Distribution

Only certain geographically confined populations appear to be venomous, as the envenoming syndrome does not occur throughout the entire area of distribution of a particular species!

Chief causative species and zones of tick paralysis:

1. Dermacentor andersoni: northwest USA and western Canada, with the highest incidence in southern British Columbia.
2. Dermacentor variabilis: responsible for multiple cases in eastern and southern USA.
3. Ixodes holocyclus: eastern Australia.

Several cases of envenoming caused by the following species of the family Ixodidae have also been described:

Amblyomma maculatum: North America

Amblyomma americanum: North America

Haemaphysalis cinnabarina: North America

Hyalomma transiensis: South Africa

Hyalomma truncatum: South Africa

Ixodes cornuatus: Australia

Ixodes rubicundus: South Africa

Ixodes hexagonus

Ixodes tancitarius

Rhiphicephalus simus: South Africa

Other countries in which tick paralysis in humans has been described:

Europe: England, Spain, France, Greece

Africa: Algeria, Somalia

Middle East: Israel

Far East: Russia

Central America: Mexico

Biology

Like scorpions and spiders, ticks also belong to the arachnids (Arachnida), where they are included in the order of the acarines (Acarina). Ticks are ectoparasites that feed on the blood of their hosts. In this way ticks are not only vectors for a number of different diseases, but some of them also possess venomous saliva and can cause paralytic envenoming in wild animals, livestock and pets as well as in humans. It is unclear why certain geographically confined populations have venomous saliva.

The roughly 800 species of ticks are divided into hard-bodied ticks (Ixodidae, 640 species) and soft-bodied ticks (Argasidae, 160 species). Almost all species that are known to have caused envenoming belong to the family of hard-bodied ticks. In contrast to the soft-bodied ticks, they possess a hard shield on their backs, the so-called scutum (Fig. 4.86). On the small capitulum ("head") are the mouthparts, which are specifically formed to bite and suck. The body is not segmented and is of an oval, pouchy form.

  Fig. 4.86 Dermacentor andersoni.

During their life cycle, hard-bodied ticks pass through 3 developmental stages, i.e. larva, nymph and adult. The adults and nymphs have 4 pairs of legs, while the larvae have 3. Before each new stage, it is necessary for ticks to feed on blood. Many species require 3 different hosts for this process, while others only need 1. Before laying her eggs, the adult female may feed for 11–12 days. During this time, her body swells enormously and her body weight increases a hundredfold.

Risk

Tick paralysis is a relatively rare form of envenoming, but in the advanced stage it can become life-threatening for humans. It also occurs in various mammals and in certain areas can lead to considerable losses of cattle and sheep. Among the Argasidae, Ornithodorus lahorensis and Otobius megnini have been associated with a few cases of tick paralysis in humans. Along with the medically most important species Ixodes holocyclus, Dermacentor andersoni and possibly Dermacentor variabilis, other species of hard-bodied ticks have also been considered responsible for isolated cases of envenoming in humans, but less commonly (see above).

Murnaghan and O'Rourke (1978) provide data regarding the incidence of tick paralysis throughout the world. According to these data, 305 cases of tick paralysis due to D. andersoni were recorded in British Columbia between 1900 and 1968, with a mortality rate of around 10%. D. andersoni was also responsible for the total of 114 cases recorded up to 1968 in the US states of Washington, Montana, Oregon, Idaho, Wyoming and Colorado. There are at least 40 known cases of envenoming due to D. variabilis from eastern and southern USA. In Australia, between 1912 and 1940, 21 cases of tick paralysis were recorded, with a mortality rate of almost 50% (Ixodes holocyclus). However, since 1945 there have been no known cases of fatal envenoming in Australia.

Most cases of tick paralysis and the overwhelming majority of fatal cases of envenoming occur in children. In North America, the incidence is highest in the spring and summer months, while in Queensland, Australia, ticks are active throughout the year due to the mild climate, and cases of envenoming may occur in any season.

With tick bites, the most important measure to be taken is to find and carefully remove the tick. In most cases the patient then recovers rapidly. Tick paralysis becomes problematic when it is not recognised as such. The correct diagnosis is difficult in regions where tick paralysis does not occur, and where patients present with envenoming that was caused while on holiday in an endemic area.

Literature (biological)

Gentile 1989, Harwood and James 1979, Murnaghan and O'Rourke  1978, Sutherland 1983, Wikel 1984, Aeschlimann and Freyvogel 1995, White 1995c