Serological data indicate a widespread presence of rabbit haemorrhagic disease in rabbit farms in Algeria

Categories: Disease & Virus

April 5, 2025

Overview of presence and geographic distribution of RHD-positive Sera in not vaccinated rabbits

We analysed 184 sera collected from 19 farms in eight provinces between October 2020 and May 2022. One hundred three sera (56.0%) were classified as negative for RHD because they were negative in both cELISAs (RHDV and RHDV2) or negative in one ELISA and doubtful in the other. The remaining 81 sera (44.0%) were classified as positive for RHD, having tested positive with a titre of 1/10 in at least one of the cELISA tests. Table 1 shows the percentage of RHD-positive sera for each province. Although in practice, RHD seropositive unvaccinated rabbits were found in all provinces, their percentage significantly varied, ranging according to the province, from low values (5–20%) in Algiers and M’Sila to intermediate values (30–50%) in Blida, Boumerdes and Tizi Ouzou, to 100% of positive rabbits in Laghouat province.

Table 1 Serological results obtained from unvaccinated rabbits at either the Province or district levels.

Table 1 also shows all results obtained on each farm and is representative of individual districts to understand the outcomes of the serosurvey better. In 5 of the 19 farms tested, the sera collected from unvaccinated rabbits (47) were all negative; interestingly, all were sampled from fattening rabbits. These farms were distributed within five provinces, precisely one per province, whereas in the remaining three provinces (Blida, Laghouat, and Tizi Ouzou), all farms tested positive for RHD. Of note, in 8 of the 14 farms found to be seropositive for RHD, sera were collected from fattening rabbits between 75 and 150 days old. Therefore, considering the high RHD titres found in most of these rabbits (see below), these farms, primarily located in Laghouat and Tizi Ouzou provinces, had been undoubtedly affected by RHD outbreaks at most 1–2 months before sampling. Considering that the farm in the Tazmalt district, Bejaia province, was the only one formally declared to be affected by RHD and that this was confirmed by serology obtained on sampled breeding rabbits (see also below), the results obtained by cELISAs established that nine of the 14 farms examined and resulted positive had been affected by RHD shortly before sampling.

Determination of the type of agent responsible for RHD outbreaks in not vaccinated rabbits

Since RHDV and RHDV2 have distinct immunogenic and antigenic profiles, it is frequently possible to compare the titres of a serum obtained in the two cELISAs to determine which lagovirus caused an RHD outbreak. Table 2 shows the percentages of the 81 RHD-positive sera collected from unvaccinated rabbits, assigned to one of five possible categories according to the specific RT value for each examined farm. All four convalescent rabbits, collected from a farm in Tazmalt, were surely or were suspected to be positive due to an RHDV2 infection.

Table 2 Rabbits’ distribution into distinct categories based on RT value (RHDV2/RHDV titre ratio).

As for RHDV, only one serum was classified as RHDV-infected in Baba Ali district (RHDV cELISA titre 1/160, negative for RHDV2 cELISA), while a second serum from the same farm was negative; both sera have been collected from rabbit breeders older than one year. In addition, we collected 63 sera on the same farm from vaccinated rabbits a few months later to check the effectiveness of vaccination. Still, the results did not indicate any other cases of possible RHDV infection (see below). Two other sera were classified as suspected RHDV infection in farms in the Thenia and Ksar El Hirane districts, but half of the rabbits collected from these farms were also found to have been infected with RHDV2. In all remaining farms, sera were classified as coming from RHDV2-infected or unclassifiable rabbits.

The average RHDV2 titres found in the farms were highly variable, ranging from low to medium titres in breeding rabbits to high titres (≥ 1/1280) in fattening rabbits. Moreover, this last result also correlated, in several cases, with a high percentage of positive sera, reaching 100% in some farms, as all those in the Laghouat province. Unexpectedly, we found significant variability among farms in classifiable and unclassifiable sera proportions. Indeed, while farms such as Makouda 1 and 2 showed a similar proportion to that of the convalescent rabbits in Tazmalt, i.e., all sera were traceable to RHDV2 infection, in Makouda 3, Makouda 4, and Laghouat Center unclassifiable sera were 89, 70 and 62% respectively, and this despite high cELISA titres (see Fig. 1) suggesting RHD outbreaks in these farms a few weeks before sampling. However, assigning outbreaks at the farm level was still possible as caused by RHDV2.

