The Eye

CHAPTER 43 The Eye

The Ophthalmic Examination

Examination of the neonate must take into account age-related variations in ocular appearance and response. Generally the eyelids open at 10 to 14 days of age, revealing a cloudy cornea that begins to clear within 24 hours. The patient should demonstrate a blink in response to a bright light when the lids open. Pupillary light reflexes are present within 24 hours of eyelid separation but remain sluggish for 3 weeks. Vision and menace responses are generally poor for the first 3 weeks and may not reach adult standards until 6 to 8 weeks of age. Reflex lacrimation begins when the eyelids open. Box 43-1 summarizes additional features of the neonatal eye.

BOX 43-1 Normal features of the immature canine and feline eye

Eyelids	Separate at 10-14 days
Globe position	Kittens demonstrate subtle divergent strabismus until 3-4 weeks
Tear film	Reflex tearing is present when eyelids separate
Cornea	Mild corneal edema begins to clear within 24 hours after eyelids open; corneal thickness continues to decrease until 6 weeks of age
Iris	Newborn blue-gray iris shows signs of adult coloration by 1 month of age; persistent pupillary membranes are seen until 6 weeks of age in puppies and 3 weeks of age in kittens
Lens	Vessels of the tunica vasculosa lentis are seen until 4 weeks of age
Fundus	The blue-gray tapetum gradually assumes adult coloration by 4 months of age; the optic disc appears smaller because of incomplete myelination, but caliber and distribution of retinal vessels are similar to adult
Vision/menace reflex	Visual reflexes are poor at 3 weeks of age, gradually improving until 6-8 weeks of age, as retina continues to differentiate
Pupillary light reflexes	Present within 24 hours of lid separation but sluggish until the retina matures during week 4
Dazzle reflex	Present when eyelids open

The Globe and Orbit

Congenital Abnormalities

Anophthalmos

Anophthalmos, or complete absence of the globe, is extremely rare. In most cases, careful examination of the orbital contents will reveal primitive ocular tissue.

Microphthalmia

Failure of the eye to develop to normal size is referred to as microphthalmia. The small globe is often associated with a correspondingly small palpebral fissure. Depending on the constellation of accompanying defects, vision may be normal, diminished, or absent. Microphthalmia with multiple colobomas is an autosomal recessive trait linked to merling in the Australian Shepherd (Figure 43-1).

Figure 43-1 Microphthalmia. Note the prominent third eyelid, small cornea, and disproportionate amount of sclera visible in this merled Australian Shepherd.

In addition to small globes, dogs with merle ocular dysgenesis may have persistent pupillary membranes, cataract, equatorial staphylomas, choroidal hypoplasia, retinal dysplasia and detachment, and optic nerve hypoplasia. Vision is frequently impaired. Similarly affected breeds include the Great Dane, Collie, Shetland Sheepdog, Dachshund, and Catahoula Leopard Dog.

Microphthalmia is associated with inherited congenital cataracts in the Miniature Schnauzer, Old English Sheepdog, Akita, and Cavalier King Charles Spaniel, as well as with retinal dysplasia in the Bedlington and Sealyham Terriers, Beagle, and Labrador Retriever. In the Doberman Pinscher, microphthalmia, anterior segment dysgenesis, and retinal dysplasia are thought to be inherited as an autosomal recessive trait.

Microphthalmia, choroidal and optic disc colobomas, retinal dysplasia, and tapetal aplasia have been attributed to heredity and in utero viral infections in Domestic Shorthair kittens. Multiple anomalies, including colobomas, have also been described in Persian cats. Administration of griseofulvin to pregnant cats may cause microphthalmia in their offspring and has also resulted in anophthalmos, cyclopia, and optic nerve aplasia.

Atypical Eye Position

Strabismus refers to deviation of the eye from the visual axis. A lateral deviation or exotropia occurs in brachycephalic dog breeds, notably the Boston Terrier, Pug, Pekingese, and Shih Tzu. Convergent strabismus or esotropia is inherited in Siamese and Himalayan cats as an autosomal recessive trait, linked to pigment-defining genes that cause aberrant routing of optic nerve fibers. The strabismus is thought to be a compensatory mechanism for the abnormal visual input. Bilateral ventrolateral strabismus occurs with hydrocephalus because of enlargement of the calvaria and its effect on the globe-orbit relationship.

