Respiratory disease is a commonly diseased organ system in snakes in captivity.
Snakes develop both upper and lower airway diseases. The range of respiratory diseases seen in captivity can have many different causes, including bacterial, viral, parasitic, and fungal infections.
Factors that contribute to the expression of disease include:
- Captive husbandry
- Stress
- Cage spacing
- Genetics
- Physiological ecdysis or shedding
- Individual species characteristics
- Cleanliness of the habitat
This article is designed to explain both disease-causing pathogens and the environmental husbandry factors that often suppress the snake’s immune system and precipitate clinical disease.
Developing a working knowledge of the physiological and anatomical differences between mammalian and reptilian respiratory systems is an important concept that affects husbandry changes, prevention, and treatment.
Diagnosing respiratory dysfunction is done using standard techniques commonly used in veterinary medicine, including:
- Cultures
- Serology
- Radiology
- Parasite examinations
- Cytology
Diagnosing a potential underlying husbandry problem is often a foreign concept that veterinarians have received little training in.
For this reason, a review of the physics of thermoregulation will be discussed.
Cage design and other husbandry considerations will also be explained.
Lastly, treatment will be discussed, including conventional and unconventional routes of administration.
Snakes are a fairly large and diverse group of reptiles that occupy niches in trees, on land, underground, in fresh water, and in the ocean.
The shape of snakes varies little, but their size can vary from inches to 30 feet.
With the diversity of niches occupied and the size variability of snakes, the challenges of captivity are related to:
- Space
- Knowledge of natural history
- Knowledge of captive manipulation
- Current health status
- Past health status
Anatomy and Physiology
While the ecological niches occupied by snakes are diverse, the respiratory anatomy of commonly kept snakes is fairly consistent.
Snakes have an obvious, rostrally located glottis that fits into the choanal slit when the mouth is closed.
When the snake is eating, the mobile glottis can be positioned to maintain active respiration.
The trachea is lined with more primitive endothelium, with reduced-function cilia. It is “C-shaped” and ends at the level of the heart.
The endothelial lining of the trachea functions ineffectively as a mucociliary escalator.
As a result, snakes seem to use gravity and body positioning to clear mucus caused by infection from the lungs and trachea.
Certain snake species have a dorsally located tracheal lung, which may function in respiration when the lung and air sac are compressed after eating large meals.
Most snakes have only a functional right lung, which is attached dorsally to the body wall.
Boids have a vestigial but functional left lung.
The normal lung is composed of a cranial portion where gas exchange occurs across peripherally arranged simple alveoli.
The right lung in the Boidae and Colubridae begins just caudal to the heart and extends to the dorsal surface of the liver.
The remaining caudal and central portion of the lung functions as an avascular air sac. In most snakes, this ends as a blind pouch as it approaches the stomach.
Snakes have no diaphragm, which prevents effective coughing.
Respiration is controlled by groups of muscles attached to the ribs.
Inspiration is controlled by muscle groups that expand the ribs, resulting in lower intrapulmonary pressure.
Air is pulled in from the atmosphere, increasing intrapulmonary pressure until the air sacs no longer expand.
The passive expiratory phase begins when the inspiratory muscles relax.
Relatively high intrapulmonary pressure and the natural recoil of the lungs, coupled with an open glottis, allow air to flow passively from the lungs.
The stimulus for reptiles to take a breath is a low partial pressure of oxygen, or pO2.
This differs from mammals, in which a high partial pressure of carbon dioxide, or pCO2, stimulates respiration.
The clinical significance of low oxygen pressure stimulating spontaneous respiration becomes apparent when snakes are anesthetized.
If too much positive-pressure ventilation is administered during surgical procedures, the partial pressure of oxygen may become high.
The patient may also be over-anesthetized.
Both conditions can result in prolonged recovery.
Temperature control is also important during anesthesia.
Snakes need to be kept within their preferred optimal temperature range while anesthetized and during recovery.
If the temperature drops during anesthesia or recovery, the demand for oxygen decreases.
Body metabolism, including the metabolism of the anesthetic drugs, also decreases, resulting in slower recovery and delayed spontaneous respiration.
If the temperature increases during anesthesia or recovery, oxygen consumption and drug metabolism increase, resulting in deeper anesthesia and prolonged recovery.
Reptiles are well known for their ability to tolerate anoxia and hypoxia.
The clinical significance of this fact is that, despite major pulmonary disease, they may continue to function in a somewhat normal physiological state.
It is unclear how much anaerobic metabolism truly occurs during hypoxic periods.
