Animalia Life

The Breed History
In Mexico an indigenous dog, the Techichi was represented in historical records beginning in the 9th century and is considered the primary source of the modern Chihuahua's genes. Some historians believe the dog was crossed with a hairless breed from the Orient at some point. In the travels of Columbus, a similar small mute dog was noted in Cuba. This type of dog was highly treasured in Toltec and Aztec civilizations, so much so that they were often buried with their masters. It is unlikely that the origin of the breed was just in the Chihuahua area of Mexico, but since many of the foundation dogs that were exported in the mid 1800s came from this state, the name was selected. Longhaired varieties may have resulted from Yorkshire Terrier or Papillion crosses. First exports to the US occurred in the early 19th century, and AKC registration began in 1908.

Breeding for Function
The Aztecs used to sometimes sacrifice the red-colored dogs, and the blue ones were considered sacred. Primarily throughout the breed history though, these were highly valued companion dogs.

Physical Characteristics
Height at withers: 6-9" (15-23 cm).
Weight: Under 6 lb (2.5 kg). They are considered the smallest dog breed.
Coat: Two coat varieties exist: smooth and longhaired. Smooth haircoats are glossy, lay close, and are soft textured, and sometimes an undercoat is present. A bit of ruff is preferred. Long coats are soft, flat, and an undercoat is preferred. Some feathering is desirable. Any color is accepted whether solid, solid with markings, or splashed colors.
Longevity: 12-14 years.
Points of Conformation: Their build is compact and well muscled, and they are longer than tall, with fine bones. They have a domed skull, large wide-set dark eyes, though in light dogs, the eye color may match the coat. Domed skulls in some dogs leads to an open fontanelle termed molera. Ears are erect when alert, and folded back when resting. They are large and triangular. The muzzle is moderately short and tapers. The nose is black or self-colored, or pink in blonde dogs. The neck is short and the topline is level, ribs are well sprung and the thorax is moderate in depth. The tail is sickle shaped and rests out or up over the back (just touching). Limbs are straight, and feet are very small with toes set well apart.

Recognized Behavior Issues and Traits
Breed characteristics reported include: Loyal, alert, and very fast movers. They express many terrier qualities, and have high activity/ energy levels. They are noted for poor cold tolerance, and love close human contact and lots of attention. May be snappy with strangers or young children, and can be dominant and aggressive. Medium to high barking tendency, some are picky eaters, good for apartment or house-not for rural outdoors, and have low grooming and exercise needs.

Normal Physiologic Variations
Molera: An open fontanel in the skull (molera) is a frequent finding in the breed, and may not always be associated with hydrocephalus. Infantile Stress Hypoglycemia: Very young Chihuahuas can be vulnerable to stress hypoglycemia once weaned. Treat with oral dextrose and warming.
Merle Coat Color: Caused by a dominant mutation in the SILV gene. Breeding two merle dogs together should be avoided, as homozygous dogs can be born with multiple defects, including blindness, deafness, and heart anomalies.
Cesarean Section/Dystocia: A high percentage of Chihuahua litters show difficult whelping and are delivered by C-section. A study in the UK shows 34.4% of Chihuahua litters are delivered via C-section.

Drug Sensitivities
None reported

Inherited Diseases
Patella Luxation: Polygenically inherited laxity of patellar ligaments, causing luxation, lameness, and later degenerative joint disease. Treat surgically if causing clinical signs. Reported 8.9x odds ratio versus other breeds. OFA reports 5.5% affected. Dorn reports a 7.91x odds ratio in Chihuahuas versus other breeds. Reported at an increased fequency in the 2009 CCA Health Survey.
Hip Dysplasia and Legg-Calve Perthes Disease: Polygenically inherited traits causing degenerative hip joint disease and arthritis. Too few Chihuahuas have been screened by OFA to determine an accurate frequency. Reported 26.8x odds ratio for Legg-Calve- Perthes versus other breeds.
Elbow Dysplasia: Polygenically inherited trait causing elbow arthritis. Too few Chihuahuas have been screened by OFA to determine an accurate frequency.

