Clinical diagnosis is not so difficult when swelling of the parotid gland is seen, and virus isolation is seldom done for confirmation. However, aseptic meningitis may develop without showing swelling of the parotid gland. In such cases, serological diagnosis or virus isolation becomes necessary for differential diagnosis from such other agents as enteroviruses.
Mumps incidence: Mumps cases reported from pediatric sentinels during the period from 1982 to 2003 under the National Epidemiological Surveillance of Infectious Disease (NESID) are shown in Fig. 1. Epidemics have occurred every 3-4 years, namely 1982-83, 1985-86, and 1988-89. After April 1989, measles-mumps-rubella (MMR) vaccine has been allowed to choose instead of monovalent measles vaccine for routine immunization. During the period from 1990 to the first half of 1993, the number of cases was kept low, below 1.0 per sentinel clinic. MMR vaccination has been discontinued since April 1993 due to the occurrence of many cases of post-vaccination aseptic meningitis (see IASR, Vol. 15, No. 9). After that, mumps epidemics recurred from the latter half of 1993 to 1994, and 1996-98, but not in such a large scale as cases per sentinel clinic exceeding 2.0. Cases increased from the latter half of 2000 and built up a large epidemic with a peak exceeding 2.0 per sentinel clinic in the 28th week of 2001. Until the first half of 2002, cases per sentinel clinic were kept over 1.0. After the 35th week of 2002 until now, the 13th week of 2003, it has been kept below 1.0.
The annual cases reported by sentinel clinics were 132,877 in 2000 (44.62 per sentinel clinic), 254,711 (84.37) in 2001, and 182,663 (60.32) in 2002. According to the report by Nagai, M. (March 2003) of the Working Group on Improvement of the NESID System organized by the Ministry of Health, Labour and Welfare (MHLW) (headed by Okabe, N.), cases were estimated at 1.17 million in the whole country in 2000 (95% confidence interval: 1.11-1.24 million), and 2.26 million (2.15-2.36 million) in 2001.
The ages of 2, 3, and 4 years of cases (Fig. 2) have been on the slight increase after 1993 (see IASR, Vol. 15, No. 9), and since 1996, cases younger than 4 years have been accounting for 45-47% without much change. Cases aged 0 year were very few; cases increased in number with age and 4-year cases were the largest in number, followed by 5-year and 3-year cases; 3-6 year cases accounting for 60% of all cases.
There were many cases in Kyushu-Okinawa district in the 1994 epidemic, spreading to Hokkaido and Tokai-Hokuriku districts in 1996; in 1997, so did to Tokai-Hokuriku and Kyushu-Okinawa districts, and in 1998, cases were spread to Tohoku and Chugoku-Shikoku districts. The epidemics in 2000-2002 are shown in Fig. 3. In 2000, there were many cases in Yamaguchi and Kumamoto Prefectures, and in 2001, cases increased in all over Kyushu district, Okinawa Prefecture and Hokuriku-Shinetsu district, particularly in Fukui (222.27) and Ishikawa Prefectures (213.00). In 2002, an increase of cases was seen in Tohoku district.
Mumps virus detection: Reports of mumps virus detection from prefectural and municipal public health institutes (PHIs) during January 1993 through December 2002 numbered at 1,728 (reported as of March 25, 2003). A total of 1,679 monthly detected cases excluding 49 vaccine-associated ones are shown in Fig. 4. Clinical diagnosis was mumps in 778 and aseptic meningitis in 549 cases. Reports of detection of mumps virus from aseptic meningitis cases numbered at 34 and 56 in 1999 and 2000, respectively, which increased to 110 in 2001. The reports numbered at 71 in 2002. Detection from mumps cases in 2002 increased from the preceding year.
The virus-positive specimens of 1,679 cases were nasopharyngeal swabs in 1,086 cases, cerebrospinal fluid in 620, urine in six, stools in three and sputum in one. Detection from nasopharyngeal swabs accounted for the greater part (including cases with detection from two or more sources of specimens). Among mumps cases, the virus was detected from nasopharyngeal swabs in 729 cases and cerebrospinal fluid in 83 cases, and among aseptic meningitis cases, from cerebrospinal fluid in 455 cases and nasopharyngeal swabs in 119 cases. The specimens for virus detection differed largely depending upon the diagnosis.
Mumps vaccine: In Japan, voluntary immunization of this vaccine was begun in 1981. The Urabe strain used at the beginning was abandoned because of the occurrence of many cases of aseptic meningitis. The Torii, Hoshino and Miyahara strains are now being used in Japan. The seroconversion rate is now 90-98% (see p. 105 of this issue). Complication with aseptic meningitis after vaccination occurs now in one out of 20,000-120,000 vaccinees. The rate of complication with meningitis at natural infection is about 4-6% and complication with meningoencephalitis has also been recognized. In the USA, reports of meningoencephalitis infected naturally have become rare due to the introduction of vaccine. According to the nationwide survey conducted in 2001 by Kitamura, K. et al. of a working group of the MHLW, acute complete deafness due to natural infection was estimated at 650 (95% confidence interval: 540-760)(see p. 107 of this issue). Tamura, M. reported cases requiring artificial internal ear complicated with bilateral complete neurosensory deafness (see p. 107 of this issue). Since many deafness cases are difficult to treat, rapid improvement of vaccine coverage rate is desired.
The mumps vaccine products in this country have been constant, being about 400,000 doses per year, from the beginning, which were not changed during 1989-1993 when MMR vaccine could be chosen at the routine immunization for measles, and 400,000-500,000 doses were produced during 1997-2000. Since the yearly products of measles vaccine are about 1.4 million doses, the vaccination rate for mumps is estimated at about 1/3 of that for measles. According to Hiraiwa, M. et al., the mumps vaccine coverage at the health examination of 3-year children in Toda City, Saitama Prefecture is about 30% (see p. 106 of this issue). According to the National Epidemiological Surveillance of Vaccine-Preventable Diseases, the rate of choosing MMR vaccine for routine measles immunization is about 20-30%, the mumps immunization in 1989-1993 is estimated to be a little less than twice that at present. Since the number of mumps cases during 1990s, when the MMR vaccine-available generation was the main age group of the epidemic, was smaller than those before and after, the increase in the vaccination rate may have contributed to the suppression of epidemics.
Mutation of mumps virus: The subgenotype of mumps virus strain isolated from cases of natural infection during 2000-2002 is different from those of the domestic isolates before 1989 and the antigenicity seems to be changing. No difference was found, however, in the neutralization antigen between the vaccine strain and the epidemic strain isolated in 2002 (see p. 109 of this issue). It is considered that prevention of infection with the current epidemic strains by the current vaccine strain is possible (see p. 105 of this issue). Since such a change in subgenotype is occurring in a global scale, continuous investigation seems necessary.
The vaccination rate at present is expected to be the same as that before introduction of MMR vaccine, and the epidemic status is anticipated to return to that before introduction of MMR vaccine. The prognosis of mumps is usually good, but because of acquired complete deafness and the high rate of mumps meningitis, prevention of this disease by vaccination seems urgent.