Coping with AHS

by Andrea van Rijswijk

At the time of writing this article, the African Horse Sickness virus (AHSV) epidemic reached 861 reported cases this year, according to the African Horse Sickness Trust. Out of those, 578 horses died. This is more than the Trust has ever listed since it began recording AHSV statistics in the 2005/2006 season. These numbers have given horse owners many sleepless nights worrying about what to do to save their horses.

About AHSV

First, we need to know what AHSV is. The virus is classified as an Orbivirus. It has a very high mortality rate and is spread through southern Africa by the Imicola and Bolitinos species of the vectors called Culicoides midges. These are tiny – about 1 to 2 mm in size. With changes in climate, the potential threat of all species of Culicoides to transmit AHSV is very real. Culicoides midges occur worldwide and also spread other diseases, such as encephalitis and bluetongue among sheep.

Male midges live on fruit but females feed on blood, as they need protein for egg production. This they obtain by sucking the blood of equidae and some other animals. Midges are infected with the AHS virus when they suck on the blood of AHSV-carrying equidae and spread the disease when moving on to suck blood from other equidae. The midges are attracted to horses by heat, CO2 and sweat/smell.

One female lays 450 eggs at a time and lives three to four weeks. Eggs hatch within two to ten days. If half her offspring are female, she can theoretically produce up to 6 400 midges in her lifetime. Midges need moisture to breed but breeding sites vary from mud to leaf litter, wet meadows, tall grass, standing water, irrigation areas, and dung (C. bolitinos).

Transmission

While many believe that introducing other livestock species to a yard will draw midges away from horses, it appears that this is mainly a means of reducing the bite load on each horse – more animals on a property means the bite rate is spread, reducing the number of times each one is bitten. High-lying areas are at as much risk if there are flat areas where water can accumulate. Also, the introduction of cattle can lead to the local establishment of the dung-breeding Culicoides.

The primary cycle is between Culicoides and zebra and the African donkey. The secondary cycle is between Culicoides and horses. In South Africa, zebra and probably African donkeys, if in large enough numbers, can act as a reservoir of the disease, for example, in the Kruger National Park where there is a continuous transmission cycle between midge and zebra.

Zebra and donkeys are largely asymptomatic or only get a very mild fever. Mules are less susceptible with fatalities of 50 to 70%. Clinical tests are being conducted to determine what keeps these animals from getting sick and to see if this can be of any use in combating AHS. Horses have severe symptoms with high fatalities; 70 to 95% depending on the form of the disease.

Infection

The more midges bite the horse, the higher the chance of infection. Horses do not remain carriers of the disease – viraemia is supposedly typically for about four to eight days in an infected horse. However, it is now thought that zebra may remain as reservoirs during AHSV-dormant winter months.

Dogs can contract AHSV if they feed on the carcass of an animal that died of AHSV. However, Culicoides do not readily feed on dogs, so they are not associated with spread or maintenance of AHSV. Members of the cat family are not affected by AHS.

AHSV is endemic in eastern, central, western and most of southern Africa. Progress north is blocked by the dry Sahara, although the risk of it developing in Europe does exist where it is flat, low-lying, wet and warm. In southern Africa, it appears in summer, usually in January. It spreads south, with optimum conditions being early heavy rains followed by warm, dry weather, such as experienced this season. The height of infection is usually in March and April. Vector midges die off with the first frosts around the end of April/May.

Researchers found that there are nine different strains or serotypes of this disease in tropical Africa, all occurring throughout South Africa and serotypes 3, 4 and 9 occurring outside Africa and occurring twice in Europe – the last being in Spain in 1990.

Variants of AHSV

AHS occurs in four forms:

• The peracute (pulmonary) form called dunkop, attacks the lungs and has a 95% mortality rate. Death is due to anoxia soon after dyspnoea onset. Incubation is three to five days. Duration, one to two days. Symptoms include fever (40 to 41°C), congested ocular, nasal and oral mucous membranes, increased respiratory rate (up to 60 bpm) and flared nostrils with forced expiration resulting in heave lines, wide abnormal stance with head and neck extension, profuse sweating, spasmodic cough, frothy often blood-tinged fluid from the nostrils.
• The sub-acute (cardiac) form called dikkop, attacks the heart and has a 50 to 60% mortality rate. Death is due to cardiac failure. The incubation period is seven to fourteen days. Duration three to eight days. Symptoms include swelling of face, supraorbital fosse (hollows above eyes), eyelids, lips, cheeks, tongue, larynx, jugular groove, neck, and sometimes shoulders and back, bleeding of conjunctiva and tongue, fever (39 to 41°C), depression, colic signs.
• The acute mixed form, which attacks both heart and lungs, has a 50 to 90% mortality rate. Death is due to cardiac failure. Incubation is five to seven days. Duration is seven days. Symptoms include a combination of the peracute and sub-acute syndromes, non-progressive pulmonary distress and oedema.
• Horse sickness fever is the mildest form with a very low mortality rate, and often not detected, occurring in partially immune animals such as those vaccinated with or exposed to the heterologous serotype. Incubation is similar to those above. Duration is three to eight days. Symptoms include fever (remittent, rarely above 40°C with peak fever in afternoon), and perhaps some slight congestion of conjunctivae, increased pulse, mild anorexia, mild depression.

