Sulfonamides rely on the requirement of susceptible organisms to synthesize folic acid as a precursor of other important molecular molecules in the cell. Sulfonamides act as false substrates in the synthesis of folic acid. Trimethoprim and ormetoprim (diaminopyrimidines, discussed in Section Potentiated Sulfonamides) produce a synergistic effect when used together by inhibiting the enzyme dihydrofolate reductase.

The spectrum of activity for the sulfonamides is broad, affecting gram-positive, gram-negative, and many protozoal organisms. Sulfonamides have been used clinically for approximately 50 years and many organisms once susceptible to the sulfonamides are now resistant. To increase the activity, most of the sulfonamides used in clinical practice are combinations with either trimethoprim or ormetoprim (diaminopyrimidines). These combinations (referred to in this chapter as trimethoprim–sulfonamides, but also referred to in clinical practice as trimethoprim–sulfa or simply abbreviated as TMP/SU) have increased the activity.

Administration of a single sulfonamide, or combination of sulfonamides, continues to be used in some livestock practices. In the United States, there are no approved formulations of trimethoprim–sulfonamides available for food animals, but trimethoprim–sulfadoxine is available in some countries.

The susceptibility/resistance patterns of sulfonamides and the trimethoprim–sulfamethoxazole combination against the most commonly encountered veterinary pathogens has been reported (van Duijkeren et al., 1994a, 1995; Bade et al., 2009; Winther et al., 2011). The activity of these agents has allowed for treatment of common respiratory infections, urinary tract and soft tissue infections, and intestinal infections (intestinal protozoa). Susceptible organisms include Arcanobacterium, Bacillus spp., E. rhusiopathiae, L. monocytogenes, Streptococcus spp., (Streptococcus equi subsp. zooepidemicus from horses), and protozoa (coccidia and Pneumocystis carinii).

The wild-type strains of following organisms are usually susceptible to the trimethoprim–sulfonamide (or ormetoprim–sulfonamide) combination: Pasteurella spp., Proteus spp., Salmonella spp., Histophilus (formerly Hemophilus), the protozoa Toxoplasma, and coccidia. Other bacteria that may be susceptible, but for which resistance can develop, include Staphylococcus spp., Corynebacterium, Nocardia asteroides, Stenotrophomonas maltophilia, and bacteria of the Enterobacteriaceae (Klebsiella, Proteus, Enterobacter, and Escherichia coli).

The organisms that are consistently resistant to trimethoprim–sulfonamide combinations include: Pseudomonas spp., Chlamydia spp., and Bacteriodes. One should cautiously interpret trimethoprim–sulfonamide susceptibility for Enterococcus spp. Although Enterococcus may appear susceptible to trimethoprim–sulfonamides using in vitro tests, it escapes the antifolate activity of the drug in vivo by its unique ability to incorporate preformed exogenous folates (Wisell et al., 2008). Sulfonamides alone are not active against Enterococcus spp. Clinical failures are reported despite in vitro susceptibility and microbiology laboratories should not report the susceptibilities of Enterococcus to trimethoprim–sulfonamides.

The activity of trimethoprim–sulfonamides against anaerobic bacteria can be variable. When measured in vitro, trimethoprim–sulfonamides have good activity against anaerobic bacteria (Indiveri and Hirsh, 1986), but clinical results are not as good (Dow, 1988) because thymidine and PABA (inhibitors of trimethoprim–sulfonamide activity) may be present in anaerobic infections.

Trimethoprim–sulfonamides have been used to treat infections caused by protozoa (including Toxoplasma gondii) and intestinal coccidia. Trimethoprim–sulfonamide combinations have also been used to treat equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona. (Use of pyrimethamine for treating EPM and treatment of protozoa infections is discussed in Chapter 42.)

Components found in some tissue environments may inhibit trimethoprim–sulfonamide activity. For example, thymidine and PABA present in infected tissue – may interfere with activity. This has been demonstrated in tissue cages in horses. Ensink et al. (2005) showed an inability to eliminate the infection in an infected environment, despite in vitro sensitivity. They cited inhibitors – such as PABA and thymidine – present in abscessed and infected tissues that may inhibit the effects of these drugs. In another study in which trimethoprim–sulfadoxine was administered to cattle with infected tissue cages (Greko et al., 2002), it was shown that high levels of thymidine in the tissue cage fluid inhibited trimethoprim and compromised the ability to eradicate the infection.

