Customers used to feeding their aquatic pets only dried or freeze-dried foods are sometimes hesitant to make the step to include frozen foods as part of the diet presented to their fish, amphibians, aquatic invertebrates or turtles, because ‘I’ve read on the internet about frozen feeds potentially containing parasites so no, I am not going to take the chance of infesting my aquarium with parasites. Safety first!'.

   Our answer to that myth: frozen foods contain no parasites able to infect aquatic pets (or the food handler for that matter). One can safely administer our frozen feeds without fearing to be going to infest one's aquarium with parasites able to infect one's fish.

   How can we, a manufacturer of frozen fish foods (amongst other foods), be sure of having succeeded in killing off any potentially present fish parasites? Because we strictly adhere to (and actually exceed) the legally prescribed maximum freezing temperature and minimum freezer dwell times required for achieving a 100% fish parasite kill rate. Both the FDA (United States Food and Drug Administration) and EFSA (European Food Safety Authority) have published a detailed set of processing steps manufacturers of fish products [aimed at being consumed raw by humans (think salmon, sushi, sashimi, etc.)] must follow for excluding the presence of parasites in these products.

CESTODES, TREMATODES & NEMATODES

   These are not the names of some obscure Moldavian death metal bands but classes of important parasites which have the ability to infect (and unfortunately, also kill) humans if food products have not been properly treated before being consumed raw. As such, government agencies watching over public health have spend quite some efforts on determining appropriate processing techniques for obtaining a 100% kill rate of these parasites.

   Cestodes are a class of parasitic flatworms, of which some species are commonly known as tapeworms. We all know these! Humans, cats, dogs, birds, reptiles, fish … ALL vertebrates can harbor tapeworms in their intestines or digestive tract. Most tapeworms have an intermediate host (most of the time not a vertebrate but an insect, mollusk or crustacean), so a host in which a larval tapeworm grows before moving on to its final host as an adult tapeworm.

   Brine shrimp for example are known to be intermediate hosts for certain avian tapeworms[1]. With ‘avian tapeworms’ we mean that birds are the final hosts of the tapeworm in question. One should not be surprised finding out that brine shrimp are intermediate hosts for certain avian tapeworms when considering that birds are the major, natural predators of brine shrimp in saline lakes or saline evaporation ponds. Another thing to keep in mind is that most avian tapeworms, but not all, are quite specific in their final host requirement, meaning that they require a certain bird species for maturing. A tapeworm maturing in flamingo’s will require flamingo’s to mature. If the brine shrimp the tapeworm infected is eaten by a grebe, eared grebe, or any kind of wading bird or gull, the tapeworm will not reach adulthood.

   Trematodes are another class of flatworms and are commonly known as flukes; these are discussed further.

   Nematodes, which are commonly known as roundworms, are one of the most successful phyla inhabiting our planet and can literally be found everywhere as they have colonized every conceivable habitat (soil, salt water, freshwater but also animals). Not all nematodes are parasitic but a lot are. And not all parasitic nematodes should be classified as ‘bad’ because some parasitic species are actually being used for controlling malaria, as the nematode kills mosquito larvae.

FREEZING FOR PARASITE KILLING

   Freezing can kill off parasites commonly found in fish or seafood products, but the efficiency of the freezing process depends on the following, most important factors:

• The freezing temperature.
• The time the frozen product is stored frozen at a certain temperature (this is called the freezer dwell time).
• The type of product.
• The type of parasite. Cestodes for example are more susceptible to freezing than nematodes, but trematodes are even better equipped to withstand freezing.

   The USFDA requires the following freezing temperatures with according freezer dwell times[2]:

FREEZING TEMPERATURE	FREEZER DWELL TIME & TEMPERATURE
-20°C	7 days at -20°C
-35°C	15 hours at -35°C
-35°C	24 hours at -20°C

FREEZING PROCESS

   So how does we freeze all of our products? After having been filled, blisters and flat-packs are put on aluminum trays stacked on bread carts and are rolled into the quick-freezer, which is cooled by a high efficiency air cooler. Freezing occurs at a temperature ranging from -22°C in summer on busy days, up to -33°C in winter, but mostly around -26°C, resulting in blisters being frozen solid after about 30 minutes. After having been cased out, the products are stored in pallet racks in our cold store (freezer). Ambient freezer temperature is continuously displayed and recorded every 20 minutes by a data logger. The following chart depicts the temperature (blue points) recorded in our cold storage throughout 2016 whilst the red line depicts the average, ambient cold storage temperature. Average ambient freezer temperature fluctuated around -21.6°C.

Ambient temperature in our freezer, over the course of a full year.

   All products are minimally stored for at least a month before being sold but actually, as nearly all our single ingredient foods are only seasonally available, most products spend a much larger time in our cold store before being sold. For some items freezer dwell time can reach 8 months. It is this long storage period at a low enough temperature, which results in a 100% kill rate for all cestodes, trematodes and nematodes, in whichever life stage.

   Both the USFDA and EFSA based themselves on various studies, amongst for example the results obtained by Adams et al. (2005)[3] when investigating the minimum required freezing temperature and freezer dwell times.

   All very well, but what about the parasites aquarium keepers are mostly concerned with?

ICH, AKA ICK OR WHITE SPOT DISEASE

All aquarists, hobbyists as well as professional aquaculturists, will soon or later have to deal with an outbreak of the freshwater protozoan parasite Ichthyophthirius multifiliis. Ok, so the question to answer now becomes: can one introduce ick into one’s tanks by using our frozen feeds?