Atypical Eye Motion

A wandering movement of the eyes in puppies and kittens is associated with congenital blindness. Rapid, repetitive, involuntary movement of the eyes (nystagmus) commonly occurs in Siamese kittens and is likely related to the neuroanatomic abnormalities that produce strabismus. Vision is functionally normal. The nystagmus may lessen as the kitten matures.

Acquired Abnormalities

Orbital Cellulitis and Abscessation

Orbital inflammation is usually characterized by an acute unilateral exophthalmos, protrusion of the third eyelid, chemosis, pain on opening the mouth, and fever. Swelling or discoloration behind the last upper molar may occur. Foreign objects and fight wounds are likely causes in young dogs and cats. Response to twice-daily warm compresses and a broad-spectrum systemic antimicrobial agent is generally rapid. If no response is seen within 24 hours, incise the oral mucosa behind the last maxillary molar with a blade, gently advance a closed hemostat through the pterygoid muscle, and partially open the hemostat within the orbit to establish drainage. In most instances, only a slight serosanguineous fluid appears. Damage to orbital structures occurs when the procedure is performed carelessly. Do not insert sharp instruments into the orbit that can damage the globe or its associated nerves and vessels. If exophthalmos is extreme, a bland ophthalmic ointment is applied to the corneal surface to prevent drying.

Orbital Neoplasia

Orbital neoplasia is extremely rare in animals younger than 1 year of age.

Traumatic Proptosis

Complete displacement of the eye from the orbit is most commonly seen in brachycephalic dogs, but any dog or cat may present with proptosis with sufficient cranial trauma (Figure 43-2). A completely displaced eye is a true ocular emergency. Prognosis for vision is always poor because of optic nerve injury, but the intact globe may be salvaged cosmetically.

Figure 43-2 Proptosis. This mixed Terrier’s globe is displaced in front of the eyelids and tightly positioned against the orbital rim, with minimal apparent anterior segment damage.

A soft eye, indicating rupture of the fibrous tunic, or one with extensive avulsion of extraocular muscles or optic nerve should be enucleated. Those with severe intraocular hemorrhage usually shrink over time because of irreparable damage to the ciliary body.

General anesthesia is required to reposition the globe. After flushing the eye with sterile saline solution, a blunt probe, such as a spay hook, or preplaced sutures of 5-0 nylon are used to elevate the eyelid margins. Simultaneous gentle counterpressure is applied against the cornea with a moistened cotton ball or the flat surface of a scalpel handle to push the eye back into place. The eyelids are sutured closed using horizontal mattress sutures of 5-0 nylon placed over stents. Sutures should enter the eyelid 3 to 5 mm from the margin and exit through the meibomian gland openings to prevent corneal damage.

A broad-spectrum systemic antibiotic and a tapering regimen of oral corticosteroids are recommended for 7 to 10 days. Injections should not be made into the orbital area. If the animal allows, topical antimicrobial ointment may be applied 3 or 4 times daily between the eyelid margins at the medial canthus. Warm compresses are recommended for 3 to 4 days after the replacement. One of the most frequent management errors is removing the tarsorrhaphy sutures prematurely. Sutures are left in place for 14 days or until the globe settles back into the orbit. Sequelae include lateral strabismus as a result of rupture of the medial rectus muscle, blindness, low tear production, and shrinkage of the eye (phthisis bulbi).

Enophthalmos

Enophthalmos describes an eye that has receded into the orbit. The Saint Bernard, Great Dane, Doberman Pinscher, Golden Retriever, and Irish Setter often appear enophthalmic owing to their large orbits and deeply set eyes. Congenital enophthalmos is most often associated with microphthalmia. Acquired causes include any painful ocular disorder, postinflammatory atrophy, loss of retrobulbar fat in debilitated or malnourished animals, phthisis bulbi, or Horner’s syndrome with loss of sympathetic tone in the orbital fascia.

The Eyelids

Congenital Abnormalities

Premature or Delayed Eyelid Opening

A gradual increase in tear production triggers eyelid separation in the 10- to 14-day-old dog and cat. If the eyelids open prematurely, corneal drying and ulceration are likely to follow. Treatment consists of frequent application of a topical lubricating ophthalmic ointment to prevent corneal desiccation until tear production improves. A temporary tarsorrhaphy may be indicated in instances of progressive corneal ulceration.

