Hib is a fastidious organism to grow in cultures of tissue fluids. It grows well only in media enriched with sheep blood. Many laboratories have failed to isolate Hib using chocolate agar medium or other standard media for cultures. Such laboratories has bean successful in growing Hib once they switched to enriched media for culture (4). Counter immuno electrophoresis or CIE for Hib antigen is also used to further chances of diagnosis of Hib. It can be done on CSF, blood or pus from empyema. It is positive often when culture is negative especially when done post antibiotics where the culture becomes negative but the antigen remains detectable for sometimes. The yield of Hib diagnosis goes up by 20-30% by doing CIE in addition to cultures (4).

95% of invasive disease due to H. influenza is caused by type b & 59% by type a or non-typable H. influenza. 95% of Hib occur in children < 5 yrs old. More is the % of population of children <5 yrs, more will be the number of cases of Hib per year in that country. Developing countries have both more incidence of Hib/100,000 in <5yr old children & more % of population <5 yr old. This leads to heavy disease burden on the otherwise fragile health care system. Male to female ratio in some studies is shown to be 2:1 in UK & 3:2 in India. (5)

It is estimated that 3 million cases of Hib infection occur every year world over and 0.375 million of them die due to Hib. Studies from west have indicated an annual incidence of 50-100/100000 in children <5 yrs of age as shown in Table 1 (2,6). 50-60% of total Hib presents as meningitis, 5-8% each as pneumonitis, epiglottitis, skin infections, bone infections, bacteremia & others. Whereas data from developing countries like India is meagre, the incidence varies from 100-250/100,000 in children < 5 yrs. Majority of them present as meningitis (70-90%) or pneumonitis (10-20%). Epiglottitis & skin infection of face virtually do not occur in developing countries. (2,6)

95% of invasive Hib disease occurs in <5 yrs old children. The peak occurs later in developed countries e.g. in Finland 28% of cases occur in <1 yr old & 40% in < 18 months old children. In UK, 44% occurs in <1 year old and 71% is <2 yr old children, in USA 38% occurs in < 1yr & 60% occur in <18 months old children, in Netherlands 37% occurs in <1 yr old (2,6). This means that the peak occurs at 9-18 months of age in such countries. This leads to less cases of meningitis (50-60%) & pneumonitis (<9% of total) whereas epiglottitis & facial skins infections occur in these countries. This is because older the child, more is the natural immunity & less is the severity of disease. Whereas in developing countries the peak occur early at 6-9 months e.g. in Gambia 84% of cases occur in < 1yr (49% < 6 months), in Senegal 69% occur < 1 yr (32% in < 6 months) & in Thailand 76% occur < 1 yr (50% in <6 months) (2,6). This leads to severe invasive disease as natural immunity is lacking in such young infants. This leads to more chances of meningitis (70-90%) & pneumonitis (10-20%) & virtual absence of epiglottitis & facial skin infections. It is interesting to note that epiglottitis & facial skin infections appear in such countries when the disease burden is reduced due to mass vaccination which leads to shift to older age for the peak. Even within countries there may be a difference in incidence depending upon the economical & socio cultural differences e.g. in Nerago USA, the annual incidence is upto <50/100,000 <5 yr with 81% of cases occurring in <1 yr old or Alaska where the incidence could be as high as <50/100000 <5 yr with 67% of cases occurring in <1 yr old children.(2,6)

Table I : Incidence of Hib in various countries

Country	Total Hib /  100,000<5yr	Hib  meningitis/  100,000<5yr	Mass  immunization  Year	% drop /  Year
Finland	53	16-43	1988 - 89	100%/1991
Netherlands	>60	22	1993	85%/1994
U. K.	60 - 120	25	1992	98%/1994
U. S. A.	112	60	1989 - 93	99%/1995
Iceland	63	43	1989	100%/1991
Alaska	450	280-450	-	-
Gambia	60 - 270	60 - 70	-	-
Australia	59	25	1993	70%/1995
Chile	50 - 70	25	-	-
Israel	35	18	-	-
India	80 - 100	50 - 60	-	-

In India only hospital based data is available as shown in Table II. It shows that 30-45% of cases of meningitis & 8-12% of cases of pneumonitis are due to Hib (4,7). These values are similar to those seen in European countries before mass vaccination suggesting similar epidemiology. It is estimated that the annual incidence Hib in India is 50-60/100000<5yr (4). The IBIS study has shown that 76% occurs in <1 yr with peak at 6-9 months (range < 1 months to >9 yr old individual) (8).

Table II : Hospital based data on Hib meningitis in India

City / year	Hib%	Pneumococcus %	Meningococcus  %
Delhi (1981)	25	45	6
Chennai(1994)	28	21	4
Vellore (1985)	38	33	2
Mean	31	29	4
Of culture (%)	(45)	(43)	(5)
Pondicherry(1990)	8	30	?