Meningoencephalitis – a life-threatening disease with many causes

The brain is well protected against infectious agents by the meninges and the blood-brain barrier. However, if these barriers are not intact, pathogens can enter the brain from the bloodstream and cause inflammation.

Immune cells entering the brain cause swelling. A more permeable blood-brain barrier also leads to edema (accumulation of fluid in the tissue). Both cause pressure on the surrounding structures. In addition, pathogens themselves cause tissue destruction. Depending on which area of the brain is affected, this can lead to various functional disorders, some of which are life-threatening.

There are numerous pathogens that can cause encephalitis (inflammation of the brain), meningitis (inflammation of the meninges), or meningoencephalitis (inflammation of the brain and meninges). In addition to viruses, bacteria, parasites, fungi, algae, and prions, malfunctions of the immune system can also occur:

Viruses

• TBE (see corresponding article) in dogs
• Rabies (see corresponding article)
• Aujeszky's disease (see relevant article)
• Distemper (see relevant article) in dogs
• Feline infectious peritonitis (FIP) (see relevant article) in cats
• FIV-induced meningoencephalitis (see relevant article) in cats

Bacteria

• Lyme disease (see corresponding article)
• Bacterial meningoencephalitis can develop in the case of deep bite wounds to the head. Bacteria can also spread to the CNS (central nervous system) from a site of inflammation in the head. This can happen, for example, in the case of middle ear or sinus inflammation, as well as in the case of a tooth root or retrobulbar (behind the eyeball) abscess. However, they can also be carried to the CNS via the bloodstream from sites of inflammation further away in the body. These can include inflammation of the heart valves, the urinary and reproductive systems, or spleen abscesses.
• One form of listeriosis, cerebral listeriosis, is characterized by meningoencephalitis. However, dogs and cats only contract listeriosis in exceptional cases. Listeria bacteria are commonly found in the environment and in the feces of healthy animals. The main sources of infection are raw beef, pork, and vegetables. This should be taken into account when feeding your pet a raw diet (see related article). The pathogen is also transmitted to puppies via the placenta during pregnancy. If this does not lead to abortion, the puppies born may develop meningoencephalitis, among other things.

Parasites

• Toxoplasmosis (see relevant article)
• Leishmaniasis (see relevant article)
• Neosporosis causes tissue in the cerebellum to die. Neospora caninum is transmitted to dogs via raw meat from infected cattle. Dogs excrete the pathogen in their feces and infect cattle via feed contaminated with feces. The pathogen is transmitted to puppies or calves via the placenta during pregnancy. Symptoms usually appear within the first year of life.
• Encephalitis caused by worm larvae: Larvae from various nematodes sometimes migrate to internal organs and the central nervous system. This causes considerable damage and inflammation.

1. The raccoon roundworm is becoming increasingly widespread in Europe. In Germany, 50 to 75 percent of all raccoons are already infected with it. They transmit the worm eggs via their feces, soil, and water. Larvae that migrate to the brain continue to grow there.
2. The fox tapeworm is found in 35 to 65 percent of all foxes in Germany. It is also transmitted via eggs on the ground and in water. The larvae can also migrate to the brain.
3. The French heartworm is widespread in Europe. It is ingested via snails and settles in the pulmonary artery and the right heart. In rare cases, its larvae migrate to the brain.
4. Trichinella is transmitted through the consumption of raw meat containing trichinella from pigs, wild boars, or other wild animals and rodents. Trichinellosis is a notifiable disease. It is rare in Germany, as trichina testing is carried out as part of the legally required official meat inspection. However, it is more common in Spain, Bulgaria, and Romania. The infection is usually asymptomatic. However, the larvae migrate to the muscles and, in rare cases, to the central nervous system.
5. The esophageal worm is common in the southern United States and in tropical and subtropical countries. It is transmitted by ingested beetles and its larvae can also migrate to the brain.

• Trypanosomes cause sleeping sickness, which is transmitted by the tsetse fly in Africa, Asia, Central and South America, and Chagas disease, which is transmitted by predatory bugs. Both diseases are associated with encephalitis. The insects that transmit these diseases do not occur in Europe.

Fungal infections

Fungi thrive in warm, humid climates and can cause serious diseases of the entire organism and the CNS. Infection occurs via the nose and sinuses.

• Cryptococci are yeast fungi that can infect the CNS. Cats and dogs become infected by inhaling fungal spores from the feces of pigeons and other birds.
• Aspergillus is a mold that can cause encephalitis in individuals with weakened immune systems.

