What is cat scratch fever?
Cat scratch fever, more correctly known as cat scratch disease (or catscratch disease), is an infection contracted through a scratch or bite from a cat. It is a bacterial infection caused by Bartonella henselae that tends to affect the lymph nodes in the area of the scratch or bite. Understandably children are more often affected as they tend to have closer contact with pets. Often the scratch or bite that leads to the infection is from kittens rather than adult cats. Although the infection can resolve on its own within a few months, there can be severe complications in people with a weakened immune system.
How common is cat scratch fever?
The exact incidence of cat scratch disease is unclear but it is estimated to affect about 10 in 100,000 people annually. Children under the age of 18 years are more frequently affected. The incidence largely varies based on a person’s contact with cats and kittens in particular. Understandably cat owners and workers in contact with domestic cats are at a greater risk and therefore the incidence would be higher in these groups.
Bartonella Henselae Infection
There are many different Bartonella species of bacteria. Bartonella henselae is responsible for cat scratch fever. Once the Bartonella henselae bacteria gains entry into the body, it causes enlargement of the lymph node near the site of the inoculation. It is the proximal most lymph node that is first affected – this means the lymph node that lies first in the direction of lymph flow from the point of entrance of the bacteria. The presence of the bacteria in the lymph node activates white blood cells within it and triggers an aggregation of more white blood cells. It is these white blood cells that fight off the infection.
Lymph Node Abscess
In the process, pus collects and the body attempts to wall off the infection leading to the formation of abscesses and granulomas. The abscess tends to form first and is a collection of pus containing bacteria, debris and immune cells in fluid and sometimes blood. Later a granuloma may arise at the site where immune cells become clumped together. The effects of the bacteria in the body are not limited to the lymph nodes near the site of inoculation. Other lymph nodes may also be affected. The bacteria can spread and affect the central nervous system, eyes, lungs, spleen and vertebral bones.
Skin and Bone Growths
Bartonella henselae infection has a more severe effect on people with a weakened immune system – immunocompromised patients like those with HIV/AIDS or uncontrolled diabetes mellitus. It can cause angiomatosis where there is extensive development of new small blood vessels that form a tangled mass. Since it is caused by bacteria, it is known as bacillary angiomatosis. These types of growths are mainly seen on the skin or in bones. It almost exclusively occurs in immunocompromised patients with cat scratch disease.
Signs and Symptoms
Cat scratch may appear with a classical presentation in the majority of patients or an atypical presentation in a minority of cases.
Bartonella henselae infection primarily causes superficial regional lymphadenitis. This simply means inflammation and swelling of the surface lymph nodes closest to the site where the bacteria enters the body, usually as a result of a cat scratch.
Enlargement of the proximal lymph nodes is usually seen within 1 to 2 weeks after the scratch. It can sometimes be as delayed as 8 weeks after a scratch. First the proximal most lymph node is affected and then other lymph nodes along the course are involved. These lymph nodes are painful and may suppurate.
Other symptoms that may eventually appear as a part of systemic manifestations includes :
- Fever (affects only 60% of patients at most thereby making the term cat scratch fever a misnomer)
- Malaise – feeling of being unwell
- Anorexia – loss of appetite
Sore throat and joint pain (arthralgia) are seen in a minority of cases.
An atypical presentation means symptoms that are not seen in majority of the patients with the infection. It is more likely to be present in this manner in the elderly and immunocompromised patients. Complications are more likely to be seen in these instances giving rise to the clinical presentation associated with cat scratch fever. This includes :
- Chronic fever
- Confusion and mental impairment
- Back pain and abdominal pain
- Conjunctivitis with enlarged pre-auricular lymph nodes (located in front of the ears), collectively known as Parinaud’s oculoglandular syndrome.
- Tender red nodules on the skin (erythema nodosum)
Picture of Bartonella henselae bacteria in a cardiac valve sourced from Wikimedia Commons
Causes and Risk Factors
Cat scratch fever or catscratch disease (CSD) is caused by an infection with the Bartonella henselae bacteria. Infection with other Bartonella species of bacteria (bartonellosis) can cause other diseases like trench fever. The infection with Bartonella henselae is contracted by humans primarily through a scratch or bite from a cat with the bacteria. Domestic cats are the natural reservoir for these bacteria and it may spread among cat through fleas and possibly ticks. Although rare, it is possible that humans can contract the infection through fleas and ticks after feeding on cats. Most cases are due to bites and scratches from kittens rather than adult cats.
People who are at risk of contracting the infection are :
- Children, particularly those who interact closely with cats.
- Cat owners and their family members.
- Those living in warm and humid areas, particularly during autumn and winter months.
- Family members of an infected person.
- Veterinarians and veterinary assistants.
- Pet parlor workers.
Tests and Diagnosis
Although the symptoms present and history of contact with cats may raise the suspicion of cat scratch fever, various diagnostic investigations are necessary to confirm the diagnosis, assess complications and monitor it. The only tests that can confirm the diagnosis of cat scratch fever by the presence of Bartonella henselae bacteria includes :
- Antibody and PCR (polymerase chain reaction) tests of the blood and samples aspirated from the affected lymph node(s).
- Blood cultures and cultures of lymph node aspirates. Blood cultures are more likely to be positive in immuncompromised patients with disseminated disease. However, cultures of the lymph node aspirates are not often positive.
Picture of tissue from a patient with cat scratch disease sourced from Wikimedia Commons
Treatment of Cat Scratch Fever
Antibiotics are not always necessary for the treatment of cat scratch fever in patients with a competent immune system. It can take between 2 to 6 months for the infection to resolve completely. Supportive treatment may be necessary to control symptoms. This includes :
- Antipyretics to manage fever.
- Analgesics are helpful for pain management.
- Local heat therapy may help with pain relief for swollen for painful enlarged lymph nodes.
- Tender suppurating lymph nodes may be drained (aspiration) although incision and drainage is usually discouraged. Surgical removal (excision) of a lymph node is rarely done.
- Antibiotics are indicated for immunocompromised people and particularly AIDS patients. This may include the use of :