Eye Opener: clinical signs

Showing posts with label clinical signs. Show all posts

Pathology - Common diseases

Common diseases

Conjunctivitis

Figure 1: Allergic conjunctivitis with hyperemia and mucoid discharge in a dog.

Source: Gelatt K.N. (2008) Essentials of veterinary ophthalmology, 2nd edn., USA: Blackwell Publishing.

Figure 2: Viral conjunctivitis in a dog with canine distemper.

Source: Gelatt K.N. (2008) Essentials of veterinary ophthalmology, 2nd edn., USA: Blackwell Publishing.

Figure 3: Viral conjunctivitis with chemosis and hyperemia in a cat with feline herpesvirus.

Source: Gelatt K.N. (2008) Essentials of veterinary ophthalmology, 2nd edn., USA: Blackwell Publishing.

Inflammation of conjunctival membrane

Red eye with discharge

Serous discharge

Physical irritants
Allergy
Viral agents
Treatment: Flush eye with sterile saline eyewash or artificial tears 3-4 times a day

Mucoid discharge

Follicles on underside of third eyelid
Eye irritant
Infection
Treatment: Antibiotic and corticosteroid eye ointments

Purulent discharge

Infection
May crust eyelids
Treatment: Remove mucus and pus from eyes and apply antibiotics

Bilateral - allergy or systemic disease
Unilateral - foreign body in the eye

Clinical signs

Hyperemia
Chemosis
Ocular discharge
Harmorrhage
Follicle formation
Pruritis
Pain/discomfort

Cats: twitching
Dogs: no discomfort

Classification

Infectious

Bacterial

Dogs: usually caused by Streptococcus and Staphylococcus
Cats: Chlamydophila Felis, Bordetella bronchiseptica

Viral

Dogs: canine distemper virus
Cats: feline herpesvirus-1, Feline calicivirus

Fungal
Rickettsial

Rickettsia rickettsii

Parasitic

Cats: Thelazia californiensis

Noninfectious

Allergic
Follicular

Cats

Lipogranulomatous
Eosinophilic

Ocular squamous cell carcinoma

Figure 1: Limbal squamous cell carcinoma in a horse. Carcinoma is seen as a light pink lobulated mass on the limbus extending to the cornea and conjunctiva.
Source: Hendrix D.V.H. (2005) 'Equine Ocular Squamous Cell Carcinoma', Clinical Techniques in Equine Practice, 4(1), pp. 87-94.

Figure 2: Third eyelid replaced by squamous cell carcinoma in a horse.
Source: Hendrix D.V.H. (2005) 'Equine Ocular Squamous Cell Carcinoma', Clinical Techniques in Equine Practice, 4(1), pp. 87-94.

Figure 3: Ulcerated squamous cell carcinoma in the medial canthus of a horse.
Source: Hendrix D.V.H. (2005) 'Equine Ocular Squamous Cell Carcinoma', Clinical Techniques in Equine Practice, 4(1), pp. 87-94.

Figure 4: Limbal squamous cell carcinoma in a cattle.

Source: Tsujita H. and Plummer C.E. (2010) 'Bovine Ocular Squamous Cell Carcinoma',Veterinary Clinics of North America: Food Animal Practice, 26(3), pp. 511-526.

Figure 5: Corneal squamous cell carcinoma in a cattle.

Source: Tsujita H. and Plummer C.E. (2010) 'Bovine Ocular Squamous Cell Carcinoma',Veterinary Clinics of North America: Food Animal Practice, 26(3), pp. 511-526.

Tumour of the eye and ocular adnexa

Nictitating membrane/third eyelid
Conjunctiva
Eyelids
Cornea
Limbus

Cattle

7-9 years old
Herefold

Kept in subtropcal areas - environmentally favourable
White faced and pink skin around eyes
Incidence: 0.8-5%

Holstein
Whole-carcass slaughter

80% of bovine tumors reported at slaughter

Pigs, goats and sheep - low incidence
Horses

10-11 years old
Lack of periocular pigmentation and ultraviolet light exposure
Draft breeds

Colorado
Belgians
Clydesdales

Light breeds

Appaloosa
Albino
Colour dilute

Clinical signs

Conjunctiva or limbus: small white to light pink mass
Limbal to corneal: multilobulated and compacted
Eyelid: ulcerative mass

Treatment

Surgical excision
Cryosurgery
Radiofrequency Hyperthermia
Irradiation therapy
Radiotherapy
Laser ablation
Immunotherapy
Topical and intralesional chemotherapy

Uveitis

Figure 1. Uveitis of a cow

Source: Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.

