What is Capnocytophaga? How an innocent dog lick can trigger a fatal infection

Disease name: Capnocytophaga

Affected populations: Capnocytophaga is an extremely rare condition — two large epidemiological surveys conducted in Scandinavian countries suggest that it may affect between 0.5 and 0.7 people out of every million each year. In the United States, the disease is not nationally notifiable, which means it's not tracked at a national level and so there are no annual estimates of its incidence. As of 2018, approximately 500 cases of Capnocytophaga infection caused by the bacterial species Capnocytophaga canimorsus had been reported in the medical literature.

People with weakened immune systems are at a higher risk of catching Capnocytophaga and developing serious illness. Other risk factors include not having a spleen, having alcohol use disorder, smoking and being over age 40.

Causes: Capnocytophaga infection can be caused by nine species of bacteria that belong to the same genus: Capnocytophaga. Some of these species, such as Capnocytophaga gingivalis, naturally live in humans' mouths and tend to cause infections mostly in people who have weakened immune systems.

However, most Capnocytophaga infections in humans are caused by C. canimorsus, a species that lives in the mouths of dogs and cats.

Related: A woman needed her hands and legs amputated after contracting infection from dog 'kisses'

Patients who become infected with C. canimorsus usually get it through the bite of a dog or cat, or if dog or cat saliva enters an open wound or sore on the person's body. However, most dog or cat bites do not cause Capnocytophaga or any other illness. In the U.S., for instance, around 4.5 million people are bitten by dogs every year, but only 20% of them require medical attention for resulting injuries or infections.

Symptoms: In people who do develop Capnocytophaga infection, symptoms normally begin between three and five days after a dog or cat bite or scratch. Patients may develop blisters at the wound site, as well as redness, swelling and pain. They may also experience fever, diarrhea, vomiting and headaches.

Capnocytophaga bacteria can also spread beyond the wound to other organs, triggering serious conditions such as heart attacks, kidney failure and gangrene, the death of bodily tissue. Infection may additionally lead to septicemia (blood poisoning) or meningitis (infection of the membranes over the brain and spinal cord). Patients who develop these serious conditions have a 30% or 5% chance of survival, respectively.

Patients who lack a spleen have between a 30% and 60% greater risk of death from Capnocytophaga infections than people who have the organ, and they can die within one to three days of their symptoms beginning. These patients may be particularly susceptible because the spleen plays a vital role in the immune system's response to bacteria like Capnocytophaga.

Treatments: Capnocytophaga infection can be treated with antibiotics, and starting the therapy quickly can lower the risk of developing serious complications. Patients who develop gangrene may have to have limbs amputated.

The Centers for Disease Control and Prevention recommends that anyone who is bitten by a dog or cat call their health care provider immediately or go to an urgent care clinic to be assessed. They should also wash the bite area with soap and water right away, the agency advises.