Bovine Neosporosis: Causes and Control
Prabhakar Shil, Ph.D. Scholar, Department of Veterinary Parasitology, College of Veterinary Science and Animal Husbandry, Anand Agricultural University, Anand Gujarat-388001
Abstract: Neospora caninum infection in dairy cattle is now considered as a major cause of abortion and economic losses to the farmers worldwide. Dogs and coyotes are its definitive hosts and main reservoirs of infection. The route of infection in cattle may be through vertical or horizontal transmission. Vertical transmission appears to be the major route of infection in cattle, there is also a role for postnatal transmission, involving a definitive host. At present, several IFAT, ELISA and DAT kits for N. caninum diagnosis are commercially available for diagnosis. Presently, the control of dogs and their access to bovine tissues, particularly potentially infected placentae and other foetal tissues, appear to be the most prudent control methods along with strict hygienic measures.
Key words: N. caninum, Vertical transmission, Abortion, Dogs etc.
Livestock plays a vital role for strengthening of rural economy in India. Abortion and neonatal mortality causes major loss in livestock operations. Neosporosis caused by Neospora caninum is one of the major cause of abortion in dairy and beef cattle worldwide (Dubey et al., 2007). In India, neosporosis has been reported in cattle, water buffaloes and dogs from Northern India, Mithuns from North-Eastern India and Southern Peninsular India (Meenakshi et al. 2007; Rajkhowa et al., 2008; Sharma et al., 2008; Sengupta et al., 2012). The protozoan belongs to phylum Apicomplexa and family Sarcocystidae. The parasite was first isolated from five Boxer puppies with congenital encephalitis and myositis in Norway in 1984 for the first time (Bjerkas et al., 1984).
Different species of canines such as domestic dogs (Canis familiaris), coyotes (C. latrans), dingoes (C. domesticus) and grey wolves (C. lupus) are definitive as well as intermediate host for N. caninum (Dubey et al., 2005). A broad range of domestic and wild ruminants, equids, camels, rodents and avian species serve as intermediate host (Dubey and Schares, 2011). Life cycle starts with definitive host shedding unsporulated oocysts undergoes sporogony in environment to become infective, when ingested by an intermediate host the sporozoites are released and transform into tachyzoites, proliferate and disseminate to different organs. When host develops a protective immune response, tachyzoites turns into slowly proliferating bradyzoites, develop within the tissue cyst, generally confined to nervous tissue and skeletal muscle, with a persistent phase of infection. Definitive host acquire infection by ingestion of infected tissues (Gondim et al., 2002).
Transmission may occur in two ways: horizontally by the ingestion of oocysts or vertically via the placenta in pregnant cows. Trans-placental transmission of the infection is either endogenous or exogenous. Endogenous is caused by the reactivation of tissue cysts in a previously (persistently) infected animal and reproductive failure has an endemic pattern. In contrast, exogenous occurs following a primary horizontal infection (i.e. oocysts are ingested by a pregnant animal) with an epidemic pattern of abortion (Dubey et al., 2007). Latent infection in cows may be acquired by vertical or horizontal route of transmission. However, the mechanism of reactivation of latent infection is still not known (Trees and Williams, 2005). Transmission to the definitive host occurs mainly following ingestion of foetal or placental tissues infected with tissue cysts. There is a strong association between the occurrence of abortion outbreaks, sero prevalence to N. caninum, the presence and the number of dogs with access to cattle or fodder (Hobson et al., 2005). Other risk factors such as the presence of other species of possible definitive hosts, immunosuppressive host as well as age, breed, climatic factors and herd location plays distinct role (Haddad et al., 2005).
Abortion is a predominant clinical sign of neosporosis in cattle at any age (Dubey, 2003). There is direct association between the level of antibodies to N. caninum in cows and occurrence of abortion (De Meerschman et al., 2002). Neosporosis-induced abortions generally occur at 5-6-month of gestation. Foetuses may die in utero, be resorbed, mummified, autolyzed, stillborn, born alive with clinical signs, or born clinically normal but chronically infected. Infected calves may have neurologic signs, underweight, unable to rise, hind limbs or forelimbs or both may be flexed or hyperextended, exophthalmia or asymmetrical appearance in the eyes. Neurological signs include ataxia, decreased patellar reflexes, and loss of consciousness. However, up to 95% of calves born congenitally-infected from seropositive dams remain clinically normal. Reports indicated that in persistently infected cattle vertical transmission is more efficient in younger than older cows. Neospora caninum occasionally causes birth defects including hydrocephalus and narrowing of the spinal cord (Dubey, 2003).
To diagnose bovine neosporosis, clinical history and epidemiological data are very important. Facts like abortion pattern, foetal age and status should be considered (Anderson et al., 2000). Demonstration of N. caninum in lesions is the only way to make a definitive diagnosis of N. caninum-induced abortion. However, it is difficult to find N. caninum in mummified fetuses (Wouda, 1998). In vivo diagnosing N. caninum infection aimed at detecting specific antibodies. Several assays including: Indirect fluorescent antibody test (IFAT), various enzyme-linked immunosorbent assays (ELISAs), immunoblotting (IB) and direct agglutination tests (DATs) have been developed. For diagnosis in live animals, detection of antibodies in serum or milk has been shown to be the best option both at the herd and the individual level. At present, several IFAT, ELISA and DAT kits for N. caninum diagnosis are commercially available (Atkinson et al., 2000, Reichel and Pfeiffer 2002). An immuno-chromatographic test (ICT) with recombinant NcSAG1 has recently been developed for the rapid detection of antibodies to N. caninum in cattle (Liao et al., 2005). The rate of seroprevalence may vary widely depending on factors such as the methods of serological test used, management practices, production system, geographical locations, etc. (Dubey et al., 2007). It was also observed that the affected animals may remain seropositive even up to 4 years (Chunlun et al., 2007). Dam serologic examination can aid but cannot establish diagnosis, because N. caninum antibody titers in aborting and non-aborting cattle may be similar (Dubey and Schares, 2006).
Treatment and Control.
There is no specific treatment for Bovine neosporosis nor there is any vaccine available in market. Supportive treatment as per possible is provided.
It is almost impossible to eradicate neosporosis from a herd. But measures must be taken to control the infection
- a) Farm dogs, cats other wild canids are potential source of infection. So prevention must include keeping cattle food and water away them.
- b) High hygiene standards within the farm area. Oocyst-contaminated pastures, fodder, and drinking water are regarded as potential sources for postnatal infection of cattle.
- c) Disposal of placental membranes and aborted or dead calves as soon as possible.
However, vertical transmission is far more important to control. Over 90% of calves born to mothers with antibodies to Neospora will have been infected in the womb. To control Vertical transmission, identify infected cattle with serological tests. All cattle with antibodies to N.caninum are sources of infection Additionally, cattle and 20 times more likely to abort between 90 and 270 days of pregnancy. Therefore, select only seronegative cattle for breeding.
Conclusion:
Recent studies have enlightened annual economic losses due to Neospora caninum abortions worldwide, therefore it is high time for control of bovine neosporosis. However, with no specific medication and availability of vaccines control options for N. caninum have become more restricted. Researchers are working on alternative efficacious vaccines, till than control options presently are ‘Test-and-cull’ or ‘not breeding from seropositive dams’. Treatment with a coccidiostat has been suggested as the only viable option. Dogs are the key animal to the life cycle of N. caninum and therefore avoiding them in farm premises as per as possible is advised along with strict biosecurity measures.
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