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The “Huddling Syndrome” in the southeastern US broiler chicken industry: on-going investigations

Drs. Alexis Kiers, Samantha Pohl, Holly Sellers, Monique Franca, Brian Jordan, Stephen Roney
Poultry Diagnostic and Research Center, University of Georgia, Athens, GA


Huddling Syndrome was observed at the University of Georgia Poultry Diagnostic and Research Center (PDRC) for the first time in August 2014. This disease will be referred to in this article as “huddling syndrome” but it has also been called within the local broiler industry as “dark fear syndrome”, “rotten feet syndrome” and “covered in mud syndrome.”

This syndrome seems to be quite widespread and has apparently affected more than ten broiler integrators in seven states of the eastern and southeastern parts of the United States of America. Results from nine investigated clinical cases between August 2014 and January 2015 are presented in this article (table 1). Those nine clinical cases affected five broiler integrators in three states (Alabama, Georgia and Mississippi). Clinical signs, macroscopic and microscopic lesions and laboratory analyses are presented in this article and discussed.

Huddling table 1

Table 1. Cases included in the report and requested analyses

Clinical signs

The condition usually starts between 21 and 35 days of age. Fever and a flushing episode are the first manifestation of the syndrome followed by a moderate drop in water consumption. A decreased activity with abnormal bird distribution (huddling) around feeder and water lines is always observed. It usually concerns part of a broiler house ranging from a small area as if a water line was leaking to an entire half of the building, either the brood end or the off-brood end. It has rarely been reported to expand to an entire house. On a multi-house farm, when different houses are affected, it is always on different scales.

After a feverish episode that lasts 2 to 3 days, the birds sit on the caked litter, dig holes in the litter and huddle under the drinkers and the PIP logo 550x350bfeeders. Birds appear to have flushed previously. All the birds quickly show severe footpad dermatitis with a moderate swelling seemingly coming from the footpad infection rising to the shank. No swelling of the gastrocnemius tendon has been noticed. As the chickens are reluctant to move and the litter becomes soaked, they rapidly cover themselves in mud. At that stage, the chickens are cold and start dying from dehydration. Mortality happens over a period of 7 to 10 days after the beginning of the symptoms. It varies greatly from moderate increase to dramatic losses (up to 4000/day/house). Surviving birds show severe growth retardation compared to non-affected chickens in the same house.

This syndrome manifests usually sporadically on a couple of farms at the same time within a complex; however, one of the reported cases concerned 20 farms infected at the same time and localized in a specific area of one broiler complex. Surprisingly in this case, each affected house had only one half much more affected than the other.

Gross lesions

Severe footpad dermatitis (FPD) (fig.1) is the lesion that has been encountered on all the affected birds from all the cases. Severe FPD is definitely an aggravating factor of the clinical picture and should be primarily addressed if a treatment is considered. Mild enteritis, mild to moderate conjunctivitis, mild to moderate coccidiosis lesions and mild bursa atrophy were also observed. The fact that some of those lesions might be causing the syndrome is addressed in the discussion.

Figure 1. Severe footpad dermatitis.  The footpad extends to the 4 toes and the central footpad on all the birds plus the hock on about 20% of the birds.

Figure 1. Severe footpad dermatitis. The footpad extends to the 4 toes and the central footpad on all the birds plus the hock on about 20% of the birds.


Fixed tissue from 5 cases were submitted for histopathology (table 2).

Microscopic lesions of the footpad were consistent all across the 5 cases that were submitted: marked to severe focally extensive ulcerative pododermatitis with coccoid-shaped bacteria. In more details, focal to multifocal, moderate to severe extensive epidermal ulceration covered with abundant serocellular crust containing colonies of coccoid-shaped bacteria and fibrinoheterophilic infiltration were observed. The inflammation extended to the underlying dermis. There was also granulation tissue formation in the dermis of some lesions. The pododermatitis was likely predisposed by ammonia burn due to wet litter and bacterial superinfection.

