Introduction

Ethiopia has the largest livestock inventories in Africa, including more than 38,749,320 cattle, 18,075,580 sheep, 14, 858,650 goats, 456,910 camels, 5,765,170 equines and 30,868,540 chickens with livestock ownership currently contributing to the livelihoods of an estimated 80% of the rural population (CSA 2009). In Ethiopia, sheep are the dominant livestock providing up to 63% of cash income and 23% of food substance value obtained from livestock production. Sheep play a vital role as sources of meat, milk and wool for smallholder keepers in different farming systems and agro-ecologcal zones of the country (Ehui et al 2003; Kassahun 2004; Markos 2006; Getahun 2008; FAO 2009). They are also sources of foreign currency (Berhanu et al 2006). Unlike the large potential of small ruminants in the country their productivity is low. Endo-parasitic infection is known to be the main factors that affect productivity. Helminth parasites are among the causes of substantial productivity losses in sheep production.

Helminth parasites of ruminants are ubiquitous, with many tropical and sub-tropical environments of the world providing nearly perfect conditions for their survival and development. Although these parasites are widely prevalent, the clinical signs they showed in infected animals can be less obvious than signs of other livestock diseases (Hansen and Perry, 1996). The various species of gastrointestinal and pulmonary nematodes, trematodes and cestodes are known to be prevalent in Ethiopia (Ahmed et al 2007).

Respiratory diseases resulting from helminth parasites are of a great economic concern in sheep production in the highlands of Ethiopia where sheep are important livestock units. Dictyocaulidae and certain Metastrongylidae are known to exist in East Africa including Ethiopia (Tony 2006). The three respiratory parasites that cause a significant damage in small ruminant production are Dictyocaulus filaria, Protostrongylus rufescent and Muellerius capillarius. These lungworms particularly Dictyocaulus filaria can suppress the immunity of the respiratory tract (Gelagay et al 2005) and causes death, poor weight gain or loss of body weight as well as greatly affects the potential productivity of sheep industry in the areas where it is prevalent.

The incidence of respiratory helmenthiasis varies greatly from place to place depending on the relative importance of associated risk factors. Although environmental conditions are conducive for lung worm infections in sheep in the highlands of Tigray and lungworm infection is considered an important disease in this region, very limited studies have been conducted so far. Hence, this study was conducted with the objectives of to determine the prevalence of lungworm infection in sheep in Mekelle town, to identify the species of the respiratory helminthes circulating in Mekelle town and to investigate risk factors associated with lungworm infection in area.

Materials And Methods

Study area

Study animals

Study design

Sampling method and determination of sampling size

Simple random sampling technique was used. To calculate the total sample size, the following parameters were used: 95% Level of Confidence (CL), 5% desired level of precision and with the 11% (Frewengel 1995) expected prevalence of lung worm in sheep and then inflating the number (150) by 2-3 folds it was 402, to account for the effect of randomness and representativeness in sampling (Thrusfield 2005) using the formula:

Figure 1

Where,

n = required sample size

Pexp = expected prevalence

d² = desired absolute precision

Sample collection

Data Management and Analysis

Results

Over all field prevalence under Coproscopic examination

Prevalence of lungworm infection on the basis of sex

Of the total sheep examined female (253) and male (149) the overall infection prevalence of female was significantly (P<0.05) higher than males. Of there covered lungworm species, D.filaria tend to infect predominantly female sheep with significant variation

(P<0.05) than M.capillarius and P.rufuscens as indicated in the Table 1 below.

Figure 2

Table 1: Prevalence of lungworm infection based on Sex

Prevalence of lungworm infection based on Age

The overall lungworm in lamb and adult was 20.6% and 8.7%, respectively, revealing a statistically significant variation (P<0.05) between age groups. The infection prevalence

of lungworm was higher in lambs than adults. D.filaria and M.capillarius tends to infect lambs predominantly than P.rufuscens which infects adults mostly with statistically significant variation (P<0.05) (Table 2).

Figure 3

Table 2: Prevalence of lung worms in different age groups

Prevalence of lungworm infection based on Management

The overall prevalence of sheep under extensive and semi intensive managements was 14.9% and 12.6%, respectively. This revealed that lungworm prevalence did not show a significant variation (P>0.05) based on management. D.filaria prevalence was slightly higher in extensive than the other lungworm species with no significant variation (P>0.05) as indicated in the Table 3.

Figure 4

Table 3: Prevalence of lungworm infection based on management

Monthly prevalence of lungworm infection under coproscopy

Monthly prevalence of lungworm infection was found to be higher in November (22.8%) and lower in March (7.9%) with significant variation (P<0.05). The infection prevalence was equally significant in January and February as shown in Table 4 below.

Figure 5

Table 4: Monthly prevalence of lungworm infection

Over all prevalence under post mortem examination

Post mortem prevalence based on sex

Of the total male (67) and female (33) examined at postmortem, overall infection prevalence of male and female was 9% and 27.3% respectively, with significant variation of (P<0.05) prevalence under sex groups (Table 5). In addition, the infection prevalence showed significant variation (P<0.05) between each lungworm species.

