Low-pathogenic H5N1 avian influenza virus detected in laying hens
In the first week of January 2024, GD received blood samples from an egg farm for AI-ELISA, for mandatory monitoring purposes. The blood samples from a single free-range barn tested positive for antibodies to H5. Follow-up diagnostic testing showed a low-pathogenic H5N1 virus.
The virus subsequently spread to the four other houses that were not free range. It should be noted that the laying hens in the three houses that were first infected did not exhibit any clinical symptoms. The birds in the two houses that were infected last showed very severe disease symptoms with elevated mortality, a substantial drop in feed intake and a fall in production of eighty percent. Although the virus did not mutate into an official highpathogenic avian influenza virus, it did have a significant increase in pathogenicity (causing more sickness) as it spread between the bird houses. While the virus was officially still classified as low-pathogenic the farm was not subjected to a cull, but remained under embargo during the months that the virus was present. Production hardly recovered in the two houses where there were clinical symptoms. On post-mortem egg concrements were found in the oviducts and the birds had extensive peritonitis. Histological examinations of the oviducts showed oedema, connective tissue build-up and marked loss of glandular tissue. With IHC staining the virus was found to be present in the affected oviducts. This lack of glandular tissue explains why many hens did not start producing again. Abnormalities in egg shape and poor shell quality were observed in some of the birds that did start laying again.
Septicemia by Bacillus cereus in broiler breeders (including pullets)
Septicemia caused by the bacterium Bacillus cereus was diagnosed in two boiler breeder flocks (of a slower-growing breed) at two different farms at the end of rearing and just after transfer; this led to increased mortality. At one of the breeder farms this appeared in three successive flocks, twice during the rearing period and once shortly after transfer.
Necropsy
The findings on post-mortem featured a necrotising hepatitis with haemorrhages in the liver and also in the proventriculus, caeca and spleen. Swelling of both liver and spleen were seen in some birds. The bacterium could be cultured from various organs. All the birds examined appeared to be in normal to good condition.
Genetic research
B. cereus isolates were collected from all the outbreaks except the very first. The genetic material of these isolates is being studied using whole genome sequencing. This will allow us to determine how closely related the isolates are within a single outbreak and between the various outbreaks. This will give us a better understanding of the epidemiology of these unusual outbreaks.
Results of a study of the origins of a new reovirus cluster, genotype 4.7
A large increase in the number of cases of reoviral tenosynivitis was observed in 2023. In these cases genotyping to determine the type of reovirus is standard procedure. It was striking that 99 out of 223 cases from the post-mortem room of GD were shown to be caused by a reovirus within a single new cluster, which has been named genotype 4.7 (GT4.7). This consisted of 78 primary cases, 11 at another house on the same farm, and 10 recurrences in a new flock at a previously infected farm. Of the 78 primary cases, 66 were broilers of slowergrowing breeds, 11 were regular broilers and one case was of laying birds being reared. This indicates a new strain spreading rapidly within the sector. For that reason, further tests were started on this strain: additional typing and investigation of its origins.
Genotyping
The standard typing for reoviruses is done using a single gene from the genetic material (the σ C gene). To get a more precise view of what was happening, multiple isolates were cultured and from three of them a larger proportion of the genetic material was examined. The genetic makeup of these viruses matched almost precisely, which shows that only a single strain was indeed involved.
Investigation of the origins
In addition, the hatcheries and (insofar as they were known) the parent flocks were determined for all the broiler flocks with reovirus problems. This revealed striking clusters over time among the broiler chickens of slower-growing breeds derived from three hatcheries (see hatcheries 5, 6 and 8 in the figure below).
In two of the 25 parent flocks, a striking clustering of cases could be seen among the offspring, which suggests vertical transmission of the virus from the mother bird to the offspring. In a further three of the parent flocks, limited clustering of three or four cases was observed among the offspring but in these cases there was no clear indication of vertical transmission, even though this could not be excluded entirely. All these cases involved a slower-growing breed. Among the regular broilers and the single rearing flock there was no clustering observed.
