Natamycin (Natacyn)

Natamycin is a polyene antifungal medication that serves as the primary treatment for fungal keratitis in horses, a serious and potentially vision-threatening condition affecting the cornea. Marketed under the brand name Natacyn, this ophthalmic suspension is the only topical antifungal medication specifically approved by the FDA for the treatment of fungal keratitis in humans, and it has become the gold standard for treating this condition in equine patients as well. Fungal eye infections in horses are considerably more common than in many other species, making natamycin an essential medication in equine ophthalmology.

The mechanism of action of natamycin involves binding to ergosterol in the fungal cell membrane, creating pores that disrupt membrane integrity and lead to leakage of essential cellular contents. This fungicidal activity results in direct killing of fungal organisms rather than merely inhibiting their growth. Natamycin has excellent activity against filamentous fungi, including Fusarium and Aspergillus species, which are the most common causes of fungal keratitis in horses. The medication's inability to penetrate intact corneal epithelium actually becomes advantageous in treating surface fungal infections while minimizing effects on deeper ocular structures.

Natamycin is available as a 5% ophthalmic suspension specifically formulated for application to the eye. The medication has a creamy white appearance and must be shaken well before each use to ensure uniform distribution of the active ingredient. Treatment of fungal keratitis requires frequent application, often every one to two hours initially, making owner compliance and practical administration considerations important factors in treatment planning. The suspension is well-tolerated by most horses, though the intensive treatment schedule required can be challenging for both horses and their caregivers.

The safety profile of natamycin in equine ophthalmic use is excellent when administered as directed. Systemic absorption from topical ocular application is negligible, eliminating concerns about systemic side effects. However, fungal keratitis is a serious condition requiring prompt and aggressive treatment under close veterinary supervision. Delayed or inadequate treatment can result in corneal melting, perforation, and permanent vision loss. Horse owners should recognize that natamycin therapy represents just one component of comprehensive fungal keratitis management, which may also include other medications and frequent veterinary monitoring.

The primary indication for natamycin in horses is the treatment of fungal keratitis, an infection of the cornea caused by various fungal organisms. Fungal keratitis in horses is significantly more common than in many other domestic species, partly due to horses' natural environment and behavior that exposes their eyes to plant material, soil, and organic debris containing fungal spores. The condition typically develops following corneal trauma that disrupts the protective epithelial barrier, allowing fungal organisms to establish infection in the corneal stroma. Natamycin's broad-spectrum activity against the filamentous fungi most commonly responsible for equine keratomycosis makes it the first-line treatment choice.

Fusarium species represent the most common cause of fungal keratitis in horses and are particularly well-suited targets for natamycin therapy. These organisms are ubiquitous in the environment, particularly in association with grasses, hay, and other plant materials. Aspergillus species constitute another major group of fungal pathogens causing equine corneal infections, and these too respond well to natamycin treatment. The medication also has activity against other filamentous fungi including Cladosporium, Curvularia, and Penicillium species, providing coverage against less common causative organisms.

Secondary prophylactic use of natamycin may be considered in horses with corneal ulcers at high risk for fungal superinfection. While routine use of antifungal prophylaxis in all corneal ulcer cases is not recommended due to concerns about selecting for resistant organisms, certain situations may warrant empiric antifungal coverage. These include ulcers associated with plant material trauma, ulcers that fail to respond to appropriate antibacterial therapy, and ulcers occurring in geographic regions with high prevalence of fungal keratitis. Your veterinarian will assess the specific risk factors for each case when making treatment decisions.

Natamycin may also be used for fungal conjunctivitis and other superficial ocular fungal infections, though these conditions are considerably less common than fungal keratitis in horses. The medication's limited penetration into the eye makes it most suitable for surface infections rather than deep-seated intraocular fungal disease. For infections involving deeper ocular structures, systemic antifungal therapy or antifungals with better intraocular penetration may be necessary in addition to or instead of natamycin.

