Genetic welfare problems of companion animals
Breed: Chihuahua (long and smooth coat)
Related terms: Hydrocephaly, primary hydrocephalus, ”Water on the brain”
Outline: Chihuahuas are predisposed to congenital hydrocephalus in which cerebrospinal fluid (fluid normally present in brain and spinal cord) builds up causing pressure on the brain. The skull may become abnormally large and the pressure can cause chronic pain and loss of brain function. The disease usually begins early in life and typically progresses. Treatment is difficult. It is not possible to determine which animals are likely to have affected offspring, but affected animals should not be bred from (and, ideally, those with affected relatives should not be bred from either). Predisposition to the disease may be associated with small size and, if so, it may prove to be difficult or impossible to eliminate the disease without changing conformation (body shape) or size.
Summary of Information
(for more information click on the links below)
1. Brief description
Hydrocephalus is characterised by the presence of an abnormally large volume of cerebrospinal fluid (CSF) present within the brain or between the brain and skull. In both forms, the pressure exerted by the CSF on the bones of the skull may result in the cranium becoming excessively domed in shape and to thinning of the skull bones. The disease can cause brain damage and dysfunction. Signs are often not present at birth but become apparent over the first few weeks, or months, of life. Affected dogs are often smaller than littermates, slower to learn, and can exhibit bizarre behaviours such as manic walking, repeated pressing of their heads against a wall, and to seizures (fits). It may also lead to blindness (due to brain damage). The progress of the disease is variable and unpredictable. Signs usually progress from mild to severe and then to death, but in some animals the disease may stabilise before this, meaning that they exhibit only minor signs that persist throughout life.
Treatment is difficult but can involve medication aimed at reducing fluid production and brain swelling, or surgery to implant a shunt to assist drainage of the fluid from the skull (eg into the abdominal cavity).
2. Intensity of welfare impact
Affected dogs are likely to suffer from pain, and probably also from nausea (feeling sick) and other forms of discomfort (mild to severe depending on the circumstances) as they are not able to function normally. The disease frequently leads to death.
3. Duration of welfare impact
This tends to be a relatively slowly developing condition and the duration of welfare problems is likely to be prolonged. Unless successfully treated, the problems are likely to persist and become progressively worse until the animal dies or is euthanased.
4. Number of animals affected
Chihuahua are said to be predisposed to hydrocephalus, along with other toy breeds, especially Maltese terriers and Yorkshire terriers (Selby et al 1979, Barker 1984, Skerritt 1989, Hudson et al 1990, Kornegay 1990, Fenner 1995). However, we are not aware of any published data on the prevalence of the disease. From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Chihuahuas (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 50,000.
The diagnosis of hydrocephalus is suspected in any young Chihuahua with signs of brain disease. Signs include a marked dome-shape to the cranium (the skull bones that encase the brain), unfused skull bones and ventrolateral strabismus (eye squint). Confirmation of the diagnosis requires ultrasound examination, magnetic resonance imaging (MRI) or computer tomography (CT) scans of the brain.
As far as we are aware, the genetic basis of the Chihuahua’s disposition for hydrocephalus has not been investigated. Other toy breeds are similarly prone to developing the disease and this suggests that the condition may be related to the selection for reduced body size in these affected breeds having also inadvertently caused incompatibilities between the dimensions of some internal organs and the skeleton; ie there are differences in the rate at which different bones and organs have reduced in size relative to each other.
7. How do you know if an animal is a carrier or likely to become affected?
It is not possible, at present, to determine which individuals may produce affected offspring. The best advice for conditions where the genetic basis of a condition is strongly suspected but unknown is not to breed from affected individuals and, generally, to avoid breeding from animals that have affected relatives.
8. Methods and prospects for elimination of the problem
It may be that characteristics of the Chihuahua breed such as its small size and domed head are linked to a predisposition to develop hydrocephalus. If so, eliminating the condition may be difficult or impossible without changing the conformation and size of the breed.
For further details about this condition, please click on the following:
- Clinical and pathological effects
- Intensity of welfare impact
- Duration of welfare impact
- Number of animals affected
- How do you know if an animal is a carrier or likely to become affected?
- Methods and prospects for elimination of the problem
Chihuahuas are predisposed to congenital hydrocephalus in which cerebrospinal fluid builds up in the skull of the dog causing pressure on the brain. Cerebrospinal fluid (CSF) circulates in cavities within and around the brain and spinal cord and provides a protective, cushioning effect to these organs as well as supplying them with nutrients.