Slower-growing Regular
Reared laying birds
Figure 1: Overview of the primary reovirus tenosynovitis cases and recurrences per hatchery, by date of birth. Hatchery 1 is imported birds from abroad (Belgium or Germany); a proportion of the chickens at hatchery 2 come from hatchery 4 and another fraction from hatchery 10.
Spreading within breeders
Spreading within hatcheries is not a known transmission route for reoviruses, but because of the conspicuous clustering at that level, the possibility of spreading within breeder farms was investigated further. No clear indications were found for spreading within a hatchery, although this could not be completely excluded in some cases.
Conclusion for the 2023 outbreak
The data collected show a single reovirus strain that spreads rapidly, including vertical transmission in a few cases. The extent to which spreading within the hatchery can be involved is unclear. The recommended approach is prevention of introduction of the virus in the house through good biosecurity, cleaning and disinfection. The suspected cases of vertical transmission also stress the importance of biosecurity in parent farms. Furthermore, good hygiene practices within the hatcheries should be maintained. The current investigations did not look specifically at the effect of protection from maternal antibodies; the average reoviral-titers in Ducth breeders flocks do not seem to have changed all that much since our last study on this, in 2019. While many flocks offer good titers, improvement is still possible in this regard.
Recent update: situation for the first quarter of 2024
After relatively few cases in January 2024, the number of post-mortem submissions with reoviral tenosynovitis rose sharply again in February and March. In the first quarter of 2024 there were 11 cases among regular broilers, 97 among broilers of slower-growing breeds, and one case in other poultry. Based on the genotyping, the overall picture is the same as in the second half of 2023. The majority of cases were genotype 4.7; the remainder were split between genotypes 1, 2 and 4.
Intestinal dilation in laying hens
GD received multiple signals from practicioners about laying and breeding flocks with disappointing production figures. Hens in these flocks were seen to stop producing and on post-mortem they had dilation (widening) of a specific part of the jejunum*, sometimes extending into the ileum*. This syndrome has been known for about a decade now, under the name IDS (intestinal dilation syndrome). The cause of the condition is unknown.
For a case to classify as IDS, certain characteristics have to be present that can be seen under the microscope (histological examination). During the period 2022 to 2024, GD received hens from sixteen flocks of white laying hens and three submissions from mother birds from various brown breeds with the typical gross findings
Although the intestinal abnormalities looked similar to the naked eye, microscopy revealed significant differences between these cases and not all cases fitted the diagnostic criteria for IDS. There seems to be no relationship with age: the affected birds ranged in age from 24 to 91 weeks.
Further investigations
To see whether a pathogen might be present in the intestines that had been missed so far, GD recently started using the PathoSense platform for new suspected IDS cases in the necropsy room. In addition, there is a collaboration on IDS with Utrecht University in an effort to gain more insight in this disease syndrome.
* The jejunum and ileum are the middle and final parts of the small intestine respectively.
Photo 1. Intestinal dilation in a laying hen (Source: GD)
Animal health barometer for poultry
Disease/disorder/ health characteristic
Execution decree (EU) 2018/1882 of the Animal Health Law (AHL) (EU) 2016/429 (Category A disease)
Avian influenza (AI) in the Netherlands (H5/ H7)
(Source: GD, WBVR, national government)
ND in the Netherlands
(Source: GD, WOAH)
Highly pathogenic AI (H5/H7)*:
* In commercial poultry and in non-commercial kept birds with >50 birds.
