Over the last two decades old bird
diseases, such as Coryza, Blackhead and Pasteurelle multocida increasingly reared their ugly heads among
free-range hens. These diseases barely occurred when laying hens were kept in cages.
Addressing the growth in demand for animal friendly housing systems, more and more birds are kept in free-range houses and less of them spend their days in cages. Dr Naomi de Bruijn, poultry veterinarian at GD Animal Health in the Netherlands, analysed reports on the prevalence of several poultry diseases within the Dutch borders. “What stood out were the statistics of old poultry diseases that almost didn’t appear anymore. We noticed a rise in occurrence, as a result of the switch to free-range housing systems.”
“In the Netherlands we monitor what is happening and communicate our findings to the government and the producer organisations and interbranch organisations”, De Bruijn explains. The increase in old poultry diseases was not found by GD Animal Health alone. Also international publications mention a growing number of reports about bacterial diseases that accompany loss. “Within the Netherlands it may only concern a limited number of farms, but the companies that have to deal with these diseases suffer great damage. The phenomena reaches beyond the borders of the Netherlands, with its high poultry density. It also occurs in countries such as Denmark, where poultry is kept in alternative housing systems, at large distances from each other.”
The increased occurrence can be largely explained by the changing housing styles. “We know the concerning pathogens from the past, when poultry was kept in colonies.” After a period in which one focused on safety in food production for the consumer, we went back to alternative, more animal friendly systems. This is accompanied by a rise in the occurrence of old diseases. After the switch, apart from colony housing, laying hens are no longer kept in cages. In modern housing styles birds have more contact with other birds, but also with manure, dust and pathogens. The latter, such as bacteria, viruses and parasites, will quickly spread through the house. Pathogens can be found everywhere in the birds’ environment. It is in the animals themselves, and is excreted via the gastrointestinal tract. “Particularly ill birds excrete vast amounts of pathogens with the manure. Chickens tend to examine everything, including each other’s excretions. Eating each other’s manure brings them into contact with high levels of pathogens. The disadvantage of free-range systems is that pathogens are difficult to eliminate. After all, excavating a meter of soil is not a realistic solution. As pathogens remain in the environment animals can be infected again and again.”
Also stress has a role to play in laying hens that are kept in free-range systems. When stress hormones were measured, birds that interacted a lot with other chickens did not necessarily have less stress than animals that lived in separation. Changing animal contacts and the interaction with the environment for feed and water turned out to be rather stressful.
This all led to the return of outbreaks of several old diseases, such as worm infections, blackhead (Histomonas), Salmonella gallinarum, coryza, Pasteurella multocida, E.coli and Avian influenza. “Comparing the outbreaks of high pathogen aviary influenza in 2003 and 2014, the situation 2003 was a reassortment of low pathogen strains to a high pathogen strain, spreading in a poultry dense area. In the fall of 2014 it concerned an introduction of a high pathogen virus in closed housing systems, which didn’t spread.” De Bruijn’s findings are not restricted to old diseases alone. Also a few new illnesses appeared. “Since the new decade, we saw significantly more chronic and clostridium related enteritis, Mycoplasma synoviae and AIS in laying hens.
According to De Bruijn the poultry industry ought to guard itself against diseases. “There is no link to company size or hygiene. Pathogens are widely spread and it can be considered to be an illusion we will once completely get rid of them. When a chicken will be in contact with pathogens, there is a chance of infection and possibly death. However, we can guard ourselves to diseases. Vaccinating free-range hens is a possibility for diseases such as Pasteurella multocida and coryza, although this should be done several years in a row. Unfortunately it is not an option for every disease.”
Besides vaccination, a few measures can be taken in order to stimulate a healthy immune system. “Eventually it is about the balance between immune system and infection levels. If a bird is flooded with pathogens from its environment, this has a vast impact on its immune system. Take care of the overall health of the animals. Pay attention to treating red mite, which can be an enormous stressor when a bird suffers continuous itching and is kept awake by it.”
Wild birds may be carriers of pathogens, just like other species present at and around poultry farms. Adjustments of the open-air runs may prevent wild birds from entering. The difficulty is that, once infection has taken place, a reservoir is created in the house’s environment. There is a chance the disease will return round after round. You can try to break the circle by vaccinating and try to eliminate the disease when it occurs on lower levels.”
In literature a soil crop cover is mentioned as a successful way to prevent worm infections from spreading. Yet, for a commercial house keeping the soil green may turn out to be challenging in reality. De Bruijn underlines the importance of consulting a veterinarian when a poultry farmer is dealing with losses or poor performance. “Use diagnostics to identify the root cause and to estimate the prospects for future flocks. This will help you to keep health issues at the farm under control, and it will lead to a timely solution.“