Equine respiratory infection is the most common problem that impacts the performance of the working ability of equids. In Ethiopia, respiratory disease (coughing and nasal discharge) is the most common presenting complaint at veterinary clinics and a priority concern for owners of the working equids and veterinary practitioners. Equine herpesviruses are important pathogens that are involved in respiratory problems of varying severity. In the present study, the detection of equine gammaherpesviruses (EHV-2 and EHV-5) from horses and donkeys in central Ethiopia is described. We also assessed the risk factors associated with respiratory disease and EHV-2/5 clinical outcomes.
In the present study, from a total of 58 equids, 34.5% and 32.8% were tested positive for EHV-5 and EHV-2, respectively. This shows that both EHV-2 and EHV-5 are prevalent in horses and donkeys residing in central Ethiopia. In this study, EHV-5 was recorded relatively in a higher proportion than EHV-2. This finding is consistent with previous studies conducted by Diallo et al.  and Wang et al.  in Australia and Negussie et al.  in Ethiopia, but in contrast to Dunowska et al.  in New Zealand, Nordengrahn et al. (2002)  in Sweden, Hungary, and UK and Torfason et al.  in Iceland in which EHV-2 has been recorded in a higher prevalence than EHV-5.
In this study, EHV-2 was detected in a significantly higher proportion of horses (54.5%) than donkeys (4%). In contrast, a significantly higher proportion of EHV-5 was detected in donkeys (56%) as compared with horses (18.2%). Our result demonstrated the presence of gammaherpesviruses susceptibility variation between host species. The existence of breed-specific susceptibility differences  and species-specific susceptibility differences  has been reported. However, for more validation, this variation in susceptibility should be further investigated using large sample size.
Co-infection of EHV-2 and EHV-5 was found in 6 (10.3%) equids displaying clinical signs of respiratory disease. This dual infection is consistent with other reports [9, 16, 23, 25] in which both viruses can simultaneously infect equids. This co-infection may have a synergistic effect on disease outcome and may lead to severe respiratory disease, however, need further investigation.
EHV-2 and EHV-5 were found test positive from 16 (48.5%) and 11 (33.3%) equids exhibiting clinical signs of respiratory disease, respectively. Similarly, EHV-2 and EHV-5 were tested positive from 3 (12%) and 9 (36%) equids with no signs of illness, respectively. The existence of EHV-2 and EHV-5 in apparently healthy equids is in agreement with previous studies conducted elsewhere [16, 19, 25, 26]. A significantly higher proportion of EHV-2 was detected in equids with signs of respiratory disease (48.5%) as compared to those without clinical signs (12%). EHV-2 positive equids were seven times more likely to display clinical signs of respiratory disease than EHV-2-negative equids (OR = 6.9; 95% CI: 1.72–27.60). This causal association between EHV-2-test-positive and the appearance of clinical signs of respiratory disorders may suggest the involvement of EHV-2 in the development of respiratory diseases. Negussie et al.  proposed that EHV-2 may have a possible etiological contribution either to induce or predispose equids to respiratory diseases. Ataseven et al.  stated that EHV-2 played a pre-disposing role in the occurrence of respiratory diseases. However, the presence of a statistical association between EHV-2-test-positive and displaying of clinical signs alone is not sufficient to causal linkage with the disease, thus, further detailed studies on the pathogenesis are necessary to unequivocally conclude a causal association between EHV-2 and induction of clinical symptoms.
In this study, a higher proportion of EHV-2 was detected in yearlings and young equids (42.9%; n = 6) as compared with adults (34.3%; n = 12) and old ages (11.1%; n = 1), however, the observed difference was not statistically significantly (P > 0.05). Our study is in agreement with previous studies where equids are highly infected with EHV-2 in the first years of life despite the presence of maternal antibodies in the colostrum [12, 23, 27]. Similarly, although not significant (P > 0.05), EHV-5 was detected in a higher proportion in older equids (55.6%; n = 5) as compared with adults (31.4%; n = 11) and young (28.6%; n = 4) (Table 3). Some epidemiologic evidence suggests that foals are usually infected with EHV-5 later on in life than with EHV-2 [23, 27].
The proportion of equids infected with EHV-2 and EHV-5 was varied in different geographical locations of the sampling sites. A significantly higher (P = 0.041) proportion of EHV-2 test positive was found in equids residing in midlands (52.9%) as compared with highlands (24.4%). Equids residing in the midlands were four times more likely to be exposed to EHV-2 than highlands equids (OR = 3.97; 95% CI: 1.05–14.89). In contrast, a higher proportion of EHV-5 was found in equids residing in highlands (39.0%) as compared with midlands (23.5%), however, this difference was not statistically significant (P = 0.444). This study suggests a wide distribution and varying susceptibility to the viruses in different agro-ecology. Azab et al.  reported that the variation in the prevalence of the disease among studies is attributed to geographical variability and environmental factors. However, at present, a possible explanation of why equids residing in different agroecology have varying susceptibility to EHV infection could not be given.
In conclusion, EHV-2 and EHV-5 are highly prevalent both in horses and donkeys residing in central Ethiopia. Concurrent infection with EHV-2 and EHV-5 was recorded. Species-specific susceptibility differences in EHV-2 and EHV-5 infection are observed. Our result suggests the involvement of EHV-2 in the development of respiratory diseases. However, further studies are necessary to unequivocally conclude respiratory illness associated with EHV-2.