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Rats' oral health

CONTENT WARNING
This article contains graphic and potentially upsetting imagery

Introduction to rat oral anatomy


Rat mouths are built of almost all the same components as our human mouths. The lower and upper jaw, teeth, soft tissue, taste buds, salivary glands, etc. The main differences are the following;

TONSILS & UVULA

Unlike us, rats do not have a uvula or proper tonsils.

TEETH

While our human teeth consist of incisors, canines, premolars, and molars, our rats’ teeth only consist of incisors and molars. Our pet rats do not have canine or premolar teeth. Altogether, rats have 16 teeth, but most of us will never see more than four of the 16 teeth. Four out of their 16 teeth are the incisors (front teeth), and the remaining 12 are molars! You might ask, “Rats have molars?? but I’ve never seen them, not even when my rats yawn", this is because of a large diastema, aka a gap between the incisors and molars (see image 2).
Rats do not have two sets of teeth as we humans do. The teeth they are born with will have to last them their whole life. Their molar teeth are very similar to ours in function and structure, but their incisors are very different from our human teeth. Rats' incisors are open-rooted, which means they are always growing to keep up with the intensive gnawing.

 

LIPS

Just like us, rats have an upper and a lower lip, but unlike us, rats have a naturally cleft upper lip. A cleft upper lip refers to the split in the middle of the upper lip (see image 1). In humans, this is a defect that affects around 0.14% of the world's population, while in rats a cleft lip is how the upper lip should naturally form.

Look at the oral soft tissue.

Soft tissue aka the tongue, lips, cheeks, and gums are the main building blocks of the mouth and have many functions. Alimentation, respiration, and immune defence are just some of the functions of the oral soft tissues. The blood supply in the soft tissues is very intense, hence why the smallest injury to the mouth can bleed very excessively and look much scarier than it really is.

LIPS

In humans, the lips are a major player in communication and attraction, but in rats, there is no correlation between lips and physical attraction nor do they rely on lip shaping for complex vocalisation. In rats, the main function of the lips is mastication and sensation.

 

GUMS

Just like human gums, rat gums are a major part of the teeth’s supportive foundation and provide nourishment to healthy roots. The gums also function as a physical barrier between the roots and the mouth’s bacteria

TONGUE

Just like in humans, the tongue's role is to control the food's movement during mastication, to assist in swallowing and tasting. The tongue plays a significantly smaller part in vocal communication in rats compared to us humans.

 

CHEEKS

Just like our cheeks, rats’ cheek help to keep food in the mouth and aid in enzyme digestion by secretion of enzymes from the parotid gland. Muscles in the cheek region are a major player in the movement of the whole face.

Look at the muscular structure and biting

As you might already know, rats have an extremely strong and well-controlled bite. What gives rats such raw power in their bite and the ability to gnaw down extremely hard surfaces is the way their facial muscles are placed. The muscles responsible for moving the lower jaw are attached far forwards on the nose. These same muscles’ path goes through the eye socket behind the eyeball. This is mechanism is responsible for what many rat owners know as boggling while the animal bruxes.

When gnawing the rat’s lower jaw moves forward and allows the lower incisors to make contact with the upper incisors. Because of this movement and the length of the incisors, the molars do not make contact and are never used for gnawing, they are only used for chewing. Just like in humans, the lower jaw does all the hard work when it comes to the up-and-down movement of biting. Rats’ upper incisors hold food/objects against the teeth and the lower incisors move

up and down to gnaw through the objects/food.

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Look at the muscular structure and biting

As mentioned earlier, rats' dental structure consists of 16 different teeth, 12 of which are molars and four of which are incisors.

Rat molars are formed in a very similar manner to our human molars. The crown part is coated by an enamel layer, which is the protective layer made of the strongest material the body produces. Below the enamel lays the dentine layer, which is much softer. Below the dentine lays the dental pulp that holds access to the blood supply and nerves. Even though the layers of the molars are made of the same materials, rat molars look a little different from ours (see image 3) Compared to most other mammals, human molars are actually the weird ones. Rat molars look more in line with the rest of mammals. Molars are strictly used for chewing, they are never used for gnawing as they are so far back in the mouth. Rats only have one set of molars and they do NOT continuously grow like in some other rodents.

