Phenols and Related Compounds as Antiseptics and Disinfectants for Use With Animals

Phenolic compounds used as antiseptics or disinfectants include pure phenol and substitution products with halogens and alkyl groups. They denature and coagulate proteins and are general protoplasmic poisons.

The oocysts of Eimeria tenella (a coccidian) and the eggs of Ascaris suum (a nematode) are susceptible to phenols and cresols. All disinfectants that are effective against oocysts are based on these two substances, both of which are highly toxic to cats because cats are deficient in the enzyme UDP-glucuronosyl transferase. Therefore, phenol-based disinfectants are best avoided in feline environments (home, shelter, veterinary clinic, etc).

Phenol (carbolic acid) is one of the oldest antiseptic agents. It is bacteriostatic at concentrations of 0.1%–1% and bactericidal/fungicidal at 1%–2%. A 5% solution kills anthrax spores in 48 hours. The bactericidal activity is enhanced by EDTA and warm temperatures; it is decreased by alkaline medium (through ionization), lipids, soaps, and cold temperatures. Concentrations >0.5% exert a local anesthetic effect, whereas a 5% solution is strongly irritating and corrosive to tissues. Oral ingestion or extensive application to skin can cause systemic toxicosis, manifested primarily by CNS and cardiovascular effects; death may result.

Phenol has good penetrating power into organic matter and is used mainly to disinfect equipment or organic materials that are to be destroyed (eg, infected food and excreta). Because of its irritant and corrosive properties and potential systemic toxicity, at present phenol is not commonly used as an antiseptic, except to cauterize infected areas—eg, the infected umbilicus of neonates. It is also incorporated into cutaneous applications for pruritus, stings, bites, burns, etc, because of its local anesthetic and antimicrobial properties, to relieve itching and control infections.

Cresol (cresylic acid) is a phenol derivative commonly used as a surface disinfectant. Cresol is a mixture of ortho-, meta-, and paracresols and their isomers. It is a colorless liquid; after exposure to light and air, however, it turns pink, then yellowish, and finally dark brown. A 2% solution of either pure or saponated cresol in hot water is commonly used as a disinfectant. Ingestion may cause local mucocutaneous damage and extensive systemic toxicosis including CNS, cardiovascular, hematologic, hepatic, renal, and pulmonary damage. Treatment consists of rapid decontamination (eg, using activated charcoal) and supportive care.

Hexachlorophene (a trichlorinated bisphenol) has strong bacteriostatic action against many gram-positive organisms (including staphylococci) but is highly toxic and is therefore no longer in use.

Pine tar(not to be confused with coal tar) is a viscid, blackish-brown liquid used primarily for antiseptic bandaging of wounds of the hoof and horn. Pine tar contains phenol derivatives that provide antimicrobial properties. Because of its soothing and antiseptic properties, pine tar has been used in medicine for more than 2,000 years to treat a range of skin conditions. Pine tar has been shown to be antipruritic, anti-inflammatory, antimicrobial, and antifungal. These properties make it suitable for the topical treatment of eczema, psoriasis, seborrheic dermatitis, and other dry, itchy, flaky, or inflamed skin conditions. Pine tar is thought to exert its effect by decreasing DNA synthesis and mitotic activity, thereby promoting a return to normal keratinization. Over-the-counter topical products contain up to 2.3% pine tar. Modern-day pine tar is manufactured with increased purity to eliminate toxic phenol and carcinogenic components.

Chloroxylenols are broad-spectrum bactericides with more activity against gram-positive than gram-negative bacteria. Streptococci are more susceptible than staphylococci. Chloroxylenols are active in alkaline pH; however, contact with organic matter diminishes their activity. Chloroxylenols are found in 2.6% of operating-room scrubs and disinfectants.

Parachlorometaxylenol (PCMX) and dichlorometaxylenol (DCMX) are the two most commonly used chloroxylenols. DCMX is more active than PCMX. Strong solutions of these compounds can cause irritation and have a disagreeable odor. A 5% chloroxylenol (eg, PCMX) solution (in alpha-terpineol, soap, alcohol, and water) is diluted with water for skin sterilization (1:4) and for wound cleansing and irrigation of the uterus and vagina (1:25 to 1:50).

Triclosan, a polychlorinated phenoxy phenol, has both ether and phenol functional groups. It is an antimicrobial commonly found in consumer products such as soap, handwashes, shampoos, and mouthwashes; however, in 2017 it was banned from health care antiseptics by the FDA based on concerns that it promotes antimicrobial resistance. Triclosan requires a contact time of at least 2 minutes to be effective.