Hold the horse's head up when checking above the eyes to make sure it is not due to grazing with the head down. Cushing's disease and fat ponies with metabolic illness can also show swelling above the eyes.

If a sick horse's daytime temperature drops to 36°C or less, the infected horse's body is under severe stress and battling to survive. Temperatures of 40°C and above can be dangerous to the central nervous system and should be lowered slightly. However, increased temperature above 38,5°C completely stops all viral replication, giving the immune system T-cells and macrophages enough time to track down and destroy invading viruses.

Complications

• Biliary fever – many horses harbour dormant or sub-clinical biliary and AHSV infection, which suppresses the immune system, resulting in an acute flare-up of biliary.
• Choke from dry feed getting stuck.
• Low blood platelets.
• Internal bleeding.
• Colic.

A horse with dikkop can still die suddenly two weeks after symptoms started.

Differential diagnosis

The equine encephalosis virus is carried by the same vectors and has a 5% mortality rate. It presents with the same signs, but the horse is likely to survive with or without any treatment. Unfortunately, this can cause people to assume that treatments saved the horse from AHSV.

Vaccination

There is currently no guaranteed cure for AHSV. Before vaccinations were available, as many as 70 000 horses died in one season from AHS. Vaccines were developed by the Institute for Veterinary Research at Onderstepoort in Pretoria. The current vaccine is 56 years old, but is still the best available. Numerous trials could not yet improve on it.

The vaccine consists of two parts, one containing vaccination for serotypes 1, 3 and 4 and a second with vaccines for serotypes 2, 6, 7 and 8. As yet, it has been impossible to create a vaccine for serotype 5 and 9, owing to the instability of these strains and danger of reaction in horses. Research work by Onderstepoort Biological Products shows that there is some in vivo cross-protection of serotypes 5 and 9 after vaccination with serotypes 8 and 6 respectively.

While the vaccine is indeed very effective in aiding immunity, it does not guarantee complete immunity or survival. According to world authority, Dr Rudy Meiswinkel, on warmwell.com in the UK, there is general concern of reaction to the vaccines and the risk of vaccine reversion to wild type, as well as the potential of these polyvalent vaccines to transmit AHSV strains to the vector midges, thereby transmitting the disease. However, vaccination is still the safest option available and considered useful even in Europe, because it provides a base level of immunity and is very effective.

Recombinant vaccines

There is an urgent need for developing recombinant vaccines for AHSV. Success with these vaccines for West Nile virus provides some optimism. In June 2010, DEFRA in the UK announced it would be putting £190 000 (R2 million) per year into a three-year project to develop a new, effective vaccine. The report states that nine different vaccines would be made for the nine AHSV variants and that these would be produced for the mass market. It may eventually be possible to produce one vaccine for all nine variants.

Any new animal medicine takes eight to ten years plus 200 to 500 million USD before it can be sold legally. No new vaccine or medication or treatment can be sold without due process and proper scientific testing.

When to immunise

There is possible maternal antibody interference by the vaccine in foals born to immune dams, therefore vaccination is not recommended until foals are six months old. Foals usually enjoy effective immunity in the colostrum of vaccinated dams.

Levels of antibodies depend on the immunity of the dams and foals, which only acquire low levels of antibodies and could be vulnerable when not vaccinated or only vaccinated once – it is recommended that horses should be vaccinated twice – as weanlings (six to twelve months old) and again as yearlings (12 to 18 months old), and yearly thereafter.

Horses must be vaccinated in spring, with the two vaccines given at least three weeks apart and before onset of the first outbreaks. It is unwise to vaccinate during the worst times of the AHSV outbreaks of summer, or when horses in the immediate vicinity are sick, as this lowers immunity and increases risk of succumbing to infection.

Young and weaker horses must not be worked during week two known as 'mid-week' of each vaccine. Light exercise is okay in weeks one and three of both vaccines. Pregnant mares should be vaccinated three months into gestation and again two months before the foal's birth. Older animals with a long vaccination history may have a drop in antibody levels and may need to be vaccinated every second year.

Vaccination by veterinarians is advised and is compulsory for horses moving into or out of the Western Cape AHSV-free zone and for horses competing at SANEF/FEI-affiliate shows. Owners also need to do their utmost to ensure that the cold-chain of the vaccine is maintained up until it is used. The vaccine must remain cold until use in order to be effective.

In the next issue we will look at the various treatments.

(References available from author)