CERCARIAL DERMATITIS 
197 
to produce a dermatitis by penetration of the human skin. Cort (1936) lists five such species, 
namely, Cercaria ocellata La Vallete, 1855, C. douthitti Cort, 1914, C. elvae Miller, 1923, 
C. physellae Talbot, 1936, and C. stagnicolae^ Talbot, 1936. To this list must be added 
C. tucker ensis Miller, 1927, C. longicauda Macfarlane, 1944 and C. oregonenesis Macfarlane and 
Macy, 1946. 
McMullen and Beaver (1945) consider C. elvae to be a synonym of C. ocellata and Szidat 
(1942) believes the latter to be more than one species distinguishable only if killed by heat as 
recommended by Brumpt. Talbot (1936) separated C. physellae and C. stagnicolae from 
C. elvae with which they were previously confused, merely by slight differences in time of 
emergence, taxes, method of swimming, and attachment. The measurements given for 
C. ocellata by various authors vary quite considerably. 
Brumpt (1931) by experimental infection has shown C. ocellata to be the larval form of 
Trichobilharzia ocellata (La Valette, 1854). McMullen and Beaver (1945) experimentally 
obtained the adult forms of C. physellae and C. stagnicolae and found these also to belong to the 
genus Trichobilharzia Skrjabin and Zakharov, 1920. Price (1931) experimentally infected 
laboratory mice and rats with C. douthitti and showed it to belong to the genus Schistosomatium. 
Thus it would appear that the cercariae belonging to the “ Elvae group ” do not belong to 
the same genus and that the classification of these larval forms is somewhat artificial. They 
also show a considerable degree of variation in their measurements which makes species 
determination somewhat difficult. Even where the knowledge of the structure of the cercaria 
and of its parthenita is complete, it is difficult to arrive at a final decision as to specificity except 
through a series of breeding experiments conducted in order to obtain the final adult stage. 
These itch-producing cercariae in Malaya belonging to the “ Elvae group ” are probably 
the larval forms of some member of the Bilharziellinae, sub-family of the Schistosomatidae, 
possibly belonging to the genus Trichobilharzia. There is some resemblance to the cercariae 
of T. stagnicolae (Talbot, 1936) as well as to Cercaria oregonensis. Macfarlane and 
Macy, 1946. As the post-larval development has not been studied and as the morpho- 
logical study does not extend to details necessary for species determination the procedure, so 
widely used, of giving numbers instead of names is adopted and the species is referred to as 
Cercaria malayi I. 
It is of interest to record that Macfarlane (1944) in New Zealand noted that the dermatitis 
following the first exposure to Cercaria longicauda was milder than that following re-exposure 
and suggested that it may be a sensitisation reaction resulting from repeated exposure to the 
cercariae. To test the validity of this hypothesis, Olivier (1949) carried out a series of human 
exposures to species of cercariae known to cause dermatitis in the United States. He showed 
that persons exposed the first time had only mild, innocuous and inconspicuous lesions, like 
the ones experienced by Professor Buckley and myself. When persons were given additional 
exposures to relatively small numbers of the same species, the reaction became progressively 
stronger and the dermatitis more severe. Similarly, Hunter et al. (1949) report two cases of 
what appears to be an allergic response following sensitisation a month previously. 
It is probable, therefore, that in Malaya Buckley’s sepsis caused by scratching of the lesions 
only partly explains the ill effects, allergic skin reaction playing a part as well. A careful 
history taken from the rice-cultivators may help to establish this point. Dr. Suleiman is of the 
opinion that the complaints of “ sawah-itch ” are less common from children and young adults 
working in the rice-fields. 
The question has been frequently raised as to whether dermatitis-producing schistosome 
cercariae could set up a systemic infection in man. Buckley (1938) having described the 
dermatitis caused by the cercariae of S. spindale in Malaya, mentions that there are several 
records in the literature of cases of human schistosomiasis, which from the characteristic 
MALAYA , No. 26. 1953 