The decision to use natamycin should be guided by appropriate diagnostic testing whenever possible. While the clinical appearance of fungal keratitis may be suggestive, definitive diagnosis requires laboratory confirmation through cytology, culture, or polymerase chain reaction testing of corneal samples. However, the serious nature of fungal keratitis and the time required for fungal cultures often necessitates initiating empiric antifungal therapy before confirmatory results are available. Your veterinarian will balance the need for definitive diagnosis against the urgency of initiating treatment in each individual case.

The dosing protocol for natamycin in equine fungal keratitis is intensive, reflecting the serious nature of this condition and the need to maintain adequate drug concentrations at the corneal surface. Treatment regimens are determined by your veterinarian based on the severity and extent of the fungal infection, the specific organisms involved if known, and the individual patient's response to therapy. Strict adherence to prescribed dosing schedules is critical for treatment success, as inadequate therapy can allow fungal proliferation and worsening of corneal damage.

Typical initial treatment protocols involve application of natamycin ophthalmic suspension every one to two hours during waking hours for the first several days of treatment. This intensive schedule is necessary to achieve and maintain fungicidal drug concentrations at the corneal surface, as natamycin has limited corneal penetration and is gradually diluted and washed away by the tear film. The frequency may be reduced as the infection responds, but treatment duration typically extends for several weeks and sometimes months depending on the severity and depth of corneal involvement.

Proper administration technique is essential for treatment efficacy. The bottle should be shaken well before each application to resuspend the medication uniformly. The horse's head should be positioned to allow direct application of drops to the affected eye, and the dropper tip should not contact the eye or any surface to maintain sterility. One to two drops are typically administered per application. After administration, gentle manual blinking or holding the eye closed briefly may help distribute the medication across the corneal surface, though horses often blink naturally after application.

Treatment duration for fungal keratitis is substantially longer than for bacterial infections. Most cases require treatment for a minimum of several weeks, and severe infections may require months of therapy. Treatment should continue until there is complete resolution of clinical signs and ideally confirmation of fungal clearance through repeated cytology or culture. Your veterinarian will perform frequent reexaminations to assess treatment response and adjust therapy accordingly. Premature discontinuation of treatment is a common cause of recurrence and treatment failure.

Missed doses should be administered as soon as remembered, and the regular dosing schedule should resume from that point. Given the frequent dosing requirements, occasional missed doses are almost inevitable during extended treatment courses. While single missed doses are unlikely to significantly compromise treatment, patterns of poor compliance can lead to treatment failure. If the prescribed dosing schedule proves impractical, discuss alternatives with your veterinarian rather than simply reducing frequency on your own.

Completion of the full treatment course as prescribed by your veterinarian cannot be overemphasized. Fungal organisms can persist in the corneal stroma even after apparent clinical improvement, and premature treatment cessation allows surviving organisms to repopulate and cause recurrence. Additionally, the corneal tissue damage caused by fungal infection requires time to heal, and ongoing antifungal therapy protects against reinfection during this vulnerable healing period. Follow your veterinarian's guidance regarding treatment duration even if the eye appears to have fully recovered.

Natamycin ophthalmic suspension is generally very well tolerated when applied to the equine eye, with most horses experiencing minimal adverse effects from treatment. The medication has been used in equine ophthalmology for decades, establishing an excellent safety record. Systemic absorption from topical ophthalmic administration is negligible, effectively eliminating the potential for systemic side effects. However, local reactions can occur, and owners should be familiar with potential side effects to distinguish between expected treatment-related changes and signs of worsening infection requiring veterinary attention.

The most commonly observed side effects from natamycin are mild, transient signs of ocular irritation immediately following application. Horses may exhibit increased blinking, mild tearing, or brief eye rubbing after drops are administered. These reactions typically resolve within a few minutes and do not indicate a need to discontinue treatment. The suspension formulation may cause a slightly gritty sensation, contributing to momentary discomfort. If irritation is severe or prolonged after applications, contact your veterinarian to discuss whether the reaction is within normal limits or requires evaluation.