Rat incisors are very different from ours. Rats like other rodents have open rooted incisors to allow continuous growth of the teeth as they get quickly worn down due to intense gnawing. Only the front of the incisors is coated in enamel, this is what gives rat teeth their orange/yellow appearance. The enamel on rats’ teeth, both incisors, and molars, is stronger than the one we humans have. According to the Mohs hardness scale, human enamel is 5, while rat enamel is 5.5. To compare the hardness to some materials we are all familiar with, gold is 2.5-3, iron is 4, and diamonds are 10. This is what allows rats to gnaw on incredibly sturdy materials. By having the enamel on only one surface the constant gnawing wears the incisors down in a very well-designed manner that allows them to level and sharp. Under the enamel lays the softer dentine layer just like in the molar teeth. Under the dentine layer lays the dental pulp. While in the molar teeth, dentine makes up most of the teeth, in the incisors the dental pulp is large and makes up a mass majority of the teeth. The unique structure of the incisors is an intricate design fit for its purpose.

 

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The lower incisors are longer than the upper ones as many of you might have already observed in your pet rats. The rate at which our rats’ incisors grow is impressive .The upper incisors grow an average of 2.2mm in a week and the lower incisors grow an average of 2.8mm in a week. Due to this extremely rapid growth rate, malocclusion in rats becomes traumatic and dangerous very quickly and cannot be left unchecked. This rapid growth doesn’t only allow the teeth to keep up with intense wear and tear from gnawing, but the rapid growth rate also protects the incisors from cavities as these will form significantly slower than the new tooth grows, gets gnawed down, and grows again.

In the dental pulp, we can find the tooth's blood supply and nerves. Just like human teeth, rat teeth are ectodermal organs which means they are in fact alive. For this reason, the blood supply and nerves in the dental pulp are vital for the general health and well-being of oral health.

 

A single side of a lower jaw of an adult rat and a separated full-length incisor. See the length of the incisor compared to the full-length of the jaw bone. See a set of 3 lower molars present in the middle section of the jaw bone.


Same jaw and incisor from the previous image. Dislodged incisor was placed back into its original space to demonstrate how much of the incisor is not visible to us. The “root end” of the incisor reaches towards the back of the jaw past the last molar.



Unlike us, rats do have mobility control of some of their teeth. The lower incisors have a joint made of fibrous tissue between them. This is called mandibular symphysis. Humans too possess this, but in us, this area is fused unlike in rats. The unfused fibrous tissue allows each side of the jaw to have a small amount of mobility. With this mobility, each side of the jaw rotates along its axis own causing the visible separation of the lower incisor teeth. This separation of the lower incisors plays an important part in mastication in rats. When gnawing and biting, the lower incisors adjust their separation accordingly which allows for a strong and precise bite.

A single side of a lower jaw of an adult rat and a separated full-length incisor. See the length of the incisor compared to the full-length of the jaw bone. See a set of 3 lower molars present in the middle section of the jaw bone.


Same jaw and incisor from the previous image. Dislodged incisor was placed back into its original space to demonstrate how much of the incisor is not visible to us. The “root end” of the incisor reaches towards the back of the jaw past the last molar.

Unlike us, rats do have mobility control of some of their teeth. The lower incisors have a joint made of fibrous tissue between them. This is called mandibular symphysis. Humans too possess this, but in us, this area is fused unlike in rats. The unfused fibrous tissue allows each side of the jaw to have a small amount of mobility. With this mobility, each side of the jaw rotates along its axis own causing the visible separation of the lower incisor teeth. This separation of the lower incisors plays an important part in mastication in rats. When gnawing and biting, the lower incisors adjust their separation accordingly which allows for a strong and precise bite.

Oral health problems in rats

Broken teeth

Just like us, rats can break their teeth. In rats, broken teeth are mainly seen when it comes to the incisors. As they are long and only have one surface coated in enamel, they can break surprisingly easily when the animal sustains trauma to the face. Thankfully, as rats’ incisors continuously grow, most of the time broken incisors sort themselves out. While the tooth is growing back, it is important to keep an eye on the other incisors to make sure they are getting gnawed down appropriately. If you notice the other teeth growing too long or weirdly, get in contact with a vet as wrongly growing teeth can cause major trauma if left unchecked

Symptoms;
Uneven teeth
trouble eating

Malocclusion

Malocclusion refers to the misalignment of the teeth, aka what some humans may need braces for. In rats, even minor malocclusion can cause severe issues that can result in major trauma. As the incisors of a rat are constantly growing at a rapid rate, they have to be positioned correctly, otherwise, the rat animal won't be able to maintain a safe and healthy length for its incisors. If left unchecked, overgrown teeth can and will pierce through the soft tissue and poke through the cheeks, nose, or lower jaw.

