Depending where in the country you work, you and your customers may consider ticks a mortal enemy to be loathed and feared, or little more than a nuisance. Along the eastern seaboard, from Cape York in the north to Lakes Entrance in the south, the impact of ticks is readily apparent, with tens of thousands of dogs and cats affected every year by paralysis ticks.1 Even in areas outside of this traditional range, the eastern paralysis tick (Ixodes holocyclus) makes her impact felt (and yes it is a her – paralysis is associated with feeding adult females) with recent research reporting cases in the greater Melbourne area.2 Elsewhere in the country the impact of ticks may be less overt, however they are far from benign, with a growing recognition of the importance of tick-borne diseases.
We will delve into the leaf litter to take a look into the world of paralysis ticks and the impact they have on dogs and cats in Australia.
73 different tick species are reported to be capable of causing paralysis.
Of the more than 900 species of ticks identified globally, only 73 species are able to cause paralysis in their hosts through the injection of salivary toxins.3 Compared to other arachnids (ticks are more closely related to spiders than to fleas with which they are often unjustly lumped!), this is a paltry subset, with about 99% of the approximately 47,000 recognised species of spiders being venomous.3 Speaking of venom, why do we say that a spider has venom, and yet ticks are relegated to the more general term of toxin? According to the Australian Academy of Science4, a venom is a toxic substance that has evolved for a specific purpose and is actively injected via a bite or sting. Well tick toxin is most certainly transmitted by a bite – is it denied the right to be called a venom because, like nipples on men, it lacks a purpose? Well it is not my place to question evolution, but when it comes to the role of paralysis toxins produced by ticks, who benefits? The benefit of the toxin to the tick is not clear. It has been suggested that paresis/paralysis may be beneficial in helping to inhibit grooming by the host to allow the tick to feed, although given paralysis only occurs days after initial attachment and the beginning of feeding, this seems questionable. Perhaps paralysis is an unintended consequence of molecules evolved for some other purpose. We may never know, but it is fun to speculate and regardless of whether you call it a toxin or venom, it is not a good thing to be exposed to!
Of the 73 tick species capable of causing paralysis, the eastern paralysis tick is hands down the undisputed champion when it comes to disease severity, with a single adult female tick capable of causing death in a dog or cat. Paralysis is associated with the production of salivary holocyclotoxin, or more accurately holocyclotoxins, as recent research has identified a family of almost 20 related molecules that may act together in varying ratios to cause clinical disease.5
3 different hosts are needed for the eastern paralysis tick to complete its lifecycle.
Although it may take a village to raise a child, it takes only three hosts to raise a paralysis tick to adulthood. Whilst all ticks are obligate parasites, spending at least part of their lifecycle on a host, they differ in just how reliant they are. The lifecycle of ticks, like other arthropods, consists of four life stages; eggs, larvae, nymphs and adults. Tick lifecycles in general are categorised based on the number of free-living stages that feed on a host; either one-, two-, or three-host ticks. In case you are wondering, you can’t have a four host tick as eggs come with their own ready source of nutrition and so don’t need a host to feed on! Paralysis ticks, like other hard ticks, are three-host ticks. You may be thinking that as a three-host tick, I. holocyclus would spend most of its time on a host, however, of a lifecycle that may be up to a year in duration, only three to six weeks are actually spent living on a host (with the remainder spent in the moist leaf litter or the nests of hosts).6 Speaking of hosts, I. holocyclus has a broad host range, being reported on a range of mammalian and avian hosts, however native bandicoots (primarily the northern brown bandicoot and the long-nosed bandicoot) are considered the primary host.6
There is generally a single generation of I. holocyclus ticks per year, and as a result there is marked seasonality to the prevalence of different life stages.6 As mentioned earlier, it is the feeding adult female ticks that are primarily responsible for the development of clinical signs. The lifecycle of the tick therefore explains the seasonality of clinical cases of tick paralysis, with spring and summer associated with a greater relative abundance of adult ticks, and therefore peak incidence of tick paralysis. It is incorrect to speak of an absolute ‘tick season’ however, as adult ticks may be found throughout the year. Additionally, although less common, clinical signs of tick paralysis have been associated with infestation with larval or nymphal I. holocyclus ticks.7
72 hours is generally accepted as the minimum time from tick attachment to the onset of clinical signs.
