Measles epidemiology from parks textbook
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Measles Epidemiology - Park's Textbook of Preventive and Social Medicine
Definition
Measles (Rubeola) is an acute, highly infectious disease of childhood caused by a specific virus of the group myxoviruses. It is characterized by fever, catarrhal symptoms of the upper respiratory tract (coryza, cough), followed by a typical rash. Measles occurs only in humans - there is no animal reservoir.
Problem Statement / Global Burden
- Measles is endemic virtually in all parts of the world
- Epidemics occur when the proportion of susceptible children reaches ~40%
- When introduced into a virgin community, >90% of that community will be infected
- In 1980, before widespread vaccine use: estimated 2.6 million measles deaths occurred worldwide
- By 2018: ~9.7 million cases and >140,000 measles-related deaths globally
- Measles deaths declined 73% and cases by 76% from 2000-2018 following vaccination programs
- Responsible for approximately 2% of under-five mortality worldwide
WHO 2015 Targets (milestones toward eradication):
- Routine MCV1 coverage ≥90% nationally, ≥80% in every district
- Annual measles incidence < 5 cases per million
- Reduce measles mortality by ≥95% compared to year 2000
Challenges for measles elimination:
- Weak immunization systems
- High infectious nature of measles
- Populations inaccessible due to conflict
- Increasing refusal of immunization
- Changing epidemiology - increased transmission among adolescents and adults
- Need to provide catch-up vaccination to >130 million children in India
- Gaps in human and financial resources
Agent Factors (Epidemiological Triad)
| Factor | Detail |
|---|---|
| Agent | Measles virus - RNA myxovirus; only one antigenic type |
| Survival | Cannot survive outside the human body for any length of time; retains infectivity at sub-zero temperatures |
| Source of infection | Only a case of measles; carriers are not known to occur |
| Infective material | Secretions of nose, throat and respiratory tract during the prodromal period and early rash stages |
| Communicability | ~4 days before to 4 days after appearance of rash |
| Isolation | For 1 week from onset of rash (covers the communicability period) |
| Second attack | Only one antigenic type; infection confers lifelong immunity. Most "second attacks" are diagnostic errors |
Host Factors
(a) Age: Affects virtually everyone in infancy or childhood
- Developing countries: 6 months to 3 years (poor environmental conditions)
- Developed countries: Older children, usually >5 years
- After vaccine use: disease now seen in somewhat older age groups
(b) Sex: Incidence is equal in both sexes
(c) Immunity:
- No age is immune without prior immunity
- One attack generally confers lifelong immunity; second attacks are rare
- Infants protected by maternal antibodies up to 6 months (may persist beyond 9 months in some)
- Immunity after vaccination is solid and long-lasting
(d) Nutrition:
- Measles is very severe in malnourished children
- Mortality up to 400 times higher in malnourished vs. well-nourished children
- Related to poor cell-mediated immunity secondary to malnutrition
- Severely malnourished children excrete measles virus for longer periods - prolonged spread risk
- Even in a healthy child, severe measles may precipitate malnutrition
Environmental Factors
- The virus can spread in any season
- Tropical zones: Most cases during the dry season
- Temperate climates: A winter disease (people crowd indoors)
- India: Epidemics common in winter and early spring (January to April)
- Population density and movement affect epidemicity
- Less favourable socio-economic conditions → lower average age of attack
Transmission
- Mode: Directly person-to-person via droplet infection and droplet nuclei
- Period: 4 days before onset of rash to 4 days thereafter
- Portal of entry: Respiratory tract (infection through the conjunctiva is also considered likely)
- The measles virus is one of the most communicable infectious agents known - secondary attack rates in susceptible household contacts approach 90%
Incubation Period
| Event | Timing |
|---|---|
| Exposure to onset of fever | Commonly 10 days |
| Exposure to appearance of rash | 14 days |
| Artificially induced (vaccine injection) | Average 7 days |
Clinical Features - The Three Stages
1. Prodromal (Pre-eruptive) Stage
- Begins 10 days after infection, lasts until day 14
- Fever, coryza, sneezing, nasal discharge, cough, red eyes (lacrimation), photophobia
- May have vomiting or diarrhoea
- Koplik's spots: Appear 1-2 days before the rash on buccal mucosa opposite the 1st and 2nd lower molars - small bluish-white spots on a red base. Pathognomonic of measles
2. Eruptive Phase
- Dusky-red, macular or maculo-papular rash beginning behind the ears
- Spreads rapidly to face and neck, then downwards - takes 2-3 days to reach lower extremities
- Rash may be discrete or confluent/blotchy
- Fever disappears in 3-4 days after rash onset
- Rash fades in same order of appearance, leaving brownish discoloration (persists up to 2 months)
- Virus present in tears, nasal/throat secretions, urine, and blood during prodrome and first 2-5 days of rash
- In defective cell-mediated immunity: no rash develops
3. Post-Measles Stage
- Child will have lost weight, become anaemic and debilitated
- A period of immunosuppression follows measles infection
Diagnosis: Typical rash + Koplik's spots. In developed countries: IgM antibodies by ELISA or RT-PCR of throat swabs/oral fluid/urine. (Note: diagnosis is likely incorrect if red eyes and cough are absent)
Prevention
1. Measles Vaccination (Primary Prevention)
- Live attenuated vaccine - given at 9-12 months (developing countries) or 12-15 months (developed countries)
- Schedule in India: Two doses - first at 9 completed months, second at 15-18 months (as MMR or MR)
- Reconstituted vaccine given subcutaneously (also effective intramuscularly)
- Reactions: Mild measles illness (fever + rash) in 15-20% vaccinees, 5-10 days post-immunization; fever lasts 1-2 days, rash 1-3 days
Contraindications:
- Severe immunosuppression (congenital immunodeficiency, HIV with severe immunosuppression, leukaemia, lymphoma, alkylating agents, large-dose corticosteroids)
- Known pregnancy (avoid pregnancy for 4 weeks post-MMR)
- Moderate/severe acute illness (minor illness is NOT a contraindication)
- Blood product recipients (delay 3 months after IG; 7-11 months after IV immunoglobulin)
TST and MMR: Do TST at same visit OR delay TST by at least 4 weeks if MMR given first. Measles-containing vaccines may transiently suppress TST response.
2. Herd Immunity
- High vaccination coverage (≥95%) creates herd immunity that prevents outbreaks
Eradication Prospects
- Measles is a candidate for eradication: only one antigenic type, no animal reservoir, effective vaccine available
- Global strategic plans (2012-2016, and continuing) aim for measles elimination region by region as a step toward eradication
Source: Park's Textbook of Preventive and Social Medicine, pp. 166-170
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