Transmission of and infection with Campylobacter jejuni bacteria
As discussed above, most Campylobacter infections in humans are caused by the consumption of contaminated raw milk and dairy products, food, or water. Direct contact with infected animals, including pets, especially puppies and kittens, is also a well-documented means of disease-transmission. In the fall of 2017, an outbreak of multidrug-resistant Campylobacter was linked to puppies sold through a national pet store chain. The presence of Campylobacter in playgrounds has been recognized as an emerging environmental source of campylobacteriosis, especially in children, who frequently put their hands in their mouths, favoring the ingestion of microbes. Many playgrounds are natural habitats for a variety of wildlife including birds, lizards, dogs, and stray cats.
Although less common, person-to-person transmission can also occur, especially in caretakers of small children or those exposed to other individuals in diapers. Males and females appear to be equally affected, although the prevalence of infection in otherwise healthy people is quite low. Population-based studies show that the peak incidence of infection is in children under five years of age, and in persons between 15 and 29 years of age. Sexual transmission has also been reported, especially in men who have sex with men. The incidence of Campylobacter infection in HIV-positive individuals is higher than in the general population.
The infective dose of Campylobacter—that is, the amount of bacteria that must be ingested to cause illness—is relatively small. As few as 500 organisms, an amount that can be found in one drop of chicken juice, have been shown to cause human infection. Despite this low infectious dose and the ubiquity of Campylobacter in the environment, most cases of Campylobacter infection occur as isolated, sporadic events, and are not usually part of large outbreaks. Nonetheless, very large outbreaks (greater than 1,000 illnesses) have been documented, most often from consumption of contaminated milk or unchlorinated water supplies.