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Six thoroughbred horses formed a study group which was immunized twice with a vaccine (Equigrip, Rhone-Merieux), the control group was six unvaccinated horses. Blood was collected for hematological examinations, cellular immune responses, specific humoral immunity (HI) at days 0, 17, 34, 83, 117 and 123 of the experiment. On day 117 of the experiment, 3 horses of the experimental group and 3 horses of the control group were infected intranasally with a mixture of 2 subtypes (A-l and A-2) of the influenza virus. It is worth noting that 17 days after the first vaccination the lymphocyte stimulation index increased in all horses in the experimental group, while in four of the six unvaccinated horses (control) the index decreased. During this time a statistically significant increase of the specific antibodies titres was not found in vaccinated horses. Six days after the infection, the fairly high level of specific humoral immunity was accompanied by a strong stimulation of cellular immunity, which was expressed as a statistically significant increase in the lymphocyte stimulation index (as determined by the specific blastic transformation test) and was higher in vaccinated horses. Reisolation of the virus, which was attempted five days after the challenge from the nasal cavities of vaccinated horses was unsuccessful, while influenza virus subtype 2 was isolated from each of the vaccinated horses.
Equine influenza is highly contagious and spreads rapidly among susceptible horses. The disease occurs globally and is caused by two main strains: H7N7 and H3N8. The H7N7 strain has not been isolated since the 1980s, and H3N8 circulates in equine population throughout most of the world. The H3N8 virus has diverged into two antigenically and genetically different evolutionary lineages since the 1986s: the American and European ones. Equine influenza exists in an endemic form in many countries. Transmission of the influenza virus from one host species to another is a crucial feature of its ecology and epidemiology. Two basic mechanisms of interspecies transmission are possible. One is the direct transfer of an essentially unaltered virus from one species to another. The second mechanism is a consequence of the segmented nature of the influenza genome and genetic reassortment.
Influenza virus is the main etiological agent of respiratory diseases in horses. Equine influenza virus is represented by two different serotypes: H7N7 and H3N8. The H7N7 strain has not been isolated since 1980 and H3N8 circulates in equine population throughout most of the world. Inactivated and subunit vaccines are most commonly used in order to prevent infections. These vaccines, in contrast to natural infections, do not induce either cytotoxic lymphocytes T or mucosal antibodies, nor do they provide long-lasting immunity. The introduction of new types of vaccines has become necessary and the first group of new generation vaccines is those containing live viral strains. They may be produced by attenuation in low temperature or by reverse genetics. Viral vector vaccines may be included in this group of vaccines. The next group of new generation of vaccines consists of a DNA vaccine and an inactivated vaccine with new adjuvants.
Influenza virus plays an important role in respiratory diseases in horses. Equine influenza virus is represented by two different serotypes: H7N7 and H3N8. The strain H7N7 has not been isolated since 1980 and H3N8 circulates in the equine population throughout most of the world. For preventive and prophylactic measures inactivated and subunit vaccines are most commonly used. Contrary to natural infections, traditional vaccines induce neither cytotoxic lymphocytes T nor mucosal antibodies and they do not provide enduring immunity. There is also the difference in the immune response as the natural infection induces IgA, IgGa and IgGb antibodies whereas the traditional vaccines induce IgGc and IgG(T) and no IgA. The low efficacy of the traditional vaccines also depends on an antigenic drift of the surface glycoprotein - hemagglutynin.
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