Serological monitoring by GD: (first detection in flock) (antibodies for H5/H7)
Commercial poultry:
Execution decree (EU) 2018/1882 of the Animal Health Law (AHL) (EU) 2016/429 (Categories B through E)
Avian influenza (AI) in the Netherlands (H5/H7)
(Source: GD, WBVR, national government)
Low pathogenic AI (H5/H7): Not detected Not
Campylobacteriosis No data available
Avian mycoplasmosis (Source: GD)
Mycoplasma gallisepticumA Serological monitoring by GD: Reproduction sector: Layer pullets: Layers: – not vaccinated and infected: – vaccinated and infected:
Cases in EWS C based on positive serology and/or voluntary PCR testing:
Reproduction sector: Layers: Turkeys: Non-commercial kept birds:
M. meleagridis (Source: GD)
Salmonellosis (non-zoonotic salmonella) (Source: GD)
Salmonella arizonae
Salmonella
Gallinarum (SG) Commercial poultry: Non-commercial kept birds:
Salmonella
Pullorum (SP)
West Nile fever
Article 2.1 Designation of animal diseases in the ‘Rules for Animal Health’ of the Dutch Animals Act
Avian chlamydiosis
(Source: GD)
Disease/disorder/ health characteristic
Article 2.2. Designation of zoonoses in the ‘Rules for Animal health’ of the Dutch Animals Act Salmonellosis (zoonotic salmonella) (at the flock level) (Source: NVWA)
Salmonella Enteritidis
Salmonella Typhimurium
Other types of salmonella
(S. Hadar, S. Infantis, S. Java, S. Virchow)
pullets:
pullets:
Other WOAH-list poultry diseases in the Netherlands subject to compulsory notification Duck viral hepatitis (Source: GD)
Gumboro disease (IBD)
(Source: GD; EWS)
Infectious bronchitis (IB)
(Source: GD)
Infectious laryngotracheitis (ILT)
(Source: GD; EWS)
Mycoplasma synoviae B
(Source: GD)
Reported in EWS C : Broilers:
Layer breeder pullets:
Layer pullets:
Non-commercial kept birds:
Types most commonly detected by GD:
Broilers: Layers:
Reported in EWS C :
Layer breeders:
Layer pullets:
Layers:
Broiler breeders:
Broilers:
Serological monitoring and/or dPCR GD:
Broiler pullets:
Broiler breeding:
Broiler breeder pullets:
Broiler breeders:
Layer pullets:
Layer rearing:
Layer breeder pullets
Layer breeders:
Layer pullets:
Turkey rhinotracheitis (TRT)
(Source: GD)
Detected by GD:
Broiler reproduction sector (incl. pullets):
Layer reproduction sector (incl. pullets)
Broilers:
Layer pullets:
Layers:
Meat turkeys:
QX(D388) 4/91-793B
QX(D388) 4/91-793B QX(D388) 4/91-793B
4/91-793B
Continuation of table
Disease/disorder/ health characteristic
Other poultry diseases
Avibacterium paragallinarum
(Source: GD; EWS)
Histomonosis
(Source: GD)
Pasteurella multocida
(Source: GD)
Erysipelas (Erysipelothrix rhusiopathiae)
(Source: GD)
Reported in EWS C :
Layers:
Detected by GD:
Reproduction (meat sector):
Reproduction (layer sector):
Layer pullets: Layers: Meat turkeys:
Detected upon necropsy:
Broiler breeder pullets:
Layers: Ducks: Turkeys:
Detected by GD: Layers:
Ç Increase or strong increase
Ç Limited increase
- Situation unchanged
È Limited decrease
È Decrease or strong decrease
A Based on serological monitoring
B Based on serological monitoring and/or differentiating Ms-PCR
C Early Warning System
Animal health monitoring
Since 2002, Royal GD has been responsible for animal health monitoring in the Netherlands, in close collaboration with the veterinary sectors, the business community, the Ministry of Agriculture, Fisheries, Food Security and Nature, vets and farmers. The information used for the surveillance programme is gathered in various ways, whereby the initiative comes in part from vets and farmers, and partly from Royal GD. This information is fully interpreted to achieve the objectives of the surveillance programme – the rapid identification of health problems on the one hand and the following of more general trends and developments on the other. Together, we team up for animal health, in the interests of animals, their owners and society at large.