Huddling table 2

Table 2. Organs that have been submitted for histopathology

Kidney tissues were sampled in 4 out of 5 cases and diverse lesions were observed. Microscopic examination of the kidneys of one case revealed scattered glomeruli with eosinophilic material in the glomerular loops. PAS staining revealed fibrin in some tufts of the kidneys. The presence of fibrin indicated damage to the tuft either due to viral infection or toxins. Two cases showed mild to moderate, focal to multifocal interstitial infiltration of lymphocytes which may be due to IBV infection (including vaccine virus). The kidneys of two cases have a few tubules and collecting ducts with urate deposition, which are seen in cases of dehydration and in birds with excess calcium in the diet. One case had unremarkable kidneys.

The tendon lesions (4 cases) varied from no significant findings to mild fibrino-heterophilic and histiocytic tenosynovitis, which could result from bacterial infection and acute reovirus infection.

Bursas from 2 cases were scored. Lymphoid depletion was observed both times, mild in one case and moderate to marked in the other case.

Other organs were submitted for histopathology (skeletal muscle, heart, trachea, intestine, liver, spleen, brain, sciatic nerve, bursa and thymus) and altogether, a wide variety of very mild non-specific microscopic pathology was appreciated in all those organs.

Virus isolation

Virus isolation was performed on 8 out of 9 cases. The results are summarized in table 3. Infectious bronchitis viruses (IBV) were found in the 5 cases they were looked for. Ark-DPI IBV was isolated in four cases (cecal tonsils, trachea), IBV GA-08 in one case (ceca) and IBV GA-13 in one case (cecal tonsils). In one case, two different IBV viruses were isolated (Ark-DPI and GA- 08). Virus isolation from the kidney was unsuccessful in the 4 cases they were looked for. Avian reoviruses were isolated from the tendons of 2 out of 3 submitted cases. Adenoviruses were also isolated from 2 cases (midgut and trachea).

Table 3. Virus Isolation results of 8 cases

Table 3. Virus Isolation results of 8 cases. a Similarity percentage given by comparing the amino-acids sequence of the reovirus sigma C product of the isolate to known viruses. b Similarity percentage given by comparing the nucleotides sequence of the IBV S1 LC product to known viruses. c The media used for virus isolation was either SPF chicken embryo liver cells (Li) or SPF embryos (E), the adjunct number being the total number of passage that have been performed before isolation. NA: Non available.


Serological investigations were performed for one case (10 birds, 35 days-old). ELISA results for IBV, NDV and reovirus were uniformly very low. ELISA for IBD-XR was low to moderate and heterogeneous. No paired sera was available.


Early during the disease process, some broiler managers observed that if the affected birds were pushed away from the affected area and fresh litter was added on top of it, the affection would stop spreading. In-water antibiotic and salicylic acid therapies have been tried with mitigated successes.

Chicken inoculation study

Seven non-vaccinated specific pathogen free (SPF) chickens were raised in isolator units and were given kidney homogenate by eye-drop and by oral gavage at either 10 (3 birds) or 18 daysold (4 birds). The homogenate came from case #9, in which IB virus GA 13 was isolated from the cecal tonsils. 10 days after inoculation, the birds were euthanized, posted and sampled for virus isolation (eyelid, trachea, kidney and cecal tonsils) and histopathology (duodenum, jejunum, cecum, kidney, cecal tonsils, trachea, eyelid, ureters).

On the 20 days-old birds, no gross lesion was observed. On the 28 days-old chickens, it was observed mild airsacculitis, mild tracheitis, small hemorrhage located under renal capsule and feed passage. Histologically, the tracheas have mild tracheitis associated with bacteria. The small intestines have mild enteritis with evidence of mild blunting in some samples. The eyelids have mild edema but the conjunctivas are unremarkable. The air sac is unremarkable. All the changes seen are very mild and would be considered normal for chickens raised in a commercial setting. Ideally, It would have been good to examine an age-matched negative control bird to properly determine if these histological lesions were caused by the inoculum or not.

Real time reverse transcriptase PCR (qRT-PCR) was performed on homogenates of the eyelid, trachea, and kidney of the 28 day old chickens using a universal IBV primer and probe combination (Callison et al., 2006). The eyelid and trachea samples were positive with Ct values of 30 and 32 respectively and the kidney sample was negative. A sequencing reaction was performed on the eyelid and trachea samples to determine the S1 genotype of the IBV detected, but not enough viral RNA was present in the samples to obtain an accurate sequence. Since samples were collected 10 days post inoculation, it is likely that whatever IBV was present had already run its course and was being cleared by the birds, leaving little virus to recover. This was further confirmed by the fact that no virus would grow after traditional embryo inoculation with the same samples used for qRT-PCR.


