A case of diphtheria in an eye, probably from a dog

On 12 December 2018, an immunocompromised man of 73 with non-Hodgkin's
lymphoma saw an ophthalmologist for **left eyelid redness, swelling and eye
discharge**, and was diagnosed with **ligneous (pseudomembranous)
conjunctivitis.**

The membrane was debrided and cultured. It grew Corynebacterium ulcerans
— one of **three Corynebacterium species that can carry the diphtheria
toxin gene**, alongside C. diphtheriae and C. pseudotuberculosis. On **28
December CDC confirmed it was producing the toxin.**

**This is the first reported case of conjunctivitis caused by a toxigenic
strain of C. ulcerans.**

He had **no systemic symptoms, was not hospitalised, and did not receive
diphtheria antitoxin.** After several membrane removals, **cultures on 11
January 2019 were negative.**

The detail the case turns on

**The patient was not up-to-date for tetanus-diphtheria (Td) vaccine and had
postponed vaccination because of his ongoing cancer treatment.**

That is a reasonable decision made for good reasons, and it is the gap the
bacterium found. Diphtheria toxoid vaccines target the toxin, so they
**most likely prevent toxin-mediated disease caused by all toxigenic
Corynebacterium strains** — not only the species the vaccine is named for.

Where it comes from

Unlike C. diphtheriae, C. ulcerans is zoonotic. It has been isolated
from many animal species, and **infected livestock and pets such as dogs and
cats are recognised sources of occasional human infection — particularly in
older people who are unvaccinated or overdue a booster.** Since the late
1980s it has been increasingly reported as a cause of diphtheria-like
respiratory and cutaneous illness.

**The patient lived with two dogs. Neither was reported ill or examined by a
vet, and he declined to have them tested.** So the source is unproven, and the
most likely explanation is standing in the room.

The investigation

One household contact was identified. **Paired nasal and throat swabs from
the patient (after treatment) and the contact were negative.** The contact
declined a Td booster; ophthalmology staff who had direct contact reported
wearing recommended PPE and declined prophylaxis. **No contact developed
disease.**

**Secondary person-to-person transmission of C. ulcerans has not been
verified**, though it is possible — which is why **the vaccination status of
household contacts and medical providers should be assessed**, and anyone
overdue offered vaccination.

The wider problem it points at

Both C. diphtheriae and C. ulcerans **become toxigenic through lysogeny by
beta-corynebacteriophages carrying the diphtheria toxin gene** — a virus that
infects the bacterium and hands it the weapon.

**Circulation of toxigenic C. ulcerans in animals highlights an animal
reservoir for corynebacteriophages.**

The toxin gene has somewhere to live that no human vaccination campaign
reaches. Eliminating diphtheria in people does not eliminate the gene, which is
why the recommendation ends where it does: **maintain high population immunity
through vaccination, including the recommended ten-yearly boosters.**

And for clinicians: **be aware that C. ulcerans infection can be acquired
from pets**, particularly by elderly or immunocompromised people. Treatment
follows C. diphtheriae guidelines, and **antitoxin is recommended for
respiratory infections** caused by either.

Source: Centers for Disease Control and Prevention, MMWR.