Behind the Bit: Vaccinating your horse: A primer

Harv, my older horse, has a "vaccination routine" which varies only slightly. He's a been there done that kind of horse with a lot of built-up immunity. My other horse Riley is at a tender age, and getting some vaccinations for the first time. I really have to think about what he needs and understand the risks. Another learning opportunity, a new set of google searches, and another blog entry! Here is a summary of the most popular vaccines, the diseases they prevent, and some indications for use.

Coincidentally, the AAEP updated its vaccine guidelines, and the 2008 revised AAEP guidelines are now online. In addition to helpful background information on vaccinations, there are two lists of guidelines: core (all horses should have) and risk-based guidelines (depending on region, age, use (pleasure vs. competition), and other factors.

AAEP CORE VACCINATIONS

Equine Viral Encephalomyelitis. Initial series, then every spring (or spring/fall for southern climes). Killed virus, usually combined with tetanus. Also called sleeping sickness, insects transmit this viral disease of rodents, birds, horses and man. Western Equine Encephalomyelitis (WEE) is nationwide, Eastern Equine Encephalomyelitis (EEE) is in the east and southeast. Infected horses have severe depression, weakness, incoordination, ataxia, stiffness, fever, difficulty eating, and abnormal behavior. About 25-50% of horse infected with WEE die, and the death rate is 70 to 90% of animals infected with EEE. For more information see the EEE/WEE article in The Horse.
Tetanus. Initial series, plus annual. Must be kept up annually or need to re-administer the initial series. An infection from a toxin produced by the bacterium clostridium tetani which is found in the soil. Can occur wherever skin is broken. The vaccination is a modified toxin that stimulates an immune response. Symptoms include muscle stiffness and rigidity, flared nostrils, hypersensitivity, and the legs stiffly held in a sawhorse stance. As the disease progresses, muscles in the jaw and face stiffen, preventing the animal from eating or drinking. More than 80 percent of affected horses die. See The Horse article on Tetanus, rabies, and botulism.
West Nile Virus. Initial series plus spring booster. Spread by mosquitos, 40% mortality rate in horses. Early symptoms include twitching in nose; after paralysis starts prognosis is poor. Fatality rate is 33%. For more information see the West Nile Virus article in The Horse.
Rabies. Initial series, then annual, spring. Killed virus. Rabies is transmitted through bites of skunks and raccoons--on horses bites usually occur on face or legs. Clinical signs include weakness in the limbs, loss of neurologic control of limbs, loss of ability to swallow, profound depression, or furious states where the animal aggressively attacks objects or people. It affects the central nervous system and is always fatal. Also transmissible to humans. See this University of Kentucky publication Rabies in Horses for more info.

RISK BASED VACCINES

Botulism. Initial series then annual. This is a bacterial neurologic disease causing paralysis. Can be caused when the horse eats an animal carcass, but it has been found in newly disturbed earth and in alfalfa hay. For more information see this special report in The Horse.
EPM. Initial series, then annual. Parasitic infection that affects the spinal cord and nerves. It is transmitted through possums and possibly birds. Clinical signs can include weakness, lameness, incoordination, inability to move correctly (especially in the hindquarters) or to stand up, seizures, weight loss, blindness, loss of balance, disuse of a single limb, and/or inappropriate sweating. Lack of treatment can lead to permanent nerve damage and death. For advice on the vaccination see this article from the Horse Channel.
Influenza. Killed virus, usually combined as "flu-rhino". Initial series, then every 3-4 months for horses that show or travel. A highly contagious viral respiratory disease that can be transmitted from horse to horse through coughing/sneezing -- over distances as far as 30 yards. It is not usually fatal but there is no real treatment and horses can be laid up awhile, plus the disease predisposes horses to chronic obstructive pulmonary disease - heaves. Symptoms include nasal discharge, fever, loss of appetite, Horses can be vaccinated according to exposure and need. Viral strains change constantly so frequent boosters are needed. For more information see the Merck Manual chapter on Equine Influenza.
Potomac Horse fever. Initial series, with a spring booster. A parasitic disease causing fever, severe diarrhea, colic, and often founder. High fatality. Most common on the east coast. See article Potomac horse fever more dangerous than West Nile Virus? from vetcontact.com.
Rhinopneumonitis. Killed virus, usually combined as "flu-rhino". Initial series, then every 3-4 months for horses that show or travel. This disease is caused by a herpesvirus similar to the human cold. There are two viruses involved in this disease: equine herpesvirus-1 which protects horses against abortion and possibly the paralysis form. EHV-4 protects horses against the respiratory form. The disease is not usually fatal but can cause prolonged illness. Vaccination does not guarantee protection, but symptoms in vaccinated horses are milder. Also see the rhinopneumonitis article in The Horse.
Strangles. Initial series, then annual, spring. This highly contagious bacterial disease generally appears in the summer. Horses aged 1-5 are most susceptible. Symptoms for this respiratory disease are swollen glands and nasal discharge, fever, depression, inappetance. Internal abscesses are called "bastard strangles." Not usually fatal if treated. See the strangles article in The Horse for details.