Natural Born Killers

Not surprisingly, most plants do contain pesticides of one sort or another. The plant pesticides, or phytoalexins, protect the plant against such threats as fungi and other pathogens, insects and other herbivores, and even other plants. Based on their structures, approximately 37% of these natural products are predicted to be carcinogens (Rosenkranz & Klopman 1990). In high-dose tests, from 30% to 50% of both natural and synthetic chemicals are estimated to be carcinogens, mutagens, teratogens, and/or clastogens (Ames et al. 1990).This ignores the applicability of these multi-megadose challenges to the exposure of human beings in everyday life, but does give some indication that there are tradeoffs between synthetic and natural pesticides that may not be readily apparent.

ON-ANIMAL USE. Control of ectoparasites on animals, especially food animals and pets, is a sensitive area for two reasons. For food animals there is concern about chemical residues in meat, milk and eggs. With companion animals especially dogs, cats and horses there is significant opportunity for human insecticidal exposure because of the intimacy with which owners share the pet's environment.

For this reason, there is considerable interest in the development of safer chemicals for use in these niche markets, and plant by-products have been targeted. In fact, application of plant products for suppression of ectoparasites is the second most common use of botanical insecticides (Omino & Kokwaro 1993). Because of their production from plants, these "natural" insecticides are perceived as less toxic to humans and other vertebrates.

Use of insecticides on pets presents at least three potential problem areas:

• Applicator exposure when the pet's owner treats the animal.

• Sustained release from the pet and consequent human exposure.

• Direct effects of the insecticide on the treated animal.

For both medical and veterinary reasons, compounds used on companion animals need to have minimal toxicity.

`NATURAL' FLEA KILLERS. There are a variety of flea products including collars, dips and sprays that contain "natural" compounds such as extracts of pennyroyal, eucalyptus, rosemary and citronella. Cases have been reported of dogs being treated with pennyroyal oil (typically obtained from health food stores) resulting in acute illness, vomiting and death (Sudekum et al. 1992).

Pennyroyal contains pulegone, a hepatotoxic monoterpene. In addition to damaging the liver, pulegone significantly depletes glutathione (Thomassen et al. 1990), thereby increasing the animal's susceptibility to other toxicants, including many insecticides. Eucalyptus appears to be toxic mainly when ingested (Webber et al. 1985). Even the ubiquitous oil of citronella can be toxic at high doses (Temple et al. 1991).

Flea dips containing d-limonene, when not adequately diluted, have produced necrotizing dermatitis with sloughing of the skin (Frank et al. 1992). Cats are particularly susceptible to poisoning by crude citrus extracts, as well as the refined linalool and d-limonene, with toxicosis resulting in hypersalivation, muscle tremors, ataxia, depression and hypothermia (Hooser 1990). Limonene and its derivatives have high sensitizing potential, producing allergic responses upon repeated exposure (Karlberg et al. 1991).

Another botanical ectoparasiticide appearing in the popular press is "tea tree" or Melaleuca oil. Melaleuca oil toxicosis is characterized by depression, weakness, incoordination and muscle tremors (Villar et al. 1994). In addition to being toxic, melaleuca oil contains a number of sensitizing components, producing contact dermatitis in susceptible individuals (Knight & Hausen 1994).

Many aromatic compounds produced by plants are hepatotoxic. Although they may have some repellent and insecticidal properties as well, they certainly have no advantage over synthetic insecticides when considered on a mammalian health basis.

NATURAL¹SAFER. While there is certainly a need for continued investigation and development of safer, more effective means of controlling pests, the tendency to equate "natural" with "better" or "safer" must be avoided. Natural products are not necessarily safer or more efficacious than their synthetic alternatives.

Particularly in cases where human and animal health are involved, it is imperative that we employ dependable control strategies. Traditional insecticides have made a vital contribution to health, and their withdrawal would present far more serious health problems than do their potential long-term toxic effects.

REFERENCES

Ames, B.N., M. Profet & L.S. Gold. 1990. Nature's chemicals and synthetic chemicals: comparative toxicology. Proceedings of the National Academy of Sciences of the United States of America 87(19): 7782-7786.

Blain, P.G. 1990. Aspects of pesticide toxicology. Adverse Drug Reactions and Acute Poisoning Reviews 9(1): 37-68.

Frank, A.A., J.L. Ross & B.K. Sawvell. 1992. Toxic epidermal necrolysis associated with flea dips. Veterinary and Human Toxicology 34(1): 57-61.

Karlberg, A.T., A. Boman & B. Melin. 1991. Animal experiments on the allergenicity of d-limonene the citrus solvent. Annals of Occupational Hygiene 35(4): 419-426.

Knight, T.E. & B.M. Hausen. 1994. Melaleuca oil (tea tree oil) dermatitis. Journal of the American Academy of Dermatology 30(3): 423-427.

Omino, E.A. & J.O. Kokwaro. 1993. Ethnobotany of Apocynaceae species in Kenya. Journal of Ethnopharmacology 40(3): 167-180.

Rosenkranz, H.S. & G. Klopman. 1990. The structural basis of the carcinogenic and mutagenic potentials of phytoalexins. Mutation Research 245(1): 51-54.

Sudekum, M., R.H. Poppenga, N. Raju & W.E. Braselton Jr. 1992. Pennyroyal oil toxicosis in a dog. Journal of the American Veterinary Medical Association 200(6): 817-818.

Temple, W.A., N.A. Smith & M. Beasley. 1991. Management of oil of citronella poisoning. Journal of Toxicology. Clinical Toxicology 29(2): 257-263.

Thomassen, D., J.T. Slattery & S.D. Nelson. 1990. Menthofuran-dependent and independent aspects of pulegone hepatotoxicity: roles of glutathione. Journal of Pharmacology and Experimental Therapeutics 253(2): 567-572.

Villar, D., M.J. Knight, S.R. Hansen & W.B. Buck. 1994. Toxicity of melaleuca oil and related essential oils applied topically on dogs and cats. Veterinary and Human Toxicology 36(2): 139-142.

Webber, J.J., C.R. Roycroft & J.D. Callinan. 1985. Cyanide poisoning of goats from sugar gums (Eucalyptus cladocalyx). Australian Veterinary Journal 62(1): 28.