Algae

• The single-celled algae Prototheka is found worldwide and is ingested via water and feces. It is often found in animal feces without causing any symptoms. If illness occurs, it usually begins with bloody diarrhea. The algae can then spread throughout the body and cause encephalitis.

Prions

Prions are the body's own misfolded proteins (=protein particles) that cause fatal, sponge-like brain diseases with dementia. A spontaneous gene mutation (a change in the genetic information of a gene) causes a structural change in a protein produced by the body. Prions can also be transmitted through the consumption of beef infected with BSE (bovine spongiform encephalopathy, mad cow disease).

• Feline spongiform encephalopathy (FSE) occurs in cats.

Idiopathic (=cause unknown) diseases

In dogs, the most common inflammations of the CNS are of unknown cause. This leads to an overreaction of the immune system. This can be caused by a previous infection that has already been fought off. Another cause can be a toxin that can no longer be detected. Genetic predisposition often plays a role.

• Systemic lupus erythematosus (see article on autoimmune diseases)
• Granulomatous meningoencephalitis is characterized by small, nodular accumulations of immune cells in the CNS. It occurs in dogs worldwide.
• In necrotizing encephalitis, brain tissue dies in small clusters. It occurs in various small dog breeds. Pugs are genetically more susceptible to this disease.
• Sterile purulent meningitis-arteritis in dogs is accompanied by inflammation of the meninges and blood vessels. Beagles, boxers, Bernese mountain dogs, and Nova Scotia duck tolling retrievers are genetically predisposed to this disease.
• Eosinophilic meningoencephalitis rarely occurs in dogs. It is characterized by the appearance of so-called eosinophilic granulocytes (=specialized white blood cells). These are otherwise more common in cases of parasite infestation or allergies.
• Idiopathic cerebellitis is also known as Little White Shaker, as it was first found in small white dog breeds and is accompanied by pronounced, involuntary muscle tremors. However, it also occurs in dogs with other coat colors.
• Meningoencephalitis in greyhounds only affects young greyhounds.
• In idiopathic hypertrophic pachymeningitis, the dura mater is inflamed.

The following symptoms may occur with meningoencephalitis:

• High fever, loss of appetite, vomiting
• Change in personality, apathy, disorientation
• Head tilt, head shaking, scratching the head
• Neck pain (stiff gait, stiff posture), hypersensitivity to touch
• Movement and balance disorders
• Paralysis
• Seizures (see relevant article)
• Strabismus, different pupil sizes, eye tremors/involuntary eye movements

In bacterial infections, the general condition is usually very poor. Acute severe neurological symptoms occur. They are often unilateral, depending on which area of the CNS is affected.

After taking a detailed medical history, your veterinarian will perform a general and neurological examination. Meningitis and encephalitis can be detected using an MRI or CT scan (see relevant articles) of the head. In most cases, a contrast agent (a substance that improves the visibility of structures) is administered intravenously beforehand. The distribution pattern of the inflammation can provide initial clues as to the underlying cause. However, in about 25 to 30 percent of inflammatory diseases of the CNS, no abnormalities are visible on MRI and CT scans. Your veterinarian will then perform a lumbar puncture (removal of cerebrospinal fluid) to examine the fluid. This requires general anesthesia. In this way, inflammatory changes in the CNS and, in some cases, the causative pathogen itself can be detected.

Meningoencephalitis is a life-threatening emergency and requires immediate treatment. This depends on the pathogen or cause.

The prognosis depends heavily on the time of diagnosis, the cause, and the severity. If meningoencephalitis is detected early, the chances of recovery are often good. Severe cases with convulsions, forms caused by worm larvae, or immune-related forms are associated with a high mortality rate.

Prevention against meningoencephalitis primarily consists of protection against disease through vaccination, tick protection, and deworming.

Have your dog vaccinated regularly against rabies and distemper in accordance with the recommendations of the StiKo Vet (Standing Vaccination Commission for Veterinary Medicine). Outdoor cats, breeding cats, and show cats should be vaccinated regularly against FeLV (feline leukemia, see corresponding article). This protects them from FeLV-associated diseases that can lead to meningoencephalitis.

For dogs and cats, year-round tick prophylaxis is recommended to protect against tick-borne pathogens such as TBE or Lyme disease. It is also important to check their fur daily and remove any ticks quickly. Regular deworming with appropriate effective deworming agents protects against French heartworm. Your veterinarian will be happy to advise you on this.