4 Fishery Board for Scotland. 



DoJcus adus — An Intracapillary Parasite of Fish. 



This parasite occurs not infrequently in the muscles of the haddock 

 (Gadus cealefinus) ; it gives rise to the condition known as the 

 " spotted haddock," "greasers," or " smelly haddock." The disease is 

 not to be detected on the outside of the fish. It is only when the fish 

 has been split, as, for example, in the preparation for curing, that it 

 becomes visible. The parasite is then seen to consist of little white 

 bodies (cj^sts), measuring 1 to 2 mm. in length, scattered through the 

 muscles (0, fig. 7).* It is often observed in haddocks captured off the 

 West Coast of Scotland. It is said to occur in North Sea haddocks 

 also, but that statement is questioned. When smoked the infected 

 fishes have a disagreeable smell. The fishes exhibiting the parasite 

 which have been examined for the present paper measured from 

 about 12 to 18 inches (30-43 cm.) in length. They appeared to be 

 in good condition. 



The cyst tends to lie in the run of the muscle fibres, which it has 

 pushed aside to form a cavity. Fig. 2 represents an enlarged view of 

 a cj^st in situ. It consists of what looks like a mass of spores situated 

 in chambers of various sizes. Here and there about the cyst, or 

 apart from it in the muscles, can be seen elongated larvae («). These are 

 larvae which have wandered from the cyst. The enveloping tissue of 

 the cyst seems to be connective tissue formed by the host. Such a 

 cyst as is shown in the figure is probably formed of many cysts, which 

 may, however, have been derived from a single original c}^st. The 

 apparent spores are not spores. They are branches of the parasite 

 which are situated in pouches pushed out from the original cyst. 

 The flesh of the parasite is granular and rather opaque. At two 

 points in the large cyst greenish-yellow areas were seen, viz., bl. 

 One of these regions is shown in greater magnification in fig. 3. 

 Spindle-shaped bodies are to be observed in the area. These I am 

 inclined to interpret as blood corpuscles. 



The cyst can be dissected out whole from among the muscles ; it is 

 bound only very slightly to the muscles. It is bathed in a sticky 

 plasma that coagulates in water. Figs. 11 and 16 are enlarged 

 drawings of cysts. When the cyst is teazed up the chambers may 

 break off separately, bringing with them portions of the parasite. 

 The chambers then resemble spores (cp., figs. 8, 9, 14). Fig. 8 

 shows the different parts of the chamber; w. is the wall of the 

 chamber ; pa. is the parasite, which has an outer skin ; ct. is the 

 connective tissue which forms the investment of the cyst. The 

 chambers show a pore in the wall. They are of various sizes, some 

 being very small. In a large chamber - 05 mm. in diameter the part of 

 the parasite filling it exhibited vacuoles (fig. 14). It is probably an 

 encysted larva. 



The great development of connective tissue is seen between the 

 muscle fibres (fig. 1). Where the thickened connective tissue covers 

 the muscle it hides it, rendering it opaque (ct. ib. and m' . fig. 4). Where 

 the normal amount of connective tissue is present the muscle fibres 

 are clearly visible. The muscle fibre m. shown in fig. 1 was about 

 •2 mm. in breadth. 



The wandering larvae are found in tubes either as elongated forms 

 (a. figs. 4, 10, 15, 20), or as round ovum-like bodies (la. fig. 22). A 



* A haddock exhibited, to the naked eye, black flecks through the muscles. 

 The flecks were collections of minute black granules. Some larvae of Dokus 

 adus were detected by means of the microscope. 