Figure 1 shows the cELISA, IgM, and IgA results for farms with different RT2 profiles, according to what is reported in Table 2. Tazmalt convalescent breeding rabbits showed a typical isotype profile of an infection with a high IgA titre (1/5120) and a consistently lower IgM titre (1/320), suggesting an estimated time of infection of 2–3 months before sampling. RT2 values were also typical, ranging from 4 to 32, as evident by the difference in the RHDV/RHDV2 cELISA titre in Fig. 1. Makouda 1 and 2 showed a similar profile to Tazmalt but with all sera negative for IgM and lower titres for both ELISAs. This profile suggests a time of infection (or re-infection) of more than three months before sampling, which is, however, not compatible with the claimed age of the rabbits (90 days). Farms in the last two boxes showed IgM, IgA and cELISA titres indicative of an infection that occurred a few weeks before sampling. Still, they differed in having sera with high RHDV cELISA titres, like those for RHDV2. Taken together, the data suggested that high cELISA titres for both RHDV2 and RHDV induced by an RHDV2 infection may be related to the early development of humoral immunity in young rabbits.

Serological verification in vaccinated rabbits

The survey was only possible on three farms located in different provinces. The most extensive study was conducted on a farm in the Baba Ali district (Algiers), where we collected 63 sera on several occasions from June to September 2021 on breeding rabbits of around two years old. The last vaccinations for RHDV with Cunipravac RHD (Hipra, Amer, Girona, Spain) and for RHDV2 with Eravac (Hipra, Amer, Girona, Spain) were carried out approximately 3–5 months before the sera sampling. Surprisingly, only seven sera (11%) tested positive for RHD, with rather heterogeneous ELISA results, as shown in Fig. 2.

Of the three sera collected in June, one was only positive in cELISA, one was positive for IgA but not for IgM, and the last was highly positive for both IgM and IgA, indicating a recent RHDV2 infection. Similarly, of the remaining four sera collected in September, three had a low RHDV2 titre, while one was from a rabbit recently infected with RHDV2. Note that the sera collected in July and August, corresponding to 62% of the total, resulted in all negative.

The second farm monitored was in the district of Tigzirt (Tizi Ouzou) (Fig. 3). This farm also used double vaccination of breeding stock (Cunipravac RHD for RHDV and Eravac for RHDV2), with sampling done 5–6 months later. Still, the situation was opposite to the previous one, being positive 11 out of 12 sera (92%). The ELISA titres ranged from 1/40 to 1/640 for both RHDV and RHDV2 and on average, they were somewhat higher for RHDV2, as expected from vaccination with Eravac. While all sera, except one at low titre, were negative for IgM, they showed a certain variability of IgA. In fact, four (36%) of them resulted negative, three (27%) showed low IgA titres (1/320-1/640), and three (27%) medium-high titres (1/2560–1/20480). The last farm monitored was the one in the Chabet El Ameur district (Boumerdes), which was only vaccinated with the Cunipravac RHD one year before sampling. 62% of the sera, collected from 8 breeder rabbits > 1-year-old, resulted RHD-positive but with variable and even high RHDV2 titres, ranging from 1/20 to 1/2560. In addition, three showed high titres of IgM and IhA for RHDV2, indicating that the virus was circulating in the farm shortly before sampling.

Survey on the non-pathogenic lagovirus

To search the farm for the presence of the non-pathogenic Lagovirus RCV-A1, we analysed all the sera that tested negative, doubtful, or 1/10 in RHDV cELISA with the RHDV IgG ELISA, for a total of 97 sera representative of 16 farms. Apart from a few sporadic sera that tested positive at very low titres and, therefore, not indicative of RCV-A1 infection, one positive finding occurred on a farm in Soumaa district (Blida province). All ten rabbits sampled were two years old: three were positive for RHDV2 with low to medium titres, one was negative for all ELISAs, but six were positive only for RHDV IgG ELISA with titres ranging from 1/320 to 1/10,240 (average 1/4000), a clear signature of the presence of RCV-A1.

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