John A. SMITH
DVM, MS, MAM
Director of Health and Hatchery Services,
Fieldale Farms Corp.,
Baldwin, GA, U.S.A.
Disease syndromes characterized by diarrhea and reduced growth have been recognized in commercial broilers since at least the 1970’s, and have been variously referred to as runting-stunting, pale bird, malabsorption, brittle bone, and helicopter wing syndromes. A variety of viruses have been associated with these syndromes, including reoviruses, parvoviruses, enterovirus-like viruses, rotaviruses, astroviruses, and other “small round viruses”, but a clear etiological relationship has rarely been established. Many of these viruses are commonly found in healthy chickens as well. With the exception of the reoviruses (whose association with these syndromes is tenuous at best) most of these viruses are difficult to propagate in the laboratory, and no vaccines and few diagnostic techniques are available. A new wave of an enteric syndrome commonly referred to as “runting/stunting syndrome” (RSS) has emerged in the major broiler producing areas of the United States (and possibly other areas) over the last years. This syndrome is characterized by diarrhea and severe stunting within the first one to two weeks of life, poor economic performance, and specific gross and histological lesions, and appears in many cases to be followed by secondary problems highly suggestive of immunosuppression. The purpose of this paper is to update the clinical features, temporal and geographic progression, and some research and field observations with bearing on the possible etiology, epizootiology, and management of this syndrome.
History, temporal, and geographic progression of the syndrome
The following information is not intended to be comprehensive, but serves to illustrate the fairly rapid progression of the syndrome over both time and distance. This overview was obtained via interviews with several companies known to the author to have experienced the syndrome to various degrees (1). The syndrome is reliably known to be considerably more widespread than the specific examples offered here.
Initial cases 2003-2004: In the early spring of 2003, an integrated broiler company in north Georgia (Company A) experienced a syndrome characterized by severe flushing and stunting during the first week of life and severe performance shortfalls in affected broiler flocks (2). Gross and histological lesions similar to those described subsequently herein were noted in numerous cases. The syndrome abated somewhat during the summer of 2003. To our knowledge, this is the first well-documented case of this particular syndrome. During the winter of 2003-2004 Company A was again severely affected and Company B, an unrelated company in another distinct growing region of north Georgia was sporadically affected. Company C also reported a similar syndrome in 3 of 4 Georgia locations and 1 of 2 Alabama locations in the winter of 2003-2004. The problem again lessened somewhat in the summer of 2004 and Companies A and B regained some performance, but neither entirely returned to normal. (Summer recovery in Company C was not discussed.)
2004-2005: In the winter and spring of 2004-2005 the syndrome returned with a vengeance in Companies A and B, repeated in the 3 Georgia complexes (1 mildly), the same Alabama complex, and a South Carolina complex (mildly) of Company C, and significantly affected the north Georgia and North Carolina locations of Company D. Company D also reported the syndrome to lesser degrees at complexes in Alabama and Arkansas during the late winter and spring of 2004-2005. Reports of similar signs and lesions were received from other companies, particularly in north Georgia but possibly in other areas of the country (such as Arkansas) in 2004-2005.
2005-2006: Company A, which was severely affected from 2003-2005, appears to have recovered in 2006 and is experiencing excellent performance. Company B experienced the syndrome essentially unabated in the winter of 2005-2006. In Company B the overt clinical signs in 2006 were generally less severe but the syndrome appeared to be even more widespread among flocks, and the final effects on complex live performance and bird health were at least as severe as in 2004-2005. Company C reports that only 2 Georgia complexes were affected in 2006, but they were severe, the Alabama complex was mildly affected, and the South Carolina complex appeared to have recovered. Company D reports that the North Carolina complex has recovered, the Georgia and Alabama complexes were only mildly or sporadically affected, but the Arkansas complex was severely affected; that complex appears to have improved considerably over the summer of 2006. Company E reported the syndrome in Louisiana in 2005-2006. There are also reasonably reliable reports of other cases in Mississippi, Tennessee, and other complexes in Georgia, Arkansas, and Alabama. There are companies in the north Georgia area that appear to be unaffected to date. The author is not aware of any confirmed reports in breeders, and only limited, sketchy reports in breeder pullets. The author has consulted with colleagues who indicate the occurrence of a clinically similar syndrome in Great Britain, Jamaica, Mexico, and Panama. The colleague in Mexico demonstrated the characteristic cystic enteropathy on histopathology.
