Toxoplasma gondii Infection in Dogs
Toxoplasmosis infection is caused by a parasite called Toxoplasma gondii (T. gondii). It is one of the most common parasitic diseases, and is known to affect nearly all warm-blooded animals and humans.
Cats are recognized as the primary host, as the parasite completes its life cycle in the cat's intestinal tract, passing back into the environment through the feces. However, cats are not the only source of infection.
In the U.S., the main source for T. gondii transmission is raw meat and unwashed fruits and vegetables. Both acute and chronic forms of toxoplasmosis exist, where the chronic form is usually a low-grade disease without any clinical symptoms, and the acute form is more symptomatic.
Symptoms and Types
Cats are more commonly seen with clinical symptoms than dogs. Still, dogs can be made ill from this parasite, and may mimic other infections, such as canine distemper or rabies. At increased risk are young dogs with developing immune systems, and dogs that have impaired immunity. The following symptoms are known to occur in infected cats, and may also be seen in dogs:
- Neurological symptoms
- Uncoordinated gait
- Muscle weakness
- Partial or complete paralysis
- Respiratory problems like shortness of breath
- Weight loss
- Loss of appetite
- Abdominal Pain
- Inflammation of tonsils (tonsillitis)
- Inflammation of retina (retinitis)
- Inflammation of middle part of the eye including iris (uveitis)
- Inflammation of the cornea (keratitis)
Dogs become infected through contact with the T. gondii parasite, which may be acquired from rooting in infected soil or from ingesting cat feces.
You will need to give a detailed history of your dog's health, onset and nature of symptoms, and possible incidents that might have precipitated this condition, such as contact with cat feces, or prevalence of feral cats in the yard space. Your veterinarian will perform a thorough physical exam to evaluate your dog's body systems and to evaluate the overall health of your dog. Routine laboratory tests -- such as a complete blood count, biochemistry profile, and urinalysis -- are also used to confirm infection.
For example, dogs with toxoplasmosis may show an abnormally low number of white blood cells (leukopenia), low neutrophils (neutropenia), and low lymphocytes (lymphopenia) in the complete blood count.
Conversely, during recovery, the complete blood count may reveal an increased number of white blood cells, an indication of the increased activity of the infection fighting white blood cells.
The biochemistry profile usually reveals abnormally high levels liver enzymes ALT (alanine aminotransferase ) and AST (aspartate aminotransferase). Moreover, the level of albumin (the protein normally present in the blood) is also found to be at decreased levels in some dogs with toxoplasmosis; a medical condition known as hypoalbuminemia. In some rare cases, jaundice is seen with disturbed liver enzymes ALT and AST. The urinalysis may reveal abnormally high level of proteins and bilirubin in the urine sample.
Serological tests are the most reliable tests for making a definitive diagnosis. By measuring the levels of toxoplasma antigens in the body, your veterinarian can determine the type of infection, and whether it is active, dormant, recent (acute), or long term (chronic).
Serological tests will also help in determining the levels of antibodies IgM and IgG. Antibodies are proteins that are normally present in the body or produced in response to an antigen (in this case toxoplasma) for the purpose of neutralizing the antigen. Determination of antigen and antibody levels will help your veterinarian make the confirmatory diagnosis. The polymerase chain reaction test is a reliable test for verifying the presence of Toxoplasma gondii in samples.
More advanced diagnostic testing includes taking a collection of cerebrospinal fluid (CSF). Laboratory testing of CSF may reveal an abnormally high number of white blood cells (WBCs) and protein concentrations in patients with infection that has reached the central nervous system.
In case of severe disease, your dog may need to be hospitalized for emergency treatment. This is rarely the case with dogs, and is more likely only with dogs that have suppressed immune systems. Fluids can be given intravenously to dogs with poor hydration. Antibiotics may also be given to control the infection and prevent further progression of disease symptoms.
While cats are the best known transmitters for the T. gondii parasite, it is important to remember that the parasite is more frequently acquired through handling raw meat and eating unwashed fruits and vegetables. The best protection against this parasite, for you and your pet, is through prevention and hygiene. Do not feed raw meat to your dog, and do not allow your dog to have access to cat feces. That is, if you also have a cat in the home, keep the litter box in a location where the dog cannot gain access to it, as dogs are known to eat cat feces.
Other protective measures include covering outdoor sandboxes when not in use to prevent cats from using them as litter boxes, wearing gloves while gardening, washing hands after playing outside (particularly with children), wearing disposable gloves while changing litter boxes (and possibly a face mask as well, if pregnant or immune compromised), and keeping the litter box clean on a daily basis. The longer the infected feces remains in the litter box, the more likely the possibility that the eggs of the parasite will become viable and infectious. If at all possible, pregnant women should avoid cleaning litter boxes, as this parasite has been known to cause severe complications during pregnancy. If it is unavoidable, make sure that all precautions are taken to avoid contact through the respiratory tract (face mask, disposable gloves).
If you do also have a cat in your home, you may choose to have your cat tested for the T. gondii parasite, but the irony is that cats that test positive are less likely to be a threat of infectious transmission than cats that test negative, since cats that test positive are only testing positive for the antibodies to the parasite, meaning that they have already been previously infected and are now nearly immune to the infection; therefore, posing a much lower risk of being infectious. In fact, cats that have been infected with the T. gondii are generally immune to repeat infections for up to six years.
Conversely, if your cat tests negative for T. gondii antibodies, you will need to be that much more preventative in your approach to protecting your cat from infection, since they have no immunity to protect them from infection.
You can also have your dog tested for T. gondii antibodies, but the same rule applies generally. If your dog has antibodies in its bloodstream, then it has already been infected. Dogs are not regarded as a threat for transmission of this parasite.