Some horses may develop sensitivity to natamycin or its vehicle components over the course of extended treatment. Signs of developing hypersensitivity may include progressive increase in irritation with successive applications, increased redness of the conjunctiva, or swelling of the eyelids. True allergic reactions to natamycin are uncommon but possible. If you observe signs suggesting allergic reaction or increasing intolerance of the medication, contact your veterinarian promptly. Alternative antifungal medications may be substituted if natamycin hypersensitivity develops.

It is important to distinguish between side effects of the medication and progression of the underlying fungal keratitis. Worsening of corneal cloudiness, increased discharge, development of a visible stromal infiltrate or plaque, or corneal melting are signs of treatment failure or disease progression rather than medication side effects. These findings require urgent veterinary evaluation, as fungal keratitis can progress rapidly to corneal perforation without appropriate treatment adjustment. When in doubt about whether changes represent side effects or disease progression, err on the side of seeking veterinary evaluation.

Serious side effects from natamycin are rare in horses. There are no well-documented cases of significant adverse reactions in equine patients. However, any unexpected changes in the eye or the horse's general condition during treatment should be reported to your veterinarian. The intensive treatment required for fungal keratitis means horses are receiving medication very frequently, so even rare adverse effects may become apparent during extended treatment courses. Maintain close communication with your veterinarian throughout the treatment period.

The primary contraindication for natamycin use is known hypersensitivity to natamycin or other polyene antifungal agents. Horses that have experienced allergic reactions to natamycin in the past should not receive this medication again. Cross-reactivity with other polyene antifungals such as amphotericin B is theoretically possible, though clinical significance of this cross-reactivity in equine patients is not well established. Always inform your veterinarian of any previous adverse reactions to antifungal medications before beginning treatment for a new fungal infection.

Natamycin is not effective against non-fungal causes of keratitis and should not be used as the sole treatment for bacterial corneal infections. While fungal and bacterial keratitis can sometimes coexist, using antifungal therapy alone for a primarily bacterial infection will result in treatment failure and potentially catastrophic corneal damage. Proper diagnostic testing to determine the causative organisms is essential for selecting appropriate treatment. Many cases of keratitis receive combination antibacterial and antifungal therapy until culture results clarify the primary pathogen.

Deep-seated intraocular fungal infections are not effectively treated with natamycin alone due to the medication's poor penetration beyond the corneal surface. Conditions such as fungal endophthalmitis or fungal infections of the anterior chamber require systemic antifungal therapy or antifungal medications with better intraocular penetration. Natamycin may be used as an adjunct in these cases but should not be relied upon as the primary treatment for intraocular fungal disease.

Pregnancy and lactation considerations for natamycin in horses are minimal given the negligible systemic absorption from ophthalmic administration. However, as with any medication, use in pregnant or nursing mares should be guided by veterinary judgment that the benefits of treatment outweigh any theoretical risks. Fungal keratitis is a vision-threatening emergency regardless of pregnancy status, and appropriate treatment should not be withheld from pregnant mares. Your veterinarian can advise on the most appropriate approach for mares in these reproductive states.

Drug interactions with topically applied natamycin are limited due to the medication's lack of systemic absorption. The primary considerations involve interactions with other medications being applied to the same eye during treatment of fungal keratitis. Many horses with this condition receive multiple topical ophthalmic medications concurrently, and appropriate timing and sequencing of applications is important to ensure each medication has adequate contact time with the ocular surface.

When multiple ophthalmic medications are prescribed, they should generally be administered at least five to ten minutes apart to allow each medication adequate contact time with the ocular surface before dilution by the next application. Your veterinarian will provide specific guidance on the order and timing of medications. Natamycin suspension, being somewhat viscous, is often applied after aqueous solutions to prevent barrier formation that might impede absorption of subsequently applied medications. However, specific protocols vary based on the other medications being used and the individual treatment plan.