Clinical signs and impact
Signs consist of a marked unevenness that is glaringly evident by 10 days of age. Experienced observers can detect definite signs by 6 days of age, and signs were initially reported in Company A as early as 3 days of age. A portion of chicks in affected flocks will display pasting of the perineum below the vent, often resulting in scalding of the skin. In the most severely affected houses, the litter and feed lids may become wet and slick, and there may be a sour odor in the house associated with the wet litter and feed. A few birds may experience cloacal impactions or prolapses. The down on the ventrum may appear matted from sitting in wet litter. The birds typically exhibit a transient phase of huddling as if they are febrile. Many of the stunted birds appear visibly ill with a ruffled, hunched-up, pot-bellied appearance and waddling gait. Feathering is poor, with some birds exhibiting protruding “helicopter” primary feathers on the wings. The flock will fall behind on feed and water consumption, often during the early starter phase. A number of affected complexes have noted that the syndrome is followed by a wide variety of secondary conditions such as severe colibacillosis, gangrenous dermatitis, inclusion body hepatitis, air sacculitis, cellulitis, purulent arthritis, and osteomyelitis, leading to speculation that immune suppression is involved. Final feed conversion, daily gain, and uniformity are severely impacted. Direct mortality from the disease itself appears to be fairly low, but mortality from culling and from the secondary conditions can be severe. Company A experienced weeks in 2004-2005 in which the complex average livability for a small bird was as low as 84% and feed conversions suffered by at least 20 points (2). The lack of uniformity may lead to increased condemnations due to “septicemia-toxemia” and contamination issues. Secondary septic conditions also increase condemnations.
Gross lesions
Gross lesions include general paleness of the tissues, especially the shanks and intestinal serosa. The mid-gut serosa will often appear very white as opposed to the normal pink-gray color. The bone marrow is usually normal, but some affected birds are anemic. The bursa and thymus are variable but frequently normal during the acute phase, becoming atrophied in the chronic phase. The spleen is frequently very small. Livers are often small and dark, with enlarged gall bladders. Proventriculitis, pancreatic atrophy, and mild hydropericardium seem to be a feature in some cases (3). The hallmark lesion is a moderately to markedly distended, extremely thin, translucent gut with undigested feed suspended in clear, thin, watery mucus. There are no other gross serosal or mucosal lesions. The cecum is often distended with gas and brown-tinged clear watery fluid or foam. The kidneys may be mildly swollen in some cases.
Histopathology
Histopathological lesions include villous atrophy, clubbing, and fusion and a remarkable crypt dilation and hypertrophy with accumulation of amorphous material, cyst formation, and squamous metaplasia of the crypt epithelium. Pancreatic vacuolar degeneration and severe chronic lymphoplasmacytic enteritis have been noted in experimental cases (3). Dr. Fred Hoerr’s group at Auburn University has consistently identified microsporidian organisms resembling Myxospora in the amorphous debris filling the cystic crypts (4).
Clinical pathology and serology
In general, a comparison of total protein, albumen, globulin, aspartate aminotransferase (AST), bile acids, lactic dehydrogenase (LDH), and uric acid between affected and ostensibly unaffected chicks in the same houses revealed some differences, but none were considered clinically relevant and all could be attributed to general antigenic stimulation, dehydration, handling, and/or inanition (5). A comparison of affected and unaffected flocks for processing age ELISA titers to Infectious Bursal Disease Virus (IBDV), type I avian adenoviruses, Reovirus, Newcastle Disease Virus (NDV), Infectious Bronchitis Virus (IBV), Avian Encephalomyelitis Virus (AEV), Chicken Infectious Anemia Virus (CIAV), and Hemorrhagic Enteritis Virus (HEV) revealed no clinically meaningful differences. Serum samples were obtained from hen flocks in Company B producing fewer or greater numbers of affected progeny flocks, before and after the peak of the syndrome in the spring of 2005. The hen flocks were selected based on an ongoing RSS scoring system in the broilers. Weighted average progeny scores were calculated for each hen flock during the peak period of the syndrome, and flocks with the highest (worst) and lowest scores were examined. No significant trends were noted for CIAV, IBDV or Reovirus titers in these hens. Interesting associations between RSS and seroconversion to HEV in both challenged broilers and in hen flocks associated with cases in broiler progeny have been noted, but the meaning of these associations is still unclear.
Epizootiology, reproduction of the syndrome, and transmission
Analysis of the hen population in two companies implicated progeny from younger flocks to a disproportionate degree. Company A sold eggs to several companies during the peak of the outbreak. These companies reported no signs of RSS in the progeny from these eggs, casting some doubt on vertical transmission. However, the existence of vertical transmission and the importance of maternal exposure or maternal antibody remain to be elucidated. The author has not been able to make any clear associations with breed.