Figure 2. Uveitis of a guinea pig

Source: Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.

Figure 3. Uveitis of a horse

Source: Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.

Figure 4. Uveitis of a rabbit

Source: Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.

Inflammation of the uveal tract: iris, ciliary body and choroid

Anterior uveitis: Iris and ciliary body
Posterior uveitis: choroid and retina

Inflammation mediated by vascular impairment
Animals with uveitis are susceptible to secondary infection
Causes

Infection

Parasites
Fungi
Bacteria

Trauma
Metabolic disease
Cancer
Tumor

Clinical signs

Blepharospasm
Conjunctival hypermia
Chemosis
Corneal edema
Aqueous flare (proteins and cells leaking into anterior chamber due to impaired blood-aqueous barrier)
Miosis (small sized pupil)
Unique clinical signs in cats

Nodular accumulation of inflammatory cells within the iris storma

Treatment

Must be treated aggressively to prevent further development (e.g. glaucoma, scarring or loss of vision)
Aspirin
Corticosteroid- injection under the eyelid, eyedropper or oral medication
Atropine

Cataracts

Figure 4: Cataracts in a foal.

Source: Brooks D.E. (2002) 'Equine Ophtalmology', American Association of Equine Practitioners Proceedings, 48, pp. 300-313.

Figure 5: Mature senile cataract of a dog.
Source: Mitchell N. (2006) 'Treatment of canine cataracts using phacoemulsification Part 1: When to refer', Companion Animal, 11(3), pp. 83-87.

Opacity in the lens of the animals giving the animal a blurry vision
Vacuoles in equatorial region
Proceeding to lens nucleus becoming more opaque and spreading to the entire lens
Occurrence due to

Disease
Old age
Trauma to the eye
Inheritance
Can be produced by diets deficient in riboflavin or tryptophane

Clinical signs

Change in colour of the eye
Behaviour of animal changed due to increasing loss of vision i.e clumsiness

Treatment

Surgery

Phacoemulsiﬁcation cataract surgery = high success rate in foals

Topical antibiotics after surgery:

Chloramphenicol, gentamicin, ciproflaxacin, tobramycin ophthalmic solutions
1% atropine = maintain blood-aqueous barrier, minimize pain from ciliary muscle spasm and dilation of pupils.
Corticosteroid = prevent inflammation

References

Brooks D.E. (2002) 'Equine Ophtalmology', American Association of Equine Practitioners Proceedings, 48, pp. 300-313.
Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.
Eldredge D.M., Carlson L.D., Carlson D.G. and Giffin J.M. (2007) Dog owner's home veterinary handbook, 4th edn., New Jersey: Howell Book House.
Gelatt K.N. (2008) Essentials of veterinary ophthalmology, 2nd edn., USA: Blackwell Publishing.
Hendrix D.V.H. (2005) 'Equine Ocular Squamous Cell Carcinoma', Clinical Techniques in Equine Practice, 4(1), pp. 87-94.
King T.C., Priehs D.R., Gum G.G. and Miller T.R. (1991) 'Therapeutic management of ocular squamous cell carcinoma in the horse: 43 cases (1 979-1 989)', Equine Veterinary Journal, 23(6), pp. 449-452.
Kinoshita J.H., Kador P. and Catiles M. (1981) 'Aldose reductase in diabetic cataracts', The Journal of American Medical Association, 246(3), pp. 257-261.
Mitchell N. (2006) 'Treatment of canine cataracts using phacoemulsification Part 1: When to refer', Companion Animal, 11(3), pp. 83-87.
Richter C.P. and Duke J.R. (1970) 'Cataracts produced in rats by yoghurt', American Association for the Advancement of Science, 168(3937), pp. 1372-1374.
Tsujita H. and Plummer C.E. (2010) 'Bovine Ocular Squamous Cell Carcinoma',Veterinary Clinics of North America: Food Animal Practice, 26(3), pp. 511-526.
Turner S.M. (2008) Small animal ophthalmology, 1st edn., Saunders Elsevier.

