Worms are parasites that live in, or sometimes on, a host animal at the expense of the host. They can occur in the gut or other internal organs, or in some cases, in locations such as the skin. Parasitic worms include roundworms (nematodes) and flat worms (flukes and tapeworms).
Spread of infection
Because worms live inside animals, they have, by necessity developed complex life cycles to spread infection to new hosts. This almost always involves a life stage outside the host that is exposed to often hostile environmental conditions.
For locations: Worms thrive in hosts where climatic conditions support their life stages out in the environment. Consequently, different worms are predominant in different climatic regions at different seasons of the year. Worms that use more than one host in their life cycle are commonly found where the climatic requirements of the parasite and its chosen intermediate host are the same.
For Hosts: Some worms are considered to be host specific i.e. they can only develop in a specific type of host, while others are less specific and can develop in a range of hosts. For example, most sheep worm species are not found in cattle, but cattle and goats share many parasites.
Inside hosts: Once inside hosts, worms move to their predilection sites i.e. preferred locations that support their developmental needs. Lungworms, for example, cannot live in the gut but their larvae pass through the gut during part of their life cycle.
Severity of infection
The severity of infection is dependent on the number and type of parasite (worm burden, and level of challenge with larval worms), and the ability of the host to both limit (resistance) and withstand infections (resilience). Worm-related factors include the relative harmfulness (pathogenicity) of worm species, the host’s genetically-determined resistance and resilience to infection, the age and class of the host, the host’s nutritional state, and management practices that predispose animals to worm intake (such as high stocking rates of young worm-susceptible cattle).
The effects of worms on cattle
Worms in the gastro-intestinal tract (gut) have the most severe pathogenic (deleterious) effects on cattle, which include:
Reduction of feed intake: The reduction of appetite of animals affected with many worm species, and consequent decreased feed intake, is an important contributor to the reduced growth rates or weight loss seen in severely worm-affected cattle.
Changes to metabolic efficiency: The cycling of nutrients within cells, especially nitrogen and phosphorous from proteins, is impaired with many worm infections, reducing the efficiency of nutrient use and therefore growth rates.
Tissue damage: Damage to the lining of the gut by worms during feeding can cause a loss of cells and their contents, requiring replacement (and hence additional protein especially), and also may be involved with the development of diarrhoea (scouring).
Immunological effects: The immune response to worm infection has a significant nutrient cost, especially for energy, contributing to a reduction in resources for growth and tissue repair. Inflammation of the gut due to immunological reactions caused by cellular damage, and an increased efflux of fluid, is also involved in the development of scouring.
Blood feeding: Blood feeding activities of some worm species (e.g. barber’s pole worm) cause severe haemorrhage that quickly induces anaemia, bottle jaw and un-thriftiness if stock can’t replace red blood cells faster than they are being lost to the parasite.
Disease patterns in cattle
Subclinical disease: Losses in animal productivity including decreased weight gains and body condition, altered carcass composition, a fall in milk production and lower conception rates are just some of the subclinical effects that go unnoticed unless they are regularly measured. Subclinical losses occur with small and moderate worm burdens and are a major, but often unrecognised, economic loss to the producer.
Clinical disease: Visible signs such as a loss of weight and body condition, poor growth rates, diarrhoea, anaemia and roughness of coat are clinical effects due to heavy worm infections. They are seen after productivity losses have already occurred.
Immunity to worms in cattle
A strong natural immunity to worms develops in most cattle from 15 - 18 months of age (except against liver fluke infection). Immunity begins to develop by about 4 months of age and, depending on the relative exposure to worm larvae and animal genetic factors, may be completed by 12 months of age, although for others it may take many months longer. Immunity can sometimes break down in adult stock during times of stress such as pregnancy and lactation, periods of adverse conditions and a poor plane of nutrition (although this is not as marked as occurs in sheep). Adult cows develop a very strong worm immunity (drenches are rarely required on a mob scale), but bulls are far more susceptible to infection.
The immunological response in cattle develops after worm larvae are first recognised as foreign organisms by antibodies in the bloodstream. The response leads to the production of a range of specialised cells and substances that are toxic to worms. The combined response can kill worm larvae, preventing the build-up of worm numbers, and over time, also cause expulsion of the adult worms. Worms which survive the immune attack generally produce fewer eggs, which explains the very low worm egg counts of adult cows. After a period of exposure to worm larvae an ‘immune memory’ develops, so that further exposure provokes a strong on-going worm immunity. Except under conditions that can reduce the ability to mount an immune response, including nutritional, hormonal and genetic factors, most cattle maintain a life-long immunity to worms that prevents damaging worm burdens from establishing.