Marek’s disease virus (MDV) belongs to herpesviruses causes a lymphoproliferative, immunosuppressive and neuropathic disease that affects the nerves, viscera, muscle, and skin of chickens. Chickens may become persistently infected without showing any clinical signs. There are three species of MDV. Marek’s disease is primarily controlled by vaccination either in ovo at day 18, or by subcutaneous injection at day of hatch. The need for vaccination against MDV has a significant economic impact on the poultry industry.

Bivalent vaccines containing serotypes 1 and 3 or trivalent vaccines containing serotypes 1, 2, and 3 are used. These vaccines contain live virus and although they prevent tumor production they do not generate sterilizing immunity. Vaccinated chickens still get infected and can shed virulent field virus.

Several different modified live virus vaccines are available to control Marek’s disease. Turkey herpesvirus (HVT or MDV-3) is an avirulent virus that can effectively protect chickens against MDV.

Newcastle Disease is a serious respiratory disease caused by virulent strains of avian paramyxovirus serotype 1 of the genus Avulavirus. All strains of the virus (NDV) are contained in a single serotype, but they are divided into two classes, class I and class II. Class II is then divided into 16 genotypes.

 Class 1 viruses are primarily found in wild birds because NDV can infect many different avian species.

Class II NDVs vary greatly in their virulence for chickens. They are classified as velogenic—rapidly lethal; mesogenic—intermediate; and lentogenic—relatively low virulence, based on their lethality for chick embryos. For example, class II, genotype II strains are so lentogenic that some, such as Hitchner B1 and LaSota, can be used in modified live vaccines.

Inactivated vaccines are given by the intramuscular or subcutaneous routes. They are often given to layers or breeders to provide persistent high antibody levels that can be transferred to their chicks.

Modified live lentogenic or mesogenic strains are used in MLV-NDV vaccines. The live vaccines are usually grown in embryonated chicken eggs or in tissue culture. The mesogenic vaccines cause mild disease so they are generally used in countries where Newcastle disease is endemic. In countries largely free of ND only lentogenic strains are permitted. These live vaccines are given in drinking water, by coarse sprayer, or by intranasal or intraocular administration. A lentogenic strain is also available for in ovo use.

Recombinant vectored NDV vaccines using turkey herpesvirus or fowlpox vectors incorporating the hemagglutinin gene or the F gene, or both, are also available. Some of these may be appropriate for in ovo vaccination. NDV itself may also be used as a vector for other vaccines such as those against IBD or avian influenza.

Infectious bursal disease Also called Gumboro disease, the causal agent of IBD is an avibirnavirus. It infects multiple bird species, but causes clinical disease only in chickens less than 10 weeks of age. The virus destroys B cells within the Bursa of Fabricius resulting in bursal atrophy and subsequent suppression of the antibody responses. This immunodeficiency will result in a poor response to other vaccines and overwhelming secondary infections. There are two serotypes of IBDV but severe bursal disease is only associated with serotype 1. All commercial vaccines are directed against this serotype. There are no reports of clinical disease caused by serotype 2. As with all RNA viruses, IBDV is rapidly evolving and as a result there is much variation in antigenicity and virulence, features that complicate vaccine development.

Many different types of IBD vaccine are available, both monovalent and in combinations. These include live attenuated vaccines, inactivated oil-adjuvanted vaccines, live recombinant vaccines, or even immune-complex vaccines. Because this disease affects very young chicks it is important to exploit maternal immunity by vaccinating hens.

The inactivated vaccines are water-in-oil adjuvanted products. They are mainly used to induce long-term immunity in breeding stock. They are best used in birds at 16 to 20 weeks that have been primed by live vaccines at 8 weeks of age.

The viral structural protein 2 (VP2) is the major protective antigen in IBVD.

Modified live vaccines have been attenuated by serial passage in tissue culture or eggs. Depending on their degree of attenuation, live attenuated IBDV vaccines may be classified as mild, intermediate, or invasive based on their ability to replicate and cause bursal lesions. This also reflects their ability to overcome maternal immunity. Mild vaccines are used to prime broiler breeders before boosting with an inactivated vaccine. If chicks have maternally-derived antibodies, then vaccination should be delayed until this has waned. The mild vaccines show poor efficacy in the presence of maternal antibodies or against very virulent strains of IBDV. The intermediate or hot strains are more immunogenic but may induce bursal lesions. The vaccine is usually given in a spray or in drinking water.