Pathology - Ruminants

Ruminants

Keratoconjunctivitis (Pink-eye)

Figure 1. Infectious bovine keratoconjunctivitis in a calf. Note the corneal vascularization, stromal abscess, and scars.

Source: Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.

Found in sheep, goats and calves
Primary infection occurs by Chlamydiae
Chlamydiae: Gram negative, obligatory intra cellular parasites
Cytoplasmatic inclusion bodies in corneal cells found in the infected animals
Clinical signs

Watery discharge from the eye
Clouding of the cornea
Secondary bacterial infection is common due to mechanical damage of the cornea and altered local defence system allowing potential pathogenic bacteria to contribute to secondary infections

Treatment

Vaccination

Penicillins, spectinomycin or other aminocyclitols do not respond
Tetracyclines respond

High and steady plasma levels of suitable antibiotics for more than one week

Thelazia rhodesi (Eye Worms)

Figure 2: Presence of Thelazia.
Source: University of Sydney Centre for Veterinary Education (2012) Thelazia spp, Available at:http://vetbook.org/wiki/cow/index.php/Thelazia_spp (Accessed: 3rd June 2014).

Parasite of domestic animals, wild animals and less commonly horses
Occurs at surface of cornea, near nictitating membrane, in conjunctival sac and lachrymal duct
Transmission occurs through route of flies that feed on animal's ocular secretions, tears and conjunctiva.
Clinical signs

Conjunctivitis
Keratitis
Lacrimation
Ocular discharge
Ulcers
Corneal opacity
Epiphora - excessive tears production

Treatment

Mechanical removal of worms using forceps after putting anesthesia
Topical treatment with antiparasitic drugs applied to eyes directly
Irrigate with aqueous solution of Boric powder
Concurrent use with Ciplox-D (Ciprofloxacin and dexamethasone) eye drops every 8 hours for 5 days

suppression of inflammation & secondary bacterial infections.

Ivermectin to be injected subcutaneously @ 0.2mg/Kg bodyweight
Fly control

Reference

Bogaard A.E.J.M. (2011) 'Veterinary Quarterly', Inclusion keratoconjunctivitis ('pink eye') in sheep, 6(4), pp. 229 -235.
Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.
Deepthi B. and Yalavarthi C. (2012) 'Eye Worm infection in a Cattle - A Case Report', Veterinary World, 5(4), pp. 236-237.
Naem S. (2006) 'Thelazia rhodesi (Spirurida, Thelaziidae), bovine eyeworm: morphological study by scanning electron microscopy', Parasitol Research, 100, pp. 855-860.
Otranto D., Tarsitano E., Tarversa D., Giangaspero A., DeLuca F. and Puccini V. (2001) 'Differentiation among three species of bovine Thelazia (Nematoda: Thelaziidae) by polymerase chain reaction–restriction fragment length polymorphism of the ﬁrst internal transcribed spacer ITS-1 (rDNA)', International Journal of Parasitlogy, 31(14), pp. 1693-1698.
Otranto D. and Traversa D. (2005) 'Thelazia eyeworm: an original endo- and ecto- parasitic nematode' Trends in Parasitology, 21(4), pp. 1-4.
University of Sydney Centre for Veterinary Education (2012) Thelazia spp, Available at:http://vetbook.org/wiki/cow/index.php/Thelazia_spp (Accessed: 3rd June 2014).

Pathology - Companion animals

Companion animals

Cherry eye or Haws

Figure 1: Unilateral prolapse of third eyelid gland in an American Cocker spaniel dog.

Source: Mehdi M., Sepehr M., Omid, Kazemi H. and Elnaz (2012) 'Clinical evaluation of the pocket technique for replacement of prolapsed gland of the third eyelid in dogs',Turkish Journal of Veterinary and Animal Sciences, 36(4), pp. 352-356.

Figure 2: Bilateral prolapse of third eyelid gland in a mixed-breed dog.

