Equine Immunology and Health

The immune system is the integrated body system of organs, tissue, cells and cell products (e.g. antibodies) that differentiates between 'self' and 'non-self', neutralises potential pathogens and prevents development of cancer cells
Horses
1. Naturally live in small herd with little mixing
  1. New disease introduced
    1. Spreads rapidly with a high rate of infection
    2. After recovery, all members of the herd are immune from that infection
  2. New disease exposure is rare in wild horses
2. In stables, live in small groups, with mixing between other horses and humans
  1. Same-site mixing occurs regularly as well mixing with others from different sites in exercise periods and show events
    1. Increases exposure to new diseases and increases the risk of spreading to new horses
  2. New disease introduced
    1. Spreads rapidly with a high rate of infection within the yard and visiting horses
    2. Old horses are particularly susceptible
    3. Various clinical signs exhibited which may cause an accurate diagnosis to be difficult
      1. Hinders treatment and management
Common Infectious Diseases
1. Respiratory Diseases
  1. Equine Influenza Virus
    1. EIV
    2. Damage to the ciliated cells in the trachea
    3. Prevention
      1. Vaccinations
        Induce the horse's immune system
        All horses
        Tetanus, influenza and herpes
        Breeding stallions
        Arteritis
        Should contain virus strains of both American and European lineages to maximise protection
        Heterologous strains of the virus increase the risk of infection from the vaccine
        This increases the infectious period and increases the amount of virus shedding
    4. Treatment
    5. Management
      1. Full disease control measures cannot be in place all of the time
      2. In the event of an outbreak, new arrivals should be quarantined
        Handlers should be different for each horse
        Movement on and off the yard should be stopped
        Separate food and water troughs should be used for all horses
  2. Equine Herpesvirus
    1. EHV 1, 2, 4 and 5
      1. EHV-1 encodes for 76 genes
        Many would have a negative influence on immune response
        Products of these genes may come in many variations, so identifying the key ones can be difficult
        IE gene 64 of EHV-1 has been identified as a CTL target
        Vaccination against this induced specific memory cells, which when reactivated, demonstrated increased CTL activity
        Vaccinations protect horses from infection
        Future aims are to design novel vaccines that aim to stimulate CTLs in the whole horse population
  3. Equine Arteritis Virus
    1. EAV
2. Common Bacteria
  1. Streptococcus equi
  2. Streptococcus zooepidemicus
Vaccination Development
1. Successful for EIV, EAV and West Nile Virus
2. Partial protection with EHV1 and 4 vaccines
3. AHS vaccines are effective in trials
4. EIA has an experimental vaccine
5. Have to think about the individual or the herd
6. First steps in design
  1. Field observations, sequencing the virus, identifying pathological strains, studies in horses, investigations of immunity and identifications of target antigens
    1. Molecular studies of virus gene products are then applied in order to identify new antigens
7. EHV-1 causes respiratory diseases, as well as neurological diseases such as ataxia or paralysis. It can also cause abortion in pregnant mares
  1. Travels throughout the body in leukocytes and endothelial cells
    1. Example of cell-associated viraemia
      1. Gives viruses access to the rest of the body
    2. A vaccine that kills the virus in blood is therefore ineffective, as it would not kill the cells that are already infected
      1. Cytotoxic T Lymphocytes (CTLs) kill virus infected cells
      2. Although the virus is already in cells, it is useful to have the antibodies in order to reduce the amount and duration of virus shed
        Do not prevent infection
  2. In response to infection
    1. T cells increase the number of CD4+ and CD8+ lymphocytes in the blood and lung, induce memory cell production and increase the production of CTLs, which will kill the virus infected cells
      1. CTLs do not kill the free virus
  3. Correlation between immunity to EHV-1 and CD8+ CTL precursors frequency has been identified
    1. Ponies with high CTLp frequencies show reduced clinical and virological signs of the virus (O'Neill et al., 1999)
  4. EHV-1 specific CTLs ahve been shown to kill both late and early infected cells
    1. Advantage to kill them early, before tissue damage and virus spread occurs
    2. CTL target antigens have been identified
  5. High frequencies of CTL are required to lyse virus-infected cells, and high titres of antibodies are required to neutralise both the free virus and the virus released after cell lysis by CTLs
Climate Change
1. Exotic diseases
  1. Equine Infectious Anaemia (EIA)
    1. Been seen in Ireland
    2. Must prepare for outbreak
  2. African Horse Sickness (AHS)
    1. Transmitted by a midge species (Culicoides) that has already transmitted diseases in the UK
    2. Vaccine trials look promising
2. May make the UK suitable for foreign disease to become established
3. Exotic mosquitos are being imported with foreign goods
  1. Potential to transmit various viruses from Europe
    1. Level of transmission lowered due to ecological differences
4. West Nile Virus
  1. Has been transmitted to the UK via birds and mosquitoes
    1. Coordinated surveillance systems monitor these vectors
5. EU program to diagnose infectious disease in wildlife, which act as a reservoir for human and animal infections

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Equine Immunology and Health

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