Cercarial host-finding

Attachment Cercariae are larval stages of trematode worm-parasites, which emerge from snail intermediate hosts and infect other hosts. The cercariae of many species search and invade their hosts actively. We have studied cercarial host-finding and host-recognition in 4 Schistosoma-species, which infect humans and mammals (agent of human schistosomiasis), Trichobilharzia ocellata, which invades its duck host, but accidentally also humans (agent of cercarial dermatitis), 4 species, which invade fish as next intermediate hosts, e.g. Diplostomum spathaceum (the eyefluke of fish), and 4 Echinostomatid species, which invade water snails as next intermediate hosts. Our results indicate the following common characteristics of cercarial host-finding (for reviews see HAAS 1992, 1994):


CHARACTERISTICS OF CERCARIAL HOST-FINDING AND HOST-RECOGNITION

Host-finding consists of sequences of behaviour phases. Each phase may be independently stimulated by particular cues.

Many species achieve an unexpected high level of host specificity. The specificity is achieved with different strategies:

  1. Chronological succession of responses to different stimuli
  2. Responses to a combination of different stimuli
  3. Responses to individual stimuli with an extremely high specificity and sensitivity

There exists a high diversity of host-finding strategies.

Each species studied so far finds and identifies its host with mechanisms and in response to host signals which differ considerably from those of other species, even when these invade the samehost genera.

THE PHASES OF CERCARIAL HOST-FINDING

Departure from the snail intermediate host and dispersal
The swimming behavior and the orientation mechanisms (such as responses to the environmental cues intensity and direction of light radiation and temperature) are modified with age after shedding.

Localization of and remaining in the hosts habitats
The parasites select habitats, which are frequented by their hosts with defined responses to the environmental cues intensity and direction of light radiation, temperature and gravity.

Localization in the hosts time
Studies of other colleagues show, that cercariae emerge from their snail hosts at that time of the day, when a contact with the hosts is most probable. Emergence is synchronized by the photoperiod and temperature.

Host directed orientation
An approach towards the hosts occurs with very different responses to host cues such as chemical gradients, dark stimuli, water currents or touch.

Attachment to the host
The very different attachment behavior patterns are stimulated in the various species by host cues such as chemicals, warmth, dark stimuli and´water currents.

Remaining on the host (enduring contact)
Most species remain on their hosts only, when the attachment stimuli are followed by other host cues such as chemicals, warmth and hydrophobicity.

Creeping to entry sites
The entry sites are searched by responses to certain chemical and thermal gradients.

Penetration into the hosts skin
Penetration consists of another sequence of behavior phases such as penetration movements, shedding of the tail, transformation of the body wall for immune evasion and secretion of enzymes. Each of these phases can be stimulated by very different chemical components, warmth and hydrophobicity.
On the job:

Wilfried Haas, Tina Loy, Petra Stiegeler, Cynthia Wulff, Bernadett Beran, Sebastian Brachs

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