It is clear that during periods of hypoxemia, some anaerobic metabolism and hypoxic depression occur, resulting in lower oxygen demand.
This study was performed on the heart of Chrysemys scripta turtles and may be species-specific. It may not necessarily apply to snakes.
Severe pneumonia is compensated for both behaviorally and physiologically.
An inactive snake may refuse to eat and move very little.
Arching the back at the level of the lungs or resting the head and upper one-fourth of the body in a vertical position are common ways snakes keep purulent discharge away from the active respiratory surface.
Yawning is frequently observed, sometimes with productive mucus.
Cage Requirements
In captivity, cage design and available space directly affect the health of the snake.
The amount of space required depends on the snake’s genetic potential.
As a rule of thumb, captive caging for both juvenile and adult snakes should provide, at the very minimum, enough space for the snake to stretch out completely.
This requirement is important because the lung and air sac must be able to completely exchange air during a single respiratory cycle.
Cage width is not usually a problem for most snakes.
Ideally, the enclosure should be large enough to allow exercise.
Cage size affects several other husbandry factors, including:
- Thermoregulation
- Humidity
- Required sanitation
Thermoregulation
Understanding thermoregulation is central to the health and life of a snake.
Being ectothermic, or “cold-blooded,” has disadvantages, especially when the reptile relies on its owner and veterinarian to provide the correct environment.
Basic knowledge of each snake species should include:
- Continent of origin
- Climate of origin
- Period of activity
- Habitat niche
- Primary food source
More detailed knowledge about the snake’s microhabitat is also useful, including:
- Seasonal high and low temperatures
- Feeding periods
- Reproductive periods
When very little is known about a particular snake, it is essential to examine the form and function of its anatomy to make an educated estimate of the type of environment from which it comes.
For example, large snakes are generally not from the desert, and snakes with eyes and nostrils located dorsally on the head are primarily aquatic.
Consulting internet references and other text resources is also recommended for unfamiliar species.
Husbandry is a species-specific process, and knowledge develops over time.
Educating clients about this concept can benefit their other reptilian pets.
Problems can be associated with cages that are either too large or too small.
The basic goal of captive thermoregulation is to allow the reptile to control its own temperature according to its biological needs.
This is achieved through spacious caging with established microenvironments.
Cages should have:
- A dry, hot area
- A warm, humid area
- A cool, dry area
- A cool, damp area
The size and extent of these areas depend on the species.
How these areas and environments are created is up to the pet owner, with guidance from professionals such as veterinarians and experienced herpetoculturists.
House-call veterinarians may provide the best information for captive environmental manipulation.
Relatively small cages with moderate heat sources will raise temperatures throughout the enclosure.
This causes the loss of a thermal gradient, meaning the temperature remains constant throughout the cage and the reptile cannot cool down.
Loss of thermal gradients causes reptiles to drink more in order to maintain hydration.
Evaporation also occurs more quickly in small cages, which worsens dehydration and its effects.
Chronic dehydration can lead to:
- Renal disease
- Respiratory disease
- Stress
- Worsening of subclinical disease
Another potential problem associated with smaller caging is the relative increase in organic load.
The larger the snake, the more metabolic waste it produces.
More sanitation is therefore necessary to prevent the reptile from being exposed to increased organic waste.
Large cages cause fewer problems with overheating and more problems with underheating.
Large cages have significant advantages for larger snakes because thermoregulation can be achieved through temperature gradients.
A combination of heat sources must be used to achieve efficient thermoregulation.
Heat lamps, heat emitters, and room temperature can be used to provide a range of temperatures that supports the health of most large snake species.
By checking surface and air temperatures within the cage and adjusting the distance between the reptile and the light, temperature can be controlled efficiently.
Having a large enough space allows the reptile to behaviorally thermoregulate.
Heat rocks, heat tape, and heat lamps cannot be used efficiently by themselves.
Combinations of different heat sources may need to be used to achieve sufficient temperatures.
A thermometer should be placed where heat is being achieved and preferably at the coolest place in the cage.
A large range between the high and low temperatures is desirable.
The range should include the Preferred Optimal Temperature Zone, or POTZ.
Humidity
Humidity affects thermoregulation and is critical to the respiratory health of large tropical snakes.
High humidity holds heat.
Dryness allows heat to escape.
This concept is best illustrated by observing weather patterns in the tropics.
Cloud cover protects the land from evaporation.
Evaporation is a cooling process.
Less evaporation means there is more moisture in the air, and therefore humidity is higher.
