Insect populations are on the rise. RACHEL ROAN spoke with Dr Caroline Spelta to find out more.
Following last year’s heavy rains, equine owners are wise to be concerned about the rising populations of mosquitos and other insects. Protecting horses from bite-related allergies, or worse, almost untreatable viruses, is something to be taken seriously. Dr Caroline Spelta, an expert in equine medicine, warns that we need to be vigilant about the expected rise in cases of arboviruses.
Insect sensitivity and allergies
Broadly speaking, an allergy is an overreaction from the immune system to an allergen, usually a protein from the saliva of an insect. When an insect bites, their saliva is transferred to the host, causing a reaction such as redness, heat, swelling, inflammation, oedema, itchiness, or pain in the bite area. “An allergic reaction in a horse is the same as any other animal,” explains Caroline. “The level of reaction depends on what bites you and how allergic you are to that insect. The majority of horses are not anaphylactic to insect bites – whether that’s bees, ants, or other insects – and they usually experience more discomfort from the actual bite site itself, rather than the reaction.”
In Australia, insects to watch for include march flies, stable flies, buffalo flies, ticks, mites, midges, and mosquitoes. Anything that annoys humans can potentially annoy horses too.
Caroline mentions two common conditions: Black Soil Itch and Queensland Itch. Black Soil Itch occurs from a mite that lives in the grass and is usually seen after periods of rain where fresh, lush grass is in abundance. “The horses get really, really itchy – so bad that they will chew at their legs and give themselves bleeding sores,” she says. “The sores will appear over the head, ears, and from the knees and hocks down, wherever they come into contact with the grass where the mites can bite them.” Warm areas with black-soil and cracking clays that contain a lot of organic matter provide the perfect conditions for the mites to thrive.
Spikes in arboviral diseases – transmitted by insects such as mosquitoes and ticks – are expected following excessive rainfall.
Horses allergic to bites can go on to develop an intense hypersensitivity reaction. Queensland Itch (sometimes called Mane and Tail Disease) is a chronic allergic reaction to midge bites that results in hypersensitivity. It occurs anywhere in Australia in conditions that foster the survival of these midges and horses will experience severe itching, causing them to rub their hair out.
Treating the bites can be challenging. If a horse experiences an acute allergic reaction, such as hives or a swollen face, steroids and antihistamines are usually quite effective. Cold water on the bites can also provide temporary relief. A diagnosis is often straightforward, especially if the area is prone to cases of Queensland Itch.
Unfortunately, the most effective treatment for horses with severe allergies or Queensland Itch is to remove them from that environment. For chronically impacted horses, treatment options are limited. While some horses can be given long-acting steroid injections, which are effective in most cases, vets often prefer to avoid this option, and the side effects need to be considered. In small animals and people, monoclonal antibody therapy is highly effective against allergies, however it is currently cost-prohibitive for an animal as large as a horse. “Like people, some horses are more predisposed to a reaction than others,” Caroline adds.
Encephalitis risk
With an unseasonably wet winter and expectations of a wet summer to follow, the insect load across Australia is higher than normal. Spikes in arboviral diseases – transmitted by insects such as mosquitoes and ticks – are expected. “In Australia, Japanese Encephalitis Virus (JEV), Murray Valley Encephalitis, West Nile Virus (Kunjin Strain) and Ross River Virus are four viruses that cause neurological encephalitis in horses,” Caroline explains. “They are all spread by mosquitoes, and are zoonotic, meaning they can be transmitted from horse to human. I expect in the first few months of this year to see a spike in the number of mosquito-borne diseases.”
March flies (also known as horse flies) are one of the insects you should be alert for.
Controlling virus spread is extremely difficult. “Mosquitoes spread these diseases, so trying to control mosquitoes is really, really important, as Australia does not have access to vaccines,” Caroline says. “Australia also has a large feral pig population, so horses and humans near feral pigs are at a higher risk.” Cases of Murray Valley and West Nile viruses often display similarly, with acute sickness for one to two weeks before the horse progressively improves over the following six weeks. “JEV and Ross River have a much longer recovery time, can stay in the acute phase for months, and can require months of intensive management before recovery.”
For many horses, these viruses are fatal. “Once a horse is down, they are very hard to treat. Horses can display a range of clinical signs from fever and lethargy to gait abnormalities, headaches, behavioural changes and in worst case scenarios, recumbancy. If they can stay standing, then their prognosis to survive and return to work is good, but it takes a long time,” Caroline says. “Some will take a lot of management and intensive supportive care. To use antivirals in horses is horrifically expensive. We can’t kill the viruses. We just have to manage and support the horse as best we can until the virus runs its course.”
Slings to keep the horse standing are essential, but very few clinics in Australia have the necessary equipment. The geographical distance between owners and these clinics is also prohibitive, with horses often unable to make the long trip. Some of the viruses are also thought to be very painful, which adds to the challenge. Sadly, the prognosis is guarded to poor. “Even for horses that can be treated in state-of-the-art facilities, the strain placed on the horse and costs associated with treatments often means euthanasia is the most humane thing for them, particularly the severe cases,” Caroline concedes.
In this Liftex sling, all parts of the sling attach to a central anchor above the horse (courtesy of the Liftex Corporation, Warminster, Penn.)
Prevention is best
Horse owners can take multiple steps to minimise the risk of their horse getting bitten. A combination of preventative measures usually ensures the best results. Topical insect repellents are successful but need to be reapplied daily. Rugging is highly effective, and there are now rugs on the market with the insecticide permethrin bonded to the fibres. “Many of these are water resistant, and the repellent will last up to 200 days,” says Caroline. “Rugs are really helpful for those particularly itchy horses in wet summers.”
Prevention is better than cure – protect your horse with rugs, bonnets and veils.
Look for good quality bonnets, masks or veils that cover the ears and face, and consider purchasing fly boots. Reduce the breeding ground of mosquitoes and midges by cleaning water troughs regularly, removing puddles or stagnant water in buckets. Moving horses away from bogs, creeks, or marsh areas will also reduce their risk of getting bitten. Insects are most active at dusk, so stable where possible. “Midges dislike flying under a roof, so stable your horse from dark to daylight to reduce exposure,” Caroline adds.
Stopping the insects from getting to your horse is the best option, and people who live near water or on the coast need to be especially careful to keep insects away, which is not always possible. If treatment and prevention is not an option, relocating the horse is the last resort. For the arbovirus threat, horse owners are advised to stay alert and aware of the signs.
For more information, see our article ‘Insect protection’ in this issue.