Delayed eyelid opening may be associated with an accumulation of mucus or infectious exudate. If distended, the eyelids should be gently pried apart with firm digital pressure. The ocular surface is liberally flushed with sterile saline, and a fluorescein dye test is used to rule out corneal ulceration. Triple antibiotic ointment is recommended in the puppy, and erythromycin ointment is recommended in the kitten to treat Staphylococcus spp. and Chlamydophila felis, respectively.

Eyelid Agenesis

Eyelid agenesis occurs primarily in cats as a unilateral or bilateral anomaly. The condition is characterized by absence of the eyelid margin, almost always involving the lateral one third to two thirds of the upper eyelid (Figure 43-3).

Figure 43-3 Eyelid agenesis. The superior temporal lid margin is absent in this 6-month-old Domestic Shorthair cat.

Mild defects may be misinterpreted as entropion because of misdirection of adjacent hairs. Large defects lead to chronic irritation from direct contact of the cornea with facial hairs and from exposure secondary to imperfect lid closure. Small defects may be successfully managed with an ophthalmic lubricant ointment applied several times daily to reduce ocular irritation. Misdirected hairs can be cryoepilated or surgically everted using a modified Hotz-Celsus technique as for entropion. If one third or more of the eyelid is missing, pedicle grafts from the forehead or the temporal aspect of the lower eyelid are combined with rotational conjunctival grafts from the nictitans or inferior conjunctival cul-de-sac to repair the defect.

Entropion

Inversion of the eyelid margin occurs commonly in dogs but infrequently in the cat. The lower eyelid is more often affected. Accompanying clinical signs include increased tearing, squinting, corneal vascularization, and ulceration (Figure 43-4).

Figure 43-4 Entropion. The inferior temporal eyelid rolls inward, rubbing against the cornea and causing secondary vascularization in this young Chesapeake Bay Retriever.

Several genes that define the eyelid structure, globe-orbit relationship, and facial skin are likely to influence the degree of entropion. The narrow palpebral fissure of the Chow Chow, the deeply set eye of the Golden Retriever, and the redundant facial folds of the Shar-Pei are examples of the variables that determine lid conformation. Painful ocular disorders also cause entropion secondary to spasm of the orbicularis oculi muscle. This mechanism is commonly implicated in cats with chronic herpesvirus infection.

Young animals with entropion but without corneal disease may be treated palliatively with an ophthalmic lubricant ointment to postpone surgery until the patient matures. Delaying surgery in the Shar-Pei may not be possible because of the severity of the breed’s entropion and the risk of corneal ulceration. Temporary “tacking” is used to evert the lid margins and forestall corneal damage. The procedure is most effective when performed in the 3- to 4-week-old puppy. The older the animal is when tacking is first performed, the less likely it will be to correct the entropion without additional surgery. Local anesthetic blocks can be used in the very young animal, but masking the patient with isoflurane expedites the procedure. The first bite of a 5-0 nylon vertical mattress suture enters 2 mm from the eyelid margin and engages about 4 mm of eyelid skin and subcutaneous tissue. Avoid suturing the eyelid margin directly because postsurgical notching can irritate the cornea. The needle is then reinserted into the skin and deeper tissue overlying the orbital rim, adjusting the tension to evert the eyelid margin in a slightly overcorrected position (Figure 43-5).

Figure 43-5 Eyelid tacking. Vertical mattress sutures are placed to temporarily evert the lid margin in this Shar-Pei puppy.

The number of required sutures varies with the individual patient. A topical lubricant ointment is applied twice daily. Ideally the sutures are left in place for weeks. Surgical staples may also be used, with the advantages of rapid placement and excellent retention.