There are four cavities (ventricles) within the brain where most of the CSF is produced. It flows from the larger, paired lateral ventricles into the smaller third and fourth ventricles and then to the spaces between the tissue of the brain and spinal cord and the bones that surround them (the skull and vertebrae). The connecting passages between the ventricles, and between these internal brain cavities and the outside of the brain and spinal cord, are all relatively narrow. Any excess of CSF is absorbed into the blood circulation at special blood vessel sites called arachnoidal villi, although absorption into capillaries throughout the brain and spinal cord also occurs (Dewey et al 2006).
Figure 1. This diagram of the human brain illustrates the location of the four ventricles/ chambers and is similar to that seen in the dog. Cerebrospinal fluid (CSF) is produced in, and circulates through, these chambers and in the space between the brain and skull (the subarachnoid space) and between the spinal cord and spinal canal.
(This figure, for which we are grateful, is from the Canadian Institutes for Health Research website at http://thebrain.mcgill.ca).
Hydrocephalusis characterised by the presence of an abnormally large volume of CSF within the skull. There are two forms of hydrocephalus, communicating and non-communicating, and the commonest form in dogs is congenital, non-communicating (primary) hydrocephalus (Thomas 2010). This is usually caused by a narrowing, or lack of connection, between the third and fourth ventricles of the brain (the Aquaduct of Sylvius) or the absence of arachnoidal villi (Averill 1978, Sahar et al 1971). An over-production of CSF has also been mentioned as a possible cause (Skerrit 1989, Dewey et al 2006).
Figure 2. A CT scan of a human patient showing ventricles excessively dilated with CSF (darkest areas on the scan). This occurs in both communicating and non- communicating forms of hydrocephalus (see text).
In a healthy brain, the ventricles are much smaller.
(Figure available from Wikipedia: http://en.wikipedia.org/wiki/Hydrocephalus )
The result of these abnormalities is a build-up of CSF within the brain causing it to swell. When this occurs in young individuals, whose skull bones are still growing and have yet to fuse together, this swelling causes the skull bones to be pushed further apart than usual so that the head enlarges. Further, this build up of fluid causes the tissue of the brain to become compressed between the fluid trapped inside it and the inner surface of the skull. In the second form of hydrocephalus (communicating hydrocephalus) the tissue of the brain is squeezed between excessive fluid, both from within the brain and without (due to a build-up of CSF between the brain and skull). In both forms, this increase in pressure can result in the cranium becoming excessively domed in shape and thinning of the skull bones.
Figure 3. In a young dog the build up of CSF can cause distension of the brain and skull resulting in a domed head shape, as shown by the severe case in figure 3.
(Image property of www.pethydro.org, to whom we are grateful for the permission to reproduce it here).
Obstruction to the flow of CSF, or loss of function of the arachnoid villi, can occur with some acquired problems, such as cancer or inflammation with swelling or scarring of tissue after trauma or infection. These lead to ‘secondary’ hydrocephalus.
It is possible that some cases of hydrocephalus may be related to events occurring around birth. Birth difficulties can lead to bleeding in or around the brain which can cause permanent malfunction of the arachnoid villi and ‘secondary’ hydrocephalus that is present from birth (Simpson 1989). It is not known how common these events are in Chihuahua.
Hydrocephalus can cause brain damage and dysfunction. Signs are often not present at birth but become apparent over the first few weeks or months of life. This is partly due to progressive fluid accumulation and compression of the brain, and partly due to loss of brain tissue and function. Detection of brain impairment becomes easier as the puppies grow. The canine brain normally takes months to mature and abnormal behavioural development may be the first sign to owners that significant disease due to hydrocephalus is present.
Affected dogs are often smaller than littermates. Typically, they are slower to learn and, for example, may not house train or may have behavioural problems. More severe signs are also often linked to damage to the cerebral hemispheres (the forebrain area that is involved with higher brain functions). Affected puppies can exhibit bizarre behaviours such as manic walking, repeated pressing of their heads against a wall, and to seizures (fits). It may also lead to blindness (due to brain damage). The progress of the disease is variable and unpredictable. Signs usually progress from mild to severe and then to death but in some animals the disease may stabilise before this, meaning that they exhibit only minor signs that persist throughout life.
Treatment is difficult but can involve medication aimed at reducing CSF production and brain swelling or surgery to implant a shunt to permanently aid draining away of excess fluid from the skull (eg into the abdominal cavity).
In brain diseases generally, there is not a close correlation between the volume of brain loss and degree of dysfunction observed and this is also true in hydrocephalus. The degree of hydrocephalus seen (for example on a MRI or CT brain scan) does not predict the extent of disability or the prognosis.
Hydrocephalus is known to be painful in varying degrees in humans and it may be expected that affected dogs experience headaches. In humans, hydrocephalus can also cause nausea (feeling sick/wanting to vomit) and distress as a result of not being able to function normally. It is therefore considered to be a serious welfare issue.
