Total infection rate of strongyle nematodes in the sheep
In this study, a total of 4014 sheep from 180 family-owned farms located in the southwestern region of Ordos, Inner Mongolia, China, were investigated between March 2017 and April 2019. The number of sheep investigated in 3 consecutive years was 1496, 1256, and 1262, and the overall prevalence of nematode infection in each year was 84.4% (1262/1496), 36.9% (464/1256), and 42.3% (534/1262), as shown in Table 1. Therefore, the prevalence of sheep nematodiasis in this area is relatively severe.
In 2017, the epidemic of GINs in the fine-wool sheep in the investigated area was very severe; however, in 2018 and 2019, the survey results showed that the nematode infection rate decreased significantly in the sheep (Table 2). The main reason is that, during the first epidemiological investigation, we performed a comparative study of the deworming effects of several different anthelmintics; 2 types of antihelminthics with high efficacy against the predominant nematode species were screened, and the farmers were encouraged to use these drugs.
Table 1 Total gastrointestinal nematode infection rate in Ordos fine-wool Merino sheep
Investigation period
|
Number of investigated sheep
|
Number of infected sheep (EPG ≥ 300)
|
Infection rate (%)
|
March to April 2017
|
1496
|
1262
|
84.4
|
March to April 2018
|
1256
|
464
|
36.9
|
March to April 2019
|
1262
|
534
|
42.3
|
Table 2 Dosing regimen of 6 anthelmintics commonly used for controlling nematodes
Group
|
n
|
Drug
|
Dose (mg/kg)
|
Route of administration
|
1
|
20
|
Albendazole tablets
|
15
|
po
|
2
|
20
|
Ivermectin injection
|
0.2
|
sc
|
3
|
20
|
Doramectin injection
|
0.2
|
sc
|
4
|
20
|
Levamisole tablets
|
7.5
|
po
|
5
|
20
|
Nitroxynil injection
|
0.3
|
sc
|
6
|
20
|
Closantel injection
|
5.0
|
sc
|
7
|
20
|
Negative control
|
——
|
——
|
On the basis of the egg morphology, typical adult structure, and third-stage larva (L3) morphology, a total of 7 nematodes were initially identified: Haemonchus contortus, Nematodirus spp., Oesophagostomum spp., Trichostrongylus spp., Chabertia spp., Ostertagia spp., and Trichuris spp. Haemonchus contortus and Nematodirus spp. were the predominant species in the infected sheep. The different nematodes and infection rates recorded in 2017 are listed in Table 3. Images of the eggs and adult nematodes observed using a microscope and Haemonchus contortus on the surface of the abomasum mucosa after the autopsy are shown in Figs. 1 and 2, respectively.
Table 3 Nematode species and infection rates in Ordos fine-wool Merino sheep in 2017
Nematode species
|
Total number of samples
|
Positive samples (total EPG ≥ 300)
|
Infection rate (%)
|
Parasitic site
|
Haemonchus contortus
|
1496
|
1261
|
84.3
|
abomasum
|
Nematodirus spp.
|
1496
|
980
|
65.6
|
small intestine
|
Oesophagostomum spp.
|
1496
|
290
|
19.4
|
colon
|
Trichostrongylus spp.
|
1496
|
254
|
17.0
|
small intestine
|
Chabertia spp.
|
1496
|
196
|
13.1
|
large intestine
|
Ostertagia spp.
|
1496
|
106
|
7.1
|
abomasum, small intestine
|
Trichuris spp.
|
1496
|
79
|
5.3
|
caecum
|
The eggs of Haemonchus contortus (84.3%) were predominantly found, followed by those of Nematodirus (65.6%), Oesophagostomum (19.4%), and Trichostrongylus (17.0%). The eggs of Ostertagia (7.1%) and Trichuris (5.3%) were the least prevalent. Therefore, nematodes that adversely affect fine-wool sheep in the southwestern region of Ordos are Haemonchus contortus, which parasitizes the abomasum, and Nematodirus, which parasitizes the small intestine.
GIN infection rate in sheep that graze in different types of pasture
Because of the large differences in pasture characteristics and landform types in the southwestern region of Ordos, the species and distribution of nematodes in this region are affected to some extent. Therefore, the experimental sheep were divided into 4 different groups: sheep that grazed freely in sandy pasture, low-lying pasture, and hilly pasture as well as sheep raised in a pen. Then, the epidemiology of sheep nematodiasis was investigated (Table 4).
The survey results for 2017 showed that the nematode infection rates in the sheep from 4 different investigated areas were quite high, and the highest nematode infection rate (88%) was found in sheep that grazed in low-lying pastures (Table 4). However, the nematode infection rate in the sheep raised in a pen was relatively low. The survey results for 2018 and 2019 showed that the nematode infection rates in all 4 sheep groups decreased significantly, mainly because of the administration of suitable anthelmintics under our guidance.
