Safety of Nicarbazin in Raptors in Relation to Pigeon Eradication Programs

The feral pigeon has adapted well to the urban environment, and an increase in overpopulated pigeon colonies has increased their economic, environmental and social impacts. Methods to mitigate these impacts include culling, toxicants and non-lethal methods such as fertility control. Although lethal control methods, including toxicants, are increasingly unacceptable to the general public, contraception is considered a humane tool to mitigate the impacts of pigeons [1,2].

Nicarbazin (NCZ) is one of the most common anticoccidial drugs used as a feed additive in poultry and other animal industry under the EU Additives Directive 70/524/EEC and is the only oral contraceptive permitted in the EU, USA, Hong Kong and Singapore for in free-living birds. While NCZ has no negative effects on reproduction in the poultry industry, feeding NCZ to laying birds results in reductions in hatchability and egg production, and NCZ has proven to be a successful fertility control method for conflictive pigeon colonies in Italy, Spain and USA [3-7]. 

Two veterinary medicinal products s (Ovistop®, ACME Drugs Srl Italy and R-12®, ACME S.r.l., Italy) containing NCZ at 800 mg/kg, mixed with corn kernels and protected by stearic acid and dimethicone are available for the fertility control of pigeons [1,6]. The daily treatment rate of pigeons (average body weight = 300 gr) is 6.5-8 mg of NCZ corresponding to a maximum of 25 mg/kg bw, given in a daily single dose throughout the entire egg-laying period. The product is administered with an automatic feeder or hand broadcast [6]. As with any other substance placed into the environment, the use of NCZ raises issues regarding the health and safety of non-target animals including raptors, which are predatory birds positioned on the top of the food chain. Poisoning events in raptors have been reported from feeding on rodents exposed to anticoagulant rodenticides and carbamate and organophosphate insecticides, ingestion of poison baits used to control predators and feeding on domestic livestock carcasses treated with veterinary pharmaceuticals [8-10]. In contrast, there are no reports either in pharmacovigilance or in toxicovigilance of adverse reactions in raptors to the use of NCZ to control the fertility of pigeons.

The aim of the present paper is to evaluate the risk of NCZ for European birds of prey, considering the potential pathways of exposure to NCZ following its use for the fertility control of pigeons.

• NCZ pharmacology 

NCZ is an equimolecular complex of 4,4’-dinitrocarbanilide (DNC), the pharmacologically active component, and 2-hydroxy-4,6-dimethylpyrimidine (HDP) (specification 70% DNC, 30.0 % HDP) [11]. NCZ prevents embryo development, disrupting the vitelline membrane between the egg albumen and the yolk. Birds have to absorb NCZ continuously before (for 5-7 days) and during egg-laying to obtain contraceptive effects, while single or intermittent doses will not affect hatchability. The contraceptive effect is completely reversible, and the bird returns to normal reproduction within five days after the treatment is withdrawn [12]. After ingestion, nicarbazin splits into its two components (HDP and DNC), which undergo independent pharmacokinetics and metabolism. DNC alone is very poorly absorbed, and for absorption and to achieve a contraceptive blood level, the HDP component is required in a 1:1 molecular ratio as in nicarbazin. DNC is unable to re-complex with HDP and when administered alone or as a mixture of DNC+HDP, it shows a bioavailability that is 33 to 65 times lower than when administered as NCZ [4,5,13]. Residue decline rapidly after NCZ withdrawal [5,14]. In the chicken, which is the species with the highest NCZ absorption rate, after one-day withdrawal, concentrations of total residues (expressed as mg equivalent DNC/kg wet tissue) following administration of 125 mg [14C]-DNC-nicarbazin/kg feed for seven consecutive days were as follows (Table 1): 

Table 1: Concentrations of total residues in different tissues after NCZ withdrawal expressed as mg equivalent DNC/kg wet tissue. 

On this basis, an average NCZ tissue residue concentration of 13.5 ppm can be calculated after a one-day withdrawal. DNC and acetyl-DNC were the major residual compounds in all tissues, accompanied by a number of minor metabolites, each representing less than 10 %. DNC and its metabolites are mainly excreted in the feces (46 % unchanged DNC), while HDP appears in the urine, 90% as unchanged HDP) [4,5,13]. 

