First isolation of WNV in the Caribbean from sentinel chickens and mosquitoes

ProMedMail carries an interesting report on the first isolation of WNV from IgM seropositive sentinel chickens and mosquitoes in Puerto Rico (see the original paper in the American Journal of Tropical Medicine and Hygiene as well as in the Pan American Journal of Public Health). According to these sources, seroconversions started on June 4th 2007, peaked at 45% during June and July, and then fell steeply in August to 2%. However, seroconversions continued to occur at a low rate (2-6%) until October. Quite surprisingly, the article states that only 3 out of 4370 previously analyzed blood samples from dead birds, horses, pigs, monkeys and people were seropositive for IgG antibodies. Thus, sentinel chickens seem to be a highly valuable tool to detect the local activity of West Nile virus. ProMedMail also refers to an earlier review article in the Pan American Journal of Public Health on the spread of West Nile in Latin America. That paper invokes an interesting hypothesis for the apparent lack of human and equine cases of neuroinvasive West Nile in Latin America. They conjecture that a virulent strain of West Nile would be less likely to reach South America as it inhibits the reservoir bird to complete its difficult journey through the Caribbean. Apparently, a similar pattern is documented for St Louis Encephalitis virus where South American strains are less viremogenic than North American strains.

Latest update on West Nile situation in North America

In Canada, no human cases or seropositive dead birds were reported so far this year. However, in the US, according to CDC and ProMedMail,  5 states (Arizona, Mississippi, Oklahoma, Tennessee and Texas) reported a total of 8 West Nile cases in humans up to the 10th of June 2008. Six of these cases were classified as West Nile fever while 2 were neuroinvasive. Fortunately, no fatalities occurred so far. On ArboNet, the most recent case appears to be in May 2008 (21st epidemiological week) in Texas. Additionally, Alabama reported an equine infection (March 2008, 10th epidemiological week), while California and South Carolina reported WN-positive dead birds (the latest confirmed cases on the 22nd and 18th week, respectively). California and Florida reported WN seroconversion in sentinel animals (5th and 12th week, respectively). West Nile-positive mosquito pools have been reported from California (21st week), Illinois (23rd week), Indiana (23rd week), Louisiana (23rd week) and Texas (22nd week).Weeks given are the latest week for which West Nile positive pools have been found for that state.  Louisiana had a total of 251 positive mosquito pools reported up to 9th June 2008.

First case of West Nile fever in Texas in 2008

The first case of West Nile Fever in 2008 has been reported in Texas, Montgomery County, according to ProMedMail and the Houston Chronicle. The report states that although West Nile season typically starts in the summer, with mid-August through
mid-September being peak months because they're the hottest and driest months, it is not unusual to see cases as early as March and as late as December. However, one wonders how many cases of asymptomatic or mild West Nile infections must have been already gone unreported in the same area, given that 80% of infected people do not have any symptoms, and whether this early start signals an increased West Nile activity for Texas this year. Last year, Texas reported 170 neuroinvasive and 90 fever cases, and 17 deaths, according to state health records. The Centers for Disease Control and Prevention map of West Nile virus activity (as of 6 May 2008;  http://www.cdc.gov/ncidod/dvbid/westnile/Mapsactivity/surv&control08Maps.htm) as well as ArboNet shows that 2 human cases have been reported in Mississippi, one in Arizona, and one in Tennessee.

West Nile virus detected in birds in San Diego

According to ProMedMail, West Nile virus was found in 6 birds in San Diego County, California. This is unusual as evidence of viral infection in birds usually doesn't show up until the summer or fall. Fifteen cases have been reported in humans in 2007, 14 of them in people aged over 50. [Actually, ArboNET reports 16 total human cases in San Diego County, California in 2007 up to March 4, 2008]. The report mentions that mosquitoes usually die off in the winter, but a wet winter and a warm spring may have helped them survive in recent months. While this might be true, it is also possible that WNV is maintained by direct transmission between birds during the winter period. Interestingly, looking at reports of WNV-positive birds in 2007 in California, a few cases occured as early as January and February. Undoubtedly, human cases were only reported starting in June. Interestingly, WNV-positive mosquitoes appeared in earnest in July as well, while a few were reported in January [a warm spell?] and May. This could support the idea that cases of West Nile in birds are due to direct transmission between birds, and not vector-borne. Agreeing with Mod TY in the ProMedMail report, it would be very interesting (and important) to know whether there are any WNV-positive mosquito pools in San Diego County at the moment.

