Eastern Equine Encephalitis (EEE) in Alpacas
Jack Dibb
In September and October, 2005, five alpacas were confirmed to have been fatally stricken with Eastern Equine Encephalitis. Three of these animals were from two different farms in south eastern New Hampshire, the other cases were from New York and New Jersey. NEAOBA and researchers from the Cummings School of Veterinary Medicine at Tufts University conducted a limited study in New Hampshire and New York during October to determine the prevalence of natural antibodies against EEE in alpacas living in areas with a high incidence of EEE associated death. In November a proposal to conduct a trial of an equine EEE and WEE vaccine in alpacas was approved for funding by the Alpaca Research Foundation (ARF). This trial began in December, 2005, results are expected in Spring 2006.
Eastern equine encephalitis is a rare but serious disease caused by the Eastern equine encephalitis (EEE) virus, a member of the family Togaviridae, genus Alphavirus. Eastern equine encephalitis is found mainly along the eastern seaboard and Gulf coast of the United States. EEE is a mosquito borne infection of wild birds that can be transmitted to other birds, humans, horses and other animals by mosquitoes.
Infection with Eastern equine encephalitis virus can cause a range of illnesses. Most people infected with the virus have no symptoms; others get only a mild flu-like illness with fever, headache, and a sore throat. In rare cases, infection of the central nervous system occurs, causing sudden fever, muscle pains and a headache of increasing severity, often followed quickly by seizures and coma. In these rare instances, about one third of patients die from the disease. Of those who survive, many suffer permanent brain damage. Clearly, it is hard to know about mild cases in animals, but the severe neurological effects, including death, are known to occur in some infected horses (and now camelids). Reported clinical signs of EEE infection in llamas and alpacas are consistent with brain (CNS) disease and may include dullness, fever, incoordination, mentally inappropriate behavior, seizures, inability to rise, persistent tilting of the head, rapid eye movement and a swan-like backward flexion (opisthotonus) of the head and neck. Unfortunately, there are no disease specific findings for EEE infection, as these signs may mimic other CNS and spinal problems such as West Nile Virus infection, Equine Herpes Virus-1, rabies, trauma, bacterial meningitis, Thiamine (Vitamin B1) deficiency, tick paralysis and meningeal worm infection. Therefore, additional diagnostics such as a spinal tap and blood analysis are commonly required to make a diagnosis. To date, most reported cases have involved young camelids.
The Eastern equine encephalitis virus has a complex life cycle involving birds and a specific type of mosquito, Culiseta melanura, which lives in acid water swamps. These mosquitoes feed only on birds; they do not normally feed on humans or other mammals. Infected wild birds do not usually become ill with the disease. The natural cycle for EEE virus occurs nearly every year in some swamp habitats. In certain years the virus may become amplified in this bird-mosquito cycle, however, and other species of mosquitoes which feed on both birds and mammals act as bridge vectors and are capable of transmitting the virus to mammals, including horses and humans. Late summer of 2005 was such a year, with an outbreak of EEE in the north east causing several human fatalities in Massachusetts and New Hampshire. It is possible that 2006 may also be a severe year for EEE in the northeast.
Prior to September, 2004, it was felt that camelids were not infected by the EEE virus, but necropsy of an alpaca at the Cummings School of Veterinary Medicine at Tufts University at that time confirmed EEE as the cause of death. In the course of preparing a publication on this case, researchers at Tufts discovered several other cases where EEE had been confirmed as the cause of death in camelids. The fact that alpacas were (and are) at risk from EEE was not widely disseminated; early in the 2005 outbreak described below ICI was strongly asserting that camelids could not be infected.
In late September, 2005 a cluster of cases raised the visibility of EEE in alpacas significantly. Four animals from three different farms in south eastern NH presented to a single veterinary practice within a 2 week period. The first case was a 10 day old cria that had progressed to recumbency and seizures before reaching the vet. This animal died soon after being admitted to the clinic, no postmortem analyses were conducted. However, when a 16 day old cria from a nearby farm presented at the same clinic with similar symptoms, alarm bells went off and intensive supportive measures were initiated. Despite these efforts, case 2 succumbed within 48 hours. Days later cases 3 (another cria from a third farm) and 4 (healthy yearling from same farm as case 2) also presented at the same clinic. These animals died within 4 and 3 days, respectively. PCR analysis (polymerase chain reaction, a DNA test) of brain tissue from cases 2-4 confirmed EEE for all cases, The first death is also suspected to be due to EEE but was never confirmed.