The signs and/or lesions have been transmitted numerous times by several researchers using gavages of crude and filtered (0.22 µ) gut homogenates, chloroform-treated filtrates, chick- and embryo-passaged materials, and by exposure to contaminated litter. The condition has been transmitted to SPF leghorns, broilers, late stage chicken embryos, and turkey poults. A study by Drs. Cloud and Rosenberger described transmission of signs and lesions with a pure culture of a single viral agent. In these experiments, transmission to uninoculated contacts was extremely rapid and efficient, suggesting rapid and copious shed by the inoculated chicks and/or a very low infectious dose.
Dr. Guillermo Zavala (3) at the University of Georgia obtained contaminated litter and severely affected 13-day-old chicks from a broiler house at Company B and placed them in two isolated colony houses. The birds were kept for 13 more days and then removed. The following day, 150 new day-old chicks from Company B were placed in each house. These were kept for 13 days and removed and the process repeated using chicks from a single, young hen flock of the same breed each time. The same specific hen flock was not used each time. Subsequent groups were kept for 10 to 18 days, with one or two days down time and no clean out or other treatment. Beginning with the fourth batch, 150 controls from the same hen flock as the principals were placed in a clean house each time. This house was cleaned and disinfected between each successive batch. The body weights of the principals were repeatably approximately 50% that of the controls, uniformity was terrible, and gross and microscopic lesions became increasingly apparent with each succeeding batch. At least 20 batches have been placed in these houses for testing of various management interventions, with similar results each time. These findings strongly suggest that the agent takes up residence and persists in the house, but that regular cleaning and disinfection may be able to exclude the agent(s). Most of the batches placed in these houses came from Company B. The fact that the control house remained free of signs and lesions in all batches (except one described subsequently) also suggests that vertical shed is likely transient if it occurs at all.
Further examination of vertical transmission and possible maternal immunity: Dr. Guillermo Zavala (3) reared and housed 4 groups of 25 broiler breeders in clean facilities. After peak production, eggs were collected separately from each group to produce 9 progeny hatches every other week. Each group of progeny was reared to 10-14 days of age, at which time individual body weights were determined and post-mortem examinations performed. The first 4 hatches were produced without exposure of the hens or progeny to RSS. After the fourth hatch, the hens in 2 groups were gavaged with 1.5 ml of intestinal contents from severely affected broilers from the contaminated colony houses on 2 consecutive days. The other 2 groups served as uninoculated controls. Five more hatches were produced. The first 4 of these hatches (numbers 5-8) were reared in clean facilities without exposure to RSS, while the fifth hatch (hatch 9 overall) was divided among the two highly RSS-contaminated colony houses and the clean control house. The objective of hatches 5-8 was to observe for signs of possible vertical transmission, while hatch 9 was designed to assess the effects of possible maternal immunity. Since the eggs for hatch 9 were produced approximately 10 weeks post inoculation, it was hypothesized that any potential shedding should have stopped and maternal immunity might be present. The progeny from the exposed hens in hatches 6 and 7 (the second and third hatches post inoculation) did exhibit weight depression compared to the progeny from the unexposed hens, but the characteristic signs and lesions of RSS were not observed. Weights were comparable in hatch 8. It must be noted that the inoculum was derived from one of the later batches in the contaminated colony houses, which were likely heavily contaminated with not only the putative RSS agent(s) but also any number of other pathogens such as CIAV and Reoviruses. Any of these vertically transmitted agents could have been present in the crude inoculum and could have contributed to progeny weight depression without overt RSS.
In hatch 9, maternal exposure did not appear to offer any protection to exposure of progeny to the contaminated houses. The chicks from the exposed hens were as severely affected as those from the unexposed hens. Again, after approximately 20 batches in rapid succession, the challenge in the contaminated houses may have been overwhelming. Interestingly, the chicks from both the inoculated and uninoculated hens in the clean control house also developed characteristic signs and lesions of RSS in hatch 9. This house had been used for approximately 17 control batches, had been cleaned and disinfected each time, and until this batch had never produced signs or gross or microscopic lesions of RSS. These events suggest that the inoculated hens may have shed the agent at 10 weeks post inoculation, and that their progeny transferred the infection laterally to the progeny of the uninoculated hens. The questions of vertical transmission and maternal immunity therefore remain tantalizing ones.
