clear to diagnose (Figures 3 and 4). However, many times the lesions are either early or late in development, making diagnosis difficult. If the birds are less than 11 days old, this may require resubmission of tissues in 7-10 days to allow the lesions time to develop. PCRs on intestinal content are difficult to interpret because the majority of birds, with and without the clinical signs of runting and stunting, will normally become positive for a number of viruses by the end of grow out. These viruses include reovirus, rotavirus, chicken astrovirus, parvovirus, avian nephritis virus, etc. Since the birds will likely be exposed to and become positive for these viruses, it is impossible to determine if they are involved in development of disease.

Normal small intestine
Figure 3: Normal small intestine
Cystic crypt consistent with RSS (Photo: Dr. Emily Martin)
Figure 4: Cystic crypt consistent with RSS (Photo: Dr. Emily Martin)

The RSS case definition, as defined by the Ontario Industry Broiler Health (IBH) working group has the following main points:

  • Underperforming broiler chickens or uneven flocks between 5 and 21 days of age.
  • Postmortem lesions can include dilated, thin-walled, watery, ballooned intestines.
  • When there are no visible gross lesions, other production signs or clinical presentations may be reported, such as: diarrhea, underperforming birds (uneven weight) and pasty vent.
  • Birds are submitted for histopathology between 8 – 11 days. If no lesions identified from the first submission, a second submission is advised 7 to 10 days later.
  • Other common diseases should be ruled out: coccidiosis (E. acervulina, E. maxima), Infectious Bursal disease (IBDV), Reovirus (ARV), Infectious Bronchitis virus (IBV), and Chicken anemia virus (CAV).

Submissions to the AHL of confirmed runting and stunting syndrome caused by astrovirus were increased this quarter.   If you think your flock has clinical signs consistent with Runting Stunting Syndrome, please contact your veterinarian for assistance.

OMAFRA Avian Influenza industry update

The new HPAI variant, H5N8 has been sweeping across Western Siberia and Europe causing the death of over 6 million birds. Please work with your producers to review the key biosecurity measures to help with reducing the risk of infection in their flock as outlined at http://www.omafra.gov.on.ca/english/food/inspection/ahw/influenzacommercial.html.

Poultry Veterinarian Survey Highlights – Q1


Infectious laryngotracheitis cases were notably reported as increased this quarter due to the outbreak in commercial poultry in the Niagara region (Veterinary Advisory: Update on ILT in Ontario – Understanding the Risks for Infection in Poultry (gov.on.ca). Early systemic bacterial infections (<14 days) were stable to increased this quarter as reported by practitioners. Escherichia coli and E. coli with Enterococcus cecorum were identified by AHL. One case with E.coli, E. cecorum and Clostridium perfringens identified was reported. Late systemic bacterial infections (>14 days) were stable to increased this quarter with E. coli and E. coli                                                                                  with Enterococcus cecorum as the main causes. AHL reported increased numbers of cases which is not unusual for the winter season. Reovirus-associated lameness cases were stable to increased this quarter as reported both by practitioners and AHL. While variant D is still the most common variant being diagnosed at AHL, a number of other Ontario variants were also identified. The AHL also reported seeing fewer suspicious cases, diagnosed by histopathology only, after an increased number of cases in Q4 of 2020. Escherichia coli and E. cecorum were identified as the main causes of bacterial lameness this quarter with most practitioners reporting stable to increased prevalence. The number of cases of nutritional and developmental lameness was stable to increased this quarter with tibial dyschondroplasia (TD) and Rickets being noted. Slippery footing was noted in a case of developmental lameness. AHL reported a decrease in bacterial osteomyelitis and tenosynovitis this quarter which were namely due to E.coli or diagnosed on histopathology. Coccidiosis and necrotic enteritis cases were reported as stable, and stable to increased, respectively, by practising veterinarians. The AHL also reported stable coccidiosis cases identifying mainly small intestinal coccidiosis. Inclusion body hepatitis (IBH) cases were reported to be generally increased this quarter by both the practitioners and AHL. Genotyping on confirmed cases identified mainly FAdVE. The number of flocks with infectious bronchitis virus (IBV) were reported as stable to increased this quarter by the poultry practitioners. Birds will develop a fibrinous tracheitis due to the decreased ventilation and increased environmental ammonia levels which occur during winter. Clinically, this can resemble ILT but is distinguished by diagnostic testing. The DMV strain is most commonly identified. The Mass and Conn vaccine strains were also identified. Several cases were suspicious on histology at AHL. Infectious bursal disease virus (IBDV) infections were reported as stable this quarter by practicing veterinarians. Submissions to the AHL of confirmed runting and stunting syndrome caused by astrovirus were increased this quarter. Proventricular dilation syndrome was reported by one practitioner, namely in 7 day old broilers with clostridial septicemia. Condemnation issues were stable to increased this quarter and included cellulitis, respiratory conditions, ascites and cyanotic birds.

