The Paralysis Tick of Australia (Ixodes holocyclus)

By Norbert Fischer B.V.Sc. (Syd)

Significance

Half-engorged female adult Australian Paralysis Tick, Ixodes holocyclus; N Fischer

^ Half-engorged female adult Australian Paralysis Tick, Ixodes holocyclus

The Australian Paralysis Tick, Ixodes holocyclus, is an important life-threatening parasite of man and animals. It is also the tick most commonly found on dogs, cats and humans on the East Coast of Australia. It's paralysing toxin has been estimated to affect as many as 100,000 domestic animals annually, with up to 10,000 companion animals being referred to veterinary surgeons for treatment (Stone, 1997). In human population health terms, tick envenomation is a greater medical problem for children than snake or spider bite (Pitt, 1999). Humans face a small but significant risk of acquiring tick-borne infectious diseases, in particular Australian Spotted Fever and a Lyme-like Disease..

Common Names

Ixodes holocyclus belongs to the Genus Ixodes within the Family Ixodidae (see Tick Structure & Biology). The use of common names has led to a plethora of colloquial expressions for Ixodes holocyclus. The most generally accepted name is the "paralysis tick" but perhaps it should be "the paralysis tick of mainland Australia". The following list gives some of the other names, but some of these names are also used for other ticks found in Australia which leads to confusion. 

 Form Alternative Name Name Usage
scrub tick  I. holocyclus adult; in Qld is also used for Haemaphysalis longicornis
bush tick I. holocyclus adult; throughout Australia also used for Haemaphysalis longicornis
dog tick I. holocyclus adult; in NSW also used for Rhipicephalus sanguineus (Brown Dog Tick)
wattle tick I. holocyclus adult; term used by Illawarra pioneers to describe the tick causing paralysis, esp in sheep
common hardback tick I. holocyclus adult; term used in the The Northern Herald, Sydney (Aug 1996)
bottle tick I. holocyclus engorged female
blue bottle tick I. holocyclus engorged female
shell back tick I. holocyclus male; in reference to the tortoise-shell appearance of the large scutum
grass tick I. holocyclus larvae and sometimes nymphs
scrub itch tick I. holocyclus larvae; in Qld used for multiple larvae and, to a lesser extent, nymphs causing rash
seed tick I. holocyclus larvae

  female adult    male adult    larva

The Linnaean name Ixodes holocyclus is derived from the New Latin word Ixodes meaning "tick genus", combined with the Latin word holocyclus meaning "complete circle" referring to the anal groove which encircles the anus in this species and other members of its subgenus. The first species name for any tick was for Ixodes ricinus, the European castor-bean tick, named by Carolus Linnaeus of Sweden, originator of the binomial classification system. Ricinus is the Latin word for the castor bean plant. The New Latin word Ixodes is in turn probably borrowed from a Greek word. In ancient Greek the word ixodes = tackiness.

Anatomy

Whilst pioneering settlers knew that ticks posed a threat to their dogs and perhaps to themselves, the paralysis tick was not scientifically identified until 1899 (by Neumann). It was further studied by Nuttal and Warburton (1911). The life cycle was studied chiefly by Ross (1924) and Oxer and Ricardo (1942) and later summarised by Seddon (1951).

General Features

Tick Stage Image No. of legs Non-engorged body size (mm) Engorged body size (mm) Other features
eggs - -  0.3-0.4 - 2000-3000 laid per adult female in a single clump
larvae 6 0.5 x 0.4 1.15 x 1.0 Larvae are neither male nor female
nymphs 8 1.2 x 0.85  3.5 x 2.5  Nymphs are sexually immature (have no genital aperture)
adult male 8 variable:
1.9 x 1.6 to
2.1 x 2.8
- Shield (scutum) covers entire dorsal body
Short head (capitulum)
adult female 8 variable:
2.6 x 1.1 to
3.8 x 2.6
up to:
13.2 x 10.2
Shield (scutum) covers only part of dorsal body
Long head (capitulum)

Relative Sizes

^ Engorged stages of Ixodes holocyclus. Un-engorged stages are much smaller (Bernie Hudson, 2002).

Un-engorged Un-engorged Un-engorged Engorged
0.5 mm long
0.4 mm wide
1.2 mm long
0.85 mm wide
3.8 mm long
2.6 mm wide
13.2  mm long
10.2 mm wide
larval ticks
(6 legs)
nymph
(8 legs)
adult
(8 legs)
engorged adult
(only female engorges)

^ Un-engorged stages of Ixodes holocyclus. Measurements refer to the body section only - see General Features for details. Absolute sizes will vary according to monitor resolution settings.

