Aug Sept 2019
Lyme disease is the most commonly reported tick-borne disease of humans in Asia, Europe and the United States.
The geographic distribution of B. burgdorferi infection in horses corresponds closely to the distribution of Lyme disease in humans. Blood tests have found 33% of horses in Austria, up to 35% in the United Kingdom and 20% to 75% in the United States are positive for B. burgdorferi infection. The organism is found in blood, urine and joint fluid of infected horses and has been shown to cross the placenta in pregnant mares.
However, ticks are not always required for transmission of Lyme disease - infection via urine and infected needles has been demonstrated in mice and dogs.
Once a horse is infected, it’s not known how long it takes for signs of disease to show up. In dogs it takes 2-5 months, and may take years in people. Around 10% of horses with a positive blood test develop clinical signs of disease – higher for foals and yearlings – and infection can persist for years to life.
Further complicating the picture is that the diagnosis of Lyme disease is difficult. Several types of Borrelia bacteria can cause Lyme disease – and the type causing Lyme disease in the USA is very different to those responsible in Europe and Asia, and hence the diagnostic tests are different. Diagnosis is usually based on a history of possible exposure, clinical signs of disease, elimination of other diseases, the response to antibiotics and blood tests – all of which produce both false positive and false negative results.
Treatment with intravenous, but not oral antibiotics can be effective – however treatment can stimulate the organism to form cysts in the body. Vaccination can block transmission from ticks to horses, but there is no commercial vaccine available.
ORIGINATING IN AUSTRALIA OR
The Lyme Disease Association of Australia (LDAA) says accurate figures of Lyme cases in people in Australia are unknown because patients are not formally counted.
They estimate there are well over 2,000 medically-confirmed cases in Australia and many more undiagnosed cases.
Surveillance in humans and animals and attempts at isolation from ticks have failed to reveal conclusive evidence of Lyme disease in Australia. Dissection and DNA testing of over 11 000 ticks from coastal New South Wales failed to demonstrate evidence of infection with B. burgdorferi. A tentative diagnosis in a cow was also reported in New South Wales but no B. burgdorferi was found, and surveys of dogs in south-eastern Queensland did not find evidence of B. burgdorferi. The only species of tick shown to be able to be infected with, and to transmit, B. burgdorferi belongs to a group of ticks called the Ixodes persulcatus/rictnus complex. No ticks of this type have been found in Australia, nor has B. burgdorferi.
Lyme disease occurs in countries approved to export horses to Australia and there are biosecurity measures in place to reduce the risk of an infected, imported horse bringing in the organism or the ticks that transmit it.
The disease is primarily transmitted by ticks (although re-using infected needles could do it too!) and Australia may have ticks that could transmit B. burgdorferi.
If the animals and birds became infected, they could act as a reservoir of infection, allowing the disease to spread to other susceptible species - including birds, cats, dogs, cattle, sheep, people, wild rodents and rabbits. Reservoir hosts rarely show signs of disease and it’s not known whether native animals could become carriers.
Australia’s biosecurity measures require that imported horses come from countries that have had no evidence of Lyme disease for two years before a horse can be exported. If the horses come from a country with Lyme disease, and because a tick must be feeding on a horse for at least 24 hours for disease transmission to occur, the horse must be held in quarantine with no potential to be infested by ticks, for two weeks before it is exported to Australia. During this pre-export quarantine period, the horse must be inspected and treated for ticks. The inspection protocol requires a close examination of ears, false nostrils, under-body areas (armpit, groin and under the jawbone), anal area, mane and tail. This process is repeated once the horse arrives in Australia and is taken to a post-arrival quarantine station. During the post-arrival quarantine period, it is again inspected for ticks followed by immediate treatment for ticks – even if none are found. If any horse in the pre-export quarantine facility is found to have ticks, all horses in the facility are treated at once and again a week later.
Lyme disease is stealthy and difficult to diagnose. In 2013, Australia’s then Chief Medical Officer (who is also a veterinarian) Prof Chris Baggoley established a committee to study Lyme disease in Australia. The study of Lyme disease continues today. World-wide, the spirochaete research community of biologists, ecologists, medical practitioners, parasitologists, scientists and veterinarians monitor, study and track the ecology and potential health impacts for humans and animals of emerging strains of spirochaetes.
Ticks have inhabited earth for around 140 million years and we have many types in Australia. Throughout the world, ticks carry more diseases than mosquitoes, harbouring a whole plethora of pathogens, parasites, bacteria and viruses that can cause yet-to-be identified diseases and illnesses.
Most tick-borne germs can be transmitted from humans to animals and from animals to humans. Some ticks feed on only one type of animal while others bite everything.
Although it hasn’t been found to contain B. burgdorferi, there may be other nasty organisms, including local spirochaetes, that it could transmit.
There are over 100 strains of Lyme borreliosis in the United States and 300 strains worldwide. It is likely that there are far more species of Borrelia than are currently identified and although they may not all cause disease, we are finding new species roughly every two years – including one recently identified in echidnas in 2017. Four species of borreliae have been identified to date in Australia. Testing and diagnosis has a difficult time keeping up with the new species and tick bite prevention must remain our central focus.
Preventing tick exposure or prolonged (more than 24 hours) attachment on a horse by clipping tall grasses, clearing shrubs and preventing the horse from entering the bush, can reduce the risk of infestation.
Topical sprays can be used when exposure to ticks is expected.
These are often recommended when adult ticks are noticeable in the late summer, autumn and the early part of winter. But infection with larval or nymphal stages should also be considered. If ticks are found on the horse, they should be identified and veterinary advice sought if a horse becomes unwell or develops a fever.