FREEZING TO CONTROL HAZARDS FROM PARASITES IN FISH - Peter Howgate
Date 10.11.1999

The first paragraph of Chapter 5 Parasites of the FDAA Fish and Fishery Products Hazard and Controls Guide lists parasites posing human health hazards in fish.  The list includes species of trematodes, nematodes and cestodes.  The following section, headed Controlling Parasites, refers to freezing as a means of killing parasites in fish.  I have commented on the freezing and frozen storage conditions required to kill nematodes in my message and attachment of 13 May.  The purpose of this note is to emphasise that my review applied only to freezing to control the hazard from nematodes.

Trematodes, on a world basis, are more serious parasites;  the WHO report on Control of Food-borne Trematode infections, Nº. 849, estimated around 40 million people infected.  Freezing might be a means for killing the parasites in fish, but there is scant experimental evidence on the required conditions.  Hamed and Elias, Journal of Food Science, 1970, 35, 386-388, examined the viability of Heterophyes spp. in frozen mullet.  They seem to have frozen the fish, 25-30 cm long, by putting them in a freezer at –10º and at –20ºC.  Fish were sampled at intervals, and the metacercariae examined in squash preparations.  Lack of movement of the parasite was used to indicate death.  On this basis, they found that 30 hours at either temperature was required to kill the parasite.  A very recent paper, by Fan.Int.J. for Parasitology, 1998, 28(4), 603-605, examined the effect of freezing, and of salting, on infectivity of metacercariae.  The fish were a cyprinid, Pseudorasbora parva, 10g in weight and were frozen in a ´freezer´, presumably a cabinet, at –12º and –20º C.  These small fish would freeze very quickly.  Samples were taken at intervals and the author recovered metacercariae from the flesh by enzyme digestion.  The infectivity of metacercariae was tested by inoculation into rats or rabbits, a more relevant test of risk than observation of movement.  The author found that metacercariae were infective up to 18 days at –12º but not at 20 days, and at 7 days at –20º.  Longer times at the latter temperature were not tested.

The evidence is slight, but what there is indicates that freezing conditions adequate to control nematodes as a hazard seem not to be adequate for control of the trematode hazard.