As long as the cornea remains healthy, definitive surgery is ideally postponed until the patient is 6 months of age or older. The modified Hotz-Celsus technique is the classical procedure for entropion repair, everting the lid margin by removing an ellipse of eyelid skin equal to the amount of lid inversion. Preoperative assessment should be performed after application of a topical anesthetic to eliminate any spastic component that exaggerates the degree of entropion. It is also critical to establish a surgical plan before anesthesia, when enophthalmos and loss of eyelid tone alter eyelid position significantly. With the eyelid supported by a Jaeger lid plate inserted into the conjunctival cul-de-sac, the initial incision is made with a scalpel blade 2 to 3 mm from and parallel to the lid margin, along the line where the eyelid hair begins, extending at least 1 mm medially and laterally to the entropic section. The distal incision gently arcs, joining with the two ends of the primary incision and outlining the tissue that must be excised to successfully evert the margin. The incised ellipse of tissue is removed with scissors. The wound is closed with 4-0 to 6-0 simple interrupted braided nylon, silk, or polyglactin sutures, placing the first suture at the midpoint of the incision and continuing to bisect the remaining wound segments with sutures until the defect is perfectly apposed. Topical antibiotic ointment is applied to the eye twice daily until suture removal. A systemic nonsteroidal antiinflammatory drug (NSAID) provides satisfactory analgesia. The wound is protected with an Elizabethan collar. Sutures are removed in 10 days, although 6-0 polyglactin sutures are commonly left to resorb. A modification of the Hotz-Celsus procedure that excises an arrowhead-shaped area of skin around the lateral canthus can be used to correct the lateral canthal inversion in the Shar-Pei, Chow Chow, and Retriever breeds. A combination of techniques may be necessary in breeds with excessive facial folds or elongated lids, prompting referral of these patients to an ophthalmologist.

Ectropion

Ectropion refers to an everted eyelid margin, with secondary exposure of the bulbar conjunctiva. The conjunctival sac accumulates particulate debris and is often chronically inflamed. The condition is common in dogs with loose facial skin, including the Clumber Spaniel, Cocker Spaniel, Basset Hound, Bloodhound, and Saint Bernard. Some degree of ectropion is considered desirable in these breeds. Surgical correction is reserved for the most severely affected patients with secondary keratoconjunctivitis. The simplest method for correction shortens the lower eyelid by excising a wedge of tissue adequate to lift the eyelid into a more normal position. Patients with severe ectropion or those combined with entropion should be referred to an ophthalmologist for correction.

Distichiasis

Distichiasis refers to an extra row of cilia that protrude from the orifices of the meibomian glands along the eyelid margins. Congenital distichiasis occurs in the English Bulldog, Poodle, Cocker Spaniel, Golden Retriever, Shih Tzu, and Pekingese breeds and may be inherited as an autosomal dominant trait. The condition is rare in cats but is reported in the Abyssinian.

The mere presence of distichia does not justify their removal. Surgical treatment is reserved for those animals in which all other causes of tearing, conjunctivitis, and keratitis have been ruled out. Cryoepilation using liquid nitrogen or nitrous oxide and a 3-mm cryoprobe is a safe and effective method for managing numerous distichia. The eyelid is stabilized with a chalazion clamp, and the probe is positioned over the base of the affected meibomian glands, allowing the ice ball to extend just beyond the meibomian gland openings along the lid margin. The time necessary to reach this point varies with lid thickness and is likely to take longer in a Bulldog than a Poodle, for example. A double freeze-thaw technique is most effective. Eyelid swelling typically resolves in a matter of days, but marginal depigmentation can last for weeks to months.

Trichiasis

Trichiasis occurs when otherwise normal eyelashes or facial hairs deviate inward, contacting the surface of the eye. Secondary corneal changes include vascularization, pigmentation, or ulceration. Management of the trichiasis should begin with correction of any predisposing problem such as eyelid agenesis. Cryoepilation may be used to remove offending cilia, or a modified Hotz-Celsus procedure may be used to evert hairs away from the eye.

Ectopic Cilia

Occasionally cilia will penetrate through the conjunctiva lining the underside of the eyelid rather than emerge from the meibomian gland opening. The classical presentation is a young dog with a nonhealing superficial ulcer in the dorsal third of the cornea. Some dogs intermittently squint and tear but never ulcerate. The upper eyelid is more commonly affected. Magnification is required to demonstrate the small dark spot at the base of the meibomian gland that represents the tip of the emerging cilia (Figure 43-6).

Figure 43-6 Ectopic cilia. The tip of ectopic cilia penetrates the conjunctiva at the base of a meibomian gland, appearing as a black dot just beneath the pinpoint central flash artifact.

En bloc resection of the palpebral conjunctiva and the affected meibomian gland is curative.