The water in humid air has a high specific heat, which allows heat to accumulate in collective molecules.
Therefore, less temperature change occurs when nightfall comes because humidity maintains heat in the water molecules.
Deserts are relatively dry and cloudless, with little rainfall.
As the sun heats the earth, evaporation occurs.
Water leaves plant life and turns it brown.
As evaporation occurs, temperatures drop significantly.
Temperature variability is greater because water has evaporated and is no longer holding heat in its molecules.
For example, temperatures may reach 110 degrees Fahrenheit during the day and 70 degrees at night in the Sonoran Desert.
In tropical rainforests, temperatures rarely drop below 75 degrees Fahrenheit and, where large snakes live, rarely rise above 90 degrees.
Dry cage environments also experience drops in temperature when the heat source is turned off at night.
This temperature drop can suppress the immune system, especially in captive environments inside homes during winter.
When tropical snakes are kept in a dry environment, insensible water loss is greater.
Respiratory secretions become thick and viscous.
Normal secretions may then accumulate in the respiratory system.
As mucus accumulates, it acts as a medium for potential infections or acute obstructions.
Cage Manipulation
Captive cage manipulation is necessary to simulate the natural environment.
The goal is to create multiple microenvironments within the cage.
This is best achieved by separating heat and water.
The warm, dry area should be confined to a large section that is separated from the cool area of the cage.
The cool area should contain water and an appropriate substrate that either holds moisture or maintains dryness, depending on the needs of the species.
If a glass aquarium is used to house a snake, several inexpensive steps can be taken to increase humidity and maintain a more constant temperature.
Screen tops can be partially covered with plastic wrap or Plexiglas.
Glass or Plexiglas can be used inside the cage to contain heat on one side.
The water may be placed over a heating pad to increase humidity.
Moisture-holding substrates, such as cypress mulch, may also be used to maintain humidity and heat.
Plastic storage boxes can also be used with high-moisture substrates such as sphagnum moss.
A more expensive and effective method of maintaining humidity is the use of humidifiers.
This method also requires appropriate cage ventilation.
Hygrometers are very effective tools for measuring cage humidity.
Cage perching is also important, even for non-arboreal snakes.
Perching areas allow larger snakes to use gravity to help expel purulent discharge or mucus from the air sacs.
Ventilation and oxygenation of the cage are essential.
Both heat and humidity escape as ventilation increases.
Experience and studies in herpetoculture and reptile keeping will help balance temperature, humidity, and ventilation.
Captive management is an oversimplification of the complexity of the natural environment.
Sanitation
More infections occur in snakes kept in cages that are difficult to clean.
A common type of cage that is difficult to clean is painted plywood.
As mentioned previously, the size of the snake relative to the cage determines the amount of organic waste the animal may produce and contact.
Many owners of Boidae have the uncontrollable urge to power-feed their snakes.
These fast-growing, power-fed snakes produce large amounts of stool.
This makes effective cleaning essential for long-term health.
Most boids live only 5–7 years in captivity as a result of relatively small cages with high organic loads that eventually lead to immunosuppression and opportunistic infections.
Larger snakes, particularly boids, are especially prone to pneumonia.
Cleansing agents recommended by the author include:
- Warm, soapy water
- Bleach
- Chlorhexidine scrub
- Quaternary ammonium compounds
Thorough rinsing and drying are important after using any cleansing agent.
Differential Diagnoses and Treatment for Respiratory Dysfunction
Clinical signs associated with respiratory dysfunction may include:
- Anorexia
- Lethargy
- Weight loss
- Vomiting
- Diarrhea
- Skin opacity
- Dysecdysis
- Rostral abrasion
- Stomatitis
- Yawning
- Cyanotic mucous membranes
- Discharge from the glottis
- Exaggerated breathing
- Upper-body positioning in the cage
- Lung elevation
- Dyspnea
- Head shaking
- Abortion
- Egg retention
- Death
Feeding, stooling, and shedding records are an excellent way to identify possible subclinical respiratory infection.
Anorexia, constipation, diarrhea, and dysecdysis are physical signs that careful keepers can monitor.
Infectious causes of respiratory problems may be bacterial, viral, fungal, or parasitic.
Bacterial pneumonia is by far the most commonly diagnosed.
This statement may be influenced by the fact that bacterial cultures are commonly performed as the first diagnostic step.
Secondary bacterial infections may be mistaken for primary infections.
It is the author’s opinion that viral, fungal, and parasitic infections are underdiagnosed.
Bacterial pathogens of the respiratory tract are primarily gram-negative aerobes.