Figure 3: Unilateral prolapse of third eyelid in a Burmese cat.

Source: Chahory S., Crasta M., Trio S. and Clerc B. (2004) 'Three cases of prolapse of the nictitans gland in cats', Veterinary Ophthalmology, 7(6), pp. 417-419.

Figure 4: Bilateral prolapse of third eyelid in a Persian cat.

Source: Chahory S., Crasta M., Trio S. and Clerc B. (2004) 'Three cases of prolapse of the nictitans gland in cats', Veterinary Ophthalmology, 7(6), pp. 417-419.

Prolapse of the third eyelid gland

Unilateral or bilateral
Unclear cause
Secondary to inflammation
Weakness in connective tissue that anchors the gland

May be heritable

Dogs

More common in dogs than cats
Beagle, English bulldog, American Cocker spaniel, Boston terrier, Poodle, Shar Pei, Pekingese, Neapolitan Mastiff, Basset hound and brachycephalic breeds more susceptible

2-3 years old

Cats

Rare in cats

Strands of smooth muscle around third eyelid provide extra anchorage for the gland to the base of the third eyelid

Burmese cats more susceptible
Adult cats - 2.5-6 years old

Clinical signs

Epiphora
Conjunctivitis
Development of red mass at medial canthus

Figure 3: Conjunctival imbrication or Pocket technique for repositioning of third eyelid gland. A: exteriorize third eyelid with forceps. B: Parallel incisions through bulbar conjunctiva anterior and posterior to the gland. C-D: reposition gland to proper position and suturing it in place.

Treatment

Surgically reposition and suture gland back into proper location

Preserves tear production
Suture gland to adjacent sclera, extraocular muscle tendon or ventral endorbita of orbital rim
Conjunctival imbrication or Pocket technique

Excision of prolapsed gland

30% of tear production
predisposes dog to keratoconjunctivitis sicca (KCS) later in life

Corneal sequestrum

Figure 1: Corneal sequestrum (black/brown pigmentation indicated by arrow) surrounded by vascularisation in a cat.
Source: Claudia Hartley (2010) 'Aetiology of Corneal Ulcers: Assume FHV-1 Unless Proven Otherwise', Journal of Feline Medicine and Surgery, 12(), pp. 24-35.

Figure 2: Central corneal sequestrum in a Burmese cat.
Source: Claudia Hartley (2010) 'Aetiology of Corneal Ulcers: Assume FHV-1 Unless Proven Otherwise', Journal of Feline Medicine and Surgery, 12(), pp. 24-35.

Necrosis of corneal epithelium and stroma in domestic cats
Usually unilateral, but can also occur bilaterally
Sequestrum

Oval or round
Brown pigmentation in affected tissue

Central or paracentral dark brown or black plaque

Susceptible breeds

Brachycephalic cats: Persian, Siamese, Burmese, Himalayan and Domestic Shorthair cats

Associated with

Corneal ulcers
Ocular trauma
Feline herpesvirus-1 infection
Chlamydia psittaci infection
Keratoconjunctivitis sicca
Entropion

Clinical signs

Pain and irritation
Bleopharospasm
Epiphora
Enophthalmos
Elevated nictitating membrane
Chemosis
Conjunctival hyperemia
Photophobia
Decreased appetite and activity

Treatment

If no discomfort or pain

Medical: topical artificial tears 2-6/day and topical broad spectrum ophthalamic antibiotics 4-6.day
Surgical: recommended to minimise duration of discomfort and recurrence rate
Lamellar keratoplasty using canine or feline donor tissue
Corneoconjunctival transposition surgery

provides physical support and blood flow to damaged cornea and grafted tissue facilitates healing

Glaucoma

Figure 4. A diffuse iris melanoma in the right eye of a 14- year old neutered male domestic shot haired cat. The intraocular pressure was 45 mmHg and confirmed the diagnosis of secondary glaucoma.

Source: Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.

Figrue 5. The right eye fo a 12- year old spayed female domestic short hair cat. the eye had been treated for a chronic idiopathic uveitis for several ears and now has secondary glaucoma, buphthalmos, and a subuxated lens.

