Gary A. Balsamo DVM MPH&TM, Christine M. Scott MSPH, Erin Stanley MPH,Susanne Straif-Bourgeois PhD MPH,Raoult C. Ratard MD MPH&TM

1st human case in 2001 (1 NID) Epidemic in 2002 (204 NID) Most cases are reported in summer

and fall• The seasonal incidence corresponds to

hurricane season

What effect do hurricanes and tropical storms have on the incidence of human West Nile virus?

Hypothesis 1: Hurricanes increase incidence of cases• Increased rainfall• Accumulations of

water: breeding grounds for mosquitoes

• Mosquito populations increased

• Increased transmission of WNV

Hypothesis 2: Hurricanes, little or no

effect on incidence • Most common vector

(LA): Culex quinquefasciatus Habitat: water w/ organic

debris• Strong winds, salt water

surge, downpours displace or kill mosquitoes

• Altered ecology: New freshly hatched

mosquitoes (disease-free)

Weeks for recovery of virus in population

By then, late season… lower transmission

• Birds displaced or killed

Tulane group Focus: Hurricane Katrina “After Hurricane Katrina, the number of reported cases” of WNV NID “sharply increased in the hurricane affected regions of Louisiana and Mississippi.”

CDC Arboviral Group•Focus: Hurricane Katrina•Based on CDC Arbonet data• “…although Hurricane Katrina disrupted WNV surveillance in Louisiana, it did not…increase the incidence of WNV NID & SLE in either Louisiana or Mississippi.”

Goal: Resolve controversy • OPH IDE data from 7 years (02-08)

11 hurricanes or tropical storms

Surveillance methods per CDC recommendations

New cases of WNV-NID • Most reliable indicators • Fever and asymptomatic cases excluded

2002, Aug 5-7, Bertha, TS 2002, Sept 26, Isidore, TS 2002, Oct 3, Lili, H1 2003, Jun 30, Bill, TS 2004, Sept 24, Ivan - 2nd Landfall, TD 2004, Oct 10, Matthew, TS 2005, July 6, Cindy, TS 2005, Aug 29, Katrina, H4 2005, Sept 24, Rita, H3 2008, Sept 1-2, Gustav, H2 2008, Sept 13, Ike, H2

National Hurricane Center Tropical Cyclone Reports

Defined affected parishes (counties)

• In path of storm eye• Parishes receiving > 2 inches rainfall

CDC week (dates) used for comparison

Compared WNV-NID

• Those

reported in 4 weeks prior to storm week (“Pre-Storm”)

• Those reported in 4 weeks immediately after the storm week (“Early Post-Storm”)

Compared WNV-NID • Those reported

in 4 weeks prior to storm week (“Pre-Storm”)

• Those reported in weeks 2 to 6 post-storm (“Late Post-(“Late Post-Storm”)Storm”)

• To identify possible late effect

Controlled for seasonal effects:• Landfall related periods were compared

Same time periods (CDC weeks) Identical locations Compared data from years affected by storms to years not affected by storms

2002-08 Overlap of storm and non-storm years was permitted

in the first week pre-storm and sixth week post-storm

Hypothesized insufficient alteration of levels of infected mosquitoes

Still counted as non-storm year

• Hurricane year, “Pre-Storm vs. non-hurricane year “Pre-Storm” … compared to assess the validity of the study

• Hurricane year, “Pre-Storm” and “Early Post-Storm” vs. non-hurricane year “Pre-Storm” and “Early Post-Storm”

• Hurricane year, “Pre-Storm” and “Late Post-Storm” vs. non-hurricane year “Pre-Storm” and “Late Post-Storm”

• The statistical analysis consisted of t-Test for Paired Two Sample for Means, one tail test conducted on SAS ® version 9.1.

Hurricane Years Mean of 8.18 cases

in the four weeks prior to storms

Non-Hurricane Years Mean of 4.78 cases in

the same four week periods

T = 0.82; prob ≤ t (one tail) 0.4340No significant difference

Hurricane Years Reduction in cases

from 90 to 35 mean annual cases. (61.1% reduction)

Prob ≤ t (one tail) 0.17

Non-Hurricane Years Reduction of mean

annual cases from 52.6 to 45.0.(14.4% reduction)

Prob ≤ t (one tail) 0.62

No statistically significant difference, but lower probability this effect is due to chance

Hurricane Years Reduction in cases

from 90 to 13 cases (85.6% reduction)

Prob ≤ t (one tail) 0.10

Non-Hurricane Years Reduction of cases

from 52.6 to 40.7 (22.6% reduction) Prob ≤ t (one tail)

0.56

No statistically significant difference, but lower probability this effect is due to chance

Whether the comparison was made between pre-hurricane and post hurricane, early or late, the results are similar: • Large reductions during the hurricane years

(61.1% and 85.6%) • Smaller reductions during the control years

(14.4% and 22.6 % reduction) It is therefore reasonable to conclude that

hurricanes do depress the transmission of WNV.

Small number of storms (eleven)… limited statistical significance

 The selection of parishes based on hurricane path and rainfall as reported in the National Hurricane Center Tropical Cyclone Reports may introduce some uncertainties.• Parish lines irregular• Rainfall reported may not be representative of the entire

area.

 Some unusual circumstances under which hurricanes may increase transmission • Hurricanes striking early during the season• Hurricanes involving relocation of people in areas of intense

transmission (Lehman 2007)

Transmission of WNV is specific to many different temporal and spatial conditions.• No consistent incidence patterns over time or area.

The only consistent patterns:• seasonal transmission heaviest from late June to early August,

ending in late October to early December• sporadic distribution throughout Louisiana with a few foci of

more intense activity, but wide differences from year to year.

This study cannot take into account the effects of post-storm mosquito abatement efforts by local MADs and /or FEMA.

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