By Dr Beth Watson Bsc DVM
How does the immune system work?
The key role of the immune system is to recognise and destroy anything it recognises as foreign and potentially harmful to the body. Once alerted to the presence of an invader, or antigen, white blood cells become active. There are several types of white blood cells in the body. Some white blood cells (called lymphocytes) make antibodies. Antibodies stick to foreign invaders (antigens), like a tagging system, marking that molecule for destruction by other white blood cells. Antibodies are specific to each antigen, attaching like a key to a lock. On the first exposure to a particular antigen, whether it be bacterial, viral, fungal or an allergen, it can take several days to develop and manufacture antibodies to identify that particular molecule. Once the body has been exposed to an antigen, the immune system can store the corresponding antibodies and develop memory. The immune system develops a library of antibodies in response to the various invaders it has encountered. On the subsequent exposure to any antigen, the immune system can pull up antibodies from a its exposure library and respond so quickly. This means you are less likely to become sick. Exposing the immune system to pathogens that might cause disease in a controlled manner is the basis of how a vaccine works.
What is a vaccine and how does it work?
A vaccine works by harnessing the memory of the immune system. There are two main active components of a vaccine. Firstly, the antigen- this is commonly either a killed pathogen, parts of a pathogen or weakened versions of pathogens. This is combined with an adjuvant; these are agents that stimulate the immune system. When these two things are introduced to the body, the immune system is stimulated to respond by making antibodies to the pathogen. The antibodies are added to the immune library. On the next exposure, the body can recall the antibodies from storage, mark the antigen and remove it, in most cases without outward signs of disease.
What are the key vaccines of horses?
Tetanus
Tetanus is a commonly occurring spore producing bacteria that lives in soil, dust and animal faces. The spores of this bacteria produce a neurotoxin that causes rigid muscle paralysis. While signs may appear non-specific at first, the disease quickly progresses to generalised spasticity. Horses may display prolapsed third eyelid and extension of the limbs, head and neck. Due to muscle paralysis, eventually the horse is unable to breathe and will die of respiratory failure.
As this condition is most often fatal and is widespread in the environment, this vaccine is recommended for ALL horses.
Strangles
Strangles is a bacterial infection of the upper airway in horses. Horses present with flu like symptoms of nasal discharge and fever. The disease causes enlargement of the lymph nodes, which can on occasion impair breathing. Although mortality is rare, it is highly contagious. Rest and recovery periods after disease can mean significant time out of work for competition horses. Containment of the disease can pose a significant biosecurity challenge for large farms or areas where there are dense populations of horses.
As this condition is very challenging to manage in an outbreak and is present in the environment, this vaccination is recommended for ALL horses. Unfortunately, the strangles vaccine is not as effective as the tetanus vaccine. If your horse is likely to be exposed to
Strangles, it may be sensible to administer a booster at 6 monthly intervals to optimise its efficacy. However, in many cases a yearly booster is sufficient. It is possible for disease to occur in horses which have been vaccinated, however vaccination is still recommended to reduce spread and severity of disease.
Hendra
Hendra is a potentially fatal virus that is spread from flying foxes to horses. It may then be passed from infected horses to humans. Hendra virus is highly lethal in both horses and humans, with mortality rates approximately 79% and 60% respectively.
Bats known to carry the disease reside from Northern Queensland to Adelaide and also in the North West of WA. Hendra infection is most likely to occur in areas where there are both large numbers of horses and bats, as in the case with the ‘Hendra belt’ located from Queensland to Northern NSW.
Hendra is challenging to diagnose in the field as the signs are non-specific and may range from acute febrile illness to lethargy and depression with progression to respiratory and neurological disease. In its acute stage, Hendra may be indistinguishable from colic. Some practices on the East Coast of Australia consider that the risk of examining unvaccinated horses is too high given the potentially fatal consequences of infection.
