Influenza Canarypox Vaccine Reduces Clinical Signs and Shedding

Researchers found that clinical signs of influenza and virus shedding were significantly reduced in horses vaccinated with a recombinant canarypox-vectored influenza vaccine and experimentally exposed to influenza compared to unvaccinated horses, explained Jules Minke, DVM, PhD, at the 2006 AAEP Convention. Minke was the project leader of Biologicals at Merial, who completed the study with the help scientist at the Animal Health Trust in Newmarket, England.

A vectored vaccine uses a specific piece of DNA from the pathogen against which you want immunity. It is carried into the animal's cells by a carrier or vector that does not cause disease itself. Merial introduced the vaccine in the European market in 2003 and has received USDA approval to market the vaccine in the United States.

Canarypox platform Minke said, "These viruses (canarypox) can't replicate in horses, reversion is not possible (it doesn't mutate into an influenza virus), and horses previously vaccinated with the killed vaccine and subsequently vaccinated with the canarypox vaccine mount a very strong anamnestic response (where the body recognizes an invading antigen and produces antibodies against it)."

A strong feature of this vaccine platform is that vaccination does not induce a neutralizing response against the vector (a common problem with most other vector vaccines), meaning that the vaccine can be used to booster horses over and over again without the body becoming unresponsive to the vectors.

Study Researchers challenged 12 control horses and 12 horses vaccinated with the canarypox vaccine. Vaccinated horses received two injections at Day 0 and Day 36. They were challenged two weeks after the second vaccination with a common strain (related to the Ky/02 virus) of the flu virus They scored horses on the severity and duration of coughing, nasal discharge, anorexia, dyspnea (difficulty breathing), and depression.

"All of the controls developed coughing, and it was very pronounced and prolonged," he said. "Of the vaccinates, there were only two horses that showed coughing for one day."

The most predominant clinical sign in the vaccinated group was nasal discharge. However, compared to the control group, clinical signs were minimal.

"When you look at the nasal discharge in the controls, it was very prolonged and severe," Minke explained. "The duration of the discharge was significantly different (greater) in the controls than in the vaccinated horses."

Minke said dyspnea is a very important and severe sign of influenza. It is indicative of lower airway disease. Dyspnea was only found in the control group (five of 12 horses were positive).

Viral shedding Protection against viral shedding is an important characteristic of any influenza vaccine. Each of the control horses shed virus for about 4½ days. No virus was recovered from any of the vaccinated horses on any occasion during the post-vaccination period.

Humoral vs. cell-mediated Researchers also found that horses vaccinated with the recombinant vaccine had humoral (in the serum) and cell-mediated (in the cells) immunity. Humoral immunity involves antibodies--specific proteins made by B-cells (B-lymphocytes, a type of white blood cell) that physically attach to foreign material and help the body do its job of removing the foreign material. This is the type of immunity stimulated by most killed-virus vaccines.

Cell-mediated immunity protects the body against intracellular organisms using T-cells (T-lymphocytes). The T-cells will recognize a horse cell that has been infected by a virus before the virus has had a chance to replicate.

Most vaccines are able to produce a humoral immune response, but Minke said this is the only flu vaccine he knows of that has been demonstrated to produces a cell-mediated response.

Duration Researchers found that the vaccine mounted significant immunity for at least six months. They also discovered horses vaccinated with the canarypox vaccine were able to produce antibody titers within 14 days after the first vaccine dose, indicating an onset of immunity not typically seen with inactivated vaccines.

Take-home message "We have developed a state-of-the-art influenza vaccine," Minke explained. "This vaccine provides significant clinical and virological protection against the recent highly virulent U.S. strain of influenza (N/5/02).

The vaccine was capable of stimulating both humoral and cell-mediated immune responses, and significant antibody titers were already present 14 days after the first vaccination. Protection lasted at least six months after the initial vaccination program."

The study was funded by Merial.


Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for TheHorse.com .

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