Chickens kept in litter-based housing systems, including free-range chickens, are more prone to disease than chickens that are kept in cages, according to a study published in the Jan. 15 Acta Veterinaria Scandinavica.
In 1988, Sweden passed the Swedish Animal Welfare Act that mandated a switch from battery cages for laying hens to alternative types of housing - including free-range and indoor litter-based systems - that were designed to allow "natural" behaviors to be expressed.
Subsequently, between 2001 and 2004, there was a large increase in the number of flocks being kept in alternative housing systems.
Researchers led by Oddvar Fossum at the National Veterinary Institute (SVA) in Uppsala, Sweden, noted that during the switch in housing from battery cages to enriched cages and litter-based systems, including free-range systems, there was an increase in the number of chickens that were dying.
Therefore, Fossum et al. compiled the results from necropsies taken between 2001 and 2004 to analyze the effects of the change in housing systems on bird health.
They noted that their results will "probably have relevance" outside Sweden as well since conventional battery cages will be banned in the European Union beginning in 2012.
To conduct the study, Fossum et al. looked at the results of necropsies of commercial laying hens submitted to SVA for routine examinations connected to diseases associated with increased flock mortality.
Other diseases, parasite infections or lesions that did not lead to a manifest increase in mortality were not included. Between Jan. 1, 2001, and Dec. 31, 2004, hens were submitted from all parts of Sweden.
Fossum et al. said the criteria for inclusion in the study were:
(1) The laying hens must come from a flock with increased mortality.
(2) The number of submitted hens should be three or more.
(3) There should be information about the housing system, age of the birds, flock size and disease history in the flock.
The cause of death was recorded for 914 hens from 172 flocks. Flocks included in the study varied in size from 400 to 37,000 birds, and the age of the birds ranged from 18 to 78 weeks.
The researchers divided birds into three categories: birds housed in conventional/furnished cages, birds housed in litter-based systems (both single and multilevel) and birds housed in free-range systems with access to outdoor pens and/or pastures (including organic laying hens).
The main disease categories of the included flocks are shown in the Table.
Bacterial diseases appeared to be the most common causes of mortality during the study period. The predominating disease in all housing systems was an infection caused by Escherichia coli, i.e., colibacillosis, which was found in 85 flocks.
Coccidiosis was observed in 18 of 129 flocks housed in litter-based systems indoors, in five of 23 free-range flocks and in two of 20 flocks from cage systems. Most of the outbreaks (76pc) occurred in birds younger than 24 weeks.
In five of 129 flocks kept in indoor litter-based housing systems, increased mortality caused by infestation with the poultry red mite was demonstrated.
In addition, mites were found in the plumage of birds from another seven flocks from litter-based housing systems, including free range. Mites were not detected in birds submitted from cages.
In the necropsied birds, the most common traumatic injuries leading to increased mortality were wounds in the cloacal region that were indicative of vent pecking (cannibalism).
Cannibalism was observed in all housing systems and was the main cause of mortality in five of 129 flocks housed in litter-based systems indoors and in four of 23 free-range flocks.
Cannibalism was not observed as the main cause of death in any of the flocks housed in cages, Fossum et al. noted.
The researchers pointed out that it is important that the results reflect the unique situation in Sweden during 2001-04, when the change of housing systems away from conventional battery cages was at its peak.
According to Fossum, "Birds kept in indoor litter-based and free-range housing are more prone to disease, but measures can be taken to counter this."
Several new types of aviary systems were introduced during that time, the researchers said, and the knowledge and experience of keeping large flocks of laying hens in aviaries were limited. Additionally, during the study period, many new egg-laying farms were established by people who had no or little prior experience with keeping commercial laying hens, and these producers often chose the new aviary systems.
Hence, the health status of birds in free-range and litter-based systems should improve as more experience and knowledge are gained.
Fossum added, "The health of Swedish laying hens kept in these systems has improved as the farmers have become more experienced in managing the new housing systems."
To increase the safety of egg production in litter-based housing systems and free-range systems, it is necessary to pay much attention to management and preventive measures such as biosecurity and vaccinations. Adequate education of personnel responsible for running the operations is crucial.
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