Obligate intracellular parasites without cell walls. Includes C. pneumonia, C. psittaci, C. trachomatis. "Humans were originally infected zoonotically by animal isolate(s) of C. pneumoniae which adapted to humans primarily through the processes of gene decay and plasmid loss, to the point where the animal reservoir is no longer required for transmission (PubMed)."
C. pneumonia: human to human.
C. psittaci: birds, rarely cows, goats and pigs. The bird is not necessarily ill and coughing and saying "Polly want a Vicks."
C. trachomatis: human to human, usually sexually. It is one of the Sexually Transmitted Diseases.
C. pneumonia: upper respiratory tract infections and atypical pneumonia. Most common in the elderly.
C. psittaci: atypical pneumonia (often with a predominant headache), often with bradycardia and splenomegaly. Common for new bird owners and those that raise them to have a mild illness shortly after purchasing the bird (Emerging Infections).
In men who have sex with men, it can be often found in the pharynx and routine therapy for urethral diseases does not eradicate it (PubMed).
C. trachomatis LGV: doxycycline 100 mg po bid x 21 days (preferred) OR erythromycin 500 qid x 21 days. Although clinical data are lacking, azithromycin 1 g orally once weekly for 3 weeks is probably effective based on its chlamydial antimicrobial activity.
C. trachomatis urethritis: preferred: azithromycin 1 g orally in a single dose OR doxycycline 100 mg orally twice a day for 7 days. There may be a slight edge for doxycycline (PubMed): "There may be a small increased efficacy of up to 3% for doxycycline compared with azithromycin for the treatment of urogenital chlamydia and about 7% increased efficacy for doxycycline for the treatment of symptomatic urethral infection in men (PubMed)".
Alternative Regimens: erythromycin base 500 mg orally four times a day for 7 days OR erythromycin ethylsuccinate 800 mg orally four times a day for 7 days OR levofloxacin 500 mg orally once daily for 7 days OR ofloxacin 300 mg orally twice a day for 7 days.
When you treat chlamydia, always treat gonorrhea.
USPSTF 2007 recommendations (PubMed):
Screen for chlamydial infection in all sexually active nonpregnant young women age 24 years or younger and for older nonpregnant women who are at increased risk. (A recommendation)
Screen for chlamydial infection in all pregnant women age 24 years or younger and in older pregnant women who are at in- creased risk. (B recommendation)
Do not routinely screen for chlamydial infection in women age 25 years or older, regardless of whether they are pregnant, if they are not at increased risk. (C recommendation)
Current evidence is insufficient to assess the balance of benefits and harms of screening for chlamydial infection for men. (I statement).
Women should be screened yearly, but, because we do such a half assed job of health care in the US, it is estimated that 2 million, yes million, cases go undiagnosed each year because physicians do not think the patient is at risk. Are they human? Then there are at risk for Sexually Transmitted Diseases and for some odd reason people are hesitant to admit to sexual activity.