Acquired Abnormalities

Lacerations

Eyelid lacerations in dogs and cats are usually perpendicular to the lid margin. The lid’s exceptional blood supply provides for excellent healing even when surgical correction is delayed. Minimal debridement and meticulous suturing are advised to preserve the lid margin and eyelid function. A two-layer closure will better control wound distraction by the orbicularis muscle. A deep layer of continuous 6-0 absorbable suture is used to reappose the conjunctiva and tarsus, taking care to bury knots that can damage the cornea. The lid margin is precisely apposed with 4-0 to 6-0 braided nylon, silk, or polyglactin in a figure-eight pattern, and a second layer of interrupted sutures is placed in the skin. Postoperative therapy includes topical broad-spectrum antibacterial ointment several times daily, oral antibiotic for 7 days, a systemic nonsteroidal for analgesia, and an Elizabethan collar to prevent trauma to the surgical site. Sutures of 4-0 and 5-0 material should be removed in 10 to 14 days, but 6-0 polyglactin resorbs without complication.

Allergic Reactions

Immediate hypersensitivity caused by insect stings, vaccines, or medication may initiate an IgE-mediated response that causes sudden and severe eyelid and conjunctival swelling. Clinical signs usually subside over 12 to 24 hours with warm compresses and oral antihistamines or corticosteroids. If the corneal surface cannot be visualized to rule out ulceration, a topical antibiotic or preservative-free artificial tear ointment is preferred over a topical corticosteroid to protect swollen and exposed conjunctiva.

Bacterial Infections

Acute eyelid swelling and pustule formation may precede facial pyoderma and lymphadenopathy in puppies with juvenile pyoderma. A hypersensitivity component to Staphylococcus spp. is thought to underlie the dramatic clinical signs. Topical antibacterial or corticosteroid medications are ineffective because insufficient drug levels are achieved in the eyelids. An oral antibiotic with efficacy against Staphylococcus spp. must be coupled with an oral corticosteroid for best response. Warm compresses of the eyelids may also help reduce secondary swelling. Complete resolution may take weeks.

Parasitic Infestations

Generalized or local infestation with a variety of ectoparasites causes alopecia, erythema, and pruritus of the eyelids. The more common parasites involved are Demodex canis, Notoedres cati, Otodectes cyanotis, and Sarcoptes scabiei.

Neoplasia

Eyelid tumors are rare in young dogs. Viral papillomas are usually pedunculated, with a cauliflower-like appearance. Oral papillomas may develop concurrently. The histiocytoma is smooth, tan to pink in color, and broad-based. One or several masses may arise quickly along the lid margin. Occasionally the eyelid mass accompanies systemic histiocytosis; thus careful systemic examination is advised. With either tumor, solitary masses may spontaneously regress in weeks to months. Surgical excision or cryotherapy is necessary only if the tumor is damaging the eye.

The Conjunctiva

Congenital Abnormalities

Dermoid

A dermoid is a congenital mass of tissue containing skin, hair follicles, and sebaceous glands (Figure 43-7). It most commonly occurs in the temporal perilimbal conjunctiva and may also involve the adjacent eyelid and cornea.

Figure 43-7 Dermoid. Aberrant nonpigmented skin with several surface hairs overlies the temporal conjunctiva and cornea in this Dachshund puppy.

Dermoids often cause ocular irritation and epiphora as a result of surface hairs contacting the ocular surface. Treatment involves careful dissection of the dermoid from the surrounding conjunctiva and underlying sclera. If the cornea is involved, a superficial keratectomy is also indicated.

Medial Canthal Trichiasis

Long hairs that arise from the medial canthal caruncle may wick tears over the lid margin, staining the periocular fur and predisposing to facial fold dermatitis (Figure 43-8).

Figure 43-8 Medial canthal trichiasis. A tuft of hairs arising from the medial canthal caruncle can damage the cornea and misdirect tears in this young Shih Tzu.

Chronic contact also damages the corneal surface. The condition is most commonly seen in the Cavalier King Charles Spaniel, Lhasa Apso, Maltese, Shih Tzu, Pekingese, Poodle, and Pug. Cryoepilation is a simple, effective method of destroying the hair follicles within the caruncle. The caruncle may also be surgically excised, taking care to avoid damage to the adjacent nasolacrimal system.

Acquired Abnormalities

Allergic Conjunctivitis

Ocular allergy is rarely substantiated by other than response to topical antiinflammatory therapy or an association with systemic signs suggestive of atopy. Moderate conjunctival hyperemia, mild chemosis, serous ocular discharge, and pruritus are typical signs that occur bilaterally. Eosinophils and neutrophils are seen cytologically. Chronic antigenic stimulation may cause hyperplasia of lymphoid follicles in the conjunctival cul-de-sacs and on the bulbar surface of the third eyelids. A seasonal incidence is common. Symptomatic treatment with a topical corticosteroid is both beneficial and cost effective.