PRO/AH/EDR> West Nile virus, equine, camel - United Arab Emirates

Date: Fri, 18 Jan 2008
Source: The Horse [edited]
< http://www.thehorse.com/ViewArticle.aspx?ID=11186>


Antibodies to WNV common in Arab Emirate horses
-----------------------------------------------
Researchers found antibodies to West Nile virus (WNV) in nearly 20 per cent
of horses recently tested in the United Arab Emirates (UAE), according to
Ulrich Wernery, DVM, PhD, scientific director of the Central Veterinary
Research Laboratory in Dubai and author of the Dubai-based study published
in Wildlife Middle East [<http://wmenews.com/>].

The testing was initiated after one horse showed clinical signs of
encephalitis. "We were really astonished, especially to see only one
clinical case," Wernery said. "That means to us that we are dealing with a
very mild strain." The horse that showed clinical signs was treated and
recovered within a week.

A widespread survey of 750 horses in the UAE -- a country that previously
had no history of WNV -- was launched in late 2007 following diagnosis of
the index case in the city of Ghantoot. Results of the enzyme-linked
immunosorbent assay (ELISA) antibody test showed that 19.2 per cent of
these 750 horses had WNV antibodies. Within the immediate Ghantoot area, 84
per cent had antibodies for the disease. Serum from 11 Ghantoot horses was
sent to Cornell University College of Veterinary Medicine for further
testing, which revealed that all the horses in the sample had been exposed
to the virus at least 6 weeks prior to the survey. Exact numbers were not
released, but Wernery said all the horses showed "very high levels" of
antibodies.

"At the beginning we were shocked to see that many horses positive, but now
we are relieved (because) they are protected," Wernery said, cautioning
that this could not be considered lifelong immunity.

Wernery recommended that veterinarians vaccinate all horses traveling in or
out of the UAE -- particularly those in transit between the UAE and North
America -- regardless of their antibody levels.

About two-thirds of American horses have been vaccinated against WNV,
making a similar serosurvey in the United States unfeasible, said Frank
Hurtig, DVM, MBA, associate director of equine veterinary medical affairs
at Merial. However, an estimated 3-10 per cent of unvaccinated American
horses develop clinical signs of disease, he said, and 20-30 per cent of
those cases result in death.

Wernery said 30 per cent of UAE camels tested were also positive for WNV
antibodies. None of the camels have shown any clinical signs of encephalitis.

[byline: Christa Lest'e-Lasserre]

--
communicated by:
ProMED-mail rapporteur Joe Dudley

[Clinical signs of the neurologic disease caused by WNV in horses may
include anorexia, depression, ataxia, muscle twitches, partial paralysis,
impaired vision, head pressing, teeth grinding, aimless wandering,
convulsions, circling, and an inability to swallow. Attitudinal changes
including depression, somnolence, listlessness, apprehension, or periods of
hyperexcitability may be seen. Weakness, usually in the hind limbs, is
sometimes followed by paralysis. Coma and death may occur. Fever has been
seen in some but not all cases. Fatal hepatitis developed in one donkey
with neurologic signs in France.

However, the high incidence of sub-clinical inflections (manifested in the
high prevalence of seropositive horses) in the UAE is not that exceptional.
During an investigation in an equine research facility at the University of
California, Davis, in 2004, a (clinically) apparent to inapparent ratio of
1:4 among infected horses was seen. In 2005, when an equine infection
incidence of 62 per cent was seen, the apparent to inapparent ratio was of
1:17. See Carrie F Nielsen, William K Reisen, et al. High subclinical West
Nile virus incidence among nonvaccinated horses in Northern California
associated with low vector abundance and infection. Am J Trop Med Hyg 2008;
78(1): 45-52.

As to the findings in camels: these are known to be one of the mammal
species readily infected by WNV, though naturally occurring clinically
manifested cases are not known to this moderator. - Mod.AS

The United Arab Emirates can be located on the HealthMap/ProMED-mail
interactive map at <http://healthmap.org/promed?v=23.9,54.3,5 >. - CopyEd.MJ]