Optimal broiler production and health are highly dependent on the health status of the parent flocks. A link between broiler quality and their parents has been investigated previously but primarily based on physical, nutritional, endocrinologic and environmental aspects. A study was performed under PROHEALTH where the causes of normal mortality in broiler breeders was investigated to identify possible critical periods during production in relation to health of the breeder flocks and potential risks to the offspring (Thøfner et al., 2019a).

Four breeder flocks were followed throughout a full production cycle (20-60 weeks of age) where up to 10 random dead-on-farm birds per week per flock were collected and examined. The selection criteria for the flocks included requirements of high similarity in flock size (appr. 20.000 birds), no history of prior disease problems in the flocks or need for medication in the three rotations prior to the present one, housing, feed regimes, lighting programs, biosecurity, vaccination strategy and management routines. The birds originated from grandparents of the same age. The flocks were vaccinated against Mareks disease, Infectious bronchitis, Gumboro, Newcastle disease, Chicken anemia, avian encephalitis virus, pneumovirus, adenovirus and coccidia. All flocks were free of salmonella throughout the production cycle. The percentage of male birds was around 7% and all flocks received new males once between week 45 and 48. The cumulative mortality was below 8% in all flocks which is considered within normal range.

In total, 997 collected birds underwent full post-mortem assessment. Bacteriological investigation was performed if macroscopic lesions typical of infection were present. In figure 1, the top five causes of mortality observed in this study are shown. The proportion of birds dying from non-infectious disease was 41%, whereas infectious disease was observed in 55%. The remaining birds did not demonstrate any gross lesions. No significant difference between flocks was observed in the proportions of non-infectious mortality and infectious mortality.      

The authors are aware that only a small number of flocks were investigated, however, they represent what is typically observed in Danish broiler breeder flocks under non-outbreak situations. Although farm bias cannot be ruled out, we believe that that the present study is of interest to others, in particular when considering the overall uniformity of Danish broiler parent flocks with regard to management, feed regimes, and biosecurity strategies and strict national policies on antimicrobial and feed additive use for livestock.

Regarding the non-infectious mortality approximately 40% of the birds in this category died from fatty liver or egg bound and approximately 18% died from circulatory distress, such as heart failure or sudden death syndrome.

Figure 1. Top 5 causes of death in broiler breeders (primary lesions) in relation to age- overall

Total (997)Weeks 20-29Weeks 30-39Weeks 40-49Weeks 50-
Salpingitis-Peritonitis 29.2%Salpingitis-Peritonitis 22.9%Salpingitis-Peritonitis 32.8%Salpingitis-Peritonitis 26.8%Salpingitis-Peritonitis 33.5%
Egg bound 8.3%Egg bound 13.7%  Egg bound 9.1%Fatty liver 15.8%Salpingitis (often Chronic) 8.1%
Fatty liver 8.0%Emaciation 9.3%Cannibalism 8.4%Arthritis 8.2%Uraemia/
nephropathia 7.2%
Arthritis 6.4%No lesions 9.3%Fatty liver 6.2%Salpingitis (often chronic) 7.2%Septicaemia 6.8%  
Cannibalism 5.5%Sudden Death Syndrome 8.3%Heart failure 6.2%Amyloidosis 5.8%Cannibalism 6.8%

There were significant age related distributions of these manifestations. The majority of the young birds died from non-infectious causes such as egg bound, emaciation, sudden death syndrome (20-29 weeks old) as well as fatty liver or cardiovascular collapse (30-39 weeks). In addition, cases where no gross pathology could be observed also were more common in this group. All manifestations, which may be related to increased physiological stress in the period from onset of lay to peak production. In the age interval 40-49 weeks old, the rate of fatty liver increases dramatically to about 15% of the total mortality in the age interval. Increased mortality due to deposition of fat in the liver in post peak lay birds suggested that the birds were relatively overweight. Post peak lay body weight gain has been associated with higher cumulative mortality rates due to the increased body weight.  None of the flock managers performed regularly routine body weight measurements during the observation period. Cannibalism occurred at a rate of 5.5% of the overall mortality throughout the whole observation period.

In contrast to the non-infectious mortality, the ratio of the infectious causes of mortality increased significantly during the production period to more than 64%. In birds aged 40-49 weeks, four out of the five most frequent overall causes of mortality were infectious, that is salpingitis-peritonitis, arthritis, salpingitis and systemic amyloidosis, comprising 48% of the total mortality in that age interval. Salpingitis-peritonitis prevalence was high and relatively constant within age intervals throughout production. Salpingitis alone was mostly seen as chronic manifestations with large accumulations of exudate in the magnum of the oviduct. In the present study E. coli was isolated at a high rate both from salpingitis (84%) and salpingitis-peritonitis (88%).

These findings underline that E. coli infections, in particular are related to the oviduct, and are of major importance in both broiler breeders and broilers and have significant impact on animal health and welfare, and production economy. Arthritis was overall demonstrated to be the fourth most common lesion leading to mortality. In particular in 40-49 weeks old birds where arthritis was the third most common cause of mortality. The increased frequency of septicemia over time may be the result of poor foot health in the older birds as indicated in a recent study also resulting from PROHEALTH (Thøfner et al., 2019b).

An investigation as the present one shows the importance of obtaining specific data over the entire production period in order to identify problems related to age and management routines. In this study, the age interval 40-49 weeks was identified as a critical period in relation to mortality and possible vertical transmission of E.coli. Egg production is gradually declining from around 40 weeks of age (www.aviagen.com). The hens have then been in intensive production for more than half of their adult life, with physical and physiological stress that are part of being housed in intensive production units. Consequently, improvement of management routines throughout the production, particular emphasis on feeding management in certain periods is considered essential. Especially, when the birds are in transition from onset of lay to peak and again at post peak decline, an increase in mortality due to fatty liver, egg bound, sudden death syndrome may occur if the energy level in the feed is too high, however, further research is needed to document this. Attention to minimize the risk of birds getting an infection should also get priority as this period contains the highest number of bird dying from infections. Infections with E. coli give rise to more than 80% of the infections related to the oviduct and are also highest in number in this period; therefore this period is regarded as a period of risk in vertical transmission of E. coli to the progeny. This underlines the importance of prophylactic measures such as hygiene of hatching egg and vaccination against E. coli infections (Poulsen et al., 2017).

References used and resulting from the PROHEALTH project:

Poulsen, L.L., Thøfner, I., Bisgaard, M., Christensen, J.P., Olsen, R.H. & Christensen, H. (2017). Longitudinal study of transmission of Escherichia coli from broiler breeders to broilers. Veterinary Microbiology, 207, 13–18.

Thøfner, I. C. N., Poulsen, L.L., Bisgaard, M., Christensen, H., Olsen, R.H. & Christensen, J.P. (2019a). Longitudinal study on causes of mortality in Danish broiler breeders. Avian Diseases, 63, 400-410.https://doi.org/10.1637/12006-113018-Reg.1

Thøfner, I.C.N., Poulsen, L.L., Bisgaard, M., Christensen, H., Olsen, R.H. & Christensen, J.P. (2019b). Correlation between footpad lesions and systemic bacterial infections in broiler breeders. Veterinary Research, 50, 38. https://doi.org/10.1186/s13567-019-0657-8