Abstract
JFMS is a truly international journal, and so, as well as covering disease conditions of cats that are present the world over, it has a duty to cover important regional diseases. One such disease is tick paralysis in cats.
In the USA, there are two main species of tick capable of producing paralysis, Dermacentor variabilis (American dog tick) and Dermacentor andersoni (Rocky Mountain wood tick), with most cases occurring in the Rocky Mountain States, the Pacific Northwest and south-eastern states between late spring and early summer. The tick paralysis that results from such envenomations seems relatively mild based on the paucity of reports of feline disease in the literature. It is fair to say that, in the USA, there is more focus on the diseases that can be transmitted by ticks, rather than diseases due to direct effects of ticks on the host.
A very different situation exists along the east coast of Australia where tick paralysis caused by Ixodes holocyclus is an incredibly important disease in companion animals, including the cat (Figure 1), and is one of the most common preventable causes of death of dogs and cats in this region. Typical hosts for the paralysis tick are native Australian marsupials such as bandicoots, macropods and possums. Under usual circumstances the ectoparasite generally causes few adverse effects in these hosts as they are exposed to small numbers of nymphs and adult ticks at the beginning of the tick season (spring), thereby generating acquired resistance to the toxin (largely due to antibodies). Cats, dogs and people, however, normally have little in the way of immunity to a tick’s salivary neurotoxin, especially at the start of the tick season, and are thus vulnerable to envenomation. Indeed, one tick can be sufficient to paralyse a cat.
Two cats with tick paralysis in the intensive care ward of Veterinary Specialist Services and Animal Emergency Service, the practice of lead author of the JFMS paper, Ellie Leister. (a) This case is less severe and, after clipping and administration of tick antitoxin, the patient is stable in sternal recumbency with a pulse oximeter probe attached to the toe for monitoring. (b) This patient is much more severely affected, and after similar preliminary therapy has been placed on a ventilator. The treatment of such cases is intensive and expensive; often they are referred to 24 h intensive care facilities for handling
The specific neurotoxin in the saliva of I holocyclus that causes tick paralysis is holocyclotoxin. This is a cystine knot neurotoxin that acts presynaptically to reduce quantal release of acetylcholine from the motor nerve terminals at the neuromuscular junction. Typically, after 3–4 days there is sufficient envenomation for muscle weakness and eventually paralysis to develop.
Cats become agitated, perhaps as they do not understand ‘what is happening’; in addition, there are other issues such as weakness of the pharynx, and the propensity to aspirate saliva due to laryngeal and oesophageal weakness. This is a life-threatening condition in many cats, especially when owners are not ‘switched on’ to early signs of the disease.
A really important piece of clinical research on the subject of feline tick paralysis appears in this issue of JFMS. Ellie Leister and her mentor Rob Webster have assembled an enormous number of cases, which they have analysed carefully in collaboration with veterinary epidemiologist extraordinaire John Morton and Professor Emeritus Rick Atwell. Their findings are published in the paper ‘Clinical presentations, treatments and risk factors for mortality in cats with tick paralysis caused by Ixodes holocyclus: 2077 cases (2008–2016)’ on pages 465–478. 1
This is a really important piece of clinical research
Historically, most work concerning tick paralysis has been carried out in the dog, going back to the seminal studies of Sir Ian Clunies Ross and later work by Barry Cooper and Jan Ilkiw. The cat has been neglected in terms of clinical research, although it is affected in comparable numbers to the dog in clinical practice. In addition, what little information we have in the literature concerning tick paralysis in cats tends to be based on either surveys or review articles by experts, rather than publications underpinned by solid verifiable clinical data.
Dr Leister and colleagues, however, have made up for lost time with their work spanning 2008–2016, initially reviewing 2077 cases seen by a single group of practices, and providing mortality data on a more refined group of 1742 cases.
It is rare in feline medicine to have so much evidence upon which to draw conclusions and make recommendations
It is interesting that such a rigorous and scholarly study was performed from a private practice base, rather than a university veterinary teaching hospital. The size of the caseload, high standard of case management and presence of defined practice guidelines for case management and clinical decision-making underpin the rigor and scholarship of the work. I don’t want to say much more about what they found, however – because it’s all in the paper!
In feline medicine, prevention is always more important than cure
The recent availability of three novel prophylactic medications has produced a huge step forward for canine patients. The first drug to be released was fluralaner (Bravecto; MSD Animal Health), from a new class of isoxazoline antiparasitic drugs that act on gamma-aminobutyric acid- (GABA-) and glutamate-gated chloride channels in arthropods (ticks and insects including fleas). Because of highly favourable pharmacokinetics, one fluralaner tablet of the correct size will protect dogs against tick paralysis for 4 months or longer, and will also be effective against flea infestation for 3 months (Australian studies show 100% efficacy against paralysis ticks at 4 months and 96% efficacy at 5 months in dogs 2 ). Merial Animal Health has a similar drug, afoxolaner (NexGard), which needs to be given once a month, and a third drug is sarolaner (Simparica) from Zoetis, another once-a-month prophylactic. Veterinarians up and down the east coast of Australia have observed a sharp reduction in the number of dogs presented for tick paralysis since the widespread availability of these new drugs in the market, and this has been mirrored by the observation of veterinary insurance companies (fewer claims!). We are therefore pretty sure these new products are doing exactly what they are supposed to do.
Unfortunately none of these are currently available for cats, and so we are left with fewer effective propositions such as the clever Seresto collar from Bayer Animal Health, fipronil sprays (which cats invariably hate) and fipronil top-spot products, and old standbys such as combing the cat once a day with a flea comb to locate and remove any attached ticks (remember it takes 3–4 days for the cat to develop signs).
In the USA, Bravecto is available for cats as a topical solution, but the Australian Pesticides and Veterinary Medicines Authority is proving frustratingly slow to process the registration of this important new drug to prevent tick paralysis in cats. In the interim, Australian vets are forced to use the same doses of fluralaner off label to afford protection to cats in endemic areas. It is absolutely vital that isoxazolines are made available to prevent the morbidity, mortality and enormous cost of this devastating disease to the Australian feline population.