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Author affiliation: Université Mannouba Tunisie, École Nationale de Médecine Vétérinaire, Sidi Thabet, Tunisia (A. Amara, R. Andolsi, A. Malek); Istituto Superiore di Sanità, Rome, Italy (M.A. Di Bari, R. Bruno, B. Chiappini, I. Vanni, E. Esposito, G. Riccardi, O.A. Ben Abid, S. Marcon, R. Nonno, U. Agrimi, G. Vaccari, L. Pirisinu); Arrondissement de Production Animale de Siliana, Siliana, Tunisia (K. Emehatli); Regional Delegation for Agricultural Development in Tataouine, Tataouine, Tunisia (B. Ben Smida); Arrondissement de Production Animale de Sousse, Sousse, Tunisia (H. Kessa); Regional Delegation for Agriculture in Medenine, Medenine, Tunisia (W. Chandoul); Institut Pasteur de Tunisie, Tunis, Tunisia (M. Handous); Direction Générale des Services Vétérinaires, Ministère de l’Agriculture, Tunisia (R. Khorchani) Ministry of Agriculture, Tunis (M. Zrelli)
In 2018, a novel animal prion disease was identified in Algeria, termed camel prion disease (CPrD), affecting a previously unreported host species, the dromedary camel (Camelus dromedarius) (1). After identification of CPrD in Algeria, an epidemiologic surveillance network was set up in Tunisia to monitor neurologic diseases in dromedaries. We describe results of investigations of suspected cases and report the detection of 6 CPrD cases.
The Study
Tunisia hosts ≈57,000 dromedaries, 75% of them in the south. Accordingly, investigations focused on Tataouine, the southernmost governorate. Over 3 years, we identified 8 dromedary camels showing neurologic and behavioral signs consistent with CPrD (1), raising suspicion of prion disease (Table).
Figure 1
Figure 1. Detection and characterization of the PrPres of scrapie prion protein (PrPSc) from brain tissues of dromedary camels, Tunisia, 2019–2021. A) PrPresdetected using TeSeE Western…
In addition to a histopathologic examination for spongiform changes, we examined all brains for the presence of the scrapie isoform of prion protein (PrPSc) by using Western blot (WB) and immunohistochemistry (IHC) (Appendix). We performed WB analyses by using the TeSeE western blot kit (Bio-Rad Laboratories, https://www.bio-rad.com) and the ISS (Istituto Superiore di Sanità) discriminatory WB method, validated for the animal prion diseases surveillance in Europe. Six animals tested positive, revealing the presence of the pathognomonic protease-resistant PrPSc (PrPres), characterized by the classical 3-band pattern in the 18–30 kDa range, in all available brain regions (Appendix Table 1), providing evidence of the involvement of several brain areas (Table; Figure 1, panels A, B). Conversely, 2 animals tested negative.
After confirmation of PrPSc, we conducted in-depth analysis of the 6 positive cases to investigate PrPres features, including protease cleavage site, presence of additional fragments, and glycosylation pattern. We treated samples with high concentrations of proteinase K to clearly define the cleavage site and probed them with a panel of monoclonal antibodies spanning the PrP sequence (Appendix Table 2, Figure 1).
PrPres from dromedary isolates displayed a higher apparent molecular weight of the unglycosylated band than the scrapie control, and we detected no additional C-terminal or internal fragments (Figure 1, panel C; Appendix Figure 1). PrPres exhibited lower overall glycosylation levels compared with scrapie, mainly in cortical areas relative to subcortical regions (Figure 1, panel D; Appendix Figure 2). The PrPres characteristics of the cases, including the electrophoretic profile, molecular weight and glycoprofile, are consistent with those previously reported in CPrD cases in Algeria (1).
We initially performed histopathologic and immunohistochemical analyses on the medulla oblongata and cerebellum, the primary target regions for animal TSE surveillance. We subsequently extended the analyses to other areas of the brains available (Appendix Table 1).
Figure 2
Figure 2. Histopathologic and immunohistochemical analyses of medulla oblungata and cerebellum of dromedary camels, Tunisia, 2019–2021. Images show the analyses performed on medulla oblongata (A, E, C, G) and cerebellum (B, F,…
Histopathologic examination revealed spongiform changes in only 2 animals (P81/16 and P81/65), in both obex (mainly in the dorsal vagal nucleus and the nucleus of the solitary tract) and cerebellar cortex (molecular layer), whereas we observed no lesions in the other animals (Table; Figure 2, panels A–D). Immunohistochemistry performed on the medulla oblongata and cerebellum demonstrated the presence of PrPSc in all samples except P81/15 and P81/64. In the obex, we observed intense PrPSc immunoreactivity in the dorsal vagal nucleus, the nucleus of the solitary tract (Figure 2, panels E, G), the hypoglossal and olivary nuclei, and the reticular formation. In the cerebellum, we detected extensive PrPSc immunolabeling throughout all layers of the cerebellar cortex (Figure 2, panels F, H) and in the white matter.