Figure 6

Table 5: Prevalence of lung worm infection on the basis of sex during post mortem examination

Post mortem prevalence based on age

Of the total lamb (40) and adult (60) examined, the overall prevalence was 17.5% in lamb and 13.3% with no significant variation (P>0.05) between two age groups. In addition, the prevalence of each lungworm species is higher in lamb than adult with statistically significant variation (P<0.05) (Table 6).

Figure 7

Table 6: Prevalence of lung worm infection on the basis of age during post mortem examination

Discussion

Of the total sheep examined the overall prevalence rate of 13.4% was recorded by coprology in this study. This finding agrees with data reported by Frewengel (1995) in Tigray where the prevalence rate was found to be (11.24%), and Teferra (1993) at Deseie and Kombolcha where the prevalence was (15.47%). The overall prevalence recorded is lower than that of Mengestom (2008) in Atsbi woreda where the recorded prevalence was (21.5%), Eyob (2008) in Assella, where the prevalence rate was (72.44%), Abdukadir (2009) in and around Kombolcha, (42.96%) and Mekonen et al (2011) in Gondar town, (33.83%). Such variation in the infection rate could be attributed to the variation in altitude, rain fall, humidity and temperature in different areas of the country.

Three respiratory nematodes: D. filaria, M. capillaris and P. rufescens were identified. Dictocaulus filaria was the most prevalent (5.9%) of the total samples examined in the study area and agrees with the previous report of Uqubazgi (1990) in Hamase Awraja and Mekonen et al (2011) in Gondar town but it disagree with the report of Sissay (1996) in Bahirdar (39.3%) and Mezgebu (1995) in Addis Ababa who reported as M. capilaris is the most prevalent. These variations could be due to different transmission dynamics P. rufescens and M. capillaries and susceptibilities of the P. rufescens and M. capillaries in the two hosts. Inaddition, dry season is not suitable to intermediate host snail species (Urquhart et al 1996) and Dictocaulus filaria reach the infective stage within less time (Soulsby 1986).

In the current study female animals are more susceptible to lungworm infection than males (p < 0.05) and concur with earlier observation of Abebe (2008), Alemu et al (2006) and Mekonen et al (2011). The high prevalence in females could be related to physiology of reproduction (Craig 1998) and difference in management.

With regard to age, the overall prevalence of lungworm infection studied was higher in lambs (< 1year) than adult (>1year). This finding agrees with data reported by Radostitis et al (2007), Urquhart et al (1996) and Mekonen et al (2011) who reported that young sheep were found to be infected more than adults and this was contrary to the report by Alemu et al (2006) and Alemayehu et al (2010). This might be associated with the naturally acquired immunity against infection slowly develops with age and D. filaria infection decrease with increasing age of the animal.

The overall prevalence of lung worm infection was higher under extensive than semi intensive management system and in line with reports of Sissay (1996) and Mekonen et al (2011), this could possibly be associated with receiving an inadequate keeping and poor conditions of sheep in extensive management system, animals under well nourishment and watering lead to less risk of helminth (ILCA 1993).

Regarding the distribution of infections according to months the highest worm load of D. filarial, P. rufescens and M. capillarius were observed during November and decrease and reach to minimum during March, which coincides with previous reports (Alemayehu et al 2010; Frewengel 1995 and Bekele et al (1981). This could imply that the distribution of lungworms shown that a cool environment is very suitable for the development of D. filaria and third stage larva (L3) is resistant to cold (Kusiluka and Kambarage 2006). Inaddition survival and development of larvae is favored by low moisture and high humidity (Ayalew et al 1993).

The prevalence of lungworm infection during postmortem examination of slaughtered sheep was higher (15%) than the result obtained at coprology (13.24%). This finding is in line with the observations of Abebe (2008), Alemu et al (2006), Oncel (2000) and Yildiz (2006) but not in agreement with reports of Alemu et al 2006 and Mekonen et al 2011. This difference could possibly be attributed to the difficulty to detect these nematodes by coproscopic methods (Girisgin et al 2008) and lower number of animals examined in the postmortem examination (Alemayehu et al 2010).

Conclusion

In the present study the infection prevalence of lungworm has no significant association with management system of sheep; other epidemiological risk factors such as age, sex and season have great contribution to the prevalence of lungworm infection in the study area. Though both coproscopic and postmortem examination results confirmed the existence of lungworm infection, the coproscopic examination had limited value in terms of estimating the prevalence of lungworm infection in live sheep as compared to the postmortem examination. Grazing management and Regular strategic deworming of the whole flock (especially when infected sheep are present) with broad spectrum anthelimenthics rather than treating individuals is recommended.

Acknowledgements

The authors would like to acknowledge all staff members of Mekele Regional Veterinary Laboratory.