Two batches of chicks were placed on the litter used for the RSS-challenged hens. The hens were removed approximately 20 weeks post challenge, and the first batch was placed within the week and held for 25 days. The second batch was placed within 48 hours of removal of the first batch and held for 11 days. No signs or lesions of RSS were noted, suggesting that any shedding that had occurred had stopped and the agent(s) had disappeared from the environment.
Etiology
Bacteria: E. coli carrying genes associated with attaching-effacing strains have been isolated from some cases. However, challenge of SPF and broiler chickens with these strains caused only minor alterations in weight gain, and did not reproduce any signs or lesions whatsoever. Prestarter antibiotic programs do not seem to offer any relief to the syndrome itself. Antibiotics, including neomycin/oxytetracycline (Neo-Terra), ormetoprim/sulfadimethoxine (Rofenaid), enrofloxacin (Baytril), and penicillin seem to ameliorate the secondary complications, but do not significantly affect the progress of the syndrome itself. The consensus opinion seems to be that the primary agent is likely viral, and while this may result in adverse alteration of the gut flora (“dysbacteriosis”), bacterial agents are probably only secondarily involved.
Viruses: Reoviruses, coronaviruses, type I adenoviruses, and a small unenveloped virus have been isolated, and EM and PCR studies have implicated rota, corona, reo, astro, and parvoviruses. Dr. Guillermo Zavala at the University of Georgia, using the contaminated colony house system described earlier, compared the responses of chicks with high maternal antibody titers for IBDV, Reovirus, and CIAV, to those with low titers. Chicks with higher maternal titers fared no better than those with low titers (3). Company B produced an autogenous vaccine to one of these novel Reoviruses. Chicks from hens that received the autogenous vaccine in addition to the company’s standard Reovirus program were compared to those from hens receiving the company’s standard Reovirus program only in the contaminated colony house system described earlier. The autogenous Reovirus vaccine offered no protection. A similar comparison was made in Horsfal isolation units using gavage with homogenized intestines, with a similar outcome (6). These observations were supported by field observations in the company as well. In the author’s opinion the direct or primary involvement of a Reovirus appears unlikely. Extensive testing of affected broilers, associated breeders, and hatchery vaccines in Company B has effectively ruled out a role for Avian Leukosis Virus. Again, Drs. Cloud and Rosenberger reported on studies with the small unenveloped virus that currently appears to be the most likely candidate.
Interventions
The number of interventions attempted are literally too numerous to recall. Company A, which was severely affected for 3 years and has apparently recovered, made adjustments in IBDV, Reovirus, and CIAV vaccination programs, including inclusion of autogenous IBDV and reovirus vaccines in the pullet programs. They also obtained additional housing to increase down time, gave added attention to incubation, brooding, litter management, diet formulation, and mill sanitation and maintenance. The manager admits that he cannot attribute the apparent success to any specific item, although down time is high on the list. Other interventions by other companies include attempts at HEV vaccination of pullets, prestarter antibiotic programs, litter treatments, water acidification, dietary adjustments and supplements, etc. Of the long list, few have produced measurable results individually. At Company B, heating of houses to extreme temperatures (up to 119 F) for extended periods (up to 5 days) appears to be helpful in many (but not all) cases, while clean out has sometimes failed to prevent recurrence.
Specific observations bearing on management of the syndrome
Down time and heat sensitivity: The studies in the contaminated colony houses at the University of Georgia clearly indicate that built up litter and short down times exacerbate the syndrome (3). In laboratory studies conducted by Dr. Holly Sellers at the University of Georgia, heat treatment of gut homogenates reduced the deleterious effects of the inoculum on body weight of gavage challenged chicks. Room temperature for 4 days and 37o C for 24 hours had no effect, but 45o C for 4 hours and 60o C for 2 hours resulted in significant amelioration (6). Dr. Guillermo Zavala, using the contaminated colony house system described previously, held one contaminated house out for one 17 day cycle, and heated that house to 92-105o F for days 11-15 of the down time. The house was otherwise not cleaned or disinfected. The other contaminated house grew an affected flock as usual, and the clean house also grew a flock as usual and was cleaned and disinfected as usual. On the subsequent flock, the heat-treated house had body weights of 90.6% and 60.5% of the controls at 5 and 14 days respectively, while the untreated house had body weights of 73.9 % and 53.9% of the controls. These differences were statistically significant (3). On another occasion, both contaminated houses were allowed to sit empty for 22 days, but with no further interventions (heating, cleaning, etc.). The body weight depression in these trials was roughly 35%, compared to about 50% in the other trials (3). These results suggest that down time and heating of the houses may be helpful, but cannot be counted upon to eliminate the problem.
