To the Editor: Echinococcosis is a serious parasitic zoonosis in the Northern Hemisphere.
In Japan, it is characterized by alveolar, hepatic, and cerebral disorders in humans
caused by the larval form (metacestode) of the tapeworm Echinococcus multilocularis.
The life cycle of the parasite is maintained in the wild by gray-backed voles, Clethrionomys
rufocanus, as intermediate hosts, and by red foxes, Vulpes vulpes, as definitive hosts.
Humans are infected by ingestion of the parasite eggs, mainly through water contaminated
with the feces of wild red foxes, which have an estimated infection prevalence of
54%–56% (
1
).
The echinococcosis-endemic area in Japan is restricted to the northern island of Hokkaido,
although sporadic human cases have been reported on other islands (
2
), and infected pigs have been documented on the main island of Honshu (
3
). While the threat of echinococcosis spreading into Honshu had raised fears, an emergent
concern is the possible role of domestic dogs in dispersing the disease from disease-endemic
areas during relocation of residences by owners or when accompanying owners during
domestic travel.
In September 2005, a stray dog in Saitama prefecture in mainland Honshu was found
to be positive for E. multilocularis infection by PCR (mitochondria 12S RNA gene)
(Y. Morishima, pers. comm.). The sequence was identical to the Hokkaido isolate (GenBank
accession no. AB244598). This raised an alarm because the area in which the infection
was found is adjacent to the Tokyo metropolis, the most populous zone in Japan. Reports
also claimed that 2 of 69 dogs moved from Hokkaido to Honshu were positive for E.
multilocularis by coproantigen examination (
4
).
Nearly 10,000 pet dogs were estimated to have been transported in 1 year to and from
Honshu and Hokkaido by planes and ferries; this presumably included up to 30 E. multilocularis–infected
pet dogs per year (
5
). Even so, no compulsory quarantine or Echinococcus examination is enforced for dogs
transported within Japan. A compulsory requirement of a certificate from a veterinarian
stating that the animal has been treated with praziquantel 3–4 days before traveling
would be a helpful preventive measure. As part of an amendment to the Infectious Disease
Law in Japan, E. multilocularis infection was included among the 4th Category Diseases
(
6
). Thus, since October 2004, it has been mandatory for veterinarians who have diagnosed
echinococcosis in dogs to report each case to health authorities, the first national
reporting system of its kind worldwide.
Our laboratory established the Forum on Environment and Animals (FEA) to meet the
demand for accurate and rapid diagnosis of echinococcosis in domestic dogs. FEA is
a hub for veterinary practitioners around the country for confirmation of E. multilocularis
infection in definitive hosts, especially dogs but also cats. Feces submitted are
from dogs and cats that are suspected to be infected and that wander or walk in parks
and woodlands and likely prey on wild rodents. Examinations are performed weekly,
and results are immediately forwarded to the submitting veterinarians. Before examination,
fecal samples are sterilized by heating for 12 hours at 70°C. Fecal egg examination
is conducted by using centrifugal flotation (
7
) with sucrose solution with a specific gravity of 1.27. Sandwich ELISA using a monoclonal
antibody EmA9 (
8
) is used for E. multilocularis coproantigen detection. Egg- and ELISA-positive fecal
samples from dogs are subjected to PCR amplification (mitochondria 12S RNA gene) (
9
).
The Table presents data of samples from both dogs and cats examined by FEA from April
2004 through August 2005. A total of 1,460 domestic dogs were examined, and 4 (0.27%)
were confirmed positive to echinococcosis by PCR, all from Hokkaido. Test results
from eggs detected in cat feces suggested these animals were infected with Taenia
taeniaeformis, a cat tapeworm, rather than E. multilocularis, because coproantigen
ELISA results were negative and an ELISA-positive sample did not contain eggs.
Table
Prevalence of echinococcosis in definitive hosts subjected to fecal egg examination,
ELISA coproantigen test, and PCR copro-DNA detection, Japan
Species
No. samples
Positive samples (%)
Egg examination
ELISA
PCR
Dogs
1,460
3 (0.20)
6 (0.41)
4 (0.27)
Cats
128
4 (3.12)
1 (0.78)
ND*
Total
1,588
7 (0.44)
7 (0.44)
–
*ND, not done.
To our knowledge, this survey registered the greatest number of domestic dogs examined
recently in Japan for echinococcosis. Confirmed cases of infection in dogs further
showed the potential threat of domestic dogs transmitting E. multilocularis to humans
in this region, as well as the potential for dispersal to other islands of Japan if
proper preventive measures are not implemented.
A previous report of necropsy examinations of 9,849 dogs from 1966 to 1999 showed
a prevalence of 1.0% (
10
). Although necropsy is considered the most reliable method to diagnose E. multilocularis
in definitive hosts, it is not applicable for live animals such as domestic dogs and
cats. Fecal egg examination is generally used; however, infection is difficult to
confirm because the morphology of taeniid eggs is indistinguishable from those of
E. multilocularis, and eggs are excreted intermittently even after the worms mature.
Coproantigen detection had proven useful for primary screening and was documented
to have 94.9% sensitivity and 100% specificity for echinococcosis in wild red foxes
in Hokkaido (
1
). The combined egg examination, ELISA, and PCR methods we used showed an accurate
and rapid diagnosis in domestic dogs, which is important for immediate reporting,
treatment, and action to safeguard dog owners.