Zoonoses: infections you catch from your pets

While animals offer many health benefits to people, some pets can carry infections that humans can catch (zoonoses). A few simple care rules will help you avoid getting ill and ensure a happy and healthy relationship with your feathered, furred or scaly friend.

The list of infections is long, but we have focused here on the most common conditions:

• cat scratch disease
• psittacosis (from birds)
• toxocariasis (from cats or dogs)
• toxoplasmosis (from cats).

Cat scratch disease

While quite common, cat scratch disease is rarely serious and is usually limited to swollen lymph glands and mild flu-like illness. The infection occurs when a person acquires Bartonella henselae bacteria from their cat, often via a scratch or bite.

Many people do not remember being bitten or scratched by a cat when they develop symptoms of cat scratch disease. The bacteria that cause the infection are found in cats’ saliva and can be spread anywhere on their bodies by licking. Some people may acquire the infection simply by stroking their cat and then rubbing their eye, or by infected saliva getting into an existing cut.

Signs and symptoms of cat scratch disease may include:

• a cat bite or scratch that does not heal in the usual time
• a red area around the cat bite or scratch that continues to get bigger three days or more after the injury
• a sore that develops around the area of a cat bite or scratch three to 10 days after it has happened
• fever
• headache
• chills
• painful, swollen lymph glands near the site of a bite or scratch
• bone or joint pain
• abdominal pain
• unusual fatigue.

In most cases, cat scratch disease requires no treatment. In severely affected people, or those with lowered immunity, antibiotics may be used.

To prevent cat scratch disease it is a good idea to avoid teasing or provoking cats. Don’t let cats lick any open cuts that you have. Wash your hands after stroking a cat and wash any cat scratches or bites thoroughly with soap and running water.

Psittacosis

Psittacosis, also known as parrot fever or avian chlamydiosis, is an infection of the lungs. It is caused by the bacterium Chlamydia psittaci, which is found in the droppings and nasal discharge of many species of bird. Now relatively rare, the infection can cause anything from no symptoms to severe pneumonia and even death.

Psittacosis infection in humans usually occurs through breathing in the dust from the cage of an infected bird. The incubation of the disease is five to 14 days, after which symptoms may appear. These are generally flu-like, including:

• fever
• chills
• sweating
• weakness
• headache
• muscle pains
• sensitivity to light.

As with many infections, people with reduced immunity are more likely to develop severe symptoms and pneumonia.

As it is caused by a bacterium, psittacosis can be treated effectively with antibiotics.

Psittacosis in humans can be prevented by regularly cleaning out birds’ cages or aviaries. As an extra precaution it is a good idea to damp down the dust at the bottom of the cage, before disturbing it. This prevents the dust from becoming airborne. You may also wish to wear a face mask, particularly if you suspect your birds are infected.

Toxocariasis

Toxocariasis is infection with the dog or cat roundworm. It can be acquired through contact with soil contaminated with the infected faeces of dogs or cats. While in most cases symptoms can be mild or not present at all, in a few people they can be very serious.

Most commonly it is young children who are affected by toxocara infection, as they are more likely to play in areas where dogs have defecated and are less vigilant at washing their hands after playing and before eating. Puppies are common carriers, producing large numbers of eggs in their faeces.

Toxocariasis causes illness when the larvae of the Toxocara canis or Toxocara catis worm are ingested and travel around the body, trying to find the gut. They may cause severe inflammation in the organs they pass through. Depending on their route, they can cause:

• sight problems 
• inflammation of the body’s organs
• fever
• coughing
• pneumonia.

Toxocariasis is diagnosed using a blood test that looks for high levels of eosinophils (cells involved in killing parasites) in the blood and for antibodies to the toxocara larvae. Infection can be treated with anti-helminthics (medicines that kill worms). If the eyes are involved, the person may also need to take anti-inflammatory drugs to help prevent the eye being damaged by an immune response to the dying worms.

Good hygiene practices will prevent toxocariasis. Wear gloves when gardening and wash hands thoroughly when preparing food, after gardening or handling dogs, cats and their faeces.

Toxoplasmosis

Toxoplasmosis is contracted mainly through contact with cat faeces contaminated by the parasite Toxoplasma gondii. It can also be acquired by eating certain raw or undercooked meats that are infected by the organism. Toxoplasmosis most commonly affects young children, who are more likely to play in and eat soil that has been in contact with cat faeces. 

A paper in medical journal The Lancet  estimates that up to one third of the world population may carry a toxoplasma infection. Most healthy people show no symptoms. This is known as latent toxoplasmosis.

In acute toxoplasmosis symptoms are often flu-like: aches, pains and swollen lymph nodes that last for a month or more.

Severe toxoplasmosis damages the brain or eyes. It is more likely to affect young children and those with lowered immunity, such as people having chemotherapy or those with HIV/AIDS.

Acute toxoplasmosis in a pregnant woman can lead to infection of the baby in the womb and cause congenital toxoplasmosis. These babies can be severely affected with brain or eye damage. Many are born with no apparent symptoms but the damage from the infection may become apparent in later years. In a few cases toxoplasmosis causes stillbirth.

Toxoplasmosis can be treated with a combination of medicines. People whose immunity is suppressed long-term, may have to take the medicines for as long as they are immunocompromised.

Avoid toxoplasmosis infection by using good hygiene practices:

• cook meats well
• wash all food preparation equipment thoroughly
• feed your cat dried or canned foods; not raw meat
• wear gloves when gardening
• wash hands thoroughly when preparing food, after gardening, handling cats and their faeces
• change your cat’s litter box every day. Pregnant women should avoid this task altogether.

Pets are known to reduce stress and blood pressure and to play a part in improving fitness and mental health. In fact the advantages are so great that animals are increasingly being brought into hospitals to aid patients’ recovery.  Observe a few sensible hygiene practices and enjoy a long and healthy relationship with your animal companion.

Original material prepared by everybody. Reviewed by Dr Mark Thomas, Infectious Diseases, Auckland City Hospital. July 2009.