Horses, susceptibility

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The use of antimicrobial agents for relatively trivial infections encourages the selection of resistant organisms. Consequently, in the absence of evidence of a susceptible pathogen, antimicrobial use is irrational and may expose treated horses to unnecessary risks. 

The macrolide antibiotics include erythromycin, clarithromycin, azithromycin, tylosin, tilmicosin and tiamulin. Clindamycin and lincomycin are related lincosamides. Susceptible bacteria include staphylococci, streptococci, Campylobacter jejunii, Clostridium spp., R. equi, Mycoplasma pneumoniae and Chlamydia spp. Drugs in this group are only effective against a few Gram-negative bacteria in cattle, namely some strains of Pasteurella and Haemophilus spp. Macrolides and lincosamides are associated with causing colitis in horses, so their use is usually restricted to p.o. erythromycin for the treatment of R. equi infections in foals. Subantimicrobial doses of erythromycin are administered i.v. to horses for gastrointestinal prokinetic action. 

Agents in this group, especially clindamycin and lincomycin, are associated with bacterial overgrowth in the colon. Serious and potentially fatal diarrhea may occur in humans, rabbits, ruminants and horses. Foals appear to be less susceptible to erythromycin-induced diarrhea than adult horses and the ethylsuccinate formulation seems to be the least likely to induce diarrhea. 

Resistance involves reduced intracellular drug activation and is rare among usually susceptible bacteria. There is one report of metronidazole-resistant isolates of B. fragilis from a horse with pleuropneumonia. 

B. caballi can be treated effectively but there is not a comparable chemotherapeutic approach to the treatment of B. equi, where drugs may control the clinical signs of infection but horses often remain life-long carriers. In regions where piroplasmosis is endemic, elimination of the infection may not be desirable because these horses will then be susceptible to reinfection. In these areas, premunition is used the horses are infected or allowed to become infected and are then treated with sufficient chemotherapy to control the clinical signs but not to eliminate the infection. Horses that have been premunised are thus infected but not affected. This technique permits the development of the carrier state and as a result some resistance (immunity) to reinfection. 

Horses are susceptible to several Trypanosoma spp. with various transmission vectors ... 

Equids are highly susceptible to T. brucei infection, with a very high mortality rate within 14-90 days in imtreated horses. The prepatent period, the period from infection to parasitemia, is approximately 6-10 days. Locomotor ataxia is the earliest clinical sign. Remittent high fever (41.7°C/107°F) occurs between intervening periods when the... 

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