Zoonotic Diseases

Rebecca Dablow, CVT
Associate Veterinary Information Specialist 

The benefits of having animals in our lives is great and the list long. However, for this blog post I would like to talk about one of the potentially serious drawbacks of co-existing with animals; zoonotic diseases are those which can be passed from animals to humans.  These can range from mild to severe cases and include viral, bacterial, and parasitic infestation. There is much to be aware of as far as prevention and therapy, as we have come a long way in both veterinary and human medicine in effort to learn about this subject.  Here are just a few of my favorite examples of zoonoses and the dangers it can present to humans.

Boy With Pet King Charles Spaniel Puppy

Rabies:  Caused by a virus, rabies settles itself in the brain and spinal cord of the affected mammal and results in inflammation. Infected animals will display behavioral changes such as agitation, nervousness, fear, aggression, and sometimes even excessive affection or uncharacteristic docile behaviors. As this disease progresses to a paralytic state, we can see disorientation and seizure activity.  Loss of muscle control–which includes the throat and jaw– leads to excessive salivation or “foaming at the mouth”–a classic term used for a rabid animal.  Our domestic pets and wildlife can transmit the virus each other and to people via saliva, and typically through a bite.  Initially, humans often experience a burning or irritation at the site of the bite along with flu-like symptoms early in the infection. We then see CNS progression which includes confusion, agitation, hallucinations, insomnia. Rabies is treated symptomatically and supportively.  Once the CNS signs are present, it is very rare to survive the disease.  When caught early and treated promptly with postexposure vaccines and wound care management, we can often avoid serious outcome.  If animals are suspected rabid, they are often quarantined under observation, and/or euthanized.  Testing for Rabies requires brain tissue samples and must be performed during a necropsy/autopsy. Vaccination is extremely important in control of this disease.  All pets in the US are required by law to keep a current vaccine status.

Leptospirosis: This infection is caused by the bacteria Leptospira.  It is excreted and spread via urine, which can leach into soil or standing water, and is typically how a domestic animal may come into contact with it initially. Pets may or may not show an array of symptoms such as lethargy, fever, vomiting/diarrhea, anorexia, weakness, and joint pain.  Alternatively, they can simply carry the bacteria, quietly spreading it through the urine for months after exposure. Humans often obtain it by inadvertently handling their dog’s urine or drinking contaminated water.  It can absorb across the skin and mucous membranes but is most often attained via ingestion. Lepto in humans will present with common aches, pains, fever, chills, vomiting or diarrhea.  These initial signs may be self-limiting and require no therapy.  However, severe cases of Lepto can develop into damage/failure of the kidneys and liver, and meningitis has also been reported. Diagnosis is typically sought via ELISA or serology testing.  It can be difficult to diagnose based on signs alone, as it often presents with a differing and individual symptom onset.  It is treated symptomatically and supportively and with the use of antibiotics. There is a vaccine available for Leptospirosis in dogs.  This is an elective vaccine, often recommended with hunting dogs or those who live in highly wooded areas where the urine of wildlife may be present in standing puddles or via direct exposure.  It is often given as a Lepto/Lyme combination vaccine. There is current research being done for a human vaccine, however not available at this time.

Bovine Spongiform Encepholopathy:  More familiar as “Mad Cow Disease”, this zoonotic illness is especially interesting as humans obtain the infection via proin transmission.  Proins are naturally occurring proteins found on cell surfaces in the body.  In the case of BSE, these proins mutate within the nervous tissue–destroying brain tissue.  The affected cow may obtain the disease initially via feed or genetic mutation. BSE causes a progressive neurological disease in adult cattle.  The incubation period is long–4-6 years from the time of exposure to the development of symptoms!  The affected cow begins to lose muscle coordination and may display agitation or violent behavior.  When humans ingest the beef of diseased cows, they may also be exposed to the proin, resulting in a condition known as variant Creutzfeldt-Jakob disease (vCJD).  This condition is not unlike BSE in that the proins cause changes in the nervous system, resulting in memory loss, incoordination and death within about one year or so. This disease is still in its early stages of research.  Currently, there is no vaccine or known treatments to stop its progression.  Proins are extremely resistant to heat and irradiation.  Luckily, it is actually quite rare despite all the headlines made in the early 2000s.  All cases have been confirmed as originating in the UK.  The FDA has implemented many meat inspection protocols in high risk specimens and these animals are not used for beef.

My story: During the summer of 2016, my 9-year-old niece began to feel ill.  She experienced swelling of the lymph nodes and fever and was evaluated by her primary MD.  Thought to be a viral bug, she was provided with some steroid therapy.  However, within a few days, all of the joints in her body became very swollen and were in extreme pain.  She also started experiencing a skin rash on the bottom of her feet and palms of the hands. She was then admitted to Children’s Hospital.  The pain became more and more severe.  It seemed especially significant in her left shoulder.  After an ultrasound revealed bony changes in the shoulder, it was recommended to obtain and evaluate a joint fluid sample.  In the end, she was diagnosed with a Streptobacillus moniliformis infection, AKA “Rat Bite Fever”.  She had several pet rats over the years without issue. Rodents carry these bacteria asymptomatically and transmission occurs via handling of the animal, urine contact, bite, or scratch.  After diagnosis was made, she was treated with the specific antibiotics necessary.  The pet rats in the home had to be euthanized as they were now known to carry the bacterium.  She did need some physical therapy for the affected shoulder joint but has made a complete recovery.