|Michelle Kutzler DVM, PhD
College of Veterinary Medicine
Oregon State University
Corvallis, Oregon 97331
|David E Anderson, DVM, MS
College of Veterinary Medicine
The Ohio State University
Columbus, Ohio 43210
In recent years, West Nile Virus infection has been spreading westward
the eastern USA. The WNV was introduced into the USA in the late 1990's
has become a vital concern to the avian and equine industries. Although
rare, WNV may infected humans and can cause fatal encephalomyelitis. The
case fatality rate in humans infected with WNV is less than 5%. Many
species of mammals have been infected with WNV including cattle, sheep,
goats, llamas, alpacas, and camels but clinical disease is rare. In
WNV countries, seropositive rates vary widely among ruminant species
cattle, sheep, goats, camels, and water buffalo). Although seropositive
rates may approach 75 %, clinical WNV infection in ruminating species
appears infrequent. Llamas and alpacas are considered to be a low-risk
species. This assertion was based on preliminary data from the USA in
approximately 1 clinically infected llama or alpaca has been documented
each 1500 horses documented. However, several cases of fatal WNV have
diagnosed in llamas and alpacas.
What is WNV?
West Nile Virus is a type of virus termed "Flavivirus".
Where does it come from?
West Nile Virus is endemic to North Africa. The virus is
ubiquitous in the Nile River Valley and epidemiological studies have
seroconversion in a large range of native species.
How is it spread?
Birds serve as the natural reservoir for WNV. Mosquitoes are the
most importrant vector for transmission of the virus. Horses are viremic
for a relatively short period of time but may remain viremic for a long
enough period to allow infection of the insect vectors. This does not
to occur in the ruminant species. The physiology of South American
(SAC's: llama, alpaca, vicuna, guanacoe) most closely resembles that of
ruminants. Thus, we anticipate that llamas and alpacas will not serve as
reservoir for infection of herdmates and the insect vectors.
What species can become infected?
WNV is most pathogenic in avian species and horses. Humans and
other mammals can become infected but usually remain assymptomatic.
Serological surveys of sheep, goats, cattle, and camels indicate that up
75 % of the population may have circulating antibodies to WNV and these
antibodies may remain detectable for at least 12 months after exposure.
However, clinical disease in these species is rare.
What are the clinical signs of the disease?
Clinical signs are variable in horses, but neurological signs are
most common. These signs include ataxia, recumbency, tachycardia,
tachypnea, seizures and death. At Ohio State University's Veterinary
Teaching Hospital, two cases of WNV infection in alpacas were diagnosed
the fall of 2002. The infection progressed from head tilt and ataxia to
recumbency and seizures over a period of 72 hours. Death occurred within
hours despite intensive medical therapy.
How is WNV infection diagnosed?
A number of diagnostic tests have been developed. In living
animals, the virus neutralization (VN) test is used to detect antibodies
bloodstream and is not species specific. In horses, an ELISA test for the
immunoglobulin type IgM specific to WNV has been developed. This test is
specific to horses and can not be applied to ruminants or SAC's. In
that have died, immunohistochemistry tests can be performed on frozen
tissues (e.g. brain, heart muscle, etc.).
What is the treatment for affected animals?
No specific anti-viral treatments are available for WNV. Clinical
management of infected animals is achieved through symptomatic treatment.
These may include anti-inflammatory drugs or steroids, IV fluids and
electrolyte therapy, physical therapy, and nutritional and environmental
How can the disease be prevented?
The single most important tool in WNV prevention is elimination of
insect vectors. Since mosquitoes are the most prevalent vector, control
measures should include management practices to eliminate mosquitoes.
Mosquito control includes drainage of standing water sources, mowing
vegetation around ponds and lakes, and aerating water sources to decrease
mosquito larvae survival (e.g. mosquito larval survival is significantly
reduced if oxygen dissolved in water can be maintained above 400 ppm).
Is there a safe vaccine against WNV?
A killed virus vaccine has been developed for use in horses (Fort Dodge
Animal Health). The manufacturer's recommendation includes an initial
vaccination followed 3 to 6 weeks later by a booster vaccination. This
vaccine was recently tested for safety by Oregon State University where
male llamas and alpacas were innoculated and at The Ohio State University
where 14 female llamas and alpacas, including pregnant females, were
inncoluated. These studies found no adverse reactions or abnormal health
events after the initial dose of the vaccine in adult llamas and alpacas.
The initial vaccine dose was not inflammatory and no adverse tissue
reactions could be found. In the Ohio State study, the booster dose 3
later stimulated a mild injection site swelling. The reaction was milder
and resolved faster than that typically seen after Clostridial
No other adverse effects occurred.
Is there an effective vaccine against WNV?
The efficacy of the vaccine to stimulate antibody production is
variable. In studies at Ohio State University, no antibody response was
observed after a single dose of the WNV vaccine. After a booster dose,
given 3 weeks after the initial vaccination, serum titer rose
during the three
weeks following the booster. However, serum titers had decreased
dramatically by week 10 (7 weeks after the booster). The results of this
study suggest that the WNV vaccine may stimulate immunity but that this
immunity may be short-lived. The efficacy of the vaccine against actual
infection with WNV has not been investigated and no data exists to
determine if these antibody concentrations will prevent or lessen the
severity of infection. However, the antibody response is appropriate for
this vaccine. Thus, the WNV vaccine likely can be administered safely to
llamas and alpacas, but no information is available regarding its
against natural infection or the frequency of
vaccination required to maintain protective immunity. Oregon State
University is currently studying a 3 dose protocol to attempt to gain
greater and longer lived antibody concentration in the blood stream.
What about the Epidemiology of WNV?
The true risk factors for WNV in llamas and alpacas remain
unknown. Both Oregon State University and Ohio State Univerity are
gathering epidemiological data at this time. Oregon State University will
closely track the disease as it moves to the western USA. Ohio State
University will evaluate the effect of established WNV infection in the
environment on resident camelid populations. This data will reflect
that occurred in 2002 as well as the progression of the disease
West Nile Virus is a new disease in North America and can be
rapidly spread by mosquitoes. Although extremely unlikely, WNV may infect
llamas or alpacas. No information is currently available regarding
of vaccination against natural infection with WNV in llamas or alpacas.
Preliminary vaccination results warrant further investigation.
Prevention of WNV infection should center on elimination of risk factors
such as mosquitoes and environments conducive to mosquitoes. The issues
surrounding vaccination against WNV must be made on a farm-specific and
geographical basis. Discuss the risks and benefits of vaccination with
veterinarian to determine if this is right for your herd.
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