The occurrence of elephant endotheliotropic herpesvirus infection in wild and captive Asian elephants in Thailand: Investigation based on viral DNA and host antibody

Abstract

Background and Aim: Elephant endotheliotropic herpesvirus (EEHV) is a serious disease, threatening the life of young elephants. Many elephants have been infected with no clinical signs and may serve as carriers spreading this disease. It is important to monitor the disease through clinical signs and molecular diagnosis. In this study we investigated the occurrence of EEHV and the efficiency of different techniques used to monitor EEHV infection in various samples and populations of Asian elephants. Materials and Methods: Blood and trunk swabs were collected from live elephants, while visceral organs (lung, digestive tract, spleen, lymph nodes, and kidney) were collected from dead elephants. EEHV was detected by polymerase chain reaction (PCR) in whole blood, trunk swabs, and visceral organs as samples, while elephant anti-EEHV immunoglobulin G (IgG) in serum was detected by enzyme-linked immunosorbent assay (ELISA). A total of 162 samples were analyzed in this study: 129 from healthy, 26 from dead, and 7 from sick elephants. Results: The present study showed that the overall incidence of EEHV was 40.1% (n=65/162). Approximately 46.2% (n=12/26) and 85.7% (n=6/7) of dead and sick elephants were positive for EEHV by PCR, respectively. All sick elephants that were young and affected by EEHV clinical disease tested negative for the IgG antibody ELISA, suggesting primary EEHV infection in this group. In addition, 2.3% (n=3/129) of subclinical infections were detected using PCR, and trunk swab samples showed slightly higher sensitivity (5.3%, n=2/38) to detect EEHV than whole blood (1.2%, n=1/84). As many as, 48.4% (n=44/91) of healthy elephants were EEHV seropositive (ELISA-positive), suggesting that many elephants in Thailand had previously been infected. Overall, 30% of dead wild elephants had been infected with EEHV (n=3/10). Moreover, statistical analysis revealed no significant differences in the EEHV detection rate between different age groups or sexes (p>0.05). Conclusion: PCR is better than ELISA to detect EEHV active infection in dead/sick elephants and to monitor EEHV in young elephants. ELISA is suitable for detecting previous EEHV infection and carriers, particularly adults. Theoretically, we could use both PCR and ELISA to increase the sensitivity of testing, along with observing abnormal behavior to efficiently monitor this disease. Identification of EEHV carriers within elephant populations is important to prevent transmission to healthy individuals, especially young elephants with high mortality from EEHV. This is the first report from Thailand regarding EEHV infection in wild elephants, showing the importance of preventing disease transmission between captive and wild elephants.

Development of Nonstructural Protein-Based Indirect ELISA to Identify Elephant Endotheliotropic Herpesvirus (EEHV) Infection in Asian Elephants (Elephas maximus)

“…A serological study for EEHV detection using ELISA tests has been implemented in elephants in European zoos, as well as those living in captivity in North America and Thailand, using EEHV structural proteins such as gB and gH/gL [ 21 , 23 , 24 ]. In contrast to the results of previous reports, however, the seropositivity results for the EEHV-DNApol ELISA test in this study were lower than those of previous reports, indicating 42–44% seropositivity for EEHV-gB ELISA in Thai captive elephants [ 22 , 24 ]. Moreover, a serosurvey of EEHV infection using the EEHV-gH/gL ELISA revealed that more than 97% of elephants from European zoos were determined to be EEHV seropositive [ 23 ].…”

“…Viral genome detection includes PCR, qPCR, loop-mediated isothermal amplification (LAMP), and in situ hybridization [ 12 , 13 , 14 , 15 , 16 , 17 ]. Viral protein and viral particle detection involves histopathology, Western immunoblotting, immunohistochemistry, immunofluorescence, and electron microscopy [ 2 , 3 , 4 , 18 , 19 ], while anti-EEHV antibody detection involves enzyme-linked immunosorbent assay (ELISA) and the luciferase immunoprecipitation system (LIPS) [ 20 , 21 , 22 , 23 , 24 ].…”

“…Generally, the determination of viral infections using antibody detection has become preferable over other diagnostic tools because it is considered to be more cost effective, simple, less laborious, and has the capacity to screen a large number of samples. To date, most of the antibody tests developed for EEHV detection have been based on EEHV structural proteins (SP), namely, gB and gH/gL [ 21 , 22 , 23 ]. The presence of antibodies against viral SPs cannot be used to determine active viral replication in the host, since the presence of these antigens can be a sequence of either viral replication or degradation of the virus, during phagocytosis of antigen-presenting cells (APCs) [ 25 , 26 ].…”

Elephant Endotheliotropic Herpesvirus 1, 4 and 5 in China: Occurrence in Multiple Sample Types and Implications for Wild and Captive Population Surveillance

“…The risks posed by the EEHVs to captive and wild Asian elephants are widely recognized. Surveys of captive, semi-captive, and, to a lesser extent, wild elephants have been conducted in several countries, and surveillance for captive and semi-captive elephants have been developed to suit local conditions [2,14,15]. China is an Asian elephant range country and has a captive Asian elephant population; however, the EEHV status of China's wild and captive elephants is not known.…”

“…In this study, we used PCR assays to detect active infections in the elephants that we studied instead of screening for infection using serology, as the only population of elephants that we could obtain blood samples from was the captive population housed at the Wild Elephant Valley. Using molecular screening also allowed us to compare the sequences of the detected types and subtypes to those reported in other countries [15]. Screening elephants with PCR assays, however, has its limitations, as it only detects elephants with active infections and not latently infected elephants that are not viremic or shedding EEHV.…”