The likelihood of Hendra virus infection occurring in WA is considered low, because of the low numbers of horses where flying foxes occur and the relatively low concentrations of flying foxes. However, any horses traveling to Hendra endemic areas should be vaccinated. For competition horses, Hendra vaccination may be mandatory at some venues on the Eastern Coast of Australia. Horses travelling from Hendra endemic areas should also be vaccinated to facilitate safe examination should they become ill on arrival to Western Australia.
Horses imported from interstate, especially Queensland or Northern New South Wales or which have had contact with horses from these areas, should be isolated for 20 days and monitored for signs of Hendra virus.
Equine Herpes Virus
Equine Herpes Virus 1 and 4 are the common strains seen in Australia. Affected horses may suffer respiratory disease, neurological disease and abortion (in pregnant mares). Young foals are also at risk and can die from the infection. Due to the risk of abortion and illness in young foals, this virus is particularly problematic for stud farms.
This vaccine is commonly used in areas with dense populations of breeding horses. Vaccination may be mandatory for mares travelling to stud as stipulated by the stud in the breeding contract. We recommend vaccination against H
Herpes in breeding mares to reduce the risk of late term abortion due to Herpes Virus.
Rota Virus
Rota Virus is a viral disease of foals causing diarrhoea. It is one of the most common causes of diarrhea in foals under 6 months. Early detection and treatment of the disease often leads to a positive outcome, however on some farms where Rota Virus is endemic, vaccination of mares prior to foaling may assist in management of this disease. Several doses are required to facilitate passive transfer of antibodies to foals against Equine Rota Virus. This vaccine is not commonly used in Western Australia.
Salmonella
Salmonella is a bacterial cause of diarrhoea in both adult horses and foals. Salmonella can also cause joint ill in foals. Foals living on densely populated farms are most at risk from this disease. This vaccine can be used on farms where the disease is endemic. It can be both administered to mares to increase the antibodies in colostrum or to foals themselves from 4 months of age. This vaccine is not commonly used in WA.
Equine influenza
Equine influenza is a highly contagious cause of upper respiratory disease in horses. Thankfully it has been irradicated from Australia. As an exotic disease, there is no vaccine available. There is one exception to this, that is any horse to be exported overseas may be granted a permit for vaccination against equine influenza. This may be administered by a registered veterinarian prior to export.
How are vaccines given to horses?
A small volume of liquid is injected into the muscle of the horse, often the neck, rump or pectoral region. Depending on the age of the horse, previous vaccination history and what vaccine is given will depend on the number of doses and dosing interval required to develop immunity.
Recommendations for your horse:
Deciding on a vaccine regime for your horse will require your veterinarian to carry out a risk assessment. This assessment will depend on several factors such as:
- Exposure (weather; where you live; presence of other animals, age, use, sex and reproductive status of your horse)
- Potential risk of adverse reactions (for example- does your horse have concurrent disease?)
- Severity and consequence of the disease (is there potential for the disease to infect humans? Is there potential for the disease to cause harm or death to the horse?)
Recommendation in WA
See attached info graphic
A final word on vaccination:
No vaccination can guarantee 100% immunity against disease. Owners should always be on the lookout for signs of disease in their horses. All horse properties should have a biosecurity and quarantine plan. For further advise please consult your veterinarian.
Resources:
https://www.agric.wa.gov.au/livestock-biosecurity/strangles-horses
https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0020/150815/horse-health-vaccination-against-tetanus-and-strangles.pdf
https://aaep.org/guidelines/vaccination-guidelines
https://aaep.org/risk-based-vaccination-guidelines/equine-herpesvirus-rhinopneumonitis
Wang J, et al. (2021). A new Hendra virus genotype found in Australian flying foxes. Virology journal 18(1): 1-13.
Hume E Field,(2016) Hendra virus ecology and transmission, Current Opinion in Virology,16:120-125
https://www.zoetis.com.au/diseases/horses.aspx
https://www.zoetisus.com/products/horses/equine-rotavirus-vaccine.aspx
https://www.zoetis.com.au/product-class-new/vaccines/equivac-est-equine-salmonella-vaccine.aspx
https://www.vetvoice.com.au/ec/diseases/equine-influenza/