Malocclusion can be caused by a few different things.
Genetics, Trauma, Growths and abscesses, or simply just bad luck.

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Genetic malocclusion will present itself in a certain lineage of rats on regular basis. Presenting regularly is what allows us to determine this type of malocclusion as a problem in that specific lineage of rats. Certain rat varieties are said to be associated with a higher chance of malocclusion, such as Marble. If malocclusion presents itself in a line to a point it may be genetic, a breeder should not ignore this, but try to resolve the issue, as rats cannot have a decent quality of life with constant dental problems throughout their life.

Malocclusion caused by trauma refers to the teeth’s positioning getting altered due to some kinda injury to the area. In most cases, this injury means some sort of blow, such as falling off of a platform or something and hitting their head. After even minor falls, it is important to keep a close eye on your rats’ teeth for the next few weeks in case some level of undesired shift has happened in the teeth that prevent the animal from gnawing them down properly.

Malocclusion caused by growths in the mouth and jaw forcefully pushing the teeth to the wrong position. As the growth gets larger, the malocclusion gets more and more severe. Not just growths, but something as “little” as an abscess near the mouth can cause just enough pressure on the teeth to slightly shift them. Facial growths and abscesses shouldn’t be left unchecked for this reason.

Unfortunately, sometimes malocclusion is just bad luck. Sometimes an anomaly is born in a litter and that rat's teeth didn’t develop in the right position.

What can be done to malocclusion? Unfortunately, the options are pretty scarce. In humans, we can opt for orthodontic treatment to correct occlusion, but this isn’t an option in rats. If the malocclusion is caused by something like trauma or an abscess, if the shift is very minor, once the animal has recovered there is a chance that the teeth will shift back in place, but this is often not the case.
If you do notice your rat's teeth being alarmingly long after something like this, a vet will have to trim them down. DO NOT do this at home yourself. Some places will tell you to do it at home with nail clippers or a Dremel. DO NOT trim your rat's teeth at home yourself. By trimming a tooth at home with improper tools, you are taking a very high and unnecessary risk of shattering the entire tooth through its whole length and this will easily cause non-vitality and complete loss of the said tooth. If after a trim and recovery from the incident, the teeth are still growing wrong, it may be time to consider humane euthanasia. It may seem like a harsh course of action, but a rat with permanent malocclusion cannot eat right and will have to see a vet for tooth trimming every two weeks or so.

For animals that were born with malocclusion that prevents them from correctly grinding their teeth down and maintaining a safe length, it is kinder to humanely euthanise them.

Symptoms;
Overgrown teeth
Teeth growing in odd directions
Uneven teeth
Trouble eating
wounds inside the mouth
Blood around and inside the mouth

Dental caries and cavities

Cavities in rats are mainly seen in the back molar as these do not continuously grow like the incisors. Just like in us humans, in rats, the S.mutans bacteria is the main cause of caries in the teeth. Studies also show that C.albicans, a naturally occurring fungus in the body is also capable of causing occlusal caries in rats’ molar teeth. The main cause of dental caries in rats is the same as us, free sugars in the diet. Free sugars refer to sugars that have been added to food and drink and the sugars that are already in honey, syrups, and fruit juices. These are called “free” sugars because they’re not inside the cells of the food we eat.

An example of free sugars is; “When fruit is turned into fruit juice, the sugars come out of their cells and become free sugars. The fibre is lost and it's easier to consume extra sugar without realising.” - The British Heart Foundation [BHF]

When the bacteria in the mouth metabolize sugars, it produces an acid that demineralizes the enamel temporarily, leaving the softer layers underneath vulnerable for the time being. As rats cannot brush their teeth, a dental biofilm known as plaque builds up easier on their teeth trapping sugars and acids against the teeth surfaces.
Each time something other than clean, plain water goes inside the mouth, this cycle of acid and demineralization begins leaving the teeth vulnerable.

Dental caries aren’t seen in the incisors in rats due to the extremely rapid growth rate. As dental caries cumulative aka it takes time to form, it is mainly seen in elderly rats’ back teeth.

Once caries penetrates the outermost layer (enamel) it progresses faster as the second layer (dentine) is softer and easier to break through. Once caries breaks into the last layer, aka the dental pulp, bacteria now have a clear path to access the vulnerable roots of the tooth

Symptoms:
Most rats will not present obvious symptoms until caries has reached the final layer [continue to read – Dental abscesses for more]

Trouble eating hard foods
Signs of pain and discomfort
Out-of-character refusal of sweet foods

 

Dental abscesses

The next step from severely caries is dental abscesses. Poor diet resulting in caries, trauma, or broken teeth are ways bacteria can infect a tooth. When bacteria enter the roots, they use the nerve tissue and blood vessels as a food source. They begin to multiply and spread and causing a dental abscess.