Veterinarians have so far hit the brick wall on this huddling syndrome as no pathogen has yet been formally identified yet. The clinical picture looks very much like a farm management issue, however the multiplication of the cases and the apparent spread to the eastern part of the US led us to presume that there is an infectious agent involved. This syndrome has shown similarities with two well described disease conditions and this will be discussed below: nephropathogenic IB (NIB) and viral arthritis (VA).

“Severe flushing in 25-35 day-old broilers was reported on several farms in Northeast Georgia” (PIP, 03/2008) was the clinical description caused by the NIB virus GA-07 back during the Summer 2007 and it seems like a very good description of our “huddling syndrome” during the Summer/Fall 2014. However in our cases, histopathological lesions of the kidney are very mild and therefore non-conclusive. Indeed, similar lesions are regularly reported in apparently unaffected broiler chickens. Moreover, IBV isolates closely related to known serotypes and vaccine strains have been identified so far (Ark-DPI, GA-13 and GA-08) and none of them have been known to cause nephropathogenic symptoms. Late sampling, poorly adapted virus to embryo culture or the presence of a “vaccine type strain” could explain unsuccessful attempts to isolate an IB virus. Making farm managers and supervisors aware of this potential threat could help visiting the farms earlier and getting better samples.

The huddling syndrome shows clinical similitudes with cases of NIB that have been occurring in the Delmarva area since 2011. The cases in Delmarva have been clearly associated with a specific strain of IBV virus (DMV/1639/11) as it was reported by Dr. Gelb (Avian Diseases 57:65–70, 2013). In 2014, the NIB infection due to this particular strain seems to be widespread on the eastern shore even if the recent isolates have expectedly exhibited some antigenic drift since 2011. Interestingly, infection with this virus may result in respiratory or renal disease forms. It seems like there are certain triggers that favor the development of NIB in Delmarva broiler flocks. However in Delmarva, they do observe clear NIB lesions in the kidneys, both grossly and microscopically and it has not been difficult to isolate the virus.

Variant and undefined reoviruses have been researched and readily isolated directly from tendons in two cases. Due to the great ubiquity of reovirus, the presence of characteristic gross and microscopic lesions is mandatory to blame this virus. The age of the affected birds and the clinical picture, i.e immobilized and recumbent birds, are in favor of a reovirus hypothesis. However the absence of swollen hocks, ruptured and enlarged gastrocnemius and digital flexor tendons as well as the lack of typical microscopic lymphocytic infiltrations of the tendon sheaths and synovial membrane put the reovirus hypothesis at the bottom of the pile. However in regards to the recent and brutal resurgence of viral arthritis due to variant reoviruses all over the world, this needs to be considered every single time a new case occurs and systematically ruled out. Ultimately, the isolated reoviruses could be used for a reproduction study for further investigation.

Other conditions that are responsible for wetting the litter and causing pododermatitis (classical IBDV, coccidiosis, salt toxicity, nutritional deficiencies…) should also be part of the rule out lists and be kept in mind if new cases appear, however those are unlikely. Considering that usually, only part of the house is affected, it is likely that we are facing another multifactorial disease such as runting and stunting syndrome where the management, the environment and the seasonal changes might play an important role for triggering the affection.

In future cases, serological analyses should be used more as they can be of great support to screen efficiently and at minor costs causative agent, especially if it involves IB viruses and reoviruses. Blood sampling at the plant of the affected flocks could be compared with serological results for “normal” counterpart flock used as a baseline.


This huddling syndrome has a high potential for causing high mortality, severe growth retardation without showing any characteristic lesion but severe footpad dermatitis. Unfortunately, no disease agent has formally been identified yet. A new strain of NIB virus seems to be the primary hypothesis but the characterization by sequencing of the IB virus positive PCR product was unsuccessful. Other disease agents must be kept in mind if new cases occurs. Making farm managers and supervisors aware of this potential threat could help getting earlier to the farm and get better samples. In all cases, blood samples should be taken at the plant and have their serum run on ELISA kits for IB virus and reovirus.


Article courtesy of The Poultry Informed Professional
Published by the Department of Population Health, University of Georgia
Editor: Dr Stephen Collett, Associate Professor


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