During this period the NEAOBA board of directors initiated several efforts to learn more about EEE in camelids. First step was to ascertain whether the outbreak was localized or more widespread. Mass emails to all NEAOBA members requested “private” reporting of any similar cases. This revealed one additional fatal EEE case in an alpaca from central New York, as well as a cria in south western NH that displayed neurological problems including seizure, but survived. Analysis of serum from this animal subsequently confirmed high titer of EEE antibodies. It was also during this time that NEAOBA learned of the earlier cases at Tufts and established initial contact with Dr. Daniela Bedenice (also Drs. Karen Baum and Patricia Craven, members of the ARF BOD).
News of the outbreak spread beyond the north east through forwarded email messages and several of the industry chat sites. In October a breeder in New Jersey reported that a breeding male had died from EEE, in this case the attending vet felt that stress from elective surgery had weakened the animal’s immune system and made it easier for the virus to overcome his defenses.
NEAOBA decided to launch two studies, in collaboration with Dr. Bedenice and Dr. Amy Bright (a clinical vet at the practice that treated the four cases in south east NH described above). To establish seroprevalence (fraction of the population with natural antibodies) of EEE antibodies in naturally exposed alpacas we collected serum samples on the 22nd of October from 14 surviving animals on the NY farm that lost an animal to EEE, 27 survivors on the two NH farms that suffered losses, and 46 animals living on 4 different NH farms that were within 10 miles of the impacted farms. Serum neutralization (by PRNT) tests were conducted at the US Department of Agriculture National Veterinary Services Laboratory (NVSL). The seroprevalence of EEE antibodies (PRNT titer > 1:10) was as high as 29% on one of the farms with an overall prevalence of 10% in this study.
The second study is the vaccine trial, “Humoral response to EEE vaccination in healthy alpacas” supported by ARF. Dr. Daniela Bedenice is the Principal Investigator, collaborating researchers include: Jack Dibb, NEAOBA treasurer, Drs. Tim Fallon and Amy Bright, Cornerstone Vet Hospital, and Dr. Doug Pedersen, National Veterinary Services Laboratories (NVSL). NEAOBA breeders provided 39 subject animals, spread between two farms in Maine, two farms in NH and one in Vermont. We selected a bivalent (eastern and western equine encephalitis) inactivated vaccine because no monovalent EEE vaccines are currently commercially available. Dr. Craig Barnett at Intervet arranged for donation of the Intervet vaccine sold as Encevac™.
Specific objectives declared in the proposal are reproduced here:
1. Evaluation of EEE serum-antibodies via a semi-quantitative plaque reduction neutralization test (PRNT) following 3 intramuscular vaccinations with Encevac™ at 4-week intervals in 45 healthy alpacas.
2. Assessment of local and systemic reactions following EEE vaccination.
We hypothesize that alpacas will develop virus-neutralizing antibody titers similar to those reported in horses following repeated EEE vaccination.
In plainer English, objective 1 is to determine whether the vaccine leads to antibody titers that are similar to those proven to provide protection against EEE infection in other animals and objective 2 states that we want to be sure the vaccine has minor or no side effects. It is important to recognize that this trial does not intend to and will not prove that the Encevac™ vaccine is actually protective against EEE virus infection in alpacas, since proving this would require deliberately exposing vaccinated animals to the live virus. Such a study would have to be performed on a small scale in specialized and approved facilities due to the risks involved when working with the EEE virus.
As noted in the objectives, each subject animal will receive an initial vaccination, followed by boosters 4 and 8 weeks later. Serum samples will be collected immediately before the first vaccination, and then at 2 week intervals for a total of 12 weeks. It is possible that some of the subject animals will be sampled through the summer to monitor the persistence of any titers that are developed.
The vaccine trial began in mid December, and is nearing the end of its scheduled course. None of the animals were withdrawn from the trial due to adverse side effects. Per the advice of Dr. Pedersen, serum samples will be analyzed as a single batch at the end of the study to ensure the highest possible precision.
Alpacas first moved in with Jack Dibb to found Sallie’s
Fen Alpacas in Barrington, NH in 1994. Nicola Blake joined this happy obsession
in 1999. When not on cria watch, Jack and Nicola
research air quality around the world in their capacity as atmospheric
chemists. Being so geographically and emotionally close to the NH farms
stricken by EEE in 2005, they are eagerly anticipating the results from the
vaccine trial described above.