FORMULATION: concentrate for topical administration to sheep by «jetting»


CHEMICAL CLASS of the active ingredient(s):


PARASITES CONTROLLED * (spectrum of activity)

* Country-specific differences may apply: read the product label.

  • Protects coarse wool sheep against blowfly strike by common blowflies (incl. Lucilia cuprina, Lucilia sericata and Calliphora stygia).
  • Variable protection of up to 14 weeks can be expected depending on fly pressure and application technique


* Country-specific differences may apply: read the product label.

  • Dilute the product 1:500 in water, i.e 1 L of product in 500 L water (equivalent to 1000 ppm=mg/L cyromazine and 30 ppm=mg/L ivermectin). 
  • Use a min. of 2 L solution per sheep. For longer wool use 0.5 L for each month of wool growth, to a max. of 4 L.
  • Sheep should preferably have a min. of 6-8 weeks wool from birth or shearing. Not for use off shears.
  • Local treatment: The product will control maggots in existing strikes in treated areas. Clip wool from the lesion. Treated areas must be saturated to skin level.

Read the product label for further details on dosing and administration.


  • LD50 (acute oral) in rats:
    • cyromazine: 3387 mg/kg for the a.i.
    • ivermectin: 25 mg/kg for the a.i.
  • Estimated hazard class according to the WHO classification of pesticides for cyromazine: II, moderately hazardous

Suspected poisoning? Read the articles on cyromazine safety and  ivermectin safety this site.

Withholding periods (=withdrawal times) for meat, milk & wool (country-specific differences may apply: read the product label)

  • Meat: New Zealand: 21 days
  • Milk for human consumption: New Zealand: Milk intended for sale for human consumption must be discarded during treatment and for not less than 35 days following the last treatment.
  • Wool: New Zealand: 2 months.

WARNING !!!: Never use on humans, dogs or cats.

You may be interested in the following articles in this site dealing with the general safety of veterinary products:


Risk of resistance? LOW

Cyromazine is a particular case regarding resistance development. Blowflies developed resistance to most chemical classes and active ingredients that were successively used for their control in Australia (and to a large extent in New Zealand too):

  • Organochlorines (e.g. DDT, dieldrin): introduced in 1946 in Australia, field resistance detected in 1957. Withdrawn for safety reasons in the 1970s.
  • Organophosphates (e.g. diazinon, malathion): introduced in 1957 in Australia, field resistance detected in 1965. Withdrawn for safety reasons in Australia in the mid 2000s.
  • Benzoylphenyl ureas (e.g. diflubenzuron): introduced in 1993 in Australia, field resistance detected in 2001. Claim for blowfly strike prevention removed in Australia in 2008. Since then approved only for lice control.

First cases of blowfly field resistance to the chemical classes mentioned above appeared usually about 10 years after product introduction. Other chemical classes such as synthetic pyrethroids (e.g. cypermethrin) and macrocyclic lactones (e.g. ivermectin) have been used only marginally for blowfly strike prevention during these years, i.e. it can be assumed that the selection pressure on blowflies exerted by chemicals of these two classes has been rather low.

In 2012 a first report on Lucilia cuprina cyromazine tolerance was reported (Resistance Factor = RF ~3) almost 40 years after the introduction of the first cyromazine product (VETRAZIN Liquid for dipping and jetting) in the late 1970s. In 2020 field resistance of blowfies to cyromazine has been reported for Lucilia cuprina in Australia with a RF of about 25. These blowflies have been reported to be also resistant to dicyclanil and ivermectin treatment.

To our knowledge no reports on resistance or tolerance of blowflies (Lucilia cuprina, Lucilia sericata) to cyromazine have been reported in New Zealand, UK (and other EU-countries) or South Africa, regions where cyromazine has been also vastly used against blowfly strike for decades.

As mentioned above, a first report on resistance of blowflies to ivermectin has been published in 2020, a field population also resistant also to cyromazine and dicyclanil.  the sameor other macrocyclic lactones. This is likely to be due to the fact that this chemical class has been ued very scarcely against sheep blowfly strike so far.

This means that if this product does not achieve the expected efficacy against the mentioned parasites, it is likely to be due to incorrect use and not to resistance. Incorrect use is the most frequent cause of product failure.

Alternative chemical classes/active ingredients to prevent resistance of blowflies through product rotation:

These alternative products may not be available in all countries, or may not be available for jetting.

Learn more about resistance and how it develops.


Are the active ingredients of this product ORIGINAL* or GENERICS**?


*Meaning that they are still patent protected and generics are not yet available
**Meaning that they have lost patent protection and may be acquired from manufacturers of generic active ingredients other than the holder of the original patent.

COUNTRIES where this brand/product is marketed: New Zealand
GENERIC BRANDS available? Rather few ones so far with such a particluar composition.

Click here to learn more about GENERIC vs. ORIGINAL drugs.

Click here for an overview on the most used antiparasitic BRANDS witoncentrates for dipping, spraying, or jetting.


CYRAZIN KO for sheep is a so far unusual combination of active ingredients with different modes of action: cyromazine (a specific flystrike preventative, ineffective against lice) and spinosad (effective against flystrike and lice).

Cyromazine is a so-called Insect Growth Regulators (IGR) belonging to the group of the Chitin Synthesis Inhibitors (CSI). It was introduced in the late 1970s (by CIBA-GEIGY → NOVARTIS → ELANCO). It is narrow-spectrum larvicide. It is abundantly used in sheep, moderately in poultry, marginally in horses, but not in other livestock or pets. It is also moderately used in agricultural pesticides.

Chitin is a component of the cuticle of insects, which is an essential part of their outer skeleton. If chitin is not properly produced, larvae die when they attempt the next molt. However, cyromazine does not really inhibit chitin synthesis, but interferes with its correct deposition. The consequence is the same: Fly maggots cannot complete molting and die. Other CSIs such as the benzoylphenyl ureas (BPUs, e.g. diflubenzuron, triflumuron) do actually inhibit chitin synthesis. But whereas BPUs exert this effect an almost all insects, cyromazine is quite specific for Dipterans (flies, mosquitoes, etc.) and some beetles. This makes it much less harmful for the environment, but also ineffective against other sheep pests such as lice.

As all IGRs, cyromazine has no knockdown effect. This means that flowfly maggots will die only at their next attempt to molt to the next developmental stage, which may take several days to occur, depending on age of the maggots at the time of treatment, humidity, temperature, etc. For this reason, IGRs are usually not used for curing established infestations, but for preventing their development by killing the larvae, and thus preventing the development of parasite populations in the fleece.

Cyromazine is quite soluble in water, in contrast with many other parasiticides that are rather lipophilic. This means that heavy rains may significantly shorten the length of protection of this and other cyromazine-based products.

Spinosad is a natural insecticide obtained from soil bacteria introduced for veterinary use by ELANCO in the 1990s. It is effective against numerous insects pests (e.g. lice, fleas and flies, etc.) and also against ticks and mites. In contrast with cyromazine, spinosad does have adulticidal effect, i.e. it will kill adult parasites as well as their immature stages within a few hours after treatment. This means that it can be also used as a curative agent. It is still scarcely used in livestock so far. It is also used against fleas on dogs and cats, and against poultry mites. It is moderately used in agriculture as well as against household pests. One of its major benefits is its rapid degradation in the environment and the low residues it leaves in animal tissues. Its major weakness for use on livestock is the rather short residual effect after topical administration (against flystrike: 4-6 weeks in Australia, 2-4 weeks in New Zealand).


This article IS NOT A PRODUCT LABEL. It offers complementary information that may be useful to veterinary professionals and users that are not familiar with veterinary antiparasitics. 

Information offered in this article has been extracted from publications issued by manufacturers, government agencies (e.g. EMEA, FDA, USDA, etc.) or in the scientific literature. No guarantee is given on its accuracy, integrity, sufficiency, actuality and opportunity, and any liability is denied. Read the site's DISCLAIMER.

In case of doubt contact the manufacturer or a veterinary professional.