Carriers of Animal Diseases

Gerard Dikmans, A. O. Foster, C. D. Stein, L. T. Giltner.

Flies, ticks, and other arthropods spread and perpetuate many livestock diseases. Most are of comparatively minor concern as direct causes of injury or annoyance but, like the fever ticks of cattle, are important as reservoirs and vectors of disease-causing organisms. Some, like the tsetse flies in Africa, are of no consequence as pests, yet are a limiting factor in the production of livestock.

The remarkable ways of insects and their allies in transmitting diseases are as varied and spectacular as the diseases themselves and the vectors that transmit them. Common house flies, however benign and unspectacular they may seem, often carry on their feet and mouth parts or in their bodies the contaminating germs of everyday skin and generalized infections and sometimes even the dreaded bacilli of anthrax or the fatal toxin of botulism. Other insects, mites, and bugs of various kinds transmit the infective stages of numerous parasites, final infection usually resulting from accidental swallowing of the infected vector by a susceptible animal. Disease caused by tropical warble flies, which are next in importance to the cattle fever tick among the external parasites of livestock in Latin America and analogous to the better known warble flies of Temperate Zones, is essentially insectborne the flies attach their eggs to captured mosquitoes (Psorophora), which in turn transport the infection to cattle and other animals.

The afore-mentioned examples are illustrative only of the ways in which insects carry diseases. With the notable exceptions of tick-borne fever of cattle and tsetse fly disease, they are scarcely typical of the principal arthropod-borne diseases of livestock. As might be suspected, the chief vectors are predominantly bloodsucking species, and the diseases transmitted by them are essentially blood infections. Those vectors ordinarily spread and propagate disease in two ways. One, mechanical transmission, is the direct transfer (or its equivalent) of infective blood from diseased to healthy animals. The other, biological transmission, represents a specialized and complex relationship among vector, organism, and host, which is characterized by reproduction and structural change of the disease-causing organism within the body of the vector. Some biting flies function naturally in both ways. For a short period, probably not more than 2 hours, after feeding on the blood of a diseased animal, the dangerous organisms may be carried to healthy, susceptible animals on which the fly may chance to feed. For a longer period thereafter, from a few days to several weeks, the fly is incapable of transmitting the infection. Then it may again become infective in consequence of a biological reconstitution of the organism, culminating in the production of new infective stages in its salivary glands or other tissues.

THE ARTHROPOD-BORNE DISEASES of domestic animals are of two main kinds: Those caused by plant microorganisms and those caused by animal micro-organisms. The former comprise bacteria, spirochetes (Borrelia), Rickettsiae (Coxiella), and viruses. The latter, in part, are pathogenic protozoa, including piroplasms (Babesia), Theileriae, the trypanosomes, Leishmaniae, Leucocytozoa, and a species of Haemoproteus. Animals that recover from disease caused by some of these organisms may remain carriers, or apparently healthy animals that are dangerous seedbeds of infection, for long periods or for life.

THE BACTERIAL DISEASES that are sometimes carried by arthropods are anthrax, tularemia, swine erysipelas, and botulism (limber neck of birds) . All are spread by other means, and the role of arthropods is accidental and mechanical.

Anthrax, an acute disease caused by Bacillus anthraces, affects all classes of mammals, including man. Infections in livestock are generally acquired during grazing. Incidence is especially high during the fly season, and outbreaks in cattle have been ascribed to fly transmission. The vectors are the black horse fly and other horse flies, the stable fly, mosquitoes (Psorophora sayi and Aedes sylvestris) , and several non-biting species, including the house fly and blow flies (Calliphora). The ear tick and even ants also have been suspected.

Tularemia, caused by Bacterium tularense, is primarily an infection of small wild animals, such as rabbits, squirrels, rats, mice, woodchucks, opossums, and grouse, but it can be transmitted to man, sheep, swine, dogs, and cats. The disease is transmitted commonly by contact, sometimes by the ingestion of contaminated food and water, and occasionally by the bites of ticks, flies, lice, and bed bugs.

Swine erysipelas is a prevalent infectious disease that biting flies may spread from pig to pig. We do not know the extent to which it is insect-borne, but such transmission has been demonstrated experimentally with the stable fly. The infectious organism, Erysipelothrix rhusiopathiae, is an invader of the blood, joint membranes, and other tissues. Death is uncommon, but condition and marketability are seriously affected.