For susceptibility testing, trimethoprim–sulfame- thoxazole (1:20 ratio of trimethoprim:sulfamethoxazole) should be used, even when trimethoprim–sulfadiazine is used for therapy (CLSI, 2013, 2015). There are no quality control (QC) ranges developed for trimethoprim–sulfadiazine, and tests using trimethoprim–sulfamethoxazole are expected to give equivalent results. Winther et al. (2011) showed that there were no significant differences observed between the minimal inhibitory concentration (MIC) of sulfadiazine and sulfamethoxazole for individual bacterial strains, confirming that sulfamethoxazole is an effective surrogate for susceptibility testing of sulfadiazine. The CLSI susceptibility testing standards state that Mueller–Hinton agar containing excessive amounts of thymidine or thymine can reverse the inhibitory effect of sulfonamides and of trimethoprim, which may result in false-resistant reports (CLSI, 2013). Susceptibility testing agar that is as thymidine free as possible should be used. The current CLSI interpretive categories (CLSI, 2015) do not provide veterinary-specific interpretations; therefore, the human breakpoint is used by laboratories to predict susceptibility. For Staphylococcus spp. and the Enterobacteriaceae the susceptible breakpoint is ≤2/38 (trimethoprim/sulfonamide) and for Streptococcus spp. the breakpoint is ≤0.5/9.5 (trimethoprim/sulfonamide).

Resistance by many bacterial and protozoal organisms has become widespread due to the extensive use of sulfonamides over many years (Huovinen, 2001). Resistance occurs via efflux pumps, failure to penetrate the organism, and changes in target enzymes. Resistance can be transferable. Chromosomal resistance tends to occur slowly and confers resistance via impaired drug penetration into the microbial cell, producing an insensitive dihydropteroate enzyme and an increased production of PABA. Plasmid-mediated resistance, the most commonly encountered form of sulfonamide resistance, occurs quickly and manifests itself via the impaired drug penetration mechanism in addition to producing sulfonamide-resistant dihydropteroate synthase enzymes. If an organism becomes resistant to one sulfonamide, it is generally resistant to all other sulfonamides. Resistance to trimethoprim occurs via overproduction of the dihydrofolate reductase enzyme or synthesis of an enzyme that resists binding of the drug.

In dogs, absorption is excellent and not affected by feeding (Sigel et al., 1981). There has been considerable interest in the oral absorption of trimethoprim–sulfonamide combinations in horses and the effect of feeding. When trimethoprim-sulfonamides are administered to a horse that has not been fed, rapid absorption occurs, but is not as complete as for dogs or people. Nevertheless, oral administration is sufficient in horses to produce effective results. The fraction absorbed for trimethoprim was reported to be 67%, and for sulfadiazine 58%, but for both components the variability was high (van Duijkeren et al., 1994c). Oral absorption in another study in horses was 90.2% for intragastric administration and 74.45% for the oral paste (Winther et al., 2011). For trimethoprim in the same study it was 71.5% oral absorption for the intragastric administration and 46% for the oral paste (Winther et al., 2011). In that study the absorption of trimethoprim–sulfadiazine was likely diminished by feeding. When trimethoprim–sulfadiazine was administered to horses as an oral suspension and compared to the equine paste, the absorption from the suspension was higher for both drugs compared to the paste, that is 136% and 118% of the paste AUC concentrations for sulfadiazine and trimethoprim, respectively (McClure et al., 2015). In another study (van Duijkeren et al., 1994c) the oral paste was compared to two compounded formulations (mixed with syrup and water or carboxymethylcellulose gel). In this comparison, all three formulations were judged to be equivalent. When administered to horses that have been fed or when it is added to the horses’ feed concentrate, a delayed and biphasic absorption is observed (van Duijkeren et al., 2002, 1995). When trimethoprim sulfachlorpyridazine was administered to horses, oral absorption was delayed, with the first peak appearing 1 hour after dosing and the second appearing 8–10 hours postdosing. Dual absorption peaks were not found after nasogastric administration (van Duijkernen et al., 1995). The best explanation for this phenomenon is that that there is an initial peak of absorption in the small intestine where much of drug absorption is known to occur. However, the drug that is bound to feed (adsorption) is unavailable for absorption until it travels to the cecum and, after digestion of the carbohydrates, the drug is released, producing a delayed and biphasic peak in absorption. Trimethoprim–sulfachlorpyridazine can bind to equine cecal contents 60–90%, which supports the theory of the “double peak”. Feeding also decreased the systemic availability from 70% when fasted to 45% when fed (van Duijkernen et al., 1996).

In ruminants, age and diet can markedly affect trimethoprim and oral sulfadiazine disposition in calves (Guard et al., 1986; Shoaf et al., 1987). Orally administered sulfadiazine (30 mg/kg) was absorbed very slowly in those calves fed milk diets, with absorption slightly higher in ruminating calves. Trimethoprim was absorbed in preruminant calves, but not absorbed in mature ruminants after oral administration (Shoaf et al., 1987), probably because of inactivation in the rumen.

Sulfasalazine is not used for the antibacterial properties, but is used to treat inflammatory disease of the large intestine in small animals (discussed in more detail in Chapter 46). It is not absorbed as a whole molecule but rather is cleaved into two more active compounds by native resident colonic bacteria.

Sulfonamides distribute to most body fluids, but are not distributed to tissues as extensively as trimethoprim. Generally, sulfonamide tissue concentrations are lower than plasma concentrations (approximately 20–30% of corresponding tissue concentration), but distribution to extracellular fluids is generally high enough to produce effective concentrations against susceptible pathogens. High protein binding affects the distribution and markedly increases the half-life of sulfonamides.