Figure 3
We extended histopathological and IHC analyses to other brain regions available for each case (Appendix Table 3). We observed mild spongiform changes exclusively in the cortices of animal P81/16 (Figure 3, panel A) and scattered vacuoles in the basal ganglia of P81/13 and P81/14.
IHC confirmed the diagnosis established from the medulla and cerebellum, identifying the same 6 CPrD-positive animals. In the cortex, PrPSc deposition was predominantly localized to layers I, V, and VI (Figure 3, panels B, C). The basal ganglia and thalamus (Figure 3, panel D) showed marked PrPSc accumulation in gray and white matter. We identified multiple PrPSc deposition patterns, including punctate and diffuse, intraneuronal, perineuronal, stellate, and perivascular (Figure 3, panels E–I). Of note, in the medulla oblongata, we observed a dense intra-astrocytic PrPSc deposition that filled the entire cytoplasm (Figure 3, panel J). IHC revealed PrPSc deposits in the primary and secondary follicles of all available lymph nodes, with variable immunostaining intensity (Table; Figure 3, panels K, L).
We conducted sequencing analysis of the entire PrP coding sequence. The analysis revealed that the 7 dromedary camels shared the same genetic background, being homozygous for the wild-type prion protein allele (Table).
Conclusions
In this study, we report the detection of CPrD in dromedary camels in Tunisia, providing further evidence for the geographic distribution of the disease and supporting the hypothesis of its broader presence in North Africa. Analyses were concordant across methods; 6 of 8 suspected camels tested positive, confirming that standard tools (e.g., kits, WB, IHC, monoclonal antibodies) reliably detect PrPSc in CPrD and provide effective methods for diagnosis and surveillance.
The detection of PrPSc in all available lymph nodes, together with the relatively young age of some camels, supports the hypothesis that CPrD is a transmissible prion disease. As in scrapie and chronic wasting disease, lymphoid involvement may enable extraneural propagation and serve as a source of environmental contamination (2–5), raising concerns about prion shedding and persistence.
Despite the typically long incubation periods of prion diseases, several affected camels were relatively young, showing clinical signs at 3 years of age, when sexual maturity is only just reached (3–5 years). That finding might reflect early-life or high-dose exposure, given that extensive environmental contamination can shorten incubation (6,7). Similar patterns are reported in scrapie and chronic wasting disease, where vertical and early-life transmission contribute to infection in young animals (4,8–14).
The recent detection of sheep scrapie in the same area of Tunisia as CPrD (15) raises the question of possible links, although no similarities have yet been shown. Bioassays will be crucial to clarify strain relationships and whether environmental factors might facilitate cross-species transmission or adaptation.
Taken together, the data from Algeria and Tunisia suggest that CPrD may be endemic in certain areas. Despite the limited number of reported cases so far, the absence of active surveillance programs in those countries, unlike in Europe for scrapie and bovine spongiform encephalopathy (BSE), raises the possibility of a higher number of undetected cases. Given frequent cross-border movements and the extensive pastoral systems in which camels roam widely, contact between herds in Tunisia and herds in Algeria is frequent and has possible implications for disease circulation.
CPrD may substantially affect camel health and production, given that dromedaries are long-lived animals with extended reproductive and productive roles and are vital to pastoralist communities in arid regions. Its detection in areas where camels are central to food production underscores the need to understand its epidemiology, transmission, and long-term impact.
The emergence of a new animal prion disease also raises public health concerns. Although only BSE has shown zoonotic potential, its history highlights the necessity of monitoring all animal prion diseases. Long incubation and absence of early clinical markers complicate risk assessment, as exemplified by the decade-long delay between cattle BSE and the variant Creutzfeldt–Jakob disease epidemic in humans (2).
Given the potential implications for animal and human health and the possible socioeconomic impact in regions where dromedaries play a key role, ongoing surveillance and in-depth characterization of CPrD are essential to enhance our comprehension of its epidemiology, host range, and zoonotic potential.
Dr. Amara is emeritus professor of histology and pathological anatomy at the National School of Veterinary Medicine of Sidi Thabet in Sidi Thabet, Tunisia. He is also vice president of the Expert Committee on Animal Health at the National Center for Veterinary Surveillance in Tunisia. His primary research interests include prion diseases in ruminants and dromedaries.
