Production system: (a) Broiler and other types of breeder flocks

(b) Commercial egg laying flocks

1. INTRODUCTION

Marek’s disease is a transmissible lymphoproliferative disease caused by a herpesvirus. The malignant tumours occur in internal organs and peripheral nerves. Feather follicles in the skin are the only site of virus replication. Feathers, dander, dust and litter contain the virus, which is spread by the airborne route. Good hygiene practices are important to control the disease.

Vaccination is an effective way of controlling Marek’s disease. However, "vaccine breaks" do occur. There are many different strains of the Marek’s disease virus and vaccines may not protect all birds against very virulent strains of the virus. Vaccines may also fail to protect vaccinated birds if they are exposed to the virus before vaccinal immunity develops or if exposure to other diseases (eg, infectious bursal disease) depresses their antibody response to the vaccine. In some cases, poor vaccination procedures are responsible. Some strains of chickens have considerable resistance to Marek’s disease.

Very virulent strains of Marek’s disease have been identified in the USA, Italy and some other countries but I have not yet determined whether they are present in the UK. Marek’s disease vaccines currently licensed for use in the UK are not considered very effective against the very virulent strains of the virus (Powell & Lombardini, 1986).

2. PREVALENCE/INCIDENCE

2.1 Population at risk

Marek’s disease may occur in chickens from 3 weeks of age but is most common in birds between 12 and 24 weeks of age. Therefore, it mainly affects sexually immature birds in broiler breeder flocks and commercial egg laying flocks.

Losses from Marek’s disease in broilers in the UK are thought to be very low and most producers do not find it worthwhile to vaccinate (Ansell, 1988).

The low level of losses from Marek’s disease in broilers is supported by data from a recent survey of causes of carcase rejection in a poultry processing plant in the UK (Yogaratnam, 1995). Around 5 in every 10,000 broilers with high carcase rejection rates were condemned because of tumours. As only a proportion of the tumours would be due to Marek’s disease, the level of losses is very low.

In view of the above, it is suggested that the cost of Marek’s disease is estimated for breeder and commercial egg laying flocks only.

2.2 Prevalence of infection

Marek’s disease virus is common where chickens are raised. Therefore, most chickens will have antibodies to the virus.

2.3 Prevalence of clinical disease

Clinical signs of Marek’s disease include depression, weight loss, starvation and paralysis. Most birds with clinical signs of Marek’s disease die. Therefore, mortality rates from Marek’s disease roughly equate to the incidence of clinical disease.

Since the introduction of vaccines against Marek’s disease in the early 1970s, losses in the UK have been very low. Data on mortality rates are given in section 5.1.

Over the last 10 years, the VI Centres have diagnosed around 60 cases of Marek’s disease each year (range: 45 to 93 cases). The background to these cases is not known.

3. CONTROL METHODS

Day old chicks are injected intramuscularly or subcutaneously with 0.2 ml of a live Marek’s disease vaccine usually before they leave the hatchery.

Marek’s disease vaccines licensed for use in the UK are of two types:

(a) HVT vaccine (herpesvirus of turkeys)

(b) Rispens CVI 988 strain of Marek’s disease virus

Vaccination with HVT is considered 80 to 100% effective in preventing clinical Marek’s disease. To increase the effectiveness of vaccination, HVT may be used in combination with Rispens CVI 988 strain. Details of the vaccines are given in table 1.

For the purpose of this project, it may be assumed that all day-old chicks destined for breeder and layer flocks are vaccinated.

The frequency of use of the different vaccines is not known.

Suggested approach to estimating the cost of vaccination:

Minimum cost is the cost of vaccinating all day-old chicks destined for breeder and layer flocks with a single dose of HVT vaccine.

Maximum cost is the cost of vaccinating all day-old chicks destined for breeder and layer flocks with a single dose of HVT vaccine combined with a single dose of Rispens CVI 988 vaccine.

Cost of vaccination should include the cost of the vaccine(s) and the labour cost involved in injecting the birds with the vaccine(s).

Data required:

Cost of vaccines can be obtained from the manufacturers or distributors.

Need information on number of chicks that can be vaccinated per hour. Vaccination can be done individually or using a vaccine machine. If two vaccines are used, they can be combined for administration to the birds.

Need hourly labour cost.

4. CASE TREATMENT

None.

5. DISEASE EFFECTS

5.1 Mortality

5.1.1 Commercial egg laying flocks

Randall et al. (1977) carried out a survey of the causes of mortality in caged layers from the point of lay to slaughter around 13 months later. The survey involved 281,000 birds from 51 flocks on 37 premises in northern England, East Anglia and East Midlands. It was carried out from 1971 to 1973. Marek’s disease was diagnosed in 10.6% of the 2,615 carcases examined. Extrapolating this figure to the population of birds on the farms gave a mortality rate of 1.23% from Marek’s disease. Approximately, 75% of the flocks were vaccinated against Marek’s disease.

In the USA, Marek’s disease accounted for a mortality rate of 2.8% in layers (Ansell, 1988 quoting Purchase, 1977).

of commercial egg laying birds.

5.1.2 Broiler breeder flocks

From a survey of the causes of mortality in broiler breeder flocks on three premises in Scotland, around 0.57% of the birds bought in for laying died from Marek’s disease between 25 and 60 weeks of age (Jones et al., 1978). The birds had been vaccinated against Marek’s disease as day-old chicks. (Note that this is my estimate based on extrapolating their diagnostic sample to the chicken population on the premises. Of 542 birds subjected to laboratory examination, 4.98% had Marek’s disease.)

In the USA, Marek’s disease accounted for a mortality rate of 5.0% in broiler breeders (Ansell, 1988 quoting Purchase, 1977).

5.2 Carcase condemnations

Birds with tumours are condemned at slaughter.

In the USA, 0.07% of slaughtered layers (spent hens) were condemned because of Marek’s disease (Ansell, 1988 quoting Purchase, 1977).

In a survey of condemnation returns from poultry slaughterhouses in England and Wales in 1992/93 (Bremner, 1994), 2.62% of hens were condemned because of disease. The data are not broken down by disease condition but the figure of 2.62% could be looked upon as a maximum level of condemnations from Marek’s disease.

5.3 Drop in egg production

Drop in egg production is considered negligible.

6. REFERENCES