Concomitant topical corticosteroid use requires special consideration in fungal keratitis cases. While corticosteroids are often used in various forms of keratitis to reduce inflammation and scarring, their use in active fungal infections is controversial and potentially dangerous. Corticosteroids can impair local immune responses and potentially exacerbate fungal proliferation. If corticosteroids are deemed necessary for controlling inflammation, they should only be initiated after the fungal infection is clearly responding to antifungal therapy and only under close veterinary supervision with frequent monitoring.

Subconjunctival or intracorneal injections of antifungal medications may be used in conjunction with topical natamycin for severe fungal keratitis. These injections provide higher local drug concentrations and may use medications with better corneal penetration. When such combined approaches are used, natamycin typically continues as maintenance therapy between injections. Systemic antifungal therapy may also be employed for severe or refractory cases, and there are no significant interactions between topical natamycin and systemic antifungals such as fluconazole or itraconazole.

Monitoring requirements during natamycin treatment are intensive due to the serious nature of fungal keratitis and the potential for rapid progression or development of complications. Horses receiving treatment for fungal keratitis should be reexamined frequently, often every few days initially, with examination frequency adjusted based on response to therapy. Monitoring includes assessment of corneal clarity, size of any ulceration or infiltrate, anterior chamber inflammation, and overall comfort level. Diagnostic testing including repeat cytology or culture may be performed to assess treatment response.

Special populations requiring additional consideration include foals with fungal keratitis, which is uncommon but can occur, and horses with concurrent systemic illness that may impair local or systemic immune function. Immunocompromised horses may have delayed treatment response and may require more intensive or prolonged therapy. Horses with concurrent equine recurrent uveitis present unique challenges, as the inflammatory nature of their condition complicates both diagnosis and management of superimposed fungal infection.

Competition horses undergoing treatment for fungal keratitis face practical challenges beyond regulatory considerations. The intensive treatment schedule required, often including applications throughout the day, is incompatible with competition schedules. Additionally, the painful nature of active keratitis makes competition inappropriate regardless of medication rules. While natamycin is unlikely to cause positive drug tests, horses with fungal keratitis should not compete until the infection has fully resolved and the cornea has healed. Consult with your veterinarian regarding appropriate return-to-competition timing.

Safe handling and administration technique deserves emphasis given the critical importance of treatment compliance for successful outcomes. Caregivers should be thoroughly trained in proper eye medication administration technique before the horse is discharged from veterinary care. The medication bottle should be protected from contamination by avoiding contact between the dropper tip and any surface. Human handlers should wash hands before and after medication administration. While natamycin does not pose significant risks to human health, general hygiene practices are appropriate.

Long-term considerations for natamycin therapy relate primarily to the extended treatment duration required for fungal keratitis. Horses may receive natamycin for weeks to months, and owner fatigue with the intensive treatment schedule is a real concern that can compromise outcomes. Honest discussion with your veterinarian about the practical demands of treatment and identification of strategies to maintain compliance throughout the treatment course is important. Additionally, horses that have experienced fungal keratitis are at increased risk for recurrence and may benefit from protective measures such as fly masks to reduce risk of future corneal trauma.

Proper storage of natamycin ophthalmic suspension is essential to maintain medication efficacy throughout the extended treatment courses typical for fungal keratitis. The medication should be stored at room temperature, typically between 36°F and 77°F (2°C to 25°C). While refrigeration is not required, some practitioners prefer to refrigerate the medication, which may improve comfort upon application. Avoid exposing the medication to temperatures above 77°F, as this can affect the stability of the suspension. In barn environments, store the medication in a climate-controlled area rather than in unheated or non-air-conditioned spaces.

The suspension must be shaken thoroughly before each use to ensure uniform distribution of the active ingredient. Natamycin is not soluble in the vehicle, so the medication separates between uses. Failure to resuspend properly before each application results in inconsistent dosing and potentially inadequate drug delivery to the cornea. A good practice is to shake the bottle for at least one minute before the first use of the day and for thirty seconds before subsequent uses. Proper shaking should produce a uniform, opaque suspension without visible settling.