Hysteria in pullets

You can hear the wave of panicked birds flow from the back of the barn to the front and back again just before the barn begins to shake. That’s the experience when standing outside a barn housing “hysteric” pullets. Hysteria is not common, with only a few flocks reported yearly, and not much is know about what causes it.  However, there are some common characteristics amongst affected flocks:

  • Birds reaching sexual maturity, between 10-15 weeks of age, are affected.
  • After the birds reach sexual maturity (15 weeks of age), the incidents cease, leading some to believe that sex hormones contribute to the reactivity of the birds.
  • Episodes are more common in the spring when the ground is wet.
  • Some type of disturbance in the barn triggers an episode.
  • Hysteric pullets have not become hysteric layers.

Trickle or stray voltage has been identified as an inciting cause in some episodes of hysteria. Electricians have found measurable voltage in the ground outside affected barns. An isolator placed on a barn can reduce the stray voltage but may drive the stray voltage to the next barn. Voltage can also travel through water lines since some of the black PVC  piping contains carbon particles which conduct electricity. The outcome of an episode of hysteria is not insignificant with some flocks experiencing 1-2% mortality. Mortality is secondary to scratches or wasting secondary to crushing .However, in general, there is little overall affect on growth and uniformity. Episodes of hysteria can be blunted with feed treatments such as VEO premium. These are all natural products that can “take the edge off” the panic. If your flock suffers an episode of hysteria, please contact your veterinarian. For more information on dealing with stray voltage, please contact Hydro one. stray voltage (hydroone.com) Hysteria was reported as increased by one practitioner this quarter.


Early systemic bacterial infections (<14 days) with and without associated yolk sacculitis were reported as stable this quarter by practitioners. One practitioner reported an overall improvement in early mortality due to bacterial infection. E. coli with/without E. cecorum continues to be the most common isolate. In-lay bacterial septicemia was reported as stable to increased by practitioners, mainly due to E. coli. Similarly, AHL reported mainly E.coli with or without E. cecorum, G. anatis or C. perfringens with one case due to S. aureus. Lameness was stable this quarter. S. aureus was the most common bacterial cause of lameness reported by practitioners and AHL along with some mixed infections with E. coli and E. cecorum. Occasionally, only E. coli was isolated. IBV infections were reported as stable by practitioners and mildly increased at AHL, mainly due to the DMV and California strains. Cystic oviducts are still being identified. The number of coccidiosis and necrotic enteritis cases reported by practitioners was stable to increased this quarter, typical of the winter season. Generally an increase in the number of enteric parasites is being reported by practitioners. One veterinarian reported seeing more Heterakis sp. Poultry practitioners reported a variety of Salmonella isolates including S. Kiambu, S. Livingstone, S. Putten, S. Hadar. Multi-drug resistant E.coli ( resistant to >3 drugs) was reported resistant to ; penicillin, trimethoprim sulpha, tetracycline and apramycin.


Bacterial peritonitis/salpingitis due to E. coli was stable. Early systemic bacterial infections (<14 days) were stable to increased. Osteoporosis responsive to phosphorus supplementation was reported by one practitioner. Focal duodenal necrosis (FDN) was reported as stable by practitioners. One case of IBV due to the DMV strain was reported in pullets showing respiratory signs. ILT was reported as stable to increased. ILT is rare in commercial layers, however, there was an infected flock in the Niagara region this quarter which showed rare to minimal clinical signs. The vaccine strain of ILT was implicated. Necrotic enteritis and coccidiosis cases were reported as stable to decreased by practitioners. One case of necrotic enteritis was secondary to coccidiosis vaccination and another practitioner reported the prevention of coccidiosis secondary to vaccination with amprolium treatment. Hysteria was reported in 10-15 week old pullets. The etiology of this condition is unknown but trickle voltage has been suspected in several cases. Hysteria occurs when the birds reach sexual maturity and does not impact growth rate or uniformity. Administration of calming agents in the feed is useful.

Blackhead (Histomonas meleagridis): Lifecycle Review

Histomonas meleagridis, the causative agent of Blackhead disease (histomoniasis), is a protozoan parasite with a complex life cycle. Avian species that are susceptible to natural infection with Histomonas include, in order of susceptibility, turkeys, pea fowl, guinea fowl, chicken, chukar, pheasant, and bobwhite quail. Histomonas is a fragile organism and can only last minutes to hours in the environment, therefore, an intermediate host is needed to survive. Direct transmission is thought to be possible only in turkeys, by direct contact or ‘cloacal drinking’. Ducks can also be asymptomatic carriers of Histomonas.