Common isolates in the author’s practice include:
- Pseudomonas species
- Klebsiella species
- Aeromonas species
- E. coli
- Proteus
Atypical bacteria are occasionally found, including:
- Mycoplasma species
- Chlamydia species
- Mycobacteriosis
Performing culture and sensitivity testing on snakes suspected of having bacterial disease is important for both diagnosis and treatment.
The best samples for culture are obtained through a transtracheal wash.
The mouth-opening device preferred in the author’s practice is a cookie spatula.
Using sterile technique, an appropriately sized red rubber catheter is inserted into the trachea.
A saline-filled syringe containing up to 1% of the snake’s body weight in fluid volume is connected to the catheter.
The saline is introduced into the snake.
The snake is then rolled up and down briefly.
The fluid is re-aspirated.
It is common to recover only a small amount of fluid from this procedure.
Cultures and cytology should be performed on the fluid.
Antimicrobial therapy should be started immediately and changed if sensitivity testing indicates that it is necessary.
Because most respiratory diseases are chronic by the time the snake is presented, the author’s treatment for bacterial pneumonia is very aggressive and continues for at least three weeks.
The preference is to use the same technique as the transtracheal wash.
Instead of using a red rubber catheter, a sterile intravenous catheter is used.
The normal injectable antibiotic dose is mixed in the syringe with saline and introduced into the snake while it is held vertically.
Viral Causes
Viral causes of respiratory disease are underdiagnosed because few tests have been developed to identify them.
Many virally infected snakes develop secondary bacterial pneumonia.
The secondary pneumonia may be diagnosed through culture or treated with antibiotics.
Inclusion Body Disease
Inclusion Body Disease of Boidae is caused by a retrovirus.
It is currently thought to be transmitted through bites from snake mites, Ophionyssus natricus.
The author also suspects vertical, or transovarial, transmission.
The virus causes many signs and symptoms, including:
- Anorexia
- Intermittent or chronic vomiting
- Diarrhea
- Decreased movement
- Respiratory disease
- Neurologic disease
The clinical signs vary and probably depend on the amount of virus present and the location of the infection.
Secondary bacterial infections commonly occur in the respiratory tract, bones, and gastrointestinal system.
Diarrhea and anorexia are usually due to enteritis caused by gram-negative bacteria, anaerobes, or protozoal overgrowth.
Neurologic signs are variable.
Signs may range from slight paresis to the classical “star-gazing” posture.
Several snakes observed by the author developed severe spondylosing osteomyelitis similar to what Frye calls Paget’s disease.
There is currently no serologic test available for virus detection because funding has been limited.
Current antemortem diagnosis is performed using a combination of methods, including:
- Complete blood counts
- Endoscopy
- Biopsies
White esophageal plaques are sometimes present in snakes that test positive for viral inclusions.
A confirmatory biopsy should be performed to identify inclusions.
A laparotomy is necessary to perform liver and kidney biopsies.
Eosinophilic intracytoplasmic inclusions are evident in most positively infected animals.
This virus is certainly underdiagnosed because generalized immune suppression tends to cause further manifestations of more common problems, including respiratory disease and enteritis.
Inclusion Body Disease of Boids is usually not evident until neurologic signs develop.
Paramyxovirus
Paramyxovirus is a virus that targets the respiratory system.
Central neurologic disease occasionally accompanies clinical respiratory disease.
Clinical signs may be subclinical and nonspecific or may include:
- Flaccidity of the body
- Open-mouthed breathing
- Tracheal or pharyngeal hemorrhage with mucus
- Seizures or convulsions
- Acute death
Cases involving a fer-de-lance involved four stages during an outbreak.
Stage 1
Stage 1 lasted 5–12 days and was characterized by loss of muscle tone and linear body positioning.
Stage 2
Stage 2 was characterized by restlessness in the cage with partially open mouths and lasted 1–2 days.
The eyes were dilated, and the tongues were incompletely withdrawn into their sheaths.
Stage 3
Stage 3 lasted several hours to one day and was marked by complete mouth opening and purulent glottal discharge.
Stage 4
Stage 4 occurred from one minute to one hour before death.
Signs included excessive activity, dilated pupils, and a wide-open mouth.
Paramyxovirus is primarily found in viperid snakes.
Titers and clinical disease are seen less commonly, but increasingly, in boids.
Inclusion Body Disease of Boids should be considered as a primary differential diagnosis.
Severe lower respiratory disease, including mucus and secondary bacterial invaders, may be present.