Table 4 Nematode infection rate in sheep that graze in different types of pastures
Pasture type
|
Infection rate (%)
|
March to April 2017
|
March to April 2018
|
March to April 2019
|
Sandy pasture
|
82.7 (311/376)
|
35.0 (112/320)
|
38.4 (123/320)
|
Low lying pasture
|
88.0 (330/375)
|
43.9 (137/312)
|
51.0 (158/310)
|
Hilly pasture
|
85.0 (323/380)
|
36.4 (113/310)
|
42.9 (133/310)
|
Raised in a pen
|
76.3 (286/375)
|
32.5 (102/314)
|
36.9 (118/320)
|
Anthelmintic efficacy
According to the epidemiological survey for GINs in the sheep, nematode infections were very severe in the study area. Therefore, to examine the deworming effects of extensively used anthelmintic drugs and screen for drugs that effectively kill predominant GIN species in the investigated area, a total of 140 severely infected sheep (PGE > 2000) were selected and randomly allocated to 7 groups and administrated different drugs (Table 1). On the 14th day after administration, fecal samples were collected and quantitatively analyzed using the modified McMaster technique (Table 5).
Table 5 Fecal egg count reduction in each sheep group on the 14th day after drug administration
Group
|
Drug
|
n
|
Pre-treatment EPG (mean)
|
14th day post-treatment EPG (mean)
|
FECR (%)
|
1
|
Albendazole
|
20
|
2513
|
2272
|
9.6
|
2
|
Ivermectin
|
20
|
3523
|
3396
|
3.6
|
3
|
Doramectin
|
20
|
3212
|
3026
|
5.8
|
4
|
Levamisole
|
20
|
3135
|
2476
|
79.0
|
5
|
Nitroxynil
|
20
|
2359
|
23
|
99.0
|
6
|
Closantel
|
20
|
2705
|
22
|
99.2
|
7
|
Untreated
|
20
|
3330
|
3753
|
-12.7
|
The fecal egg count reduction (FECR) on the 14th day after administration showed that albendazole, ivermectin, and doramectin had no anthelmintic effects on GINs predominant in the investigated area (Table 5). The average FECR in these 3 sheep groups was less than 10%. Levamisole also had a poor deworming effect on the nematodes, with an FECR of 79% after administration. However, nitroxynil and closantel injections had strong anti-nematodal effects, and the FECR was more than 99% after administration.
GIN drug resistance
The anthelmintic efficacy indicated that avermectins, which are widely used anthelmintics, have no deworming effects on predominant GIN species in the examined area. To verify the resistance of sheep GINs to ivermectin and doramectin, 60 sheep naturally infected with GIN (eggs per gram [EPG] ≥ 2000) were selected and randomly allocated to 6 groups, with 10 sheep in each group. Then, the experimental sheep were subcutaneously injected with 3 different doses of ivermectin and doramectin. Fecal samples were collected on the 14th day after administration for quantitative detection of EPG to evaluate the anthelmintic efficacy.
The results showed that doubling the ivermectin dose had almost no deworming effects on GINs predominant in the sheep of the examined area (Table 6). On the 14th day after ivermectin administration, the FECR in both the therapeutic dose group and increased dose group was less than 15%. Therefore, GINs in this area have become highly resistant to ivermectin.
Table 6 Comparison of the anthelmintic efficacy of different doses of ivermectin
Dose (mg/kg)
|
n
|
Pre-treatment EPG (mean)
|
14th day post-treatment EPG (mean)
|
FECR (%)
|
0.2
|
10
|
2007
|
1907
|
5.0%
|
0.3
|
10
|
3075
|
2690
|
12.5%
|
0.4
|
10
|
3420
|
2948
|
13.8%
|
The therapeutic dose and increased dose of doramectin had no deworming effects on the predominant GIN species in the examined area (Table 7). The anthelmintic effect of the therapeutic dose of doramectin was less than 10%, and that of the increased dose of doramectin was less than 20%. This indicates that, although doramectin has no history of clinical use in the area, it also has no anthelmintic effects on digestive tract nematodes because both doramectin and ivermectin belong to the same class of avermectins with the same anti-parasitic mechanism. Thus, the predominant GIN species already possess high cross-resistance to ivermectin and doramectin.
Table 7 Comparison of the anthelmintic efficacy of different doses of doramectin
Dose (mg/kg)
|
n
|
Pre-treatment EPG (mean)
|
14th day post-treatment EPG (mean)
|
FECR (%)
|
0.2
|
10
|
2550
|
2322
|
7.0%
|
0.3
|
10
|
2293
|
1893
|
17.4%
|
0.4
|
10
|
2963
|
2430
|
18.0%
|