Intermediate hosts for Histomonas include the cecal roundworm, Heterakis gallinarum, as well as earthworms. Heterakis eggs are resistant to commonly used disinfectants and can live in the soil for months to years, harbouring Histomonas.  Mechanical transmission of Heterakis eggs is also possible. The best bird hosts for Heterakis gallinarum that are most likely to contaminate soil are pheasants, guinea fowl and chickens.

Lesions of histomoniasis are primarily in the ceca. Liver lesions are common in turkeys but not in other birds. Protozoa may also migrate through the bloodstream to other organs (spleen, kidney, pancreas).

Prevention of histomoniasis is based on three principals:

1)  Controlling Heterakis gallinarum

2)  Separation of susceptible avian species

from birds that are reservoirs of Histomonas

  meleagridis and/or Heterakis gallinarum

3)  Stringent biosecurity measures.

Please refer to ‘AHL LabNote 54 – Blackhead (histomoniasis)

in small turkey flocks’ for a more information and list of

preventative measures to take for this disease:




Abdul-Aziz T., McDougald L.R., Barnes H.J., Histomoniasis Slide Study Set #33, AAAP, 2012.

Life Cycle of Heterakis gallinarum and Histomonas meleagridis

One case of blackhead was reported this quarter by a practitioner in 20 week-old toms.


Early systemic bacterial infection <14d were stable to increased. E.coli and Salmonella sp. were reported by practitioners. Other causes of early mortality < 14d were stable this quarter. Litter impaction, baler twine impaction and excessive beak trimming were noted as causes by practitioners. Late systemic bacterial infection >14d were stable to increased. E. coli air sacculitis and infections due to E. coli and S. aureus were identified   by practitioners.   ORT was reported as decreased to increased by practitioners. One mild case was reported by a practitioner. Necrotic enteritis and coccidiosis were reported as stable to decreased by practitioners. Enteritis was reported as stable to this quarter by practitioners One cases of blackhead was reported this quarter by a practitioner in 20 week-old toms on the same premises, but different barn, as a case last summer. The flock also had a mild Ascarid infection and was treated with fenbendazole. All healthy birds were moved to an empty barn on same premise and the mortality ceased. Veterinarians reported tenosynovitis due to reovirus was stable to increased this quarter. One practitioner and AHL reported that Turkey_PA_00659_14_genotype2 partial sequence was identified. Poultry practitioners reported a variety of Salmonella isolates including S. Liverpool, S. Hadar, S. Uganda, S. Schwarzengrund, S. Senftenberg, and S. Infantis.

Rural/Backyard/Non-Quota Flocks

AHL continues to receive increasing submissions from backyard flocks. The number of backyard/urban poultry has increased due to COVID-19. Marek’s disease continues to be the most common diagnosis. A number of flocks continue to have external and internal parasitism diagnosed including lice, mites, coccidiosis, ascaridia, and cryptosporidia. Intestinal parasitism predisposes these birds to develop necrotic enteritis. Three cases of ILT were diagnosed by AHL. One case of ORT was diagnosed by AHL. Various other diagnoses in chickens included: IBV (3 positive by histology, 1 by PCR) , 3 sinusitis cases (1 due to Avibacterium sp. ), Mycoplasma gallisepticum, fowl pox, intussusception, emaciation and predation. Diagnoses in other species included: Ascaridiasis (pheasant), Mycoplasma gallisepticum (pheasant), emaciation (pheasant), and Marek’s disease (quail).

Events and News

Poultry Industry Council events: https://www.poultryindustrycouncil.ca/events Poultry Health Research Network information, events, and lectures can be accessed on the PHRN website: https://phrn.net/ or on the PHRN YouTube channel: https://www.youtube.com/user/PoultryHRN

Thank You! We thank the following poultry veterinarians who completed the veterinary survey: Dr. Elizabeth Black, Dr. Peter Gazdzinski, Dr. Shahbaz Haq, Dr. Genevieve Huard, Dr. Mike Joyce, Dr. Anastasia Novy, Dr. Mike Petrik, Dr. Cynthia Philippe, Dr. Joanne Rafuse, Dr. Fernando Salgado-Bierman, Dr. Kathleen Sary, Dr. Ben Schlegel, Dr. Chanelle Taylor, Dr. Lloyd Weber, Dr. Alex Weisz, and Dr. Jessalyn Walkey.

Report 25


2020 – 2021

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