Botulism, or limber neck of chickens, is a fatal condition induced by the potent toxin of Clostridium botulinum. Ordinarily it follows ingestion of canned vegetables that have become contaminated with the organism. At times chickens become ill and die from ingesting blow fly maggots that have developed in contaminated meat.

SPIROCHETES affect all animals and get from host to host in many ways. They are minute, spiral organisms, which show some affinities to the protozoa but are commonly regarded as bacteria. We mention two examples, both tick-borne.

Borrelia theileri is responsible in South Africa for a benign, febrile disease of cattle, sheep, and horses. It occurs in the blood stream and is transmitted biologically by one- and two-host ticks, and possibly by others.

Borrelia anserina causes relapsing fever, or spirochetosis, in chickens, turkeys, ducks, and geese. It occurs in Asia, Africa, South America, and elsewhere. It has been found in a few epizootics of turkeys in the United States. It causes a rapidly fatal blood infection. The fowl tick and probably the chicken mite are vectors. Mosquitoes also are suspected.

RICKETTSIAE, which are intermediate between bacteria and viruses, cause many serious diseases of man as well as animals. One of these, Q fever, caused by Coxiella burnetii, is a disease of man, but cattle probably are the source of most human infections. The disease is recognized now in many parts of the world, including the United States. Ticks carry the organisms, and natural infections have been found in numerous species (Rocky Mountain wood tick, Pacific Coast tick, lone star tick, brown dog tick, and others).

A few other rickettsia) infections of livestock, such as heart water fever of ruminants, occur outside the United States. Ticks, as far as we know, are the only vectors.

VIRUS DISEASES are numerous, and many are mechanically transmitted, wholly or in part, by arthropods, particularly biting flies.

The ones transmitted mainly by these agents are equine infectious anemia, infectious equine encephalomyelitis, African horse sickness, Japanese B encephalitis, louping ill of sheep, Nairobi disease of sheep, blue tongue of sheep, and rift valley fever. At times arthropods are presumably instrumental also in the transmission of fowl pox, swine pox, myxomatosis of rabbits, and infectious enteritis of cats.

Equine infectious anemia, or swamp fever, occurs throughout the world. It destroys the working efficiency of thousands of horses, mules, and donkeys. Its natural spread is imperfectly understood, but the disease is readily produced experimentally in susceptible animals by injection of infectious material, such as blood or other tissue fluids from infected animals. Under experimental conditions, the virus has been transmitted by horse flies, stable flies, mosquitoes (Psorophora columbine), and biting lice. The probability that direct mechanical transmission by biting flies commonly occurs is emphasized both by the summer intensity of the disease and the persistence of the virus in the blood of infected hosts.

Infectious equine encephalomyelitis is caused by so-called Eastern, Western, and Venezuelan types of virus in North and South America and neighboring islands. Man is susceptible to all types. Natural reservoirs, particularly birds, are a probable source of infection to mosquitoes, which are the common vectors of the virus. The virus types have been recovered from or experimentally transmitted by a large number of arthropod species, among them the yellow-fever mosquito, salt-marsh mosquitoes (Aedes), other mosquitoes (Culex, Culiseta, and Mansonia), the bloodsucking conenose, the Rocky Mountain wood tick, the chicken mite, and chicken lice (Menopon pallidum and Eumenacanthus stramineus). In the tick, the virus is present at all stages of development, but scientists do not know yet whether the virus passes through the egg.

African horse sickness, an acute and virulent infection of equine species in central and southern Africa, is presumed to be transmitted by arthropods, mainly because of apparently convincing evidence that the disease does not pass directly from animal to animal. Mosquitoes, horse flies, midges, and other insects have been suspected as vectors.

Japanese B encephalitis, a fatal virus infection of man, is not a disease of domestic animals, but domestic animals, especially the horse, are dangerous reservoirs of the virus. It occurs in the Far East, where it is transmitted biologically by hibernating culicine mosquitoes (the southern house mosquito and others).

Several virus infections of sheep, all of which affect other animals in some degree and most of which are transmissible to man, are biologically and exclusively spread by arthropods. They are serious diseases in several parts of the world. Louping ill, transmitted by the castor bean tick (Ixodes ricinus), is prevalent in the British Isles. Rift valley fever, transmitted by mosquitoes (Eretmopodites), and Nairobi disease, carried by ticks (Rhipicephalus appendiculatus), occur in Kenya, British East Africa. Blue tongue, a more widespread disease, is carried by midges (Culicoldes).