Infectious bronchitis is an economically significant respiratory disease of chickens that also causes nephritis, decreased egg production, poor growth, and high morbidity. It is caused by a gammacorona virus, avian infectious bronchitis virus (IBV). The combination of high morbidity, and loss of performance, together with secondary bacterial infections can lead to unsustainable losses. As a result, almost all commercial poultry are vaccinated against this virus.

Inactivated vaccines may be used alone or in combination with modified live virus (MLV) vaccines in layer/breeder flocks to induce maternal immunity and thus protect chicks from an early age.

Modified live IBV vaccines containing three common serotypes are administered in the drinking water, or by coarse spray, and given at day one or within the first week. Some short-lived broilers receive only this single dose. For longer-lived broilers, a second dose is generally given two to three weeks later. Long-lived broiler breeders and layers receive multiple vaccine doses at two, four, and six weeks. Revaccination after that depends upon the local threat assessment.

Infectious laryngotracheitis An economically important respiratory disease caused by herpesvirus 1, ILT affects chickens, peafowl, pheasants, and partridges. The principle lesion is tracheitis and the disease can vary in severity from lethal asphyxiation to very mild or subclinical infection. As with other herpesviruses, infected birds may become healthy carriers. The disease is usually prevented by the use of either live attenuated vaccines or recombinant vectored vaccines.

As a result of residual virulence in modified live vaccines, efforts have been made to generate safer vaccines by developing viral vectored vaccines. They are also administered by subcutaneous vaccination to one-day-old chicks or in ovo.

Avian reoviruses belong to the genus Orthoreoviruses in the Reoviridae family. They cause arthritis/tenosynovitis, proventriculitis, a runting-stunting syndrome, and “blue-wing disease” in broilers. Because these diseases affect very young birds, reovirus vaccines are often administered to breeding hens to stimulate maternal immunity and protect the newly hatched chicks. Both inactivated and modified live vaccines are available.

The inactivated oil-emulsion adjuvanted vaccines may contain multiple strains and different pathotypes. They are used in replacement and breeder hens and are often used in combination with NDV, Marek’s or bronchitis vaccines.

Avian influenza vaccination against HPAI is actively discouraged or banned because it interferes with the detection of infected flocks. Vaccination alone is not a solution to the problem of HPAI, or H5/H7, low pathogenic avian influenza (LPAI) viruses because it raises the possibility that these strains could become endemic in avian populations.

Vaccination may be used in control programs for both HPAI and LPAI. The recent emergence of pandemic influenza A strains such as H7N9 and H5N1, reveals the tremendous challenges to our current influenza control strategies.

Fowl pox is caused by an Avipoxvirus, a large, complex DNA virus. They are transmitted through aerosols or by biting insects. It is a slowly spreading infection characterized by proliferative skin lesions (dry pox) on unfeathered skin, or by diphtheritic lesions in the mucosa of the mouth, esophagus, larynx, or trachea (wet pox). The latter can result in asphyxiation of young chicks. In general mortality is low but may reach 50% in stressed flocks. Modified live fowlpox or pigeon poxvirus vaccines attenuated in cell culture or embryonated eggs are available. They may be given as monovalent vaccines or in combinations. Most are administered into the wing web after maternal immunity has waned. Some are administered subcutaneously to one-day-old chicks and there is also an in ovo recombinant vectored vaccine available that expresses ILT antigens. They may be used in situations where the disease is endemic because the infection spreads relatively slowly and may be administered in the face of an outbreak. They have also been used in pigeons, turkeys, and quail, in addition to chickens.

Avian encephalomyelitis: The cause of epidemic tremor, avian encephalomyelitis virus is a picornavirus that affects the central nervous system. In young chickens it induces paralysis, ataxia, and muscular dystrophy. In older chickens, infection is usually subclinical but causes a decline in egg production and hatchability. Several modified live vaccines are available. Some may be combined with fowl pox. Most are administered by wing web vaccination using a double needle applicator. Breeder chickens are vaccinated at 10 to 16 weeks of age, at least 4 weeks before start of lay. The site of inoculation should be examined for “take” at 7 to 10 days postvaccination. A positive take is indicated by a swelling or scab at the site of inoculation. If given to laying flocks this vaccine can cause a serious drop in egg production.

Egg drop syndrome (EDS) is caused by an adenovirus infection in laying hens. It is characterized by production of soft-shelled and shell-less eggs and also a 10% to 40% drop in egg production. An inactivated vaccine containing EDS’76 virus strain BC14 in a water-in-oil emulsion may be available. It should be administered intramuscularly to layers and breeders no later than 4 weeks before the expected onset of lay.