   Answer: no, nobody will most definitely introduce ick into your tank(s) by using our frozen foods because none of this parasite’s development stages can withstand freezing[4]. Actually, its inability to withstand freezing is a bit of a problem for the scientific community, as researchers need to have regular access to the parasite for running reproducible experiments or tests. As no method has been found yet to store the parasite in a frozen state, researchers need to maintain cultures of infected fish. Not only is this very labor and time consuming (and as such quite costly), it also raises moral questions (who or what gives us the right to introduce healthy fish in aquaria in which the parasite rules?). So no, nobody will be introducing ick into one's tank by using any of our frozen foods. Guaranteed!

   Keeping with protozoan ciliates, Trichodina spp. are very commonly found as commensals on both wild-caught and farmed fish, but they can become parasitic when settling in such large amounts on the gills of fish that infected fish start having problems breathing. Trichodina spp. have a very simple life cycle and reproduce by binary fission (basically said, they split themselves in two equal daughter cells) and contain no dormant life stage such as a cyst. As such, they can not withstand the freezing & freezer storage process our food items undergo.  

FLUKES

   When speaking about flukes, aquarium hobbyists mostly refer to various species of parasitic flatworms belonging to either the class Monogenea, either the class Digenea.

Monogenea

   The class Monogenea comprises ectoparasites (sometimes visual with the naked eye) on the gills or skin of fish. As the name Monogenea implies, monogeneans have no intermediate hosts and only infect fish, so they do not infect our food items such as bloodworms, brine shrimp, copepods etc. Monogeneans contain no dormant life stage. Some species, e.g. Gyrodactylus spp., are live-bearing; the newborn larvae either quickly find a suitable host, either they die. Other monogeneans, such as Dactylogyrus spp., are not live-bearing but release eggs. After the eggs have hatched, the free-swimming larvae either find a host, either they die. Whether we talk about the eggs or the larvae, as mentioned earlier in this document flatworms are unable to withstand the freezing process followed by a proper storage time in a freezer. So nobody will be introducing external flukes to one's tank by using any of our frozen foods.

Digenea

   The class Digenea consists of parasitic flatworms which are mainly to be found in the digestive tract of vertebrates, but which can also infect other organs. As the name implies, these flatworms have a more complex lifecycle which involves more than one host. In most cases snails or other mollusks are the intermediate hosts. Digeneans are able to produce cysts, so a dormant or near-dormant life stage. But again as mentioned earlier: the freezing and frozen storage process we use kills off all digenean life stages, including dormant cysts.

VELVET, GOLD DUST OR RUST DISEASE

   This disease is caused by a couple of genera of the freshwater, parasitic dinoflagellate genus Piscinoodinium. The infectious life stage of this parasite is called a dinospore and is free-swimming; a dinospore either quickly finds a suitable host (most fish species), either it dies. The dinospores can’t withstand the process of being frozen and stored in a cold store for a prolonged period. As is the case with ick, scientists are actually trying to find suitable cryogenic storage techniques for preserving dinoflagellates. Not necessarily so for storing Piscinoodinium, but for storing important dinoflagellates tied to reefs, which are in decline worldwide due to oceanic acidification, increase in water temperature, increase in nutrient load, overfishing and other reasons. Cryogenic storing of certain dinoflagellate species is possible but the use of cryopreservants is required in order to protect the organisms during freezing.

FISH LICE

   Fish lice (Argulus spp.) are obligate ectoparasitic crustaceans visual with the naked eye. Although they were first recorded in Japan, fish lice are thought to be endemic to warm waters although they have nowadays been spread over almost the entire world, including colder waters. ‘Fish lice’ is a bit of a misnomer as fish lice have been found able to infect crustaceans and amphibians as well. Female fish lice are stationary on their host and attract males by producing a pheromone, males will leave a host in case no female fish louse is present on that host. Adults copulate on the infested fish but gravid females leave the host in order to deposit their eggs on submersed substrates. From each egg hatches a free-swimming postnauplius, which can go a day without food if required but goes looking for a host in order to feed. Postnauplii feed on epithelial cells whilst adult fish lice puncture the scales or skin and feed on blood. No life stage of fish lice (so egg, postnauplium or adult) is able to survive freezing.

THE REAL PARASITE SOURCES

   So where then, do the parasites in our aquaria originate from? Answer: from our purchased fish or aquatic plants. Visible or not, nearly all fish (whether in nature, commercially reared or farmed, or kept in aquaria) harbor one or more kinds of parasites. In 2015 Barisone et al.[5] analyzed 232 ornamental fish imported in Italy and found 88% of these fish to show pathological lesions, with the gills and liver being the most affected organs. The rate of parasitic infection was 40%, with monogeneans being the most encountered parasites. Other frequently found parasites were Piscinoodinium, Trichodina and Ichthyophthirius.

   And no, the situation isn’t any better with fish commonly consumed by humans. In 2014 Mousa et al.[6] investigated how much percent of fish imported into Egypt for human consumption was parasitized and these were the numbers they obtained:

• herring (Clupea harengus): 96%
• mackerel (Scomber scombrus): 92%
• horse mackerel (Trachurus mediterraneus): 88%
• striped catfish (Pangasius hypophthalmus): 16%

      So how do the Dutch (maatjes) or the Germans (Glückstädter Matjes) prevent themselves from ingesting live parasites (e.g. the dangerous nematode Anisakis spp.) when eating their famous raw, brined, pickled or seasoned young herring? Well, by freezing the seasoned herring at one of the aforementioned temperatures followed by an appropriate freezer dwell time. The brining, pickling or seasoning will cure the raw herring but it won’t kill off the parasites.

   A fish carrying a parasite does not by itself imply that this fish is going to quickly perish, as fish after all have an immune system. But when it gets severely stressed (e.g. because of excessive handling, or shipping, or being kept under adversarial living conditions) its immune system will get compromised. And when this occurs, parasites present might get the overhand over the fish’s immune system which ultimately, when not timely noticed and treated by the fish’s caretaker, will result in the death of the fish.