The time is arriving when emergency practice, for us in eastern Australia at least, becomes dominated with the care of poor animals suffering from the effects of the toxin from the Australian paralysis tick, Ixodes holocyclus.
Tick paralysis caused by I. holocyclus is unique to Australia. Different tick species exist all over the world and are well known to transmit diseases but only a few cause paralysis. Two species in the USA (Dermacentor variabilis and andersoni) cause a paralysis that animals can apparently recover from without treatment once the tick has been removed. These are vastly different to our ticks and pet owners seeking the wrong information on the internet can come away very poorly informed. Australia is well known to have the nastiest of the nasties, and our ticks fall into that category as well. Once paralysis develops from the I. holocyclus toxin, recovery is rare without the lifesaving anti-serum despite removal of the tick or ticks that caused the clinical signs.
The Australian paralysis tick is distributed along the eastern seaboard of Australia, mainly found in coastal and bushland areas. Normal hosts for ticks are the wildlife who have lived in symbiosis with these creatures for longer than white man has existed in this country. They are often resistant to the effects of the toxin due to daily exposure and innate immunity. Dogs, cats and other domestic introduced species are not normal hosts for ticks, hence they are more susceptible to the toxic effects of their venom.
Ticks will often be attached for several days before their toxin causes clinical signs. They feed in spurts rather than constantly and will inject venom during these feeding spurts, which increase in frequency after a couple of days. Once the frequency of feeding steps up, ticks will inject more venom, which also helps to numb the feeding site. When they have filled their bellies to feed their young, they will fall off and leave a dying host.
The toxin is not well defined and due to ethical reasons, good studies into the effects of tick toxins on animals have been difficult to perform. The tick toxin obviously is part neurotoxin, since the predominant effects are neurological. It is also thought to have a cardiotoxin, exerting effects on the heart which may explain the unfortunate cases of sudden death in animals that appear to have recovered from the neurotoxin effects.
Removing a tick as soon as it is found is essential, ideally without squeezing the body, though I am not convinced that a rough removal causes more problems for the pet. I do not think there is any evidence that salivary flow increases during tick removal. Acute allergic reactions have occurred in humans upon removal of ticks, but I have never seen or heard this reported in other species.
Paralysis ticks do NOT have a head, which is a common misconception, so it is impossible to leave the “head” behind. They have mouth parts which penetrate the skin and may be left behind when the tick is removed, but these no longer inject venom without the body that provides life, and a venom sack! At worst, they may cause a local reaction as a splinter may do. Tick removal may be painful and very difficult in some pets, especially when positioned around the face.
I hear lots of owners complaining about how diligent they are at applying the preventatives so how can their pet still develop paralysis? There is no tick preventative that is 100% effective. A combination of products may help but is still no guarantee. Daily searching for ticks is very helpful, but they can find tricky hiding places. Often they show up in places that have been previously checked and not found, however it would be unlikely that a fully engorged tick had only been present for 24 hours or less. Sometimes they are found under fresh tick collars! Clipping of long haired animals prior to the onset of Spring is a good idea to help tick searching and we always recommend clipping the coat on any animal coming into the hospital for treatment of tick envenomation.
Resistance to tick toxin can develop in animals that are exposed to ticks everyday, as our wildlife are. There are certainly dogs and cats I hear of who have ticks removed on nearly a daily basis and never suffer the ill effects of the toxin. The only way to guarantee that your pet will never get tick paralysis if you live in a tick prone area, is to lock them indoors 24 hours a day, although even then humans may carry them indoors allowing them exposure to indoor pets. It seems the only way to avoid tick paralysis in your pets is to avoid living in a tick prone area – way out west or Western Australia are good locations for this!
This is such a huge topic with so much important information that I feel pet owners need to know, so I am going to continue the discussion in my next post. Then I will delve into the clinical signs and treatment. In the meantime, start checking your pets daily for ticks.