Male and Female compared

Male ('shell back') Female
Does not engorge With early engorgement
Shield (scutum) covers entire dorsal body
Short head (capitulum)
Shield (scutum) covers only part of dorsal body
Long head (capitulum)

Egg

^ I. holocyclus adult female with recently deposited eggs heaped near the capitulum. Up to 2000 eggs can be deposited.

I. holocyclus eggs with human hair

^ I. holocyclus eggs with human hair (courtesy GR Turnbull, 2004)

Larva (seed tick)

Images

I. holocyclus larva; a, capitulum (dorsal view); b, scutum; c, hypostome; d, tarsus I; e, tarsus IV; f, coxae

Diagnosis Capitulum with slender palpi, hypostome rounded apically, dentition 2/2; scutum about as long as wide, with faint lateral carinae; all coxae with small, external spurs.
Body Broadly oval, 0.5 x 0.4 mm (unfed) to 1.15 x 1.0 mm (engorged)
Capitulum About 0.2 mm in length, basis triangular, about 0.16 mm wide, palpi elongate and slender. Hypostome apically rounded, 0.14 mm in length, dentition 2/2 of 10-12 teeth, the teeth of the inner fileblunt and small, some minute denticles apically.
Scutum About as long as broad, 0.31 by 0.32 mm and widest a littel anterior to mid-length, lateral carinae present but faint; anterolateral margins usually convex and posterolateral margins concave; cervical grooves short but well defined.
Anal grooves Ill defined anteriorlyand do not converge behind
Legs Coxae with small external spurs; tarsus I 0.14 mm in length, tarsus IV 0.14 mm in length

Nymph

 

Images

I. holocyclus nymph; a, capitulum (dorsal view); b, capitulum (ventral view); c, scutum; d, spiracular plate; e, sternal plate; f, hypostome; g, coxae; h, tarsus I; i, tarsus IV

Diagnosis Capitulum as in female, hypostome dentition mainly 2/2, 3/3 distally; scutum about as long as wide with lateral carinae; sternal plate present, oval; anal grooves converging posteriorly but remaining narrowly open; legs as in female.
Body Oval with fine parallel striae and some scattered pale hairs; 1.2 x 0.85 mm (unfed) to 3.5 x 2.5 mm (engorged); marginal grooves well developed and complete in unfed specimens.
Capitulum Length 0.40- 0.43 mm. Basis dorsally 0.23- 0.25 mm in width; posterior margin straight; posterolateral angles not salient; auriculae well defined. Palps as in female; articles 2 nd 3 0.30- 0.32 mm in length. Hypostome lanceolate and bluntly pointed; dentition 2/2, 3/3 distally.
Scutum About as wide as long, 0.61 x 0.63 mm - 0.71 x 0.70 mm. Lateral carinae well developed . Punctations few, shallow, scattered. Cervical grooves apparent, continuing to mid-scutal region as superficial depressions.
Sternal plate Oval, 0.27- 0.30 mm in length and a little more than twice as long as wide.
Spiracular plate Subcircular, greatest diameter about 0.14 mm.
Legs Coxae armed as in female. Tarsus I tapering gradually, other tarsi more abruptly; length of tarsi I and IV about 0.28 mm.