A questing adult female tick will migrate to a passing host unlucky enough to make contact, however they may not immediately attach, instead wandering about on the host for several hours, sometimes as long as 24 hours.2 Whether due to wandering or their initial point of contact, most paralysis ticks (>70%) are found attached on the head and neck.8
Once attached the female ticks begin to feed. This is a two phase process; the first is a slow feeding phase that lasts a few days, followed by “the big sip”, a rapid feeding phase that lasts 24 hours. It is during the slow feeding phase that the genes responsible for the production of holocyclotoxins are ramping up, with peak toxin production associated with the rapid engorgement phase.9 Holocyclotoxins inhibit the release of a neurotransmitter resulting in paralysis. The clinical signs associated with tick paralysis include:
- A loss of coordination in the hind legs (wobbly or not being able to get up) which may progress to paralysis.
- A change in voice or bark.
- You may notice your dog vomiting after a tick bite.
- Retching, coughing or loss of appetite.
- Laboured breathing or rapid breathing.
By the way, we talked about what adult female ticks feed on, what about the males? Well, they feed on the female ticks!
Why do these numbers matter?
The numbers above, despite being a rollicking introduction to some fun facts about ticks, highlight some important points associated with ticks and their control.
For paralysis ticks, an understanding of the lifecycle helps to understand the peak risk periods for tick paralysis, as this coincides with peak numbers of adult female ticks. However cases may be seen all year round, and that brings up another number – 12. This is the number of months in the year in which the Australian Paralysis Tick Advisory Panel recommends the use of isoxazoline based tick control products, such as NexGard and NexGard SPECTRA, for animals living in or travelling to a paralysis tick area. Another key number when it comes to paralysis ticks is 72 hours, the typical time taken from attachment to clinical signs. This provides a window of opportunity to avoid the consequence of ticks through two simple actions – daily tick searching by pet owners and the use of an effective acaricide. The introduction of isoxazoline based acaricides has been associated with a marked reduction in cases of tick paralysis.10 Through encouraging pet owners to be compliant, not complacent around the use of tick control products, all-year round, it is within our reach to drive these numbers down further.
Outside of paralysis tick areas throughout Australia, but particularly in the north, a change in mindset around the significance of ticks may be in order. We are fortunate that we do not have as many, or potentially as serious, tick-borne disease as seen in other parts of the world. However we do have some, with one study showing 12% of healthy client owned dogs in Queensland and the Northern Territory were infected with one or more tick-borne pathogens.11 The recent discovery of emerging tick-borne pathogens like Ehrlichia canis, not previously identified in Australia, highlights the importance of good tick control throughout the country, not just on the east coast.
- Hall-Mendelin, S., et al (2011) Tick paralysis in Australia caused by Ixodes holocyclus Neumann. Ann Trop Med Parasitol, 105(2), 95-106.
- Barker, D., et al (2020) Survey of cases of tick-paralysis and the presence of the eastern paralysis tick, Ixodes holocyclus, and the southern paralysis tick, Ixodes cornuatus, in the Greater Melbourne Area. Aust Vet J, 98(1-2), 2-10.
- Pienaar, R., et al (2018) Tick Paralysis: Solving an Enigma. Vet Sci, 5(2).
- www.science.org.au/curious/people-medicine/poison-vs-venom last accessed 24 Aug 2020.
- Rodriguez-Valle, M., et al (2018) Transcriptome and toxin family analysis of the paralysis tick, Ixodes holocyclus. Int J Parasitol, 48(1), 71-82.
- Barker, S.C., et al (2014) Ticks of Australia. The species that infest domestic animals and humans. Zootaxa, (3816), 1-144.
- Fitzgerald, M.P. (2007) Tick toxicity in a cat caused by the larval stage of Ixodes holocyclus. Aust Vet Pract, 32-34.
- Atwell, R., et al (2000) The attachment sites of the paralysis tick (Ixodes holocyclus) on dogs. Aust Vet Pract, 30(2), 68-71.
- Padula, A. (2016) Tick paralysis of animals in Australia. Clinical Toxinology: Clinical Toxinology, 1-20.
- Rodd, A., et al. Time series analysis of tick paralysis cases in south-eastern Queensland over a 9-year period. In: Australia and New Zealand College of Veterinary Scientists (ANZCVS) Science Week. 2017. Gold Coast.
- Hii, S., et al (2015) Canine tick-borne pathogens and associated risk factors in dogs presenting with and without clinical signs consistent with tick-borne diseases in northern Australia. Aust Vet J, 93(3), 58-66.