Toxocara vitulorum: Major impediment of calf rearing

Pashu Sandesh, 22 September 2021

Vivek Agrawal, A.K. Jayraw, Mukesh Shakya, G.P. Jatav, Deepak Gangil, Nirmala Jamara and Nidhi Singh

Nanaji Deshmukh Veterinary Science University- College of Veterinary Science and Animal Husbandry, Mhow(Indore) 

1. Introduction

Toxocara (syn. Neoascaris) vitulorum is a species of ascaridae nematodes that inhabits the small intestine of bovine animals including buffaloes, cattle, and zebu, and is especially common in the tropical and subtropical geographical areas. It is amongst the most destructive parasites of calves where the larvae undertake migration through to cause damages in many organs like the liver and intestines causing about 30–80% estimated losses in 2–3 months old calves. When the infection is not controlled in the field, the prevalence can reach 100% in calves and deaths frequently occur when associated with poor nutrition. Adult animals are believed to be free of mature adult worms and therefore, lack evidence of infection like eggs in the faeces unlike their calf counterparts because of immunity that help to arrest all somatically migrating larvae in their organs and tissues as well as the expulsion of adult worms. This disease in calves is recognized as the topmost cause of morbidity and mortality. The control of this parasite is not easy because the larvae migrate in the host tissues remaining as dormant or hypobiotic parasites and can survive for a long time. T. vitulorum was recorded in cattle from different geographical areas spanning Africa, East Asia, the Indian subcontinent, Europe, and North America, with various prevalence rates as 16.7% in India 

2. Clinical Symptoms

Usually, infection with T. vitulorum is subclinical, even though heavy infections with a large number of worms result in severe enteritis and diarrhoea, causing considerable morbidity and mortality particularly in the age group of 1–3 months old cattle and buffalo calves. Without proper diagnosis (usually misdiagnose with other diarrhoea-causing viral, bacterial, and protozoan pathogens) and adequate treatment, high fatality rates in bovine claves cause serious economic losses. 

3. Transmammary and transplacental routes of transmission of T. vitulorum infection

 Males and non-breeding females appear to be dead-end hosts of T. vitulorum infection. However, dormant or hypobiosed larvae within somatic tissues of adult pregnant females migrate to the udder some eight days to parturition due to changes in the dam's reproductive hormonal profiles or via a yet to be clearly understood mechanisms for their excretion in milk within the first 11 days post parturition although milk excretion 3–4 weeks thereafter has been reported. These dormant or hypobiosed larvae present in somatic tissues of infected dams can potentially infect calves over 2–3 consecutive parturitions.

Ingestion of eggs has barely any role in the transmission of T. vitulorum as larvae undergo trans-somatic migration in infected dams reaching to the mammary glands, and thus to the milk. Therefore, the galagtogenic (via milk) transmission of T. vitulorum has been reported as the main source of toxocariasis cycling between newborn calves and dams of cattle and water buffaloes. Moreover, the presence of T. vitulorum larvae in milk represents a risk factor for visceral larvae migrans, due to ingestion of unpasteurized milk of the infected animals. The transmammary route is the principal mode of infection in the case of bovine toxocariasis. Nevertheless, we cannot exclude the prenatal (transplacental) route as a second most important infection source, particularly when larvae of T. vitulorum were detected in large numbers in the fetal liver and lungs of the pregnant buffaloes. Four points of the life cycle of T. vitulorum were reported as important for the epidemiology of the disease: (i) the long period of survival of eggs in the environment; (ii) the long period of survival of larvae in the tissues of the maternal host; (iii) transmission through the mother; and (iv) the large number of eggs produced over a short period during the early life of the calf.  

4. Treatment Strategies 

Lack of knowledge among farmers is likely to accelerate the transmission of T. vitulorum based upon specific life cycle traits including dormancy of larvae in cows and galactogenic transmission. To protect cattle from T. vitulorum infection, farming conditions must be improved in addition to the use of anthelmintics or alternative control strategies including the control of dams.  Once the infection is present in a herd, it is very difficult to eliminate the infection, due to the fact that cows serve as a reservoir of larval stages and infect their calves via milk, while eggs can survive for at least two years on pasture or in stables.

If calves are treated when 10 days old with an anthelmintic effective against immature parasites, reinfection by the transmammary route is unlikely to recur. Consequently, there will be no contamination of the environment with eggs and the pathogenic effects of large numbers of immature parasites and mature parasites, will be precluded. There are, therefore, two advantages to treating calves with an appropriate anthelmintic when 10-14 days old. To handle the endemic problem of T. vitulorum, treat the herd with Ivermectin (0.2 mg kg-1 subcutaneously) and Febendazole (7.5 mg kg-1 orally) to be repeated in a fortnight thereafter, especially in calves within their first few days of life and thereafter. This treatment schedule should be continued for at least five consecutive years. However, anthelmintic treatment of adult cattle is usually ineffective due to the drug’s inability to reach the hypobiosed larvae but pregnant cows should be dewormed some three (3) weeks before parturition to guard against prenatal infection When levamisole was injected into pregnant buffalo cows between 7 and 8 months of pregnancy or 21 days before the parturition day, the level of infection of calves was reduced but this did not eliminate the infection.