Bacterial Conjunctivitis

Bacterial conjunctivitis is characterized by conjunctival hyperemia, swelling, and mucopurulent discharge. In the dog, it is not a primary ocular disease but rather a consequence of a predisposing adnexal or lacrimal abnormality. Culture may reveal one or more types of bacteria, typically susceptible to a wide range of antibacterial drugs. Response to a topical antibacterial medication is usually rapid, but clinical signs recur once treatment is discontinued. A careful examination should be performed to rule out keratoconjunctivitis sicca (KCS), ectopic cilia, foreign bodies, entropion, ectropion, and nasolacrimal disease.

In contrast, the cat may exhibit a primary conjunctival infection by C. felis. Initial infection is characterized by a mild rhinitis and unilateral purulent conjunctivitis. Second eye involvement occurs 5 to 7 days later. Lack of corneal involvement may help differentiate the conjunctivitis from that caused by herpesvirus. Basophilic cytoplasmic inclusions in conjunctival scrapings performed during the first 2 to 9 days after onset of clinical signs are diagnostic, as are indirect fluorescent antibody (IFA) and polymerase chain reaction (PCR) testing in more chronic cases. Treatment consists of topical tetracycline or erythromycin ointment 4 times daily for 3 weeks or a 3-week course of oral doxycycline (5 mg/kg twice daily) or azithromycin (5 mg/kg daily).

Mycoplasma spp. are historically implicated in feline conjunctivitis. Controversy remains with respect to the organism’s importance because infection cannot be established in the absence of other pathogens. Diagnosis is confirmed by demonstrating coccoid basophilic organisms in clusters on the epithelial cell membrane in the initial stages of infection or by PCR analysis of conjunctival swabs. Topical tetracycline is the medication of choice.

Viral Conjunctivitis

In the naive neonatal or adolescent cat, feline herpesvirus-1 (FHV-1) causes an acute conjunctival and respiratory infection. Clinical signs are directly related to the viral cytopathic effect on epithelial cells. The initial conjunctivitis is bilateral with pronounced hyperemia, accompanied by ocular discharge that changes from serous to mucopurulent. Pathognomonic dendritic corneal erosions may be overlooked if rose bengal is not used to stain the corneal surface. Kittens with viral conjunctivitis before eyelid separation will often have such severe conjunctival and corneal erosions that adjacent raw surfaces adhere together, creating symblepharon. These conjunctival adhesions can cause permanent prominence of the third eyelid, epiphora from obstruction of the nasolacrimal puncta, and corneal opacity.

Following recovery in 10 to 14 days from the initial herpesvirus infection, 80% of cats become latently infected carriers, and an estimated 45% of these will experience spontaneous viral shedding and/or recrudescence of clinical disease in the future. Subsequent episodes of conjunctivitis commonly occur without respiratory signs and affect only one eye. Diagnosis of herpetic conjunctivitis is more often based on history and clinical signs than on specific testing. Serology is predictably positive because of the widespread exposure of cats to herpesvirus. Viral detection using IFA or PCR testing fails to differentiate between wild and vaccinal virus.

Feline conjunctivitis should be considered infectious until proven otherwise. Topical corticosteroids may initially decrease hyperemia and swelling but are not recommended because they also prolong virus shedding and increase the risk of herpetic ulceration. Specific antiviral therapy is seldom used in the initial ocular-respiratory syndrome because of its self-limiting nature. If corneal ulceration is present, topical 0.1% idoxuridine may be compounded for topical use; its virostatic nature requires application 4 to 6 times daily. Topical 0.5% cidofovir requires only twice-daily application for 14 days but is much more expensive. Ocular signs improve in patients treated with oral famciclovir at a dose of 15 mg/kg every 12 hours for 14 days. Other oral antiviral medications are either ineffective or toxic to the cat. Lifelong oral L-lysine supplementation limits viral replication and may reduce the severity or frequency of relapses. The recommended dosage for kittens is 250 mg twice daily and for adult cats is 500 mg twice daily, usually supplied as a powder and mixed in food.

Dogs infected with canine distemper virus may exhibit conjunctivitis, ocular discharge, and reduced tear production. Conjunctival scrapings in the acutely infected patient may contain intracytoplasmic inclusion bodies.

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