Dirofilariasis is a fairly rare vector borne disease with a high squick factor, as it results in the implantation and growth of filarial nematodes (roundworms) in the affected host. Primarily infecting dogs, canids, cats, and raccoons – Dirofilaria occasionally finds its way into human hosts, where it can set up housekeeping in the lungs, in nodules under the skin, or even in the eye.
Dirofilaria are long, thin parasitic roundworms that infect a variety of mammals. Infection is transmitted by mosquito bites. There are many species of Dirofilaria, but human infection is caused most commonly by three species, D. immitis, D. repens, and D. tenuis.
The main natural hosts for these three species are dogs and wild canids, such as foxes and wolves (D. immitis and D. repens) and raccoons (D. tenuis). D. immitis is also known as "heartworm." D. repens is not found in the United States, and D. tenuis appears to be restricted to raccoons in North America.
Humans are accidental hosts and the worms do not grow to maturity in humans. D. immitis causes heartworm disease in dogs. In humans infected with D. immitis, the most common clinical finding is lung granulomas, often asymptomatic, which appear as coin lesions on x-rays. Among persons infected with D. repens and D. tenuis, the most common findings have been nodules under the skin and under the conjunctiva of the eye.
All of which serves as prelude to the following report from Hong Kong’s CHP on three recent cases of ocular Dirofilariasis, after which I’ll return with a bit more.
The Centre for Health Protection of the Department of Health is today (March 19) investigating three cases of ocular worm infestation, and hence reminded the public to maintain strict environmental hygiene and protective measures against mosquito bites and stray dogs.
According to the notification by the Hospital Authority, the first patient is a woman aged 58 with good past health who presented with pain, redness and foreign body sensation in her left eye on November 7, 2014. She attended the Accident and Emergency Department (AED) of Prince of Wales Hospital the following day (November 8) and a worm was extracted from her eye. The specimen was identified to be Dirofilaria species (Dirofilaria repens). Her clinical diagnosis was ocular dirofilariasis.
The second patient is a woman aged 69 who has developed foreign body sensation in her left eye since January 19, 2015. She consulted Hong Kong Eye Hospital on February 2, 2015, and a worm was extracted from her eye. The specimen was identified to be Dirofilaria species (Dirofilaria hongkongensis). Her clinical diagnosis was ocular dirofilariasis.
The remaining patient is a boy aged 15 with good past health who has presented with bilateral eye itchiness and runny nose since February 15, 2015. He has then developed low grade fever, malaise, headache and increasing eye itchiness, redness and foreign body sensation of both eyes since March 5, 2015. He attended the AED of Queen Elizabeth Hospital on March 11, 2015, and was admitted for further management on the same day. His clinical diagnosis was suspected ocular filariasis.
All patients are now in stable condition.
"Our investigations are ongoing with a view to studying if the cases were epidemiologically linked," a spokesman for the CHP said.
"Dirofilariasis is a disease caused by Dirofilaria worms which are parasites that infect a variety of mammals. Dogs are one of the main natural hosts of Dirofilaria worms. Humans may occasionally acquire the disease from infected natural hosts through mosquito bites. Infection may result in nodules under the skin, conjunctiva or internal organs, urticaria (hives), inflammation of the lymphatic system etc," a spokesman explained.
"Dirofilariasis does not transmit from person to person. It is usually treated by surgical removal of the nodules. Members of the public should consult medical professionals if they have compatible symptoms," the spokesman advised.
Members of the public should maintain strict environmental hygiene and take heed of personal protective measures against mosquito bites and stray dogs against the disease.
Dog owners are specifically reminded to adopt precautionary measures below as advised by the Agriculture, Fisheries and Conservation Department:
- Maintain their dogs on lifelong heartworm preventatives as advised by their veterinarian; and
- Be a responsible owner and not to abandon dogs so that they become stray dogs and possible reservoir of heartworm.
Ends/Thursday, March 19, 2015
Issued at HKT 20:15
As mentioned earlier, dirofilariasis is most commonly caused by three types of nematodes; D. immitis, D. repens, and D. tenuis. The second case mentioned above was attributed to Dirofilaria hongkongensis, which is a recently discovered novel Dirofilaria species.
This from a 2012 Journal of Clinical Microbiology report:
Kelvin K. W. To,a,b,c,d Samson S. Y. Wong,a,b,c,d Rosana W. S. Poon,c Nigel J. Trendell-Smith,e Antonio H. Y. Ngan,d Jacky W. K. Lam,f Tommy H. C. Tang,g Ah-Kian AhChong,h Joshua Chi-Hang Kan,i Kwok-Hung Chan,c and Kwok-Yung Yuen
Dirofilariasis is globally the commonest manifestation of zoonotic filariasis. We report the detection of a novel canine species causing human and canine dirofilariasis in Hong Kong. Three human cases occurring over 10 months were identified, one presenting with cervical lymphadenopathy, one with an abdominal subcutaneous mass, and one with a subconjunctival nodule.
Transected worms recovered from the resected abdominal subcutaneous mass were morphologically compatible with Dirofilaria. The cox1 gene sequences of the three human isolates were identical; however, they were only 96.2% and 89.3% identical to the cox1 gene of Dirofilaria repens and Dirofilaria immitis, respectively. Sequencing of the 18S-ITS1-5.8S gene cluster was successful in the intact worm, and the nucleotide sequences were 94.0% and 94.9% identical to those of D. repens and D. immitis, respectively.
Screening of the blood samples from 200 dogs and 100 cats showed the presence of the novel Dirofilaria species in 3% (6/200) of the dogs' but none of the cats' blood samples. Nucleotide sequences of the cox1 gene and 18S-ITS1-5.8S gene clusters of the dogs' samples were identical to those in the human samples. The sera of canines infected by this novel Dirofilaria species were negative when tested with the SNAP 4Dx D. immitis detection kit, except in the case of dogs with a mixed infection with D. immitis as detected by PCR.
The results from this study suggest that this novel Dirofilaria species is a cause of filarial infection in humans and dogs in Hong Kong. We propose to name this Dirofilaria species “Candidatus Dirofilaria hongkongensis.”
While the risk of contracting Dirofilariasis is exceedingly low in most places, when you add in the risks of contracting far more common mosquito borne illnesses (West Nile Fever, Dengue, Malaria, Chikungunya, EEE, STEV, etc), there’s more than enough reason to remember and follow the `5 D’s’ of mosquito safety: