Transmission

Taken from Stentiford & Shields, 2005

The exact mode of transmission of Hematodinium to new susceptible hosts is still a topic of some conjecture. Infections have been transmitted to uninfected hosts via inoculations of infected haemolymph  for Chionoecetes bairdi (Meyers et al. 1987), Callinectes sapidus (Shields & Squyars 2000), and Portunus pelagicus (Hudson & Shields 1994). Inoculations experiments have also shown that filamentous trophonts and vegetative, amoeboid trophonts are capable of establishing infections (Meyers et al. 1987, Hudson & Shields 1994, Shields & Squyars 2000). Further, both micro- and macrospores can produce infections when inoculated into C. bairdi (Eaton et al. 1993).

Perhaps the most compelling candidate for an infectious stage is the dinospore. However, in most parasitic dinoflagellates of marine crustaceans, it is not known whether the dinospore is a true transmissive stage or simply an intermediate stage preceding a resting cyst or some other non-parasitic stage (Shields 1994). The former theory is attractive because Appleton and Vickerman (1998) have shown that culture of dinospores leads directly to the development of filamentous trophonts (cf. vermiform plasmodium) in vitro. Sporulation from the trophont stage to the dinospore stage has been observed in several cultures of Hematodinium from Nephrops norvegicus, but in such cases, it was neither synchronous, nor abundant (Appleton & Vickerman 1998). In laboratory infections, sporulation in C. sapidus can be synchronous and occurs at least once or twice during an infection, with each event lasting from 2 to 4 days (Shields & Squyars 2000). During syncrhonized sporulation, dinospore density can reach extraordinarily high levels, 1.6 x 108 dinospores mL-1; levels far higher than those reported from harmful algal blooms (e.g., Hallegraeff 1993, Smayda 1997). Sporulation in C. sapidus can also be asynchronous and at low densities, possibly accounting for the low number of hosts reportedly sporulating (Shields & Squyars 2000). In N. norvegicus,  sporulation is more or less synchronous, with almost complete exsporulation from the infected host (Appleton & Vickerman 1998). Dinospores have been shown to exit the infected host via the gills in N. norvegicus (Appleton & Vickerman 1998), C. bairdi (Love et al. 1993),  C. sapidus (Shields & Squyars 2000) and C. pagurus (Fig. 5). Although, sporulation was not necessarily fatal in C. sapidus, heavily infected N. norvegicus and C. bairdi died within hours of sporulation (Love et al. 1993, Stentiford et al. 2001b). The molt stage of the host is also important to transmission, but the data are circumstantial indicating that hosts obtain infections during ecdysis or shortly thereafter (see Host Factors below). Further studies on the passive uptake and/or the natural route of entry of these stages are required to confirm this mode of transmission.

Meyers et al. (1996) found potential evidence for sexual transmission of Hematodinium in C. bairdi with parasites present in the seminal fluids of the vas deferens in a few males, but the importance of this potential route of infection needs further work.

As with many protozoan infections in crabs, cannibalism is a possible mode of transmission for Hematodinium. While Hudson and Shields (1994) were unable to transmit Hematodinium to P. pelagicus via ingestion of infected crab flesh, Sheppard et al. (2003) have reported successful transmission of disease to naïve C. sapidus via ingestion. Both C. sapidus and C. opilio are avid cannibals; conspecifics represent up to 25 % of their diet (Laughlin 1982, Alexander 1986, Wieczorek & Hooper 1995, Lovrich & Sainte-Marie 1997, Moksnes et al. 1997, Squires & Dawe 2003). Thus, cannibalism may be an efficient alternate mode of transmission for Hematodinium, at least for some host species. Transmission is clearly an important feature of the host-parasite association that requires more attention. Dinospore transmission, possible cyst stages in the life cycle, the role of cannibalism and the potential for reservoir hosts are avenues worth further research.