The disease tends to develop relatively slowly and the duration of welfare problems is likely to be prolonged. Unless successfully treated, the problems are likely to persist and become progressively worse until the animal dies or is euthanased. Some affected animals will have intermittent suffering that responds to treatment and some may spontaneously recover. Major surgery is required to treat the disease through implantation of a shunt to drain away excess fluid and this carries risk of painful infections. Such treatment can also fail in the medium term and surgery either has to be repeated or ends with the dog being euthanased (Axland 2004, Dewey et al 2006).
Chihuahuas are thought to be predisposed to hydrocephalus (Selby et al 1979), along with other toy breeds, especially Maltese terriers and Yorkshire terriers (Barker 1984, Skerritt 1989, Kornegay 1990, Fenner 1995). In a review of cases of hydrocephalus in dogs, Hudson et al (1990) observed that 6 out of the 19 cases diagnosed by ultrasound were found in Chihuahuas. However, as far as we are aware, there are no published data on the prevalence of the disease. From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Chihuahuas (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 50,000.
The diagnosis of congenital, non-communicating hydrocephalus is suspected in any young Chihuahua with signs of brain disease. Signs may include marked dome-shape to the cranium, unfused skull bones, and open fontanelles (Fontanelles are ‘soft spots’ on a puppy’s head that lack bone which, during birth, enable the bony plates of the skull to flex, allowing the puppy’s head to pass through the birth canal). However, open fontanelles may also be observed in Chihuahuas with no other signs of hydrocephalus. Another common sign of hydrocephalus is the so-called “setting-sun sign” or ventrolateral strabismus (in which the eyes look outwards and downwards rather than straight ahead; ie the opposite to being cross-eyed). Confirmation of the diagnosis requires ultrasound examination of the brain either through a fontanelle or through the closed skull bones, if they are thin enough to permit this (Hudson et al 1990). MRI and CT scans can also help to confirm the diagnosis. Radiography may reveal the thin skull bones typical of the disease, but this is a less useful technique.
As far as we are aware the genetic basis of the Chihuahua’s disposition for hydrocephalus has not been investigated.
The current breed standards for the two Chihuahua breeds, which differ only regarding their coasts, can be found at (http://www.the-british-chihuahua-club.org.uk/AboutUs/BreedStandardLC.php and http://www.the-british-chihuahua-club.org.uk/AboutUs/BreedStandardSC.php).
The Chihuahua head shape is more fully described elsewhere by the British Chihuahua club by Fitt-Savage (Savage http://www.the-british-chihuahua-club.org.uk/AboutUs/Skull.php). The head is:
“of most unusual shape. The skull of 'basic dog' is like an elongated cube, with a bony occipital peak and a slight bony ridge along the centre line of the skull, where the plates of the cranium knit together.The Chihuahua, as a legacy of the dwarfing of the breed, has a large rounded baby-like skull”.
Selby et al (1979) found that of 11 dog breeds predisposed to hydrocephalus, eight were toy breeds. They also found an inverse relationship between the likelihood of a puppy having hydrocephalus and the height of the mother at the shoulder. Within the toy breeds, smaller mothers were more likely to have offspring with hydrocephalus. This suggests that the condition may be related to the selection for reduced body size in the affected breeds having also inadvertently caused incompatibilities between the dimensions of some internal organs and the skeleton; ie there are differences in the rate at which different bones and organs have reduced in size relative to each other. The shape of head considered to be normal for Chihuahua (according to the breed standard) would be considered as suggestive for the presence of hydrocephalus if seen in other breeds.
It is not possible, at present, to determine which individuals may produce affected offspring. The best advice for conditions where the genetic basis of a condition is strongly suspected but unknown, is not to breed from affected individuals and, generally, to avoid breeding from animals that have affected relatives.
It may be that characteristics of the Chihuahua breed, such as its small size and domed head, are linked to a predisposition to develop hydrocephalus. If so, eliminating the condition will be difficult or impossible without changing the conformation or size of the breed.
UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section
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SutterNB, Bustamante CD, Chase K et al (2007) A Single IGF1 Allele Is a Major Determinant of Small Size in Dogs. Science 316: 112 – 115
Thomas TB (2010) Hydrocephalus in dogs and cats. Veterinary Clinics of North America Small Animal Practice 40 (1): 143-59
http://www.the-british-chihuahua-club.org.uk/AboutUs/BreedStandardLC.php and http://www.the-british-chihuahua-club.org.uk/AboutUs/BreedStandardSC.php accessed 1st October 2010
© UFAW 2011