Maintaining sterility of the eye medication is critical, particularly given the vulnerable state of eyes with fungal keratitis. Never allow the dropper tip to contact the eye, eyelids, or any other surface. Replace the cap immediately after each use. If the dropper tip becomes contaminated, the entire bottle should ideally be replaced to prevent introducing additional pathogens to the infected eye. Store the medication in a clean location away from dust, debris, and potential contaminants.

Discard natamycin according to the expiration date on the bottle or four weeks after opening, whichever comes first. Once opened, ophthalmic medications have limited shelf life due to potential contamination and degradation with repeated use. For extended treatment courses, multiple bottles may be needed. Dispose of expired or contaminated medication appropriately and do not use ophthalmic preparations past their expiration dates. Using degraded or contaminated medication risks treatment failure and potentially introduces additional pathogens to an already compromised eye.

Draft horses and other heavy breeds do not require specific dosage adjustments for natamycin since ophthalmic dosing is based on eye size and application volume rather than body weight. However, the physical characteristics of draft breeds may present practical challenges for medication administration. Their larger head size and height require appropriate handling facilities and may necessitate sedation for safe, reliable medication administration in uncooperative individuals. Draft breeds with heavy forelocks may benefit from forelock management to facilitate eye medication administration and examination.

Light horse breeds and warmbloods represent the majority of horses presenting with fungal keratitis. These breeds typically tolerate ophthalmic medication administration well when appropriate training and handling techniques are employed. Performance horses in these categories may face competition implications during treatment, though the primary concern should be complete resolution of the infection before returning to any athletic activity. The intensive treatment schedule for fungal keratitis is incompatible with competition participation regardless of regulatory considerations.

Ponies and miniature horses can develop fungal keratitis similarly to larger horses and are treated with the same natamycin protocols. Their smaller eye size means less medication volume may be needed per application, but standard one to two drop dosing remains appropriate. Handler positioning may differ when treating smaller equines to achieve appropriate angle for eye medication administration. The same monitoring and treatment duration principles apply regardless of patient size.

Appaloosas deserve special mention regarding ocular conditions because this breed has a significantly increased incidence of equine recurrent uveitis compared to other breeds. Horses with chronic uveitis are predisposed to various ocular complications including corneal disease. While there is no known breed-specific sensitivity to natamycin, Appaloosas presenting with fungal keratitis should be evaluated for concurrent uveitis, as this complicating factor influences treatment approach and prognosis. The presence of uveitis may require additional anti-inflammatory therapy and carries implications for long-term ocular health beyond the immediate fungal infection.

Other antifungal medications used for equine fungal keratitis represent the most closely related therapeutic alternatives to natamycin. Voriconazole has emerged as an important alternative or adjunct, offering better corneal penetration than natamycin and activity against a broad range of filamentous fungi. This medication can be formulated as ophthalmic drops and is often used for deep stromal infections or cases not responding adequately to natamycin alone. Miconazole is another azole antifungal available in ophthalmic preparations that may be used for fungal keratitis, though natamycin generally remains the first-line choice for filamentous fungal infections.

Systemic antifungal therapy may be employed as an adjunct to topical treatment for severe or refractory fungal keratitis. Oral fluconazole achieves reasonable ocular penetration and may be prescribed for cases requiring additional antifungal support beyond topical therapy alone. Itraconazole is another oral option, though its ocular penetration is more limited. Systemic therapy is particularly considered for deep stromal infections, descemetocele formation, or any case at high risk for corneal perforation where maximum antifungal coverage is warranted.

Adjunctive therapies commonly used alongside natamycin in comprehensive fungal keratitis management include topical atropine for cycloplegia and pain control, topical antibiotics to prevent bacterial superinfection, serum or plasma drops to inhibit collagenases and support corneal integrity, and systemic anti-inflammatory medications when appropriate. Protective devices such as subpalpebral lavage systems facilitate frequent medication delivery, while fly masks protect healing corneas from environmental trauma. Your veterinarian will develop a comprehensive treatment protocol addressing all aspects of the condition. Never substitute or add medications to a fungal keratitis treatment protocol without veterinary guidance, as inappropriate combinations can worsen outcomes.