Source: Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.

Figure 6. Corneal edema associated with glaucoma in a Basset Hound.

Source: Carmichael K.P., Vygantas K.R. and Whitley R.D. (2010) Ophtalmic disease in veterinary medicine, 1st edn., London: CRC Press.

Figure 7. Advanced pannus or scaring in a Wirehaired Fox Terrier with absolute or longstanding glaucoma.

Source: Carmichael K.P., Vygantas K.R. and Whitley R.D. (2010) Ophtalmic disease in veterinary medicine, 1st edn., London: CRC Press.

Optic nerve and retinal pathology resulting loss of vision

Considered as neurodegenerative disease

If prolonged, chronic glaucoma, progressive vision loss and extensive loss of retinal ganglion cell bodies from the inner retina are observed
Causes

Intraoular pressure is the most significant factor
Can be misdiagnosed to ocular hypertension

Clinical signs

Ocular discomfort
Tears in Descemet's membrane
Globe enlargement
Pupil dilation
"Laminar dot signs"

Peri-papillary pigmented or hyper-reflective halo with increased prominence of the laminar pores

Dogs:

Fulminant pain
Dense and diffuse corneal edema

Cats:

Generally maintain normal appetite and normal activity level (no obvious signs exist)
Minor corneal changes

Treatment

Need to consider type of glaucoma (primary or secondary)
Anti-glaucoma drugs (lowers intraocular pressure)

Dorzolamide
Brinzolamide

Beta blocker

Timolol maleate

Adrenergic agonist
Prostaglandin analogs
Surgical management

Chorioretinitis

Figure 8: Chorioretinitis
Source: Pet Health and Nutrition Information (2013) Chorioretinitis, Available at:http://www.petmc.com/2013/04/Chorioretinitis.html (Accessed: 3rd June 2014).

Inflammation of choroid and retina
Also known as retinitis, choroiditis, retinochoroiditis, chorioretinitis,

Difference due to inflammation from the primary tissue and the direction of spread (retina and choroid)

Causes

Pathological processes localized in the eye (infections agents)
Systemic diseases

Blepharitis, conjunctivitis, keratitis and uveitis

Clinical signs

Vitreous abnormalities
Bleeding and tearing

In severe cases

Retinal detachment
Optic nerve modifications

Diagnosis through ophthalmoscope exam & clinical exams

Modifications in tapetal area more expressive

Retina, choroid, tapetum lucidum, blood vessels

Modifications in non-tapetal area

Blood vessels

Treatment

Usually directed as the systemic disease causing the inflammation
May heal by itself in 4-8 days

References

Andrew S.E., Tou S. and Brooks D.E. (2001) 'Corneoconjunctival transposition for the treatment of feline corneal sequestra: a retrospective study of 17 cases (1990–1998)',Veterinary Ophthalmology, 4(2), pp. 107-111.
Bruce H.G., Cheryl L.C. and Robert L.P. (2004) Veterinary ophthalmology essentials, 1st edn., Philadelphia: Elsevier.
Carmichael K.P., Vygantas K.R. and Whitley R.D. (2010) Ophtalmic disease in veterinary medicine, 1st edn., London: CRC Press.
Chahory S., Crasta M., Trio S. and Clerc B. (2004) 'Three cases of prolapse of the nictitans gland in cats', Veterinary Ophthalmology, 7(6), pp. 417-419.
Claudia Hartley (2010) 'Aetiology of Corneal Ulcers: Assume FHV-1 Unless Proven Otherwise', Journal of Feline Medicine and Surgery, 12(), pp. 24-35.
Donisa A., Muste A., Beteg F. and Muste M. (2010) 'Retinochoroiditis in dog: diagnosed and incidence', Veterinary Medicine Scientific Papers, 43(2), pp. 100-103.
Gimenez P.M.T.P. and Fariña I.M. (2002) 'Lamellar keratoplasty for the treatment of feline corneal sequestrum', Veterinary Ophthalmology, 1(2-3), pp. 163-166.
McLellan G.J. and Miller P.E (2011) 'Veterinary ophthalmology', Feline glaucoma – a comprehensive review, 14(1), pp. 15–29.
Mehdi M., Sepehr M., Omid, Kazemi H. and Elnaz (2012) 'Clinical evaluation of the pocket technique for replacement of prolapsed gland of the third eyelid in dogs',Turkish Journal of Veterinary and Animal Sciences, 36(4), pp. 352-356.
Pet Health and Nutrition Information (2013) Chorioretinitis, Available at:http://www.petmc.com/2013/04/Chorioretinitis.html (Accessed: 3rd June 2014).
Raza A., Naeem A., Ahmad M., Manzoor A. and Ijaz M. (2013) 'Cherry Eye: Prolapse of Third Eyelid Gland in Dog- A Case Report ', International Journal of Molecular Veterinary Research, 3(1), pp. 1-3.
Seitz W.R. and Stegmann H. (1981) 'A multifocal serous chorioretinitis in the beagle', Veterinary Pathology, 18(1), pp. 1-12