• NCZ toxicology 

NCZ is considered “practically non-toxic” when fed to birds and mammals.Toxicity data included in the WHO-FAO Joint Expert Committee on Feed Additives (JECFA) indicate that the acute toxicity of NCZ is very low: LD₅₀ in the quail, which is the most sensitive bird, is > 2,250 mg/kg bw [15].  NCZ is considered practically non-toxic also in mammals and other species: LD₅₀ in mice and rats greater than 10,000 mg/kg [15]. Repeated toxicity studies indicated NOEL between 200 mg/kg and 400 mg/kg in 2-year rat and dog studies, rat reproduction and teratology studies [15].  On this basis, the only potential adverse effect in non-target birds following secondary exposure to NCZ is considered the reduction in egg hatchability. 

• Exposure of birds of prey to NCZ 

Birds of prey, commonly known as raptors, include vultures, eagles, hawks, kites, harriers, falcons, osprey and owls. Birds of prey are classified into two orders: Falconiformes and Strigiformes [16] the order of Falconiformes comprises five families (Accipitridae, Cathartidae, Falconidae, Pandionidae, Sagittariidae) which constitute more than 500 species of diurnal birds of prey, including hawks, eagles, vultures and falcons; the order of Strigiformes, which includes more than 200 species, comprises two families: Strigidae (typical owls) and Tytonidae (Barn- and Bay-owl). The eating habits of species of bird of prey living in the sites where NCZ is used for the fertility control of the pigeon population mean that the majority are unlikely to be exposed to NCZ. Most raptor species prey predominantly on mammals, especially rodent species. Mammals constitute 60–80% of the diets of barn owls, kestrels, Eurasian eagle-owls and red kites. Eurasian eagle-owls occasionally prey on larger mammals (e.g., hares, foxes, and deer) and red kites sometimes feed on hares, rabbits, and as carrion, foxes and deer [17-18]. Raptors also prey on other animals, such as birds, reptiles, amphibians, fish, and invertebrates. Birds constitute part of the diets in Eurasian sparrowhawks, golden eagles and red kites. In addition, the Eurasian eagle owls occasionally prey on other raptors (e.g., buzzards, falcons, tawny owls, and long-eared owls). Part of the diets of Eurasian eagle owls and bald eagles also consists of amphibians and fish, respectively. Invertebrates include some of the prey of moreporks, little owls and kestrels [17-18]. The amount of food a bird of prey ingests varies depending on many factors, including the species, weight, and sex of the bird. Smaller-sized birds have a greater surface area to body weight ratio than larger-sized birds, which results in greater heat loss. To compensate and maintain their metabolism and body temperature, smaller birds consume proportionally more food than larger birds which require a significantly lower percentage of their body mass as a daily food intake [16-18]. The daily food intake in birds of prey based on their body mass can be calculated as follows (Table 2): 

Table 2: Calculation of daily food intake in birds of prey. 

Species, prey preferences and daily food intakes of birds of prey living in the sites where NCZ could be used and that potentially can prey on or feed on urban pigeon meat [19-20], are reported below (Table 3): 

Table 3:  Species, prey preferences and daily food intakes of birds of prey that live in sites where NCZ may be used.

• Pathways of exposure to NCZ 

The primary exposure to NCZ can be excluded because birds of prey will not consume corn grain-based products (carrier in Ovistop and R-12) and it is eaten in locations where birds of prey are typically not found.

There seem to be two possible pathways of secondary exposure (E_1, E₂) by which birds of prey may potentially be exposed to NCZ:

• E₁: when they consume undigested prey gut content.
• E₂: when they prey on pigeons and feed on their meat. 

• Secondary exposure to undigested NCZ 

It is very unlikely for a predator to consume undigested NCZ. However, the consumption of NCZ through undigested prey gut content could also be considered a means of exposure.

