INFORMATION ON AFRICAN HORSESICKNESS (AHS) FROM
ONDERSTEPOORT BIOLOGICAL PRODUCTS (OBP)
(June 2008)

African Horsesickness received much attention in the media in the past months due to the mortalities it caused. OBP is the only producer of African Horsesickness (AHS) vaccine in South Africa and the vaccine came under scrutiny in the media and television. OBP would like to respond and elaborate on some issues about AHS.

During the past summer season (Jan-April 2008) several outbreaks occurred in South Africa, mostly in the Eastern Cape and Gauteng provinces. This was mainly due to very wet conditions after above-normal rains resulting in extremely high numbers of midges. The Culicoides midge is the main vector in transmitting the disease. It is currently known that AHS virus is transmitted by two important midge vectors: Culicoides imicola and Culicoides bolitinos. In South Africa, the former prefers warmer weather, low altitude and breeds in saturated or wet soils containing high levels of organic material while the latter is also widespread and feed on both cattle and horses but prefers to breed on cattle and game dung. Culicoides bolitinos is more abundant during cooler seasons which explain the reasons for some winter outbreaks. The entomology section of the Onderstepoort Veterinary Institute (OVI) reported a massive explosion in numbers of AHS midge vectors, reaching millions of midges caught on a farm in one night. Though stabling horses at night and using insecticide repellents have been advocated as tools to use for controlling the disease, vaccination is still the best practice and most effective method to both control and prevent the disease.
Sadly, up to 400 cases of horse deaths were recorded by the AHS Trust for the 2007-2008 outbreaks. OBP whose role it is to provide vaccines to minimize such occurrences, has been profoundly concerned and sympathizes with the horse owners affected, unfortunately a certain proportion of these horses were not vaccinated. Considering the fact that around 160,000 horses were vaccinated during the same year (based on vaccine sales), and given the severity of the outbreak, it can be concluded that the current vaccine played a key role in protecting more than 99% of vaccinated horses. Due to the complexity of factors that can prevent the build-up of solid immunity, no vaccine can guarantee 100% protection. OBP has embarked on investigations into available data and circumstances of the fatalities in the less than 1% vaccinated horses that succumbed to the disease (less than 400 of the 160,000 vaccinated horses)
There is general consensus that it will be impossible for South Africa to eradicate AHS due to the involvement of other hosts and the role of insect vectors. Vaccination has always been and still is one of the most appropriate ways of controlling this disease. While the efficacy and safety of the vaccine are critical for the build-up of good protection, failure to adhere to recommended vaccination schedules are often the cause of vaccination failure.
This could be the result of factors such as:
• Wrong vaccination program
• Incorrect frequency of vaccination
• Poor vaccination coverage (most animals in an area not being vaccinated, thus becoming a source of constant spread; this is complicated by the large number of other equids such as zebra and donkeys)
• Handling of vaccine; maintenance of the cold chain, etc.

Current data on AHS vaccination in South Africa indicate that vaccination timing and coverage are far from being satisfactory. Almost half of the horse population in the country is not vaccinated; these susceptible animals, together with the large population of donkeys and zebra constitute large sources of the virus that put serious pressure on vaccinated horses. Vaccine sales figures also indicate that a large proportion of horses are vaccinated during the midge season, thus compromising the efficacy of the vaccine. These factors will favour a poor build-up of immunity and constitute high challenge risk for large number of horses, including vaccinated animals. A complicating factor is the inherent inability of foals to build solid immunity after acquiring passive immunity (colostrum) from their mother at birth. Through the first 6 months of their life, which unfortunately for most cases correspond to the high risk season, foals may not respond fully to vaccination or any boosting of the immunity, thus remaining vulnerable to heavy virus challenge. OBP will always work toward improving the control of AHS, through vaccine research and also science-based recommendations for better vaccination strategies.
Despite many years of research at the Onderstepoort institutions (OBP, OVI and Veterinary Faculty) and others around the world, many areas of the horse/AHS virus interaction remain a mystery, thus contributing to the difficulties in coming up with improved control measures. The 3 Onderstepoort institutions organized the first ever AHS workshop (under OBP sponsorship) in 2006 to look into ways of improving our knowledge on this disease and develop better tools for its control. The workshop which included local and international experts identified a number of research areas which are currently being studied jointly or separately within these institutions. A better understanding of the immunological response of horses to AHS was considered as the basis to the development of improved vaccines. This ongoing research is feeding into different vaccine projects currently running at Onderstepoort. Other resolutions of the workshop included the need to improve the vaccination coverage throughout the country, hence the initiatives by institutions such as the AHS Trust to conduct blanket vaccination campaigns in critical areas of South Africa; this is done with subsidized vaccine from OBP.