Most owners might immediately think antibiotics are the next step. Due to the anatomy of the teeth, bacteria become trapped in the roots. Without complex treatment, such as through root canal treatment which is not a viable option for rats, the infection will remain and potentially spread to the jaw or even the brain.
Why not antibiotics? There are several reasons antibiotics alone will not cure the infection. The blood vessels that once supplied the inside of the tooth with the body’s antibacterial defences have been destroyed. Therefore, the antibiotics cannot reach the inside of the tooth to cure the infection at its core. Antibiotics will simply calm down and control the infection temporarily, but once the remaining bacteria trapped in the roots multiply enough, the infection will flare up once again.

Unfortunately, in small animals like rats, the only hypothetical cure would be the extraction of the tooth with the infection. But due to the structure and relationship between the jaws and the positioning of rats’ back molars, this is not an ethical option.

The skull’s structure relies on the very long roots of the rat’s molar and incisor teeth. When a tooth is removed, the bone in that site reabsorbs and collapses as there is no tooth to hold on to. To remove a tooth from a rat is not the same as in people where they are numbed and the tooth is simply taken out. To remove a tooth from a rat, you are looking at a very invasive and traumatic procedure. To remove a tooth from a rat, a trained professional would have to cut through the animal’s face and destroy a part of the jaw. Once “healed”, the removal of molar teeth permanently changes the face’s anatomy and alters the way the remaining teeth can chew. As mentioned earlier when discussing malocclusion, even the smallest shift in how the teeth come together has very quick and devastating results on the animal’s quality of life. Alongside physical changes, chewing and gnawing is an important aspect of grooming, self-soothing and social behaviour in rats, therefore losing teeth interferes with normal day-to-day function from physical, psychological, and social.

Symptoms
Trouble eating
Trouble swallowing
Weight loss
Blood in the mouth
Discharge in the mouth
Lethargy
Signs of pain
swelling in the mouth and face
Bad smell from the mouth

Oral cancer

There are many different types of oral cancer. The type of cancer tells us what type of cell the cancer started from.
The majority of mouth and oropharyngeal cancer starts in squamous cells and is called squamous cell carcinomas aka SCC. Squamous cells can be found on flat skin-like cells that cover the inside of the mouth, nose, larynx, and throat,
Other types of oral cancer include

Salivary gland cancer
Basal cell carcinomas
Lymphoma
Melanoma
Sarcoma

Not all abnormal growths in the mouth are cancerous.

Salivary gland cancer
There are minor salivary glands all around the mouth and oropharynx. It is more common for abnormal growths in these to be benign aka non-cancerous, but this does not mean that cancer never forms in these. Most salivary gland cancers start in the parotid glands. In rats, this gland is located behind and below the ear. The other site where salivary gland tumours may be seen more than in the minor glands are the submandibular glands. These are located down toward the throat

Risk factors;
Previous cancers
Lineage History


Symptoms;
Swelling on one side of the face
Trouble swallowing
Limited mouth opening
Facial palsy


Basal cell carcinoma (BCC)
BCC is a type of skin cancer that develops from basal cells that can be found on the lips. This is the most common non-melanoma skin cancer

Lymphoma
Lymphomas are cancers that start from the cells found in the lymphatic system. As rats do not have tonsils, only lymph tissues can be found at the base of the tongue and in the neck.

Risk factors
More common in males than females
Weakened immune system
obesity
Lineage History

Symptoms
Sudden and rapid weight loss
Respiratory issues
Swelling in the limbs

Painless swelling of the lymph nodes is the most common sign of lymphoma, but swelling in this area can also occur when the body is actively fighting an infection.

Melanoma
Melanomas develop from pigment-producing cells that give skin tissue its colour. It is not a very common type of oral cancer. Melanomas of the head and neck can form anywhere in the skin or inside the nose or mouth.

Risk factors
Prolonged high exposure to UV
Sunburns
Lineage History
Weakened immune system

Sarcoma
Soft tissue sarcomas are cancers that can form in the connective and supportive tissue of the body. Sarcomas are not a very common type of oral cancer, it can still develop inside the mouth.

Risk factors
Lineage History
Lowered immune system
Inguinal hernias
Obesity

Symptoms
A painless lump that grows over time
Once the lump is large, the soreness of the lump
lump returns after it has been surgically removed

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