Male adult

Images I. holocyclus male- a, body (dorsal view); b, body (ventral view); c, capitulum (dorsal view); d, capitulum (ventral view); e, hypostome; f, tarsus I; g, tarsus IV; h, spiracular plate
Diagnosis Body measurements less than 3.0 x 2.5 mm; lateral grooves completely encircling scutum, no lateral carinae; punctuations fine; basis capituli punctate dorsally, palpi short and very broad; hypostome dentition 2/2, with rounded teeth; anal plate bluntly pointed behind; adanal plate curving inwardly to a point; coxae with well-defined spurs decreasing in size posteriorly; trochanters III and IV frequently with small, ventral spurs.
Body Oval, sometimes broadly so, 1.9 x 1.6 mm- 3.2 x 2.3 mm; marginal body fold narrow but prominent; hairs dorsally sparse medianly, more numerous on marginal body fold.
Capitulum Length 0.51- 0.65 mm in width, surface punctate; posterior margin straight; no cornua; posterolateal margins slightly divergent anteriorly; basis ventrally narrowing to the straight posterior margin, surface with a short anterolateral ridge. Palps short and broad; article 1 rounded and a little salient laterally, ventrally with a transverse rounded flange continous with ridge on basis; articles 2 and 3 with no apparent suture, 0.33- 0.40 mm in length, almost twice as long as broad, rounded distally, hairs moderate in number, some long hairs ventrally. Hypostome short and borad, 0.25- 0.28 mm in length, narrowing and shallowly rounded distally; dentition 2/2 of large rounded teeth, some small teeth distally and crenulations basally.
Scutum Oval, convex, only a little smaller than body. Lateral grooves deep and completely encircling the scutum, anteriorly somewhat linear and may simulate mild lateral carinae. Punctations fine, usually most numerous submarginally and anteromedianly; pseudoscutum sometimes faintly apparent. Cervical grooves, short, shallow. Emargination moderate. Scapulae blunt.
Genital aperture On a level with anterior margin of coxa III, sometimes at level of 2nd intercoxal space.
Ventral plates Pregenital plate wider than long; median plate 1.5 x 1.2 mm, the width posteriorly about 3/4 of the length; anal plate 0.75 x 0.50 mm, anterior margin straight or mildly curved, pointed posteriorly; adanal plates curving to points near the point of the anal plate; plates with scattered punctuations and hairs.
Spiracular plate Elongate, oval, narrow posteriorly, the longer axis directed anteriorly, about 0.50- 0.53 mm in length.
Legs Length moderate. Coxae practically contiguous, with a row of long hairs near posterior margin; posterointernal angles of coxae I and II may be somewhat sharp but not salient; all coxae with an external spur, strongest and bluntly pointed on coxa I, smallest on coxa IV. Trochanters III and IV with a small, dark ventral spur, only a tuberosity on II. Tarsi ending somewhat abruptly; length of tarsus I 0.65- 0.71 mm, and of tarsus IV 0.62- 0.70 mm.

 

Female adult

Note that these images are not scaled to one another. As the legs and shield (scutum) do not enlarge appreciably with engorgement, they give some indication of relative size. See also Ixodes holocyclus - engorged with eggs.

Images

I. holocyclus female; a, capitulum (dorsal view); b, capitulum (ventral view); c, palpal article I (lateral view); d, hypostome; e, scutum; f, anal grooves; g, spiracular plate; h, tarsus I; i, tarsus IV.

Ixodes holocyclus mouthparts showing barbed hypostome (Scanning electron micrograph, courtesy Assoc Prof Kevin Broady).

Diagnosis A very large tick when fully engorged; scutum about as long as broad and broadest a little posterior to mid length, with strong lateral carinae; capitulum relatively long porose areas deep, cornua usually absent, but when present at most only mild and rounded; auriculae present; hypostome lanceolate, dentition mainly 3/3; no sternal plate; anal grooves meeting at a point behind; all coxae with an external spur decreasing in size posteriorly; trochanters III and IV usually with small, pointed ventral spurs.
Body Unfed specimens, oval , flat, yellowish, 2.6 x 1.1 mm - 3.8 x 2.6 mm; marginal groove well developed and continuous; hairs small, scattered, most numerous in region of marginal fold. Semi-engorged specimens frequently with body widest behind coxa IV and with a waist at level of spiracles. Fully engorged specimens broadly oval, attaining 13.2 x 10.2 mm, living ticks with blue-grey alloscutum, the dorsum light in colour, a dark band in region of marginal groove. 
Capitulum Length 1.00- 1.035 mm. Basis dorsally 0.60- 0.68 mm in width, the lateral submarginal fields swollen and frequently delimited from the depressed, median field by ill-defined carinae; posterior margin sinuous, posterolateral angles swollen, sometimes mildly salient; porose areas large, deep subcircular or oval, the longer axis directed anteriorly, interval frequently depressed, at most about the width of one; basis ventrally with posterior margin rounded and with well-defined, blunt, retrograde auriculae. Palps long and slender, some long hairs ventrally; article I rounded and somewhat salient laterally, inner "ring" with dorsal tongue-like prolongation and ventrally semicircular and plate-like, the posterior margin of the plate extending beyond the palp; articles 2 and 3 with no apparent suture, 0.75- 0.85 mm in length and about four times as long as wide, narrowly rounded distally. Hypostome lanceolate and bluntly pointed; dentition mainly 3/3, the innermost file of small, spaced teeth, basally 2/2.
Scutum As wide as or a little wider than long, widest a little posterior to mid length, 1.6 x 1.7 mm- 2.4 x 2.4 mm, flat medianly, convex external to the long, strong lateral carinae; anterolateral margins practically straight, posterolateral margins mildly concave; posterior anle broadly rounded. Punctations numerous , fine, sometimes a little coarser medianly and laterally, shallow rugae frequently present posteriorly. Cervical grooves well defined but short. Emargination moderate. Scapulae blunt.
Genital aperture On a level with coxa IV, but in engorged specimens sometimes just posterior to this level.
Anal grooves Rounded anteriorly, curving behind anus to meet in a somewhat elongate point.
Spiracular plate Subcircular, greatest dimension 0.40- 0.45 mm.
Legs Coxae smooth, I and II sometimes with mild rounded ridges externally, each with a row of long hairs posteriorly and an external spur, longer and more pointed than in male, and decresing in size posteriorly. Trochanter IV (and sometimes III) frequently with a small, ventral spur. Tarsi tapering a little abruptly; length of tarsus I 0.70- 0.80 mm, and of tarsus IV 0.60- 0.78 mm.