The current AHS vaccine produced by OBP contains cell culture-attenuated strains of 7 of the 9 serotypes, administered in 2 separate vials, the first containing serotypes 1, 3 and 4 and the second vial serotypes 2, 6, 7, and 8. The 2 vials should be injected at least 3 weeks apart. Serotype 9 was never included in the OBP vaccine as it never occurred in Southern Africa before 1993 and serotype 6 affords adequate cross-protection. While efforts to improve the vaccine have been continuously sustained at Onderstepoort, more studies have been directed in recent years towards the evaluation of vaccination regimes and strategies on different age groups of horses. Results obtained thus far confirmed that adequate protection can be achieved if the vaccine is used timeously as prescribed by OBP. A very small percentage of highly susceptible horses do react to the vaccine and it is for this reason that OBP continuously spend millions of rands on Research and Development to improve the vaccine and generate data on optimal vaccination strategies. All new information generated will be shared with all stakeholders.
Due to the fact that 7 different vaccine strains are included in the vaccine, each horse’s immune system may react differently towards these 7 strains. Different levels of antibodies may be induced in different horses, which may play a critical role in the overall immunity/protection against AHS. The initial vaccination regime followed at primary vaccination of foals, with the follow-up booster vaccinations will eventually determine the levels of antibodies that may be induced against the 7 different serotypes. Documented evidence of the immune status (antibody levels) of the 7 different serotypes of any of the dead horses during the past season is not known.

Different levels of antibodies vs different levels of virus challenge

Key:

A: Low virus challenge – Protection against all serotypes
B: Moderate virus challenge – Protection against some serotypes
C: High virus challenge – No protection against any serotypes

During a wet season, like we have experienced during the past summer season, resulted in a very high challenge of the virus due to the extreme high numbers of midges that prevailed. It is also known that unvaccinated horses may act as reservoir for the virus. Therefore, during high virus challenge of horses, the antibodies of specific or individual horses against any one of the 7 different serotypes may not be adequate to protect that specific individual horse against the disease, should that serotype be responsible for an outbreak.
OBP wants to assure all stakeholders that everything possible is done to expedite the research on this disease and continuously invest in improving the vaccine and vaccination regime. A sizable portion of the deaths that occurred this past summer were young foals or young horses that received no vaccinations or only one vaccination, although some exceptions were experienced. The effect of passive immunity also plays a major role in the vaccination process of foals. Currently research is focused on the primary vaccination of foals, which were the largely affected this year.
A number of actions can be taken by horse owners to improve the control of AHS.
Some of them are listed below:
• All other control measures are very critical (insect vector control, horse movement control etc.)
• Increased national immunity, by increasing vaccination coverage countrywide; all of us should assist institutions such as AHS Trust and others in contributing to blanket vaccination campaigns in different communities in the country.
• Adherence to vaccination schedules and vaccination seasons as recommended by OBP.
OBP would like to assure our customers at large that OBP vaccines are of extremely high quality as is evident in the export of our vaccines all over the world. OBP’s vaccines can be used with confidence and peace of mind. OBP is also continuously investing in research, development and improvement of all our current and new innovative vaccines. As an ISO 9001:2000 Quality Management certified company, OBP is totally committed to quality products, processes and service to our customers.

For further information please contact Dr. Jakes Modumo (082 574 1115), Dr. Baty Dungu (082 784 1359) or Mr. Willie Botha (082 857 5885).
OBP
Private Bag X07
Onderstepoort
0110
Tel: +27 12 522 1500
Fax: +27 12 522 1591
Website: www.obpvaccines.co.za