Pathology - Birds

Birds

Marek's disease

Figure 1: Left: Normal chicken eye; Right: diseased eye with grey iris and irregular pupil
Source: United States Department of Agriculture (2014) Cracking the Code of Marek's Disease, Available at: http://www.ars.usda.gov/is/AR/archive/jul01/marek0701.htm (Accessed: 3rd June 2014).

Figure 2: Chicken with leg paralysis.
Source: Gimeno I.M. (2008) 'Marek's disease vaccines: a solution for today but a worry for tomorrow?' Vaccines, 26, pp. C31-C41.

Highly cell-associated avian Alphaherpesvirus
Respiratory tract of birds = natural entry of virus

Inhaling of free virus in feather follicles in shed

Clinical signs:

Neurological clinical signs

Paralysis of legs, wings and neck

Lymphoid tumours in visceral organs

Heart, lungs, ovaries, testes, muscles

Tumours in feather follicles
Grey iris or irregular pupil
Nerve lesions leading to visual impairment

Some vaccines:

Serotype 3
Serotype 2
Attenuated serotype 1

Avian Pox

Figure 3: Sentinel domestic turkey with increasing severity of avian pox. Cutaneous lesions and tumours are observed on the eye.
Source: Thomas N.J., Hunter B. and Atkinson C.T. (2008) Infectious diseases of wild birds, 1st edn., USA: Blackwell Publishing Professional.

Skin disease
Transmitted directly between host individuals environmentally or mechanically by biting arthropods:

i.e. MOSQUITOS!

Susceptilibity of individuals determine transmission
Clinical signs:

Localized tumours in non feathered areas of skin
Lesions occur on beak, feet, claws
Begin as white /yellow foci and increase in size rapidly
Impair vision:

Tumours on eyelids
Conjunctival proliferation
Blepharitis

Sudden death
Sudden dyspnea
Upper respiratory tract disease
Pneumonia
Airsacculitis
Splenomegaly

Treatment:

Supportive therapy i.e. saline
Lesions heal in 4 weeks
Surgery to remove tumours may help
Vitamin supplementation
Immunostimulant
Anti-inflammatory drugs
Anti-microbial therapies

References

Gimeno I.M. (2008) 'Marek's disease vaccines: a solution for today but a worry for tomorrow?' Vaccines, 26, pp. C31-C41.
Mao W., Kim T. and Cheng H.H. (2013) 'Visualization of Marek’s disease virus in vitro using enhanced green fluorescent protein fused with US10', Virus genes, 47, pp. 181-183.
Raidal S.R. (1995) 'Viral skin diseases of birds', Seminars in Avian and Exotic Pet Medicine, 4(2), pp. 72-82.
Thomas N.J., Hunter B. and Atkinson C.T. (2008) Infectious diseases of wild birds, 1st edn., USA: Blackwell Publishing Professional.
United States Department of Agriculture (2014) Cracking the Code of Marek's Disease, Available at: http://www.ars.usda.gov/is/AR/archive/jul01/marek0701.htm (Accessed: 3rd June 2014).
Zylberberg M., Lee K.A., Klasing K.C. and Wikelski M. (2012), 'Increasing avian pox prevalence varies by species, and with immune function, in Galápagos finches', Biological Conservation, 153, pp. 72-79.