Ability to transmit diseases (vector competence)

In general terms a vector may simply mechanically transport a pathogen without it replicating, or it may both transmit and replicate a pathogen. Many viruses, bacteria, rickettsia and protozoal organisms are capable of replicating within ticks. This is known as "biological" transmission and this mode of transmission is the one that applies most commonly to ticks. For hard (Ixodid) ticks it is virtually impossible for mechanical transmission to occur on its own (i.e. without some replication), as they tend to not take multiple blood meals from different hosts in the one stage. Passing an organism between tick stages (trans-stadial transmission) seemingly requires that an organism can also replicate within the host.

In the case of Ixodes holocyclus there is much speculation about transmission of various pathogens, with the exception of  the spotted fevers which are now well recognised. Saying that an arthropod is a competent vector (generally) requires several findings: (1) frequent isolation of the suspected pathogen; (2) showing that the distribution of the suspected vector overlaps with a diseased population (be it human or animal); (3) showing that host-seeking and blood-feeding coincide with the disease incidence; (4) showing that disease incidence varies with host choice; and (5) showing the capacity for vector competence in the laboratory.

Bacterial Diseases

Rickettsiae

These days there is not much disputing that I. holocyclus is the main vector for R. australis (Rickettsial Spotted Fever). Unfortunately vector competence work is still required to confirm its status. (S. Doggett pers. com.).

Despite the lack of vector competence studies, the paralysis tick (I. holocyclus) is commonly mentioned as a potential vector of Q-fever (caused by the rickettsial organism Coxiella burnetii). The ornate kangaroo/wallaby ticks Amblyomma triguttatum s.l. have also been implicated in Q-fever (Russell and Doggett, 1995).

Spirochaetes

Some vector competence studies have been undertaken on I. holocyclus with respect to the Lyme disease pathogen Borrelia burgdorferi sensu stricto (with a United States strain). These suggested that the tick can not transmit this strain of spirochaete (Piesman and Stone, 1991).

Despite this, there is a strong belief that some kind of Lyme-like spirochaete causes a Lyme-like disease in Australia and that it is carried by the most common tick afflicting humans, Ixodes holocyclus. At the Royal North Shore Hospital in Sydney the Tick-borne Diseases Research Unit continues its study into this issue. Cases of Lyme-like disease are being diagnosed on the basis of clinical signs (often musculoskeletal, chronic fatigue, neurological and dermatological), exclusion of other infections, serology (which is supportive but not conclusive), and response to antibiotic treatment (initially, antibiotics may cause a worsening of symptoms [the Herxheimer reaction], presumably as spirochaetes are destroyed, and this in part supports the diagnosis).

The Australian form of Lyme-like borreliosis is associated with a different clinical picture from true Lyme disease of North America. See also the Westmead Hospital Dept of Medical Entomology site for their interpretation on the status of an Australian Lyme-like Disease. For comparison with North American Lyme borreliosis go to healthlinks.washington.edu.

Viral Diseases

So far, no viruses have been isolated from I. holocyclus (Russell and Doggett, 1995). This does not however exclude the possibility that such diseases may be found in the future.

Protozoal Diseases

So far no protozoa have been isolated from I. holocyclus. This does not however exclude the possibility that such organisms may be found in the future.