Ticks and tick-borne infections

Tick burden can be a problem in dogs as ticks can transmit a variety of diseases. This article will look at the tick life cycle, some of the associated diseases as well as ways to prevent infestation and the resultant tick borne diseases. There are hundreds of species of ticks found worldwide and only several can be found in Europe. While they can be found all year round, their numbers increase between March and November when the temperature is warmer. Ticks belong to the Acarina order of arthropods and are temporary parasites. This means that they only spend a small portion of their life on a host itself. They are typically found in grassland, scrub, shrubs and on low-hanging branches. They will climb on board an animal as it brushes past them. Tick infestation can affect all species of animals (including humans) and are relatively common in our pets. A tick bite itself can elicit a hypersensitivity response, which can be seen as a red hard swelling of the surrounding skin. Extremely heavy tick burdens can cause anaemia (a decrease in the number of red blood cells). Ticks are also able to transmit bloodborne diseases such as bacterial infections including Borreliosis (also known as Lyme Disease) and Ehrlichiosis; viral infections such as Louping-ill; as well as protozoal diseases (Babesiosis). Abroad, there have also been cases of tick paralysis, though this is mainly seen in America. Most ticks are not host specific, meaning they will consume a blood meal from a wide variety of animals and even people. The “hard” tick known as ixodid are the relevant ones found in most of Europe. Of these Ixodes ricinus (sheep tick), Ixodes hexagonus (hedgehog tick) and Rhipicephalus sanguineus (kennel tick). Ticks rely on three hosts to complete their life cycle. In other words, each parasitic stage (larva, nymph and adult) feeds from a host such as a dog once, and then drops off. They regurgitate while they feed and that is how they introduce diseases into the host if they are infected themselves. Host animals can be the same or different species. Generally, the life cycle can take up to three years to complete. This is because larvae can survive for long periods off the host and can hibernate. Female ticks can lay around 2000 eggs, which are 0.7mm long. After laying their eggs, female ticks will shrivel and die. The six-legged larvae tend to hatch from eggs in the spring or summer of the same year they are laid. They are about the size of a full stop. They remain inactive until the following year when they crawl onto grass stems and seek their first host. This is usually a rodent, but can be any animal. Once they engorged their blood meal, which takes about a week, they drop off into the environment and moult into the nymph stage. A year later the nymph then seeks larger hosts from which to feed from, and then also drop back into the environment once fed. This time the feeding takes about 11 days. They then turn into an adult, which consume their last blood meal on their final host. It takes up to two weeks for a female to become fully engorged. Thus, it can be difficult to spot a small tick between the hairs at the beginning of its attachment. They tend to hide well. The female lays her eggs in the environment after the final blood meal. And the eggs will hatch the following spring or summer, for the whole cycle to start again. Lyme disease is a multisystemic (affecting several body systems), inflammatory disorder of man and animals caused by infection through the tick-borne bacteria known as Borrelia burgdorferi. The disease can become chronic if left untreated. While there is no evidence that dogs can spread the disease directly to owners, they can bring disease-carrying ticks into the home. In dogs, the most obvious signs of Lyme disease include a “bull’s eye” red circular lesion around the site of the bite, lameness (often shifting legs), inflamed lymph nodes and fever. Lyme disease has also been associated with rapidly progressive kidney failure. In people the symptoms are often flu like. If your dog displays unusual lameness or lethargy after a tick bite, please consult your veterinary surgeon. There are many other reasons why a dog may display these symptoms, but Lyme disease should be excluded if necessary. Babesia is another infection that can affect dogs. It is a protozoal infection spread by ticks. The organism affects the red blood cells and signs are related to anaemia and include loss of appetite, weakness and fever. Affected animals will have darkened urine due to the destruction of red blood cells (haemoglobinuria). Figure 4 shows a blood smear of a dog with Babesia. It is often described as looking like paired “pears” within the red blood cells. Another, though less common tick-borne disease, is caused by infection with the bacterium known as Rickettsia, which affects the white blood cells. In dog’s symptoms can include inappetence, fever, weight loss, stiffness and prolonged bleeding. There is a lag interval for ticks to transmit disease, so regularly inspection of your dog’s coat and regular grooming will allow you to spot any ticks, which can be removed manually using commercial tick removers. Tweezers should not be used. If you try to pull the tick out directly you risk leaving parts of the mouth and head inside the dog, which can cause a foreign body reaction in the skin. After removal, the area around the bite should be disinfected. Like most things in veterinary medicine, prevention is always better than cure. There is a huge range of parasiticidal spot-ons, tablets, sprays and collars available to minimise the risk of ticks attaching to your dog. Your veterinary surgeon will advise you on appropriate tick prevention. The ideal outcome of tick control is to prevent attachment in the first place. Prophylactic tick prevention is especially important when travelling. In summary, ticks can have a significant disease impact either directly or via a tick-borne pathogen. Simple tick control measures prevent local reactions and transmission of disease. If your dog shows unexplained lameness, joint stiffness, fever, lethargy or loss of appetite or develops a rash after being bitten by a tick, please consult your veterinary surgeon.