In this scenario, on the basis of the maximum pigeon daily dose of 8 mg of NCZ, the estimated exposure (E₁) to undigested NCZ is calculated as follows:

For example, for the red kite (Milvus milvus), considering a bw of 1.2 kg, the equation is as follows:

On this basis, the maximum estimated E₁ to NCZ of the relevant birds of prey, if they consume undigested NCZ in a dosed pigeon, are as follows (Table 4):

Table 4: Estimated exposures to undigested NCZ of different species of birds of prey. 

In this scenario, Eurasian sparrowhawks are likely to be the most exposed to 22.9 mg NCZ/kg bw.

Secondary exposure by pigeon predation and feeding

Birds of prey are potentially exposed to nicarbazin when they prey on treated pigeons. Assuming a nonrealistic scenario that the entire daily NCZ dose consumed by a pigeon is available as residue, the average residue concentration (R) in the pigeon carcass is 25 ppm (worst-case scenario: NCZ residue concentration twice that extrapolated from an experimental depletion study on chickens). Considering that DNC bioavailability in the pigeon carcass is reduced by at least 33 times after ingestion and absorption, and assuming that the entire daily food intake consists exclusively of pigeon meat, the E₂ of predatory birds to nicarbazin is calculated as follows: 

For example, for the red kite (Milvus milvus), considering a bw of 1.2 kg and a daily food intake of 180 gr of pigeon carcass (15% bw) containing 25 ppm of NCZ residue, the equation is as follows:

On this basis, the estimated secondary exposures (E₂) to NCZ of the relevant birds of prey when they prey on treated pigeons and feed on its meat are as follows (Table 5).

Table 5: Estimated exposures to NCZ of different species of birds of prey by pigeon predation and feeding.

In this scenario, smaller birds (i.e., hen harrier, Montagu’s harrier, Eurasian sparrowhawk, Northern long-eared owl, common barn owl) are likely to be the most exposed to 0.19 mg NCZ/kg bw to because they eat proportionately more food than larger species such as the Eurasian eagle-owl (Bubo bubo).

• Calculation of the Margins of Exposure (MoE) 

From a toxicological point of view, the MoE_(LD) expressed as the ratio of the lowest LD₅₀ vs the highest E₁ and E₂ predatory bird exposures and the MoE_(NOEL) expressed as the ratio of the lowest toxicological NOEL vs the highest E₁ and E₂ predatory bird exposures, are:

• MoE_(LD) = 98 and 11,842 for E₁ and E_2, respectively
• MoE_(NOEL) = 9 and 1,052 for E₁ and E_2, respectively

From a pharmacological point of view, the MoE_(Pharm) expressed as the ratio of the contraceptive dose vs the highest E₁ and E₂ predatory bird exposures would be:

• MoE_(Pharm) = 1.1 and 132 for E₁ and E_2, respectively

It follows that any potential secondary NCZ exposure of birds of prey is generally acceptable in terms of both acute and chronic toxicological risks. Furthermore, the consumption of the undigested NCZ does not constitute a risk in terms of pharmacological effects because NCZ requires a sufficiently high exposure both in terms of dose and duration of treatment. In fact, a single dose does not lead to a pharmacological effect and the non-target bird would need to ingest the product for at least five consecutive days for a contraceptive effect. Furthermore, non-target birds would need to consume NCZ throughout their entire nesting season and the contraceptive effect would be completely reversible as the bird returns to normal reproduction within five days after the treatment is withdrawn. The combination of these requirements makes it extremely unlikely that non-target pharmacological effects will occur.

While primary raptor exposure by direct consumption of the NCZ product can be excluded, raptors could be potentially exposed to NCZ both by consuming undigested gut content of treated pigeons (E₁) or by preying on treated pigeons and feeding on the meat (E₂). However, in both cases, the risk of adverse effects on nontarget birds of prey is extraordinarily low, and any potential exposure is estimated to be well below the dosage necessary to have any toxicological or pharmacological effects. There are thus no risk from secondary exposure to NCZ in raptors in relation to pigeon eradication programs

The study does not require Ethics Committee approval.

This research received no external funding.

The authors declare no conflict of interest.

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