1. Field of the Invention
This invention relates to vaccines for immunizing poultry against Marek's disease (Type II leukosis), and specifically to vaccines which are immunogenic and non-pathogenic for Marek's disease, methods of producing such vaccines, and methods of immunizing poultry flocks therewith.
Neoplasia is one of the most common pathologic manifestations found in poultry. The vast majority of avian tumors have their histogenesis in the hematopoietic system and have been classified as the Avian Leukosis Complex. At least two distinct virus types, i.e., a RNA virus (sometimes called RIF virus), and a DNA herpes virus (of which the JM virus was first isolated from chickens as a prototype), can cause leukotic tumors. The etiologic separation and identification of these diseases has led to the terms of lymphoid leukosis, or Type I lymphoid leukosis, for the RNA virus tumors, and Marek's disease, or Type II lymphoid leukosis, for the DNA virus tumors.
The majority of naturally occurring tumors in poultry are Marek's disease types. Currently, the annual losses due to poultry tumors are estimated at 200 million dollars in the U.S.
Marek's disease infections in poultry populations are both common and widespread. Nearly all field flocks are infected to some degree. Most chickens become infected at an early age and remain so chronically. Though infection levels are as high as 100 percent in most cases, neoplasia resulting from such infections may range from a negligible amount in some flocks to well over 50 percent in other flocks.
2. Description of the Prior Art
Much research has been devoted to developing treatments and preventions for Marek's disease, but until recently no satisfactory vaccine was available.
U.S. Pat. No. 3,642,574 describes a vaccine produced from turkey herpes virus (HVT) which is non-pathogenic in chickens and protects, by mechanisms as yet unexplained, vaccinated chickens against development of Marek's disease tumors.
The vaccine described in U.S. Pat. No. 3,642,574 gained immediate and widespread acceptance as soon as it was available to poultry raisers, due to the seriousness of the disease and the unavailability of any effective alternative treatment.
However, the HVT vaccine has recently become suspect as a satisfactory solution to the Marek's disease problem because its effectiveness in preventing tumor development currently appears in many cases to be less than originally reported. This may be due to the fact that HVT vaccine does not produce significant levels of protective or neutralizing antibodies against field infections. It has been determined that inoculating chickens with HVT vaccine results in a chronic viremia, as distinguished from an immunity which depresses or eradicates the infection. Furthermore, chickens vaccinated with HVT vaccine are not immune to a super-infection to Marek's disease virus, and the protection provided by the HVT vaccine is more "cosmetic" than real. Furthermore, original reports that HVT did not grow in mammalian cells have now been questioned.
U.S. Pat. No. 3,590,128 describes a vaccine for the Type I lymphoid leukosis, as distinguished from a Marek's disease vaccine.
U.S. Pat. No. 3,674,861 to Churchill describes an attenuation of any strain of Marek's disease vaccine produced by serial passage of field Marek's disease virus in avian cells. The virus attenuated by Churchill was obtained from infected field birds and includes the HPRS - 16 strain and JM strain. Churchill's objective was to rid the Marek's disease virus of the A-antigen.
It is apparent from the above that there is a tremendous need for an effective vaccine against Marek's disease, and particularly for such a vaccine that will prevent and/or eradicate Marek's disease infection in a vaccinated chicken.