Age susceptibility: In another experiment in the contaminated houses, Dr. Zavala held half of the chicks intended for the two contaminated houses in the clean control house until 11 days, at which time they were moved to the contaminated houses with their hatch mates that had been exposed since day 0. The delay in exposure significantly reduced (but did not abrogate) the effects of the exposure on body weight gain, uniformity, and expression of lesions (3). These results suggest that delaying exposure, such as by cleaning and disinfecting the brood chamber, may be beneficial. The lack of significant signs (other than a transient diarrhea) in the gavaged hens of the vertical transmission study suggests that there may be an age-associated immunity, and therefore controlled exposure might be possible should vertical transmission or protection from maternal antibody appear important in control.
Treatment with Metronidazole: Metronidazole is effective against anaerobic bacteria and protozoa. Half of the chicks in each of the contaminated houses were treated with this drug as a means of examining the role of anaerobes and protozoa in the syndrome. (Metronidazole is illegal in the United States, and the objective was not to evaluate potential treatment with this particular drug.) The treatment offered no protection (3).
Progression of the syndrome
In Company B, the overt clinical signs of RSS were less severe in the winter of 2005-2006 than in the previous year, as determined by a flock scoring system in place since February 2005. However, it appeared that more flocks were affected with a milder form of the disease (5). In other words, it appeared that severity had decreased but prevalence had increased. The effects on economic performance (livability, weight gain, feed conversion) were equally as severe in 2005-2006, and the secondary effects were worse. Company B tracks the incidence of gangrenous dermatitis (GD) via a reporting system by the broiler flock supervisors. The two winters of RSS (2004-2005 and 2005-2006) were some of the worst on record for GD, and 2005-2006 was slightly worse than 2004-2005. In addition, cases of Inclusion Body Hepatitis were seen in the winter of 2005-2006, the first time this disease had been noted since an outbreak in 1998. Condemnations for septicemia-toxemia have been steadily increasing in Company B since the onset of the clinical RSS outbreak in late 2004. There was a spike in air sacculitis condemnations in the late winter of 2005-2006 that did not occur in the previous winter. Multiple houses have been observed with a combination of these problems. This pattern suggests immune suppression. Indeed, other immunosuppressive diseases would be expected to make this syndrome worse. However, due to the extremely early onset of signs, it is difficult to conceive that immune suppression developing before 6 days of age is a prerequisite for development of this condition. The author is of the opinion that the agent(s) causing clinical RSS may be directly and profoundly immunosuppressive themselves. The suspect agent has been found in bursas (7).
The reason for the eventual disappearance in some complexes is unknown, as a literal “shotgun” approach has generally been taken. One might speculate that gradual spread eventually leads to population immunity, particularly in the hens, leading to either decreased shed and/or maternal immunity, but there is currently insufficient information about either vertical shed or maternal immunity to draw any conclusions regarding the plausibility of this hypothesis. It has been observed that the syndrome often seems worse and persists longer in small bird complexes. One might further speculate that the cycle length, irrespective of actual down time, may play a role. With a disease that strikes so early in the life of the bird, assuming the birds do seroconvert, become immune as a group, and stop shedding, the later part of a long large-bird growing cycle may be tantamount to down time. In any case, should this syndrome eventually disappear (as similar syndromes have tended to do in the past), we should continue to seek answers to this problem. Our knowledge of enteric viruses in broilers is woefully inadequate. Similar syndromes are likely to recur in the future, and we do not need to be starting from scratch each time.
References
1. Interviews with Dr. Travis Cigainero, Mr. Brad Harp, Dr. Marshall Putnam, and Dr. Charles S. Roney, September 2006.
2. Dufour-Zavala, L. Cystic enteritis: reproduction of the disease and attempted control measures. Proceedings, 40th National Meeting on Poultry Health and Processing. Pages 20-21.
3. Zavala, G. Personal communication.
4. Hoerr, FJ. Personal communication.
5. Smith, JA. Runting and stunting syndrome (cystic enteritis): a field perspective. Proceedings, 40th National Meeting on Poultry Health and Processing. Pages 6-19.
6. Sellers, H. Personal communication.
7. Rosenberger, JK. Personal communication.
From "Proceedings of the 41th National Meeting on Poultry Health and Processing", Ocean City, Maryland.