Disease Predispositions
Hydrocephalus: Congenital increased volume of cerebrospinal fluid (CSF), with a concurrent dilation of the ventricular system and reduction of brain tissue. Can cause behavior changes, visual defects, impaired motor function, or seizures. Not all Chihuahuas with an open fontanel are hydrocephalic. Dorn reports a 62.63x odds ratio versus other breeds.
Dental Issues: A high frequency of undershot bites, overshot bites. and wry mouth are reported in the 2009 CCA Health Survey.3 Vitreous Degeneration: Liquefaction of the vitreous gel which may predispose to retinal detachment and blindness. Identified in 5.70% of Chihuahuas CERF examined by veterinary ophthalmologists between 2000-2005.
Collapsing Trachea: Caused by diminished integrity of the cartilage rings in the trachea. Can produce increased coughing, stridor, and respiratory distress. Dorn reports a 5.57x odds ratio versus other breeds.
Persistent Pupillary Membranes: Strands of fetal remnant connecting; iris to iris, cornea, lens, or involving sheets of tissue. The later three forms can impair vision, and dogs affected with these forms should not be bred. Identified in 5.06% of Chihuahuas CERF examined by veterinary ophthalmologists between 2000-2005.
Canine Pattern Baldness: Progressive alopecia developing at the post- and/or pre-auricular regions, along the ventral neck, thorax and abdomen, and on the caudomedial thighs. The hair loss starts around 6 months of age and gradually progresses over the following year, but remains restricted to the described areas.
Umbilical Hernia/Cleft Palate: Both of these congenital anomalies are reported at an increased frequency in the breed.
Cryptorchidism (Retained Testicles): Can be bilateral or unilateral. Chihuahuas have an increased prevalence of the disorder.
Cataracts: Anterior and posterior cortex intermediate cataracts predominate in the breed, although nuclear and capsular cataracts also occur. Identified in 4.43% of Chihuahuas CERF examined by veterinary ophthalmologists between 2000-2005. CERF does not recommend breeding any Chihuahua with a cataract.
Chronic Valvular Disease/Endocardiosis: Thickening of the heart valves with age, leading to congestive heart disease. Reported increased frequency in the breed. Dorn reports a 3.11x odds ratio versus other breeds.
Hypothyroidism: Inherited autoimmune thyroiditis. 2.6% positive for thyroid auto-antibodies based on testing at Michigan State University. (Ave. for all breeds is 7.5%).
Portosystemic Shunt (PSS, liver shunt): Congenital disorder, where abnormal blood vessels connecting the systemic and portal blood flow. Vessels can be intrahepatic or extrahepatic. Causes stunting, abnormal behavior and possible seizures. Tobias reports a 4.9x Odds Ratio versus other breeds. Undetermined mode of inheritance.
Progressive Retinal Atrophy (PRA): Inherited degeneration of the retina. Presumed autosomal recessive inheritance. 1.9% of Chihuahuas CERF examined by veterinary ophthalmologists between 2000-2005 are identified as affected, and 1.27% as suspicious for PRA.
Urinary Calculi: The breed is found to be at an increased risk of developing oxalate, and cystine bladder stones. Cystine calculi are secondary to a defect in cystine metabolism.
Corneal Dystrophy: Endothelial form is associated with progressive corneal edema, which can lead to bullous keratopathy and corneal erosions. Onset 6-13 years, with an average of 9.5 years. Treatment is symptomatic and palliative. Identified in 0.63% of Chihuahuas CERF examined by veterinary ophthalmologists between 2000-2005.
Pulmonic Stenosis: Inherited congenital malformation of the pulmonic valve. Causes stricture of the right ventricular outflow tract or stricture of the pulmonary artery. The Chihuahua is identified as a breed at increased risk. Undetermined mode of inheritance.
Acquired Myasthenia Gravis: Chihuahuas are a breed at increased risk of developing generalized or focal acquired myasthenia gravis. The most common presenting signs were generalized weakness, with or without megaesophagus. Diagnosis is by identifying serum acetylcholine receptor antibodies.
Necrotizing Meningoencephalitis: A non-suppurative acute to chronic necrotizing meningoencephalitis is identified in Chihuahuas, similar to that seen in the Pug and Maltese breeds. Genetic research suggests different genetic causes between breeds. Affected dogs present with seizures, blindness and mentation changes from 1 to 10 years of age.
Color Dilution Alopecia, Demodicosis, Factor VIII Deficiency, Glaucoma, Keratoconjunctivitis Sicca, Methemoglobin Reductase Deficiency, Myelodysplasia, Neuroaxonal Dystrophy, Osteochondritis Dessicans-Shoulder, Patent Ductus Arteriosus, Retained Primary Teeth, and Spina Bifida, are reported.

Isolated Case Studies
Primary Hypoparathyroidism: Two Chihuahuas were diagnosed in a 15 year study in Australia. Affected dogs presented with Seizures, muscle tremors and fasciculations due to hypocalcemia. Treatment with calcium supplementation.
Leiomyosarcoma: A 2 month-old female Chihuahua had a 3-week history of progressive disorientation, left-sided circling, and incessant whimpering. Necropsy revealed a primary leiomyosarcoma in the midline of the posterior thalamus with secondary obstructive hydrocephalus of the lateral ventricles.
Narcolepsy: A two-year-old male Chihuahua suffered paroxysmal attacks of muscle weakness and immobility elicited by stimulation, such as feeding. Low CSF levels of hypocretin provided a diagnosis of hypocretin-deficient narcolepsy. The dog responded to treatment with imipramine.
Ceroid-Lipofuscinosis: A two-year-old, female chihuahua presented with a six-month history of visual dysfunction. Histologically, swollen neurons possessing yellowish pigment granules in the cytoplasm were observed throughout the CNS. These storage materials stained positively for ceroid-lipofuscinosis. There was a concurrent hydrocephalus.
Esophagobronchial Fistula: A 10-month-old, intact male Chihuahua presented for a 7-month history of regurgitation and coughing. Survey radiographs revealed a soft-tissue opacity within the distal esophagus. A contrast study confirmed the presence of an esophagobronchial fistula.
Cardiac Fibrosarcoma: A 6-year-old Chihuahua dog presented with dyspnea, cyanosis, and pleural effusion. At necropsy, primary cardiac fibrosarcoma of the right atrium was diagnosed.
Absence Seizures with Myoclonus: An eight-month-old Chihuahua was reported with recurrent episodes of head and nose twitching lasting one to two seconds, multiple times per day. EEG confirmed epilepsy.

Genetic Tests
Tests of Genotype: Direct tests for color alleles and coat length are available from VetGen.
Tests of Phenotype: CHIC Certification: Required testing includes congenital cardiac evaluation, CERF eye examination (minimum 1 year of age), and patella evaluation (minimum 1 year of age). (See CHIC website; caninehealthinfo.org).
Additional recommended tests are hip and elbow radiographs, and thyroid profile including autoantibodies.

Miscellaneous
- Breed name synonyms: Teacup, Chi, Chihauhau
- Registries: AKC, UKC, CKC, KCGB (Kennel Club of Great Britain), ANKC (Australian National Kennel Club), NKC (National Kennel Club)
- AKC rank (year 2008): 12 (15,985 dogs registered)
- Internet resources: Chihuahua Club of America: chihuahuaclubofamerica.com/